Download to read offline
![3
Introduction
reliability is impaired. In systems where fibre loss is the limiting factor, an
in-line optical amplifier can be used instead of a regenerator. As the in-line
amplifier has only to carry out one function (amplification of the input
signal) compared to full regeneration, it is intrinsically a more reliable and
less expensive device.
Ideally an in-line optical amplifier should be compatible with single-mode
fibre, impart large gain and be optically transparent (i.e. independent of the
input optical signal properties).
In addition optical amplifiers can also be useful as power boosters, for
example to compensate for splitting losses in optical distribution networks,
and as optical preamplifiers to improve receiver sensitivity. Besides these
basic system applications optical amplifiers are also useful as generic optical
gain blocks for use in larger optical systems. The improvements in optical
communication networks realised through the use of optical amplifiers
provides new opportunities to exploit the fibre bandwidth.
There are two types of optical amplifier: The SOA and the OFA [1-6]. In
recent times the latter has dominated; however SOAs have attracted renewed
interest for use as basic amplifiers and also as functional elements in optical
communication networks and optical signal processing devices.
1.2 BRIEF HISTORY OF SEMICONDUCTOR OPTICAL
AMPLIFIERS
The first studies on SOAs were carried out around the time of the invention
of the semiconductor laser in the 1960’s. These early devices were based on
GaAs homojunctions operating at low temperatures. The arrival of double
heterostructure devices spurred further investigation into the use of SOAs in
optical communication systems. In the 1970’s Zeidler and Personick carried
out early work on SOAs [7-8]. In the 1980’s there were further important
advances on SOA device design and modelling. Early studies concentrated
on AlGaAs SOAs operating in the 830 nm range [9-10]. In the late 1980’s
studies on InP/InGaAsP SOAs designed to operate in the 1.3 and 1.55
regions began to appear [11].
Developments in anti-reflection coating technology enabled the
fabrication of true travelling-wave SOAs [12]. Prior to 1989, SOA structures
were based on anti-reflection coated semiconductor laser diodes. These
devices had an asymmetrical waveguide structure leading to strongly
polarisation sensitive gain.
In 1989 SOAs began to be designed as devices in their own right, with the
use of more symmetrical waveguide structures giving much reduced
polarisation sensitivities [13]. Since then SOA design and development has](https://image.slidesharecdn.com/87866-250225213249-f35702e2/85/Semiconductor-Optical-Amplifiers-1st-Edition-Michael-J-Connelly-16-320.jpg)
![5
Introduction
It is forecast that the consumption of SOAs will expand rapidly from $48
million in 2000 to $903 million in 2010 [14]. The main near-term use of
SOAs will be as basic amplifiers in wavelength division multiplexed
(WDM) and other digital optical communication links, and as switching
elements in all-optical switches and optical crossconnects. As the above
figures show, the future for SOAs is bright, and even more applications of
the device will arise as the technology matures and manufacturing costs
decrease.
REFERENCES
1. S. Shimada and H. Ishio, Eds., Optical Amplifiers and their Applications, John Wiley
(1992).
2. H. Ghafouri-Shiraz, Fundamentals of Laser Diode Amplifiers, John Wiley (1995).
3. Y. Yamamoto, Ed., Coherence, Amplification and Quantum Effects in Semiconductor
Lasers, John Wiley (1991).
4. E. Desurvire, Erbium-Doped Fibre Amplifiers: Principles and Applications, John Wiley,
New York (1994).
5. M. J. O’Mahony, Semiconductor optical amplifiers for use in future fibre systems,
IEEE/OSA J. Lightwave Technol., 6, 531-544 (1988).
6. N. A. Olsson, Semiconductor optical amplifiers, IEEE Proc., 80, 375-382 (1992).
7. G. Zeidler and D. Schicetanz, Use of laser amplifiers in glass fibre communication systems,
Siemens Forch. u. Entwickl. Ber., 2, 227-234 (1973).
8. S. D. Personick, Applications for quantum amplifiers in simple digital optical
communication systems, Bell Syst. Tech. J., 52, 117-133 (1973).
9. Y. Yamamoto, Characteristics of AlGaAs Fabry-Perot cavity type laser amplifiers, IEEE J.
Quantum Electron., 16, 1047-1052 (1980).](https://image.slidesharecdn.com/87866-250225213249-f35702e2/85/Semiconductor-Optical-Amplifiers-1st-Edition-Michael-J-Connelly-18-320.jpg)
![10 Semiconductor Optical Amplifiers
consequence of the amplification process and cannot be avoided; hence a
noiseless SOA cannot be created. Stimulated processes are proportional to
the intensity of the inducing radiation whereas the spontaneous emission
process is independent of it.
2.2.1 Spontaneous and induced transitions
The gain properties of optical semiconductors are directly related to the
processes of spontaneous and stimulated emission. To quantify this
relationship we consider a system of energy levels associated with a
particular physical system. Let and
per unit volume of the system characterised
be the average number of atoms
by energies and
respectively, with If a particular atom has energy then there is a
finite probability per unit time that it will undergo a transition from to
and in the process emit a photon. The spontaneous carrier transition rate per
unit time from level 2 to level 1 is given by
where is the spontaneous emission parameter of the level 2 to level 1
transition. Along with spontaneous emission it is also possible to have
induced transitions. The induced carrier transition rate from level 2 to level 1
(stimulated emission) is given by
where is the stimulated emission parameter of the level 2 to level 1
transition and the incident radiation energy density at frequency The
induced photons have energy The induced transition rate from
level 1 to level 2 (stimulated absorption) is given by
where is the stimulated emission parameter of the level 2 to level 1
transition. It can be proved, from quantum-mechanical considerations [1,2],
that](https://image.slidesharecdn.com/87866-250225213249-f35702e2/85/Semiconductor-Optical-Amplifiers-1st-Edition-Michael-J-Connelly-23-320.jpg)
![12 Semiconductor Optical Amplifiers
2.2.2 Absorption and amplification
By using the expression for the stimulated transition rates developed in
Section 2.2.1, it is now possible to derive an equation for the optical gain
coefficient for a two level system. We consider the case of a monochromatic
plane wave propagating in the through a gain medium with cross-
section area A and elemental length The net power generated by a
volume of the material is simply the difference in the induced transition
rates between the levels multiplied by the transition energy and the
elemental volume i.e.
This radiation is added coherently to the propagating wave. This process of
amplification can then be described by the differential equation
is the material gain coefficient given by
(2.13) implies that to achieve positive gain a population inversion
must exist between level 2 and level 1. It also shows, by the presence of
that the process of optical gain is always accompanied by spontaneous
emission, i.e. noise. A more detailed description of optical gain in
semiconductors is given in Chapter 4.
2.2.3 Spontaneous emission noise
As shown above, spontaneous emission is a direct consequence of the
amplification process. In this section an expression is derived for the noise
power generated by an optical amplifier. We consider the arrangement of
Fig. 2.4 [3], which shows an input monochromatic signal of frequency
travelling through a gain medium having the energy level structure of Fig
2.3. A polariser and optical filter of bandwidth centred about are placed](https://image.slidesharecdn.com/87866-250225213249-f35702e2/85/Semiconductor-Optical-Amplifiers-1st-Edition-Michael-J-Connelly-25-320.jpg)
![14 Semiconductor Optical Amplifiers
The smallest solid angle that can be used without losing signal power [4] is
This solid angle can be obtained by the use of a suitably narrow output
aperture. In this case (2.17) can be rewritten as
The total beam power P (signal and noise) can then be described by
where the spontaneous emission factor is given by
z
The solution of (2.20), assuming that is independent of , is
where is the input signal power. If the amplifying medium has length L
then the total output power is
where is the single-pass signal gain. The amplifier additive noise
power is](https://image.slidesharecdn.com/87866-250225213249-f35702e2/85/Semiconductor-Optical-Amplifiers-1st-Edition-Michael-J-Connelly-27-320.jpg)
![15
Basic Principles
(2.24) shows that increasing the level of population inversion can reduce
SOA noise. The noise can also be reduced by the use of a narrowband
optical filter.
2.3 FUNDAMENTAL DEVICE CHARACTERISTICS
The most common application of SOAs is as a basic optical gain block. For
such an application, a list ofthe desired properties is given in Table 2.1. The
goal of most SOA research and development is to realise these properties in
practical devices.
2.3.1 Small-signal gain and gain bandwidth
In general there are two basic gain definitions for SOAs. The first is the
intrinsic gain G of the SOA, which is simply the ratio of the input signal
power at the input facet to the signal power at the output facet. The second
definition is the fibre-to-fibre gain, which includes the input and output
coupling losses. These gains are usually expressed in dB. The gain spectrum
of a particular SOA depends on its structure, materials and operational
parameters. For most applications high gain and wide gain bandwidth are
desired. The small-signal (small here meaning that the signal has negligible
influence on the SOA gain coefficient) internal gain ofa Fabry-Perot SOA at
optical frequency is given by [5],](https://image.slidesharecdn.com/87866-250225213249-f35702e2/85/Semiconductor-Optical-Amplifiers-1st-Edition-Michael-J-Connelly-28-320.jpg)
![23
Structures
current thereby reducing resistive power losses and placing fewer demands
on temperature control.
The efficiency of an SOA is improved if the light signal is confined to the
active region. In the DH structure the active region has a slightly higher
refractive index than the cladding regions and so behaves as a dielectric
waveguide with a rectangular cross-section. This helps to confine light
travelling through the device to the active region. The amount of
waveguiding is quantified by the optical confinement factor defined as the
fraction of the energy of a particular waveguide mode confined to the active
region. Waveguide modes are solutions to Maxwell’s equations for the
electric and magnetic fields in the waveguide obeying the waveguide
boundary conditions. The calculation of dielectric waveguide modes and
their associated confinement factors is not a trivial problem; more details can
be found in [1-4].
If the waveguide is sufficiently narrow, it will only support a single
transverse mode with two possible polarisations, the transverse electric (TE)
mode where the electric field is polarised along the heterojunction plane
(along the x-axis in Fig. 3.1) and the transverse magnetic (TM) mode where
the magnetic field is polarised along the x-axis. The mode is transverse
because the associated electric and magnetic fields are both normal to the
direction of propagation.
Single transverse mode operation helps to reduce modal gain dependency
as the optical confinement factor is mode dependent and also improves the
coupling efficiency from the device to optical fibre. The design of an SOA
active waveguide to support a single transverse mode is identical to that for
laser design. This topic is outside the scope of this book, but is covered in
detail in [1-4].
In the above DH SOA there is a well-defined refractive index step in the
y-direction between the intrinsic and cladding regions. However, in the x-
direction there is no such step. Wave guiding in x-direction is achieved by
the injected carriers, which change the refractive index of the intrinsic
region. This process is referred to as gain guiding. This refractive index
change is less than that in the y-direction. This implies that is polarisation
dependent, increases as the active region thickness is increased. However
if the active region is too wide single transverse mode operation will cease.
A typical profile of the light field intensity across the device cross-section is
shown in Fig. 3.2.
The polarisation dependent optical confinement factors are usually
referred to as the TE and TM optical confinement factors, i.e. and
Methods for reducing polarisation sensitivity include the use ofsquare cross-
section active waveguides and strained superlattices (Section 3.3).](https://image.slidesharecdn.com/87866-250225213249-f35702e2/85/Semiconductor-Optical-Amplifiers-1st-Edition-Michael-J-Connelly-36-320.jpg)
![24 Semiconductor Optical Amplifiers
3.2 SUPPRESSION OF CAVITY RESONANCE
We have seen in Chapter 2 that suppression of SOA facet reflectivities is
necessary to achieve travelling-wave operation in an OA. There are a
number of methods to reduce the effective facet reflectivities.
3.2.1 Antireflection coatings
The power reflectivity for normal incidence at the interface between two
dielectrics is given by
where and are the dielectric refractive indices. Semiconductor materials
have a high refractive index (typically between 3 and 4). Typical cleaved
semiconductor-air interface reflectivities are of the order of 32%. While a
reflectivity of this magnitude is suitable to achieve laser oscillation in a DH
device, it is excessively large for a TW-SOA. The effective facet
reflectivities can be greatly reduced by the application of antireflection (AR)
coatings. If a plane wave of free-space wavelength is normally incident on
a material of refractive index placed in air (refractive index = 1), then the
optimal (i.e. for lowest reflectivity) fabrication conditions for an AR coating
formed by a single dielectric layer are [5],
where and are the refractive index and thickness of the AR coating.
(3.2) only applies to a particular wavelength so a single AR coating is not
suitable for SOA operation over a wide bandwidth.
To achieve wideband low facet reflectivities requires the use of multilayer
dielectric coatings. The analysis of such coatings is complex and even more
difficult when applied to SOAs. This is because the SOA waveguide mode is
distributed in the active and cladding regions, which have different refractive
indices.
In the following analysis we follow the technique of [6]. If it is assumed
that the field distribution is uniform along the direction parallel to the](https://image.slidesharecdn.com/87866-250225213249-f35702e2/85/Semiconductor-Optical-Amplifiers-1st-Edition-Michael-J-Connelly-37-320.jpg)
![Structures 27
where and is the thickness of the i-th layer. The reflected-field
angular spectrum is
where and are the reflection coefficients for the field components
in the active and cladding regions respectively. The reflected field at z
= 0 is equal to the inverse Fourier transform of The reflectivity R for
the coated facet is given by the square of the coupling coefficient between
and , i.e.
This equation can be solved numerically. A similar technique can be applied
to obtain the reflectivity of the TM mode. An example of the use of (3.11)
for a single layer AR coating is shown in Fig 3.4. With an appropriate
an effective facet reflectivity
combination of film refractive index and thickness, it is possible to achieve
using a single layer AR coating.
The AR coating conditions for TE and TM polarisations are not equal.
However, the use of multilayer coatings can reduce polarisation dependency
and also broaden the low-reflectivity wavelength range [7]. Many dielectric
materials such as SiN, and can be used as AR
coatings. They are applied to an SOA by evaporation or sputtering. The
refractive index of the AR coating layer can be controlled by the evaporation
or sputtering conditions. Techniques are also available for measuring AR
coating reflectivity [8,9].
To achieve low facet reflectivities using AR coatings requires careful
control of the refractive index and thickness of the dielectric layers. Altering
the SOA structure can reduce the tight tolerances required. Two techniques
are commonly used: Angled facets and the window structure, which in
conjunction with AR coatings can deliver low reflectivities over a wide
bandwidth with minimal polarisation sensitivity.](https://image.slidesharecdn.com/87866-250225213249-f35702e2/85/Semiconductor-Optical-Amplifiers-1st-Edition-Michael-J-Connelly-40-320.jpg)
![28 Semiconductor Optical Amplifiers
3.2.2 Angled facet structure
In an angled facet SOA, shown in Fig. 3.5, the active region is slanted away
from the facet cleavage plane, thereby reducing the effective facet
reflectivity. The V-number of the waveguide is
where is the waveguide width and and are the effective refractive
indices of the active and cladding regions respectively. The TE mode full
power width is given by the approximation [10]](https://image.slidesharecdn.com/87866-250225213249-f35702e2/85/Semiconductor-Optical-Amplifiers-1st-Edition-Michael-J-Connelly-41-320.jpg)
![29
Structures
If the TE mode is assumed to have a Gaussian distribution, the effective
reflectivity of an angled facet is given approximately by
where is the angle between the beam propagation direction and the normal
to the end facet. The Fresnel reflectivity of a TE plane wave, confined to
the waveguide, at the angled facet-air interface is given by
The effective reflectivity of the TM mode is almost identical to that of the
TE mode. The effective reflectivity is shown in Fig 3.6 as a function of the
facet angle and active region width. The relative reflectivity decreases as the
facet angle increases. However the coupling efficiency between an SOA and
optical fibre degrades at large facet angles due to the far-field asymmetry.
AR coatings also become more polarisation sensitive as the facet angle
increases. The relative reflectivity also decreases as the waveguide width
increases. However, if the waveguide is too wide higher order transverse
modes can appear. This problem can be overcome by broadening the
waveguide near the end facets as shown in Fig. 3.7. This also preserves the
single transverse mode condition [11]. Optimal facet angles lie in the range
to . More analyses of angled facets can be found in [12-13].](https://image.slidesharecdn.com/87866-250225213249-f35702e2/85/Semiconductor-Optical-Amplifiers-1st-Edition-Michael-J-Connelly-42-320.jpg)
![30 Semiconductor Optical Amplifiers
3.2.3 Window facet structure
As noted above facet reflectivities and optimum AR coating conditions are
polarisation dependent. The effective facet reflectivity can be further
reduced and made less sensitive to polarisation by the use of window (or
buried) facets as shown in Fig 3.8 [14-15]. This structure is simply
composed of a transparent region between the active region and end facets.
This transparent region has an energy bandgap greater than the signal photon](https://image.slidesharecdn.com/87866-250225213249-f35702e2/85/Semiconductor-Optical-Amplifiers-1st-Edition-Michael-J-Connelly-43-320.jpg)
![31
Structures
energy. This means that stimulated absorption is not possible although some
intrinsic material absorption will be present. The guided field
from the waveguide propagates in the window region at some angle due to
diffraction and is partially reflected at the end facet. The reflected field
continues to broaden in space so only a small fraction is coupled back into
the active region. The effective reflectivity decreases with increase in the
length of the window region. However, the coupling efficiency from the
SOA to an optical fibre is degraded for long Effective facet reflectivities,
junction
with single-layer AR coatings, facetreflectivities
of the order of 5% are possible with window facets. Used in con
are possible.
3.3 POLARISATION INSENSITIVE STRUCTURES
Polarisation insensitive SOAs are desirable because the polarisation state of
the input signal can vary slowly with time. The main cause of polarisation
sensitivity is the difference between and Polarisation insensitive
SOA structural designs aim to reduce or compensate for this difference. In
the early stages of SOA development hybrid designs utilising two or more
SOA were used to reduce polarisation sensitivity. Those techniques have
now been superseded by single chip solutions that mainly focus on
improvements in active region design. Three common techniques used are:
Square cross-section waveguide, ridge waveguide and strained-layer
superlattice material.
3.3.1 SOAs with square cross-section active waveguide
Equalisation of and can be achieved through the use of a waveguide
with square cross-section [16-19] as shown in the buried ridge strip device of](https://image.slidesharecdn.com/87866-250225213249-f35702e2/85/Semiconductor-Optical-Amplifiers-1st-Edition-Michael-J-Connelly-44-320.jpg)
![32 Semiconductor Optical Amplifiers
Fig 3.9. In this device the potential barrier of the n-type/p-type InP
homojunction is greater than that of the InGaAsP active region/n-type InP
heterojunction. This means that there is very little carrier leakage from the
active region. This carrier confinement is further improved by the use of
highly resistive proton implanted InP regions.
However, such structures exhibit large far-field divergence, which leads
to poor coupling efficiency from the SOA to optical fibre. Tapering the
active waveguide near the amplifier facets as shown in Fig. 3.9 can reduce
the far field divergence. [16,18-19]. The guided mode is strongly confined in
the central square-cross section waveguide, but experiences less confinement
in the tapered sections and so expands. This increases the output mode size
and reduces the far-field divergence, thereby increasing the coupling
efficiency. The device also includes window regions to reduce the effective
facet reflectivity. With this type of device a polarisation sensitivity less than
1 dB was achieved over a wide range of bias currents as shown in Fig. 3.10
[18].
3.3.2 Ridge waveguide SOA
The buried ridge waveguide SOA, as shown in Fig. 3.11, has a relatively
large active region with a geometry that can be varied to equalise and
[20]. The relatively large bulk active region and ridge-waveguide
structure allow very low modal reflectivities both for AR coated and tilted
facets without the requirement for window regions.](https://image.slidesharecdn.com/87866-250225213249-f35702e2/85/Semiconductor-Optical-Amplifiers-1st-Edition-Michael-J-Connelly-45-320.jpg)
![34 Semiconductor Optical Amplifiers
the TM mode gain coefficient relative to the TE mode gain coefficient and
thereby compensate for the fact that Reductions in polarisation
sensitivity have been reported from device structures with strained tensile
barriers [21], tensile strained quantum-wells [22], alternating tensile and
compressive strained quantum-wells [23] and strain-balanced superlattices
[24]. The particular advantage of the latter structure is that it allows
simultaneous control of the polarisation sensitivity and peak gain of the
device without limitations placed on the active region thickness. The
properties of quantum-wells are covered in more detail in Chapter 4. Using
the above techniques low polarisation sensitivity can be achieved over wide
bias current and wavelength ranges with the additional advantage of high
output saturation power.
3.4 HIGH SATURATION OUTPUT POWER
STRUCTURES
High saturation output power is a desirable SOA characteristic, particularly
for power booster and multichannel applications.
3.4.1 Basic model for prediction of amplifier saturation characteristics
To determine the factors that influence SOA gain at high input powers, a
simple rate equation model can be used. The amplifier is assumed to have
zero facet reflectivities. The material gain coefficient at the signal
wavelength is assumed to be a linear function of carrier density
where is the differential of with respect to n and is assumed here to be
a constant. is the transparency carrier density. The carrier density obeys
the rate equation
The propagation of the signal intensity through the SOA is described by
the travelling-wave equation](https://image.slidesharecdn.com/87866-250225213249-f35702e2/85/Semiconductor-Optical-Amplifiers-1st-Edition-Michael-J-Connelly-47-320.jpg)
![Structures 37
3.4.2 Improving saturation output power
(3.26) shows that the saturation output power of an SOA can be improved by
increasing Inspection of (3.20) shows that this can be achieved by
reducing and . In practice is inversely proportional to carrier density, so
operating at high bias currents leads to an increase in . However, as the
carrier density increases the amplifier gain will also increase making
by reducing
resonance effects more significant. The single-pass gain can be maintained
or the amplifier length.. This may not always be necessary as
the peak material gain coefficient shifts to shorter wavelengths as the carrier
density increases.
The choice of gain material can also influence the saturation behaviour of
the amplifier via . In bulk materials is relatively sensitive to changes in
carrier density. In quantum-well material, conditions can exist where the
gain is relatively insensitive to changes in carrier density. This leads to a
high [25].
It is also possible to increase by increasing An approach based
on this concept, shown in Fig. 3.13, is to unfold the amplifier waveguide
width towards the output facet [26-28]. This increases the mode field area at
the amplifier output.](https://image.slidesharecdn.com/87866-250225213249-f35702e2/85/Semiconductor-Optical-Amplifiers-1st-Edition-Michael-J-Connelly-50-320.jpg)
![This ebook is for the use of anyone anywhere in the United States
and most other parts of the world at no cost and with almost no
restrictions whatsoever. You may copy it, give it away or re-use it
under the terms of the Project Gutenberg License included with this
ebook or online at www.gutenberg.org. If you are not located in the
United States, you will have to check the laws of the country where
you are located before using this eBook.
Title: The Mutable Many: A Novel
Author: Robert Barr
Release date: August 10, 2017 [eBook #55326]
Most recently updated: October 23, 2024
Language: English
Credits: Produced by David Widger from page images generously
provided by the Internet Archive
*** START OF THE PROJECT GUTENBERG EBOOK THE MUTABLE
MANY: A NOVEL ***](https://image.slidesharecdn.com/87866-250225213249-f35702e2/85/Semiconductor-Optical-Amplifiers-1st-Edition-Michael-J-Connelly-56-320.jpg)
Semiconductor Optical Amplifiers 1st Edition Michael J. Connelly
- 1. Visit https://ebookultra.com todownload the full version and explore more ebooks Semiconductor Optical Amplifiers 1st Edition Michael J. Connelly _____ Click the link below to download _____ https://ebookultra.com/download/semiconductor-optical- amplifiers-1st-edition-michael-j-connelly/ Explore and download more ebooks at ebookultra.com
- 2. Here are somesuggested products you might be interested in. Click the link to download Semiconductor Industry Wafer Fab Exhaust Management 1st Edition J. Michael Sherer https://ebookultra.com/download/semiconductor-industry-wafer-fab- exhaust-management-1st-edition-j-michael-sherer/ Echo Park Harry Bosch 12 Harry Bosch Universe 17 First Edition Michael Connelly https://ebookultra.com/download/echo-park-harry-bosch-12-harry-bosch- universe-17-first-edition-michael-connelly/ Design of CMOS Operational Amplifiers Dehghani https://ebookultra.com/download/design-of-cmos-operational-amplifiers- dehghani/ Operational amplifiers 2nd edition Edition Barna A. https://ebookultra.com/download/operational-amplifiers-2nd-edition- edition-barna-a/
- 3. The Book ofRevelation for Blockheads Douglas Connelly https://ebookultra.com/download/the-book-of-revelation-for-blockheads- douglas-connelly/ Semiconductor Transport 1st Edition Ferry https://ebookultra.com/download/semiconductor-transport-1st-edition- ferry/ Semiconductor Electrochemistry 1st Edition Rudiger Memming https://ebookultra.com/download/semiconductor-electrochemistry-1st- edition-rudiger-memming/ Quantum Theory of Optical Coherence Selected Papers and Lectures 1st Edition Roy J. Glauber https://ebookultra.com/download/quantum-theory-of-optical-coherence- selected-papers-and-lectures-1st-edition-roy-j-glauber/ Fundamentals of RF and Microwave Transistor Amplifiers 1st Edition Inder Bahl https://ebookultra.com/download/fundamentals-of-rf-and-microwave- transistor-amplifiers-1st-edition-inder-bahl/
- 5. Semiconductor Optical Amplifiers1st Edition Michael J. Connelly Digital Instant Download Author(s): Michael J. Connelly ISBN(s): 9780792376576, 0792376579 Edition: 1st File Details: PDF, 8.76 MB Year: 2002 Language: english
- 7.
- 8. Semiconductor Optical Amplifiers by Michael J.Connelly University of Limerick, Ireland KLUWER ACADEMIC PUBLISHERS NEW YORK, BOSTON, DORDRECHT, LONDON, MOSCOW
- 9. eBook ISBN: 0-306-48156-1 PrintISBN: 0-7923-7657-9 ©2004 Kluwer Academic Publishers New York, Boston, Dordrecht, London, Moscow Print ©2002 Kluwer Academic Publishers Dordrecht All rights reserved No part of this eBook may be reproduced or transmitted in any form or by any means, electronic, mechanical, recording, or otherwise, without written consent from the Publisher Created in the United States of America Visit Kluwer Online at: http://kluweronline.com and Kluwer's eBookstore at: http://ebooks.kluweronline.com
- 10. For my parentsMichael and Margeret and brother Brendan
- 11. Contents Preface ix INTRODUCTION 1 BASICPRINCIPLES 7 STRUCTURES 21 MATERIALS 43 MODELLING 69 BASIC NETWORK APPLICATIONS 97 FUNCTIONAL APPLICATIONS 127 Index 167 vii
- 12. Preface Communications can bebroadly defined as the transfer of information from one point to another. In optical fibre communications, this transfer is achieved by using light as the information carrier. There has been an exponential growth in the deployment and capacity of optical fibre communication technologies and networks over the past twenty-five years. This growth has been made possible by the development of new optoelectronic technologies that can be utilised to exploit the enormous potential bandwidth of optical fibre. Today, systems are operational which operate at aggregate bit rates in excess of 100 Gb/s. Such high capacity systems exploit the optical fibre bandwidth by employing wavelength division multiplexing. Optical technology is the dominant carrier of global information. It is also central to the realisation of future networks that will have the capabilities demanded by society. These capabilities include virtually unlimited bandwidth to carry communication services of almost any kind, and full transparency that allows terminal upgrades in capacity and flexible routing of channels. Many of the advances in optical networks have been made possible by the advent of the optical amplifier. In general, optical amplifiers can be divided into two classes: optical fibre amplifiers and semiconductor amplifiers. The former has tended to dominate conventional system applications such as in-line amplification used to compensate for fibre losses. However, due to advances in optical semiconductor fabrication techniques and device design, especially over the last five years, the semiconductor optical amplifier (SOA) is showing great promise for use in evolving optical communication networks. It can be utilised as a general gain unit but also has many functional applications including an optical switch, modulator and wavelength converter. These ix
- 13. x Introduction functions, wherethere is no conversion of optical signals into the electrical domain, are required in transparent optical networks. It is the intention of this book to provide the reader with a comprehensive introduction to the design and applications of SOAs, particularly with regard to their use in optical communication systems. It is hoped that the book has achieved this aim.
- 14. 1.3 Chapter 1 INTRODUCTION In thischapter we begin with the reasons why optical amplification is required in optical communication networks. This is followed by a brief history of semiconductor optical amplifiers (SOAs), a summary of the applications of SOAs and a comparison between SOAs and optical fibre amplifiers (OFAs). 1.1 THE NEED FOR OPTICAL AMPLIFICATION Optical fibre suffers from two principal limiting factors: Attenuation and dispersion. Attenuation leads to signal power loss, which limits transmission distance. Dispersion causes optical pulse broadening and hence intersymbol interference leading to an increase in the system bit error rate (BER). Dispersion essentially limits the fibre bandwidth. The attenuation spectrum of conventional single-mode silica fibre, shown in Fig. 1.1, has a minimum in the 1.55 wavelength region. The attenuation is somewhat higher in the region. The dispersion spectrum of conventional single-mode silica fibre, shown in Fig. 1.2, has a minimum in the 1.3 region. Because the attenuation and material dispersion minima are located in the 1.55 and 1.3 ‘windows’, these are the main wavelength regions used in commercial optical fibre communication systems. Systems operating in the 830 nm region are also utilised, mainly for short-haul links at moderate bit rates which do not usually require optical amplification. Because signal attenuation and dispersion increases as the fibre length increases, at some point in an optical fibre communication link the optical signal will need to be regenerated. 3R (reshaping-retiming-retransmission) 1
- 15. 2 Semiconductor OpticalAmplifiers regeneration involves detection (photon-electron conversion), electrical amplification, retiming, pulse shaping and retransmission (electron-photon conversion). This method has a number of disadvantages. Firstly, it involves breaking the optical link and so is not optically transparent. Secondly, the regeneration process is dependent on the signal modulation format and bit rate and so is not electrically transparent. This in turn creates difficulties if the link needs to be upgraded. Ideally link upgrades should only involve changes in or replacement of terminal equipment (transmitter or receiver). Thirdly, as regenerators are complex systems and often situated in remote or difficult to access location, as is the case in undersea transmission links, network
- 16. 3 Introduction reliability is impaired.In systems where fibre loss is the limiting factor, an in-line optical amplifier can be used instead of a regenerator. As the in-line amplifier has only to carry out one function (amplification of the input signal) compared to full regeneration, it is intrinsically a more reliable and less expensive device. Ideally an in-line optical amplifier should be compatible with single-mode fibre, impart large gain and be optically transparent (i.e. independent of the input optical signal properties). In addition optical amplifiers can also be useful as power boosters, for example to compensate for splitting losses in optical distribution networks, and as optical preamplifiers to improve receiver sensitivity. Besides these basic system applications optical amplifiers are also useful as generic optical gain blocks for use in larger optical systems. The improvements in optical communication networks realised through the use of optical amplifiers provides new opportunities to exploit the fibre bandwidth. There are two types of optical amplifier: The SOA and the OFA [1-6]. In recent times the latter has dominated; however SOAs have attracted renewed interest for use as basic amplifiers and also as functional elements in optical communication networks and optical signal processing devices. 1.2 BRIEF HISTORY OF SEMICONDUCTOR OPTICAL AMPLIFIERS The first studies on SOAs were carried out around the time of the invention of the semiconductor laser in the 1960’s. These early devices were based on GaAs homojunctions operating at low temperatures. The arrival of double heterostructure devices spurred further investigation into the use of SOAs in optical communication systems. In the 1970’s Zeidler and Personick carried out early work on SOAs [7-8]. In the 1980’s there were further important advances on SOA device design and modelling. Early studies concentrated on AlGaAs SOAs operating in the 830 nm range [9-10]. In the late 1980’s studies on InP/InGaAsP SOAs designed to operate in the 1.3 and 1.55 regions began to appear [11]. Developments in anti-reflection coating technology enabled the fabrication of true travelling-wave SOAs [12]. Prior to 1989, SOA structures were based on anti-reflection coated semiconductor laser diodes. These devices had an asymmetrical waveguide structure leading to strongly polarisation sensitive gain. In 1989 SOAs began to be designed as devices in their own right, with the use of more symmetrical waveguide structures giving much reduced polarisation sensitivities [13]. Since then SOA design and development has
- 17. 4 Semiconductor OpticalAmplifiers progressed in tandem with advances in semiconductor materials, device fabrication, antireflection coating technology, packaging and photonic integrated circuits, to the point where reliable cost competitive devices are now available for use in commercial optical communication systems. Developments in SOA technology are ongoing with particular interest in functional applications such as photonic switching and wavelength conversion. The use of SOAs in photonic integrated circuits (PICs) is also attracting much research interest. 1.3 SEMICONDUCTOR AND OPTICAL FIBRE AMPLIFIERS: COMPLEMENTARY TECHNOLOGIES The Erbium Doped Fibre Amplifier (EDFA) was invented in 1985. The EDFA led to a revolution in optical communications as it made possible the replacement of 3R regenerators in links limited by fibre attenuation, leading to the possibility of optically transparent networks and thereby overcoming the ‘electronic bottleneck’. The EDFA has become the optical amplifier of choice in long haul, multichannel digital and analog applications at 1.55 Neodymium Doped Fibre Amplifiers are also available for use in the 1.3 region. Fibre amplifiers are especially attractive, as they possess high gain, low insertion loss, low noise figure and negligible nonlinearities, but require an external pump laser. SOAs have poorer gain and noise and nonlinearities can be severe. However SOA technology is advancing rapidly. SOAs are compatible with monolithic integration (hence low cost potential) and offer a wide range of applications, including optical signal processing that cannot be performed by fibre amplifiers. It is expected that deployment of SOAs in evolving optical communication networks will increase. A comparison between the main features ofOFAs and SOAs is given in Table 1.1. Fig. 1.2 illustrates some of diverse range of applications possible with SOAs.
- 18. 5 Introduction It is forecastthat the consumption of SOAs will expand rapidly from $48 million in 2000 to $903 million in 2010 [14]. The main near-term use of SOAs will be as basic amplifiers in wavelength division multiplexed (WDM) and other digital optical communication links, and as switching elements in all-optical switches and optical crossconnects. As the above figures show, the future for SOAs is bright, and even more applications of the device will arise as the technology matures and manufacturing costs decrease. REFERENCES 1. S. Shimada and H. Ishio, Eds., Optical Amplifiers and their Applications, John Wiley (1992). 2. H. Ghafouri-Shiraz, Fundamentals of Laser Diode Amplifiers, John Wiley (1995). 3. Y. Yamamoto, Ed., Coherence, Amplification and Quantum Effects in Semiconductor Lasers, John Wiley (1991). 4. E. Desurvire, Erbium-Doped Fibre Amplifiers: Principles and Applications, John Wiley, New York (1994). 5. M. J. O’Mahony, Semiconductor optical amplifiers for use in future fibre systems, IEEE/OSA J. Lightwave Technol., 6, 531-544 (1988). 6. N. A. Olsson, Semiconductor optical amplifiers, IEEE Proc., 80, 375-382 (1992). 7. G. Zeidler and D. Schicetanz, Use of laser amplifiers in glass fibre communication systems, Siemens Forch. u. Entwickl. Ber., 2, 227-234 (1973). 8. S. D. Personick, Applications for quantum amplifiers in simple digital optical communication systems, Bell Syst. Tech. J., 52, 117-133 (1973). 9. Y. Yamamoto, Characteristics of AlGaAs Fabry-Perot cavity type laser amplifiers, IEEE J. Quantum Electron., 16, 1047-1052 (1980).
- 19. 6 Semiconductor OpticalAmplifiers 10. T. Mukai, Y. Yamamoto and T. Kimura, S/N and error rate performance in AlGaAs semiconductor laser preamplifier and linear repeater systems, IEEE Trans. Microwave Theory And Tech., 30, 1548-1556 (1982). 11. J. C. Simon, GaInAsP semiconductor laser amplifiers for single-mode fibre communications, IEEE/OSA J. Lightwave Technol., 5, 1286-1295, 1987. 12. C. E. Zah, C. Caneau, F. K. Shokoohi, S. G. Menocal, F. Favire, L. A. Reith and T. P. Lee, 1.3 GaInAsP near-travelling-wave laser amplifiers made by combination of angled facets and antireflection coatings, Electron. Lett., 24, 1275-1276 (1988). 13. N. A. Olsson, R. F. Kazarinov, W. A. Nordland, C. H. Henry, M. G. Oberg, H. G. White, P. A. Garbinski and A. Savage, Polarisation-independent optical amplifier with buried facets, Electron. Lett., 25, 1048-1049 (1989). 14. J.D. Montgomery, S. Montgomery and S. Hailu, Semiconductor optical amplifiers expand commercial opportunities, WDM Solutions, Supplement to Laser Focus World, 27-30 August 2001.
- 20. Chapter 2 BASIC PRINCIPLES Inthis chapter the basic principles of SOAs are explained. The processes of optical gain and additive noise are discussed along with fundamental device parameters including gain ripple, polarisation sensitivity, saturation output power and noise figure. 2.1 SOA - BASIC DESCRIPTION An SOA is an optoelectronic device that under suitable operating conditions can amplify an input light signal. A schematic diagram of a basic SOA is shown in Fig. 2.1. The active region in the device imparts gain to an input signal. An external electric current provides the energy source that enables gain to take place. An embedded waveguide is used to confine the propagating signal wave to the active region. However, the optical confinement is weak so some of the signal will leak into the surrounding lossy cladding regions. The output signal is accompanied by noise. This additive noise is produced by the amplification process itself and so cannot be entirely avoided. The amplifier facets are reflective causing ripples in the gain spectrum. SOAs can be classified into two main types shown in Fig. 2.2: The Fabry- Perot SOA (FP-SOA) where reflections from the end facets are significant (i.e. the signal undergoes many passes through the amplifier) and the travelling-wave SOA (TW-SOA) where reflections are negligible (i.e. the signal undergoes a single-pass of the amplifier). Anti-reflection coatings can be used to create SOAs with facet reflectivities The TW-SOA is not as sensitive as the FP-SOA to fluctuations in bias current, temperature and signal polarisation. 7
- 21. 8 Semiconductor OpticalAmplifiers 2.2 PRINCIPLES OF OPTICAL AMPLIFICATION In an SOA electrons (more commonly referred to as carriers) are injected from an external current source into the active region. These energised
- 22. 9 Basic Principles carriers occupyenergy states in the conduction band (CB) of the active region material, leaving holes in the valence band (VB). Three radiative mechanisms are possible in the semiconductor. These are shown in Fig 2.3 for a material with an energy band structure consisting of two discrete energy levels. In stimulated absorption an incident light photon of sufficient energy can stimulate a carrier from the VB to the CB. This is a loss process as the incident photon is extinguished. If a photon of light of suitable energy is incident on the semiconductor, it can cause stimulated recombination of a CB carrier with a VB hole. The recombining carrier loses its energy in the form of a photon of light. This new stimulated photon will be identical in all respects to the inducing photon (identical phase, frequency and direction, i.e. a coherent interaction). Both the original photon and stimulated photon can give rise to more stimulated transitions. If the injected current is sufficiently high then a population inversion is created when the carrier population in the CB exceeds that in the VB. In this case the likelihood of stimulated emission is greater than stimulated absorption and so semiconductor will exhibit optical gain. In the spontaneous emission process, there is a non-zero probability per unit time that a CB carrier will spontaneously recombine with a VB hole and thereby emit a photon with random phase and direction. Spontaneously emitted photons have a wide range of frequencies. Spontaneously emitted photons are essentially noise and also take part in reducing the carrier population available for optical gain. Spontaneous emission is a direct
- 23. 10 Semiconductor OpticalAmplifiers consequence of the amplification process and cannot be avoided; hence a noiseless SOA cannot be created. Stimulated processes are proportional to the intensity of the inducing radiation whereas the spontaneous emission process is independent of it. 2.2.1 Spontaneous and induced transitions The gain properties of optical semiconductors are directly related to the processes of spontaneous and stimulated emission. To quantify this relationship we consider a system of energy levels associated with a particular physical system. Let and per unit volume of the system characterised be the average number of atoms by energies and respectively, with If a particular atom has energy then there is a finite probability per unit time that it will undergo a transition from to and in the process emit a photon. The spontaneous carrier transition rate per unit time from level 2 to level 1 is given by where is the spontaneous emission parameter of the level 2 to level 1 transition. Along with spontaneous emission it is also possible to have induced transitions. The induced carrier transition rate from level 2 to level 1 (stimulated emission) is given by where is the stimulated emission parameter of the level 2 to level 1 transition and the incident radiation energy density at frequency The induced photons have energy The induced transition rate from level 1 to level 2 (stimulated absorption) is given by where is the stimulated emission parameter of the level 2 to level 1 transition. It can be proved, from quantum-mechanical considerations [1,2], that
- 24. 11 Basic Principles where isthe material refractive index and the speed of light in a vacuum. Inserting (2.5) into (2.2) gives In the case where the inducing radiation is monochromatic at frequency then the induced transition rate from level 2 to level 1 is where is the energy density of the electromagnetic field inducing the transition and is the transition lineshape function, normalised such that is the probability that a particular spontaneous emission event from is level 2 to level 1 will result in a photon with a frequency between and The inducing field intensity So (2.7) becomes
- 25. 12 Semiconductor OpticalAmplifiers 2.2.2 Absorption and amplification By using the expression for the stimulated transition rates developed in Section 2.2.1, it is now possible to derive an equation for the optical gain coefficient for a two level system. We consider the case of a monochromatic plane wave propagating in the through a gain medium with cross- section area A and elemental length The net power generated by a volume of the material is simply the difference in the induced transition rates between the levels multiplied by the transition energy and the elemental volume i.e. This radiation is added coherently to the propagating wave. This process of amplification can then be described by the differential equation is the material gain coefficient given by (2.13) implies that to achieve positive gain a population inversion must exist between level 2 and level 1. It also shows, by the presence of that the process of optical gain is always accompanied by spontaneous emission, i.e. noise. A more detailed description of optical gain in semiconductors is given in Chapter 4. 2.2.3 Spontaneous emission noise As shown above, spontaneous emission is a direct consequence of the amplification process. In this section an expression is derived for the noise power generated by an optical amplifier. We consider the arrangement of Fig. 2.4 [3], which shows an input monochromatic signal of frequency travelling through a gain medium having the energy level structure of Fig 2.3. A polariser and optical filter of bandwidth centred about are placed
- 26. 13 Basic Principles before thedetector. The input beam is focussed such that its waist occupies the gain medium. If the beam is assumed to have a circular cross-section with waist diameter D then the beam divergence angle is where is the free space wavelength. The net change in the signal power due to coherent amplification by an elemental length of the gain medium is A volume element, with cross-section area A and length at position of the gain medium spontaneously emits a noise power This noise is emitted isotropically over a solid angle. Each spontaneously emitted photon can exist with equal probability in one of two mutually orthogonal polarisation states. The polariser passes the signal, while reducing the noise by half. Hence the total noise power emitted by the volume element into a solid angle and bandwidth is
- 27. 14 Semiconductor OpticalAmplifiers The smallest solid angle that can be used without losing signal power [4] is This solid angle can be obtained by the use of a suitably narrow output aperture. In this case (2.17) can be rewritten as The total beam power P (signal and noise) can then be described by where the spontaneous emission factor is given by z The solution of (2.20), assuming that is independent of , is where is the input signal power. If the amplifying medium has length L then the total output power is where is the single-pass signal gain. The amplifier additive noise power is
- 28. 15 Basic Principles (2.24) showsthat increasing the level of population inversion can reduce SOA noise. The noise can also be reduced by the use of a narrowband optical filter. 2.3 FUNDAMENTAL DEVICE CHARACTERISTICS The most common application of SOAs is as a basic optical gain block. For such an application, a list ofthe desired properties is given in Table 2.1. The goal of most SOA research and development is to realise these properties in practical devices. 2.3.1 Small-signal gain and gain bandwidth In general there are two basic gain definitions for SOAs. The first is the intrinsic gain G of the SOA, which is simply the ratio of the input signal power at the input facet to the signal power at the output facet. The second definition is the fibre-to-fibre gain, which includes the input and output coupling losses. These gains are usually expressed in dB. The gain spectrum of a particular SOA depends on its structure, materials and operational parameters. For most applications high gain and wide gain bandwidth are desired. The small-signal (small here meaning that the signal has negligible influence on the SOA gain coefficient) internal gain ofa Fabry-Perot SOA at optical frequency is given by [5],
- 29. 16 Semiconductor OpticalAmplifiers where and are the input and output facet reflectivities and is the cavity longitudinal mode spacing given by . Cavity resonance frequencies occur at integermultiples of is the closest cavity resonance to The factor in (2.25) is equal to zero at resonance frequencies and equal to unity at the anti-resonance frequencies (located midway between successive resonance frequencies). The effective SOA gain coefficient is where is the optical mode confinement factor (the fraction of the propagating signal field mode confined to the active region) and the absorption coefficient. is the single-pass amplifier gain. An uncoated SOA has facet reflectivities approximately equal to 0.32. The amplifier gain ripple is defined as the ratio between the resonant and non-resonant gains. From (2.25) we get From (2.28) the relationship between the geometric mean facet reflectivity and is Curves of versus are shown in Fig. 2.5 with as parameter. For example, to obtain a gain ripple less than 1 dB at an amplifier single-pass gain of 25 dB requires that Facet reflectivities of this order can be achieved by the application of anti-reflection (AR) coatings to the amplifier facets. The effective facet reflectivities can be reduced further by the use of specialised SOA structures. These techniques are discussed in Chapter 3.
- 30. 17 Basic Principles A typicalTW-SOA small-signal gain spectrum is shown in Fig. 2.2. The gain bandwidth of the amplifier is defined as the wavelength range over which the signal gain is not less than half its peak value. Wide gain bandwidth SOAs are especially useful in systems where multichannel amplification is required such as in WDM networks. A wide gain bandwidth can be achieved in an SOA with an active region fabricated from quantum- well or multiple quantum-well (MQW) material as discussed in Chapter 4. Typical maximum internal gains achievable in practical devices are in the range of 30 to 35 dB. Typical small-signal gain bandwidths are in the range of 30 to 60 nm. 2.3.2 Polarisation sensitivity In general the gain of an SOA depends on the polarisation state of the input signal. This dependency is due to a number of factors including the waveguide structure, the polarisation dependent nature of anti-reflection coatings and the gain material. Cascaded SOAs accentuate this polarisation dependence. The amplifier waveguide is characterised by two mutually orthogonal polarisation modes termed the Transverse Electric (TE) and Transverse Magnetic (TM) modes. The input signal polarisation state usually lies somewhere between these two extremes. The polarisation sensitivity of an SOA is defined as the magnitude of the difference between the TE mode gain and TM mode gain i.e.
- 31. 18 Semiconductor OpticalAmplifiers Techniques for realising SOAs with low polarisation sensitivity (< 1 dB) are discussed in Chapter 3. 2.3.3 Signal gain saturation The gain of an SOA is influenced both by the input signal power and internal noise generated by the amplification process. As the signal power increases the carriers in the active region become depleted leading to a decrease in the amplifier gain. This gain saturation can cause significant signal distortion. It can also limit the gain achievable when SOAs are used as multichannel amplifiers. A typical SOA gain versus output signal power characteristic is saturation output power shown in Fig. 2.6. A useful parameter for quantifying gain saturation is the which is defined as the amplifier output signal power at which the amplifier gain is half the small-signal gain. Techniques for realising SOAs with high are discussed in Chapter 3. Values in the range of 5 to 20 dBm for are typical of practical devices. 2.3.4 Noise figure A useful parameter for quantifying optical amplifier noise is the noise figure. F, defined as the ratio of the input and output signal to noise ratios, i.e.
- 32. 19 Basic Principles The signalto noise ratios in (2.31) are those obtained when the input and output powers of the amplifier are detected by an ideal photodetector. Full expressions for the photocurrent signal to noise ratio in optically amplified systems are derived in Chapter 6. In the limiting case where the amplifier gain is much larger than unity and the amplifier output is passed through a narrowband optical filter, the noise figure is given by The lowest value possible for is unity, which occurs when there is complete inversion of the atomic medium, i.e. giving F = 2 (i.e. 3 dB). Typical intrinsic (i.e. not including coupling losses) noise figures of practical SOAs are in the range of 7 to 12 dB. The noise figure is degraded by the amplifier input coupling loss. Coupling losses are usually ofthe order of 3 dB, so the noise figure of typical packaged SOAs is between 10 and 15 dB. 2.3.5 Dynamic effects SOAs are normally used to amplify modulated light signals. If the signal power is high then gain saturation will occur. This would not be a serious problem if the amplifier gain dynamics were a slow process. However in SOAs the gain dynamics are determined by the carrier recombination lifetime (average time for a carrier to recombine with a hole in the valence band). This lifetime is typically of a few hundred picoseconds. This means that the amplifier gain will react relatively quickly to changes in the input signal power. This dynamic gain can cause signal distortion, which becomes more severe as the modulated signal bandwidth increases. These effects are further exacerbated in multichannel systems where the dynamic gain leads to interchannel crosstalk. This is in contrast to doped fibre amplifiers, which have recombination lifetimes of the order of milliseconds leading to negligible signal distortion. Dynamic effects are explored further in Chapter 5. 2.3.6 Nonlinearities SOAs also exhibit nonlinear behaviour. In general these nonlinearities can cause problems such as frequency chirping and generation of second or third order intermodulation products. However, nonlinearities can also be of use
- 33. 20 Semiconductor OpticalAmplifiers in using SOAs as functional devices such as wavelength converters. SOA nonlinearities are discussed in more detail in Chapter 7. REFERENCES 1. Y. Suematsu and A.R. Adams, Handbook of Semiconductor lasers and Photonic Integrated Circuits, London, Chapman and Hall (1994). 2. A. Yariv, Quantum Electronics, New York, Wiley (1989). 3. A. Yariv, Optical Electronics, New York, HWR International (1985). 4. F.L. Pedrotti and L.S. Pedrotti, Introduction to Optics, Prentice-Hall (1993). 5. Y. Yamamoto, Characteristics of AlGaAs Fabry-Perot cavity type laser amplifiers, IEEE J. Quantum Electron., 16, 1047-1052 (1980).
- 34. Chapter 3 STRUCTURES The structuraldesign of an SOA has a large bearing on its performance. SOA structures are chosen so a given device approaches the ideal characteristics listed in Table 2.1 or to accentuate a particular characteristic desirable for a given application (e.g. high saturation output power for booster applications). In this chapter the principles underlying SOA design are reviewed. 3.1 SOA BASIC STRUCTURE In the early days of SOA research, their structures were based on anti reflection coated double-heterostructure (DH) semiconductor laser diodes as shown in Fig. 3.1. In this type of structure the active region (usually intrinsic i.e. undoped) is sandwiched between n-type and p-type cladding regions. The interfaces between the active region and cladding regions are heterojunctions, as shown in Fig. 3.2. A heterojunction is an interface between two semiconductor materials having different bandgap energies. The cladding regions have higher bandgap energies and lower refractive indices compared to the active region. Carriers are injected into the device active region from the applied bias current. The injected carriers have to make their way through surrounding layers of semiconductor before they reach the active region. If there were no carrier confinement, the carriers would diffuse throughout the device. As the active region is relatively small, this means that a only a small percentage of the injected carriers would eventually provide gain to a propagating light signal. This leads to inefficient device operation. To overcome this effect, confinement of carriers to the active region is necessary. In the DH structure 21
- 35. 22 Semiconductor OpticalAmplifiers this is achieved by the bandgap difference between the active region and the cladding regions. This confines carriers to the region between the barriers. However, it is only in the central intrinsic region that the carrier density is high enough to impart gain to a propagating light wave. Carrier confinement has the additional benefit in that the device can be operated at a lower bias
- 36. 23 Structures current thereby reducingresistive power losses and placing fewer demands on temperature control. The efficiency of an SOA is improved if the light signal is confined to the active region. In the DH structure the active region has a slightly higher refractive index than the cladding regions and so behaves as a dielectric waveguide with a rectangular cross-section. This helps to confine light travelling through the device to the active region. The amount of waveguiding is quantified by the optical confinement factor defined as the fraction of the energy of a particular waveguide mode confined to the active region. Waveguide modes are solutions to Maxwell’s equations for the electric and magnetic fields in the waveguide obeying the waveguide boundary conditions. The calculation of dielectric waveguide modes and their associated confinement factors is not a trivial problem; more details can be found in [1-4]. If the waveguide is sufficiently narrow, it will only support a single transverse mode with two possible polarisations, the transverse electric (TE) mode where the electric field is polarised along the heterojunction plane (along the x-axis in Fig. 3.1) and the transverse magnetic (TM) mode where the magnetic field is polarised along the x-axis. The mode is transverse because the associated electric and magnetic fields are both normal to the direction of propagation. Single transverse mode operation helps to reduce modal gain dependency as the optical confinement factor is mode dependent and also improves the coupling efficiency from the device to optical fibre. The design of an SOA active waveguide to support a single transverse mode is identical to that for laser design. This topic is outside the scope of this book, but is covered in detail in [1-4]. In the above DH SOA there is a well-defined refractive index step in the y-direction between the intrinsic and cladding regions. However, in the x- direction there is no such step. Wave guiding in x-direction is achieved by the injected carriers, which change the refractive index of the intrinsic region. This process is referred to as gain guiding. This refractive index change is less than that in the y-direction. This implies that is polarisation dependent, increases as the active region thickness is increased. However if the active region is too wide single transverse mode operation will cease. A typical profile of the light field intensity across the device cross-section is shown in Fig. 3.2. The polarisation dependent optical confinement factors are usually referred to as the TE and TM optical confinement factors, i.e. and Methods for reducing polarisation sensitivity include the use ofsquare cross- section active waveguides and strained superlattices (Section 3.3).
- 37. 24 Semiconductor OpticalAmplifiers 3.2 SUPPRESSION OF CAVITY RESONANCE We have seen in Chapter 2 that suppression of SOA facet reflectivities is necessary to achieve travelling-wave operation in an OA. There are a number of methods to reduce the effective facet reflectivities. 3.2.1 Antireflection coatings The power reflectivity for normal incidence at the interface between two dielectrics is given by where and are the dielectric refractive indices. Semiconductor materials have a high refractive index (typically between 3 and 4). Typical cleaved semiconductor-air interface reflectivities are of the order of 32%. While a reflectivity of this magnitude is suitable to achieve laser oscillation in a DH device, it is excessively large for a TW-SOA. The effective facet reflectivities can be greatly reduced by the application of antireflection (AR) coatings. If a plane wave of free-space wavelength is normally incident on a material of refractive index placed in air (refractive index = 1), then the optimal (i.e. for lowest reflectivity) fabrication conditions for an AR coating formed by a single dielectric layer are [5], where and are the refractive index and thickness of the AR coating. (3.2) only applies to a particular wavelength so a single AR coating is not suitable for SOA operation over a wide bandwidth. To achieve wideband low facet reflectivities requires the use of multilayer dielectric coatings. The analysis of such coatings is complex and even more difficult when applied to SOAs. This is because the SOA waveguide mode is distributed in the active and cladding regions, which have different refractive indices. In the following analysis we follow the technique of [6]. If it is assumed that the field distribution is uniform along the direction parallel to the
- 38. 25 Structures junction between thefacet and AR coatings, the waveguide can be analysed using the symmetric two-dimensional model shown in Fig. 3.3. In the analysis only TE polarised waves are considered. The active layer has refractive index and thickness d. The surrounding cladding regions have refractive index and are assumed to extend to infinity. The incident field distribution at the boundary between the active region and the AR coating, is where which satisfy
- 39. 26 Semiconductor OpticalAmplifiers where the subscripts a and c denote the active and cladding regions respectively, is the propagation constant, and are wavenumbers and A is an arbitrary constant. The plane wave angular spectrum of the incident field is given by the Fourier transform of (3.3) which gives where is the angle of incidence and and are the components of in the active and cladding layers respectively. The Fresnel reflection coefficient of the multilayer AR coating is where l is the number of AR coating layers, and are the elements of the 2 x 2 transfer matrix i-th layer the refractive index of the i-th layer. is the refractive index of the active region or cladding layer. The elements of the transfer matrix of the are with
- 40. Structures 27 where andis the thickness of the i-th layer. The reflected-field angular spectrum is where and are the reflection coefficients for the field components in the active and cladding regions respectively. The reflected field at z = 0 is equal to the inverse Fourier transform of The reflectivity R for the coated facet is given by the square of the coupling coefficient between and , i.e. This equation can be solved numerically. A similar technique can be applied to obtain the reflectivity of the TM mode. An example of the use of (3.11) for a single layer AR coating is shown in Fig 3.4. With an appropriate an effective facet reflectivity combination of film refractive index and thickness, it is possible to achieve using a single layer AR coating. The AR coating conditions for TE and TM polarisations are not equal. However, the use of multilayer coatings can reduce polarisation dependency and also broaden the low-reflectivity wavelength range [7]. Many dielectric materials such as SiN, and can be used as AR coatings. They are applied to an SOA by evaporation or sputtering. The refractive index of the AR coating layer can be controlled by the evaporation or sputtering conditions. Techniques are also available for measuring AR coating reflectivity [8,9]. To achieve low facet reflectivities using AR coatings requires careful control of the refractive index and thickness of the dielectric layers. Altering the SOA structure can reduce the tight tolerances required. Two techniques are commonly used: Angled facets and the window structure, which in conjunction with AR coatings can deliver low reflectivities over a wide bandwidth with minimal polarisation sensitivity.
- 41. 28 Semiconductor OpticalAmplifiers 3.2.2 Angled facet structure In an angled facet SOA, shown in Fig. 3.5, the active region is slanted away from the facet cleavage plane, thereby reducing the effective facet reflectivity. The V-number of the waveguide is where is the waveguide width and and are the effective refractive indices of the active and cladding regions respectively. The TE mode full power width is given by the approximation [10]
- 42. 29 Structures If the TEmode is assumed to have a Gaussian distribution, the effective reflectivity of an angled facet is given approximately by where is the angle between the beam propagation direction and the normal to the end facet. The Fresnel reflectivity of a TE plane wave, confined to the waveguide, at the angled facet-air interface is given by The effective reflectivity of the TM mode is almost identical to that of the TE mode. The effective reflectivity is shown in Fig 3.6 as a function of the facet angle and active region width. The relative reflectivity decreases as the facet angle increases. However the coupling efficiency between an SOA and optical fibre degrades at large facet angles due to the far-field asymmetry. AR coatings also become more polarisation sensitive as the facet angle increases. The relative reflectivity also decreases as the waveguide width increases. However, if the waveguide is too wide higher order transverse modes can appear. This problem can be overcome by broadening the waveguide near the end facets as shown in Fig. 3.7. This also preserves the single transverse mode condition [11]. Optimal facet angles lie in the range to . More analyses of angled facets can be found in [12-13].
- 43. 30 Semiconductor OpticalAmplifiers 3.2.3 Window facet structure As noted above facet reflectivities and optimum AR coating conditions are polarisation dependent. The effective facet reflectivity can be further reduced and made less sensitive to polarisation by the use of window (or buried) facets as shown in Fig 3.8 [14-15]. This structure is simply composed of a transparent region between the active region and end facets. This transparent region has an energy bandgap greater than the signal photon
- 44. 31 Structures energy. This meansthat stimulated absorption is not possible although some intrinsic material absorption will be present. The guided field from the waveguide propagates in the window region at some angle due to diffraction and is partially reflected at the end facet. The reflected field continues to broaden in space so only a small fraction is coupled back into the active region. The effective reflectivity decreases with increase in the length of the window region. However, the coupling efficiency from the SOA to an optical fibre is degraded for long Effective facet reflectivities, junction with single-layer AR coatings, facetreflectivities of the order of 5% are possible with window facets. Used in con are possible. 3.3 POLARISATION INSENSITIVE STRUCTURES Polarisation insensitive SOAs are desirable because the polarisation state of the input signal can vary slowly with time. The main cause of polarisation sensitivity is the difference between and Polarisation insensitive SOA structural designs aim to reduce or compensate for this difference. In the early stages of SOA development hybrid designs utilising two or more SOA were used to reduce polarisation sensitivity. Those techniques have now been superseded by single chip solutions that mainly focus on improvements in active region design. Three common techniques used are: Square cross-section waveguide, ridge waveguide and strained-layer superlattice material. 3.3.1 SOAs with square cross-section active waveguide Equalisation of and can be achieved through the use of a waveguide with square cross-section [16-19] as shown in the buried ridge strip device of
- 45. 32 Semiconductor OpticalAmplifiers Fig 3.9. In this device the potential barrier of the n-type/p-type InP homojunction is greater than that of the InGaAsP active region/n-type InP heterojunction. This means that there is very little carrier leakage from the active region. This carrier confinement is further improved by the use of highly resistive proton implanted InP regions. However, such structures exhibit large far-field divergence, which leads to poor coupling efficiency from the SOA to optical fibre. Tapering the active waveguide near the amplifier facets as shown in Fig. 3.9 can reduce the far field divergence. [16,18-19]. The guided mode is strongly confined in the central square-cross section waveguide, but experiences less confinement in the tapered sections and so expands. This increases the output mode size and reduces the far-field divergence, thereby increasing the coupling efficiency. The device also includes window regions to reduce the effective facet reflectivity. With this type of device a polarisation sensitivity less than 1 dB was achieved over a wide range of bias currents as shown in Fig. 3.10 [18]. 3.3.2 Ridge waveguide SOA The buried ridge waveguide SOA, as shown in Fig. 3.11, has a relatively large active region with a geometry that can be varied to equalise and [20]. The relatively large bulk active region and ridge-waveguide structure allow very low modal reflectivities both for AR coated and tilted facets without the requirement for window regions.
- 46. Structures 33 3.3.3 Structuresbased on strained-layer superlattices If bulk material is used in the active region of an SOA, the only parameter that can be changed to achieve polarisation insensitive operation is the optical confinement factor by using either a square cross-section active waveguide or a ridge-waveguide structure. Another solution is to keep the usual waveguide geometry (i.e. rectangular cross-section) and to use strained materials in the active region to increase
- 47. 34 Semiconductor OpticalAmplifiers the TM mode gain coefficient relative to the TE mode gain coefficient and thereby compensate for the fact that Reductions in polarisation sensitivity have been reported from device structures with strained tensile barriers [21], tensile strained quantum-wells [22], alternating tensile and compressive strained quantum-wells [23] and strain-balanced superlattices [24]. The particular advantage of the latter structure is that it allows simultaneous control of the polarisation sensitivity and peak gain of the device without limitations placed on the active region thickness. The properties of quantum-wells are covered in more detail in Chapter 4. Using the above techniques low polarisation sensitivity can be achieved over wide bias current and wavelength ranges with the additional advantage of high output saturation power. 3.4 HIGH SATURATION OUTPUT POWER STRUCTURES High saturation output power is a desirable SOA characteristic, particularly for power booster and multichannel applications. 3.4.1 Basic model for prediction of amplifier saturation characteristics To determine the factors that influence SOA gain at high input powers, a simple rate equation model can be used. The amplifier is assumed to have zero facet reflectivities. The material gain coefficient at the signal wavelength is assumed to be a linear function of carrier density where is the differential of with respect to n and is assumed here to be a constant. is the transparency carrier density. The carrier density obeys the rate equation The propagation of the signal intensity through the SOA is described by the travelling-wave equation
- 48. Structures 35 In (3.17)and (3.18), t is time, z is the propagation direction (along the amplifier axis), J is the active region current density, e the electronic charge, d the active region thickness, the spontaneous carrier lifetime, h the Planck constant, the signal optical frequency and the waveguide loss coefficient. Under steady state conditions the differential in (3.17) is zero. Solving (3.17) in this case gives The saturation intensity and saturation power are given by where A is the active region cross-section area. is the amplifier mode cross-section area. Inserting (3.19) into (3.18) gives where the unsaturated material gain coefficient is given by If for simplicity we assume that , then (3.21) has the solution
- 49. 36 Semiconductor OpticalAmplifiers where and are the input and output signal intensities respectively. The amplifier gain G is the ratio of the output and input signal intensities. From (3.23) we get where and is the unsaturated gain. The amplifier gain, obtained from the numerical solution of (3.24), is shown in Fig. 3.12 as a function of the ratio of for unsaturated gains of 10, 20 and 30 dB. From (3.24) the saturation output intensity (at which the amplifier gain is halfthe unsaturated gain), is given by The saturation output power of the amplifier is given by
- 50. Structures 37 3.4.2 Improvingsaturation output power (3.26) shows that the saturation output power of an SOA can be improved by increasing Inspection of (3.20) shows that this can be achieved by reducing and . In practice is inversely proportional to carrier density, so operating at high bias currents leads to an increase in . However, as the carrier density increases the amplifier gain will also increase making by reducing resonance effects more significant. The single-pass gain can be maintained or the amplifier length.. This may not always be necessary as the peak material gain coefficient shifts to shorter wavelengths as the carrier density increases. The choice of gain material can also influence the saturation behaviour of the amplifier via . In bulk materials is relatively sensitive to changes in carrier density. In quantum-well material, conditions can exist where the gain is relatively insensitive to changes in carrier density. This leads to a high [25]. It is also possible to increase by increasing An approach based on this concept, shown in Fig. 3.13, is to unfold the amplifier waveguide width towards the output facet [26-28]. This increases the mode field area at the amplifier output.
- 51. Another Random ScribdDocument with Unrelated Content
- 55. The Project GutenbergeBook of The Mutable Many: A Novel
- 56. This ebook isfor the use of anyone anywhere in the United States and most other parts of the world at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this ebook or online at www.gutenberg.org. If you are not located in the United States, you will have to check the laws of the country where you are located before using this eBook. Title: The Mutable Many: A Novel Author: Robert Barr Release date: August 10, 2017 [eBook #55326] Most recently updated: October 23, 2024 Language: English Credits: Produced by David Widger from page images generously provided by the Internet Archive *** START OF THE PROJECT GUTENBERG EBOOK THE MUTABLE MANY: A NOVEL ***
- 57. THE MUTABLE MANY ANovel
- 58. By Robert Barr SecondEdition “For the imitable, rank-scented many, let them Regard me as I do not flatter, and Therein behold themselves? CORIOLANUS. London: Frederick A. Stokes Company 1896
- 60. He that trustsyou, Where he should find you lions, finds you hares; Where foxes, geese. You are no surer, no, Than is the coal of fire upon the ice, Or hailstone in the sun. Your virtue is, To make him worthy, whose offence subdues him And curse that justice did it.
- 61. Who deserves greatness, Deservesyour hate: and your affections are A sick man’s appetite, who desires most that Which would increase his evil. He that depends Upon your favours, swims with fins of lead, And hews down oaks with rushes.. Hang ye! Trust ye? With every minute you do change a mind; And call him noble that was now your hate, Him vile, that was now your garland.” Coriolanus. CONTENTS THE MUTABLE MANY CHAPTER I. CHAPTER II. CHAPTER III. CHAPTER IV. CHAPTER V.
- 62. CHAPTER VI. CHAPTER VII. CHAPTERVIII. CHAPTER IX. CHAPTER X. CHAPTER XI. CHAPTER XII. CHAPTER XIII. CHAPTER XIV. CHAPTER XV. CHAPTER XVI. CHAPTER XVII. CHAPTER XVIII. CHAPTER XIX. CHAPTER XX. CHAPTER XXI. CHAPTER XXII. CHAPTER XXIII. CHAPTER XXIV. CHAPTER XXV. CHAPTER XXVI. CHAPTER XXVII.
- 63. CHAPTER XXVIII. CHAPTER XXIX. CHAPTERXXX. CHAPTER XXXI. CHAPTER XXXII. CHAPTER XXXIII. CHAPTER XXXIV. CHAPTER XXXV. CHAPTER XXXVI. CHAPTER XXXVI. CHAPTER XXXVIII. List of Illustrations 0001 0007
- 64.
- 65. T CHAPTER I. he officeof Monkton & Hope’s great factory hung between heaven and earth, and, at the particular moment John Sartwell, manager, stood looking out of the window towards the gates, heaven consisted of a brooding London fog suspended a hundred feet above the town, hesitating to fall, while earth was represented by a sticky black-cindered factory-yard bearing the imprint of many a hundred boots. The office was built between the two huge buildings known as the “Works.” The situation of the office had evidently been an after-thought—it was of wood, while the two great buildings which it joined together as if they were Siamese twins of industry, were of brick. Although no architect had ever foreseen the erection of such a structure between the two buildings, yet necessity, the mother of invention, had given birth to what Sartwell always claimed was the most conveniently situated office in London. More and more room had been acquired in the big buildings as business increased, and the office—the soul of the whole thing— had, as it were, to take up a position outside its body. The addition, then, hung over the roadway that passed between the two buildings; it commanded a view of both front and back yards, and had, therefore, more light and air than the office Sartwell had formerly occupied in the left-hand building. The unique situation caused it to be free from the vibration of the machinery to a large extent, and as a door led into each building, the office had easy access to both. Sartwell was very proud of these rooms and their position, for he had planned them, and had thus given the firm much additional space, with no more ground occupied than had been occupied before—a most desirable feat to perform in a crowded city like London.
- 66. Two rooms atthe back were set apart for the two members of the firm, while Sartwell’s office in the front was three times the size of either of these rooms and extended across the whole space between the two buildings. This was as it should be; for Sartwell did three times the amount; of work the owners of the business accomplished and, if it came to that, had three times the brain power of the two members of the firm combined, who were there simply because they were the sons of their fathers. The founders of the firm had with hard work and shrewd management established the large manufactory whose present prosperity was due to Sartwell and not to the two men whose names were known to the public as the heads of the business. Monkton and Hope were timid, cautious, somewhat irresolute men, as capitalists should be all the world over. They had unbounded confidence in their manager, and generally shifted any grave responsibility or unpleasant decision to his shoulders, which bore the burdens placed upon them with equanimity. Sartwell was an iron man, with firm resolute lips, and steely blue eyes that were most disconcerting to any one who had something not quite straight to propose. Even the two partners quailed under these eyes and gave way before them if it came to a conflict of opinion. Sartwell’s rather curt “It won’t do, you know” always settled things. Sartwell knew infinitely more about the works than they did; for while they had been at college the future manager was working his way up into the confidence of their fathers, and every step he took advanced his position in the factory. The three men were as nearly as possible of the same age, and the hair of each was tinged with grey; Sartwell’s perhaps more than the others. It was difficult to think of love in connection with the two partners, yet it is pleasing to know that when love did come to them at the proper time of life, it had come with gold in one hand and a rigid non-conformist conscience in the other. The two had thus added wealth to wealth by marrying, and, as their wives were much taken up with deeds of goodness, done only after strict and conscientious
- 67. investigation, so thatthe unworthy might not benefit, and as both Monkton and Hope were somewhat timorous men who were bound to be ruled by the women they married some of their wealth found its way into the coffers of struggling societies and organizations for the relieving of distress. Thus there came to impregnate the name of Monk-ton & Hope (Limited) a certain odour of sanctity which is most unusual in business circles in London. The firm, when once got at, could be counted on for a subscription almost with certainty, but alas! it was not easy to get at the firm. The applicant had to come under the scrutiny of those searching eyes of Sartwell’s, which had a perturbing habit of getting right at the heart of a matter with astonishing quickness; and when once he said “It won’t do, you know,” there was no going behind the verdict. A private stairway led from the yard below to the hall in the suspended building which divided the large office of the manager from the two smaller private rooms of the firm. This stairway was used only by the three men. The clerks and the public came in by the main entrance, where a watchful man sat behind a little arched open window over which was painted the word “Enquiries.” Outside in the gloom the two great lamps over the gateposts flared yellow light down on the cindery roadway and the narrow street beyond. Through the wide open gateway into the narrow stone-paved street poured hundreds of workingmen. There was no jostling and they went out silently, which was unusual. It seemed as if something hovered over them even more depressing than the great fog cloud just above their heads. Sartwell, alone in his office, stood somewhat back from the window, unseen, and watched their exit grimly, sternly. The lines about his firm mouth tightened his lips into more than their customary rigidity. He noticed that now and then a workman cast a glance at his windows, and he knew they cursed him in their hearts as standing between them and their demands, for they were well aware that the firm would succumb did Sartwell but give the word. The manager knew that at their
- 68. meetings their leaderhad said none was so hard on workingmen as a workman who had risen from the ranks. Sartwell’s name had been hissed while the name of the firm had been cheered; but the manager was not to be deterred by unpopularity, although the strained relations between the men and himself gave him good cause for anxiety. As he thought over the situation and searched his mind to find whether he himself were to blame in any way, there was a rap at his door. He turned quickly away from the window, stood by his desk, and said sharply, “Come in.” There entered a young man in workman’s dress with his cap in his hand. His face was frank, clear-cut, and intelligent, and he had washed it when his work was done, which was a weakness not indulged in by the majority of his companions. “Ah, Marsten,” said the manager, his brow clearing when he saw who it was. “Did you get that job done in time?” “It was off before half-past five, sir.” “Right. Were there any obstacles thrown in your way?” “None that could not be surmounted, sir.” “Right again. That’s the way I like to have things done. The young man who can accomplish impossibilities is the man for me, and the man who gets along in this world.” The young fellow turned his cap over and over in his hands, and, although he was evidently pleased with the commendation of the manager, he seemed embarrassed. At last he said, hesitatingly: “I am very anxious to get on in the world, sir.” “Well, you may have an opportunity shortly,” replied the manager. Then he suddenly shot the question:
- 69. “Are you peoplegoing to strike?” “I’m afraid so, sir.” “Why do you say ‘afraid’? Are you going out with the others, or do you call your soul your own?” “A man cannot fight the Union single-handed.” “You are talking to a man who is going to.” The young man looked up at his master. “With you it is different,” he said. “You are backed by a wealthy company. Whether you win or lose, your situation is secure. If I failed the Union in a crisis, I could never get another situation.” Sartwell smiled grimly when the young man mentioned the firm. He knew that there lay his weakness rather than his strength, for although the firm had said he was to have a free hand, yet he was certain the moment the contest became bitter the firm would be panic-stricken. Then, if the women took a hand in, the jig was up. If the strikers had known on which side their bread was buttered they would have sent a delegation of their wives to Mrs. Monkton and Mrs. Hope. But they did not know this, and Sartwell was not the man to show the weakness of his hand. “Yes,” said the manager, “I have the entire confidence of Mr. Monkton and Mr. Hope. I wonder if the men appreciate that fact.” “Oh yes, sir; they know that.” “Now, Marsten, have you any influence with the men?” “Very little, I’m afraid, sir.” “If you have any, now is the time to exert it; for their sakes, you know, not for mine. The strike is bound to fail. Nevertheless I don’t forget a man who stands by me.”
- 70. The young manshook his head. “If my comrades go, I’ll go with them. I am not so sure that a strike is bound to fail, although I am against it. The Union is very strong, Mr. Sartwell. Perhaps you do not know that it is the strongest Union in London.” The manager allowed his hand to hover for a moment over a nest of pigeon-holes, then he drew out a paper and handed it to Marsten. “There is the strength of the Union,” he said, “down to the seventeen pounds eight shillings and twopence they put in the bank yesterday afternoon. If you want any information about your Union, Marsten, I shall be happy to oblige you with it.” The young man opened his eyes as he looked at the figures. “It is a very large sum,” he said. “A respectable fighting fund,” remarked Sartwell, impartially. “But how many Saturdays do you think it would stand the drain of the pay-roll of this establishment?” “Not very many perhaps.” “It would surprise you to know how few. The men look at one side of this question only, while I am compelled to look at two sides. If any Saturday their pay was not forthcoming, they would not be pleased, would they? Now I have to scheme and plan so that the money is there every Saturday, and besides there must be enough more to pay the firm for its investment and its risk. These little details may not seem important to a demagogue who knows nothing of business, but who can harangue a body of men and make them dissatisfied. I should be very pleased to give him my place here for a month or two while I took a rest, and then we would see whether he thought there was anything in my point of view.” “Mr. Sartwell,” said Marsten, looking suddenly at the manager, “some of the more moderate men asked me to-night a similar
- 71. question to oneof yours.” “What question was that?” “They asked if I had any influence with you.” “Yes? And you told them——?” “That I didn’t know.” “Well, you will never know until you test the point. Have you anything to suggest?” “Many are against a strike, but even the more moderate think you are wrong in refusing to see the delegation. They think the refusal seems high-handed, and that if you were compelled to reject any requests made, you ought not to let things come to a crisis without at least allowing the delegation to present the men’s case.” “And do you think I am wrong in this?” “I do.” “Very well. I’ll settle that in a moment. You get some of the more moderate together—head the delegation yourself. I will make an appointment with you, and we will talk the matter over.” The young man did not appear so satisfied with this prompt concession as might have been expected. He did not reply for some moments, while the elder man looked at him critically, with his back against the tall desk. At last Marsten spoke: “I could not lead the delegation, being one of the youngest in the employ of the firm. The secretary of the Union is the leader the men have chosen.” “Ah! The secretary of the Union. That is quite a different matter. He is not in my employ. I cannot allow outsiders to interfere in any
- 72. business with whichI am connected. I am always willing to receive my own men, either singly or in deputation, and that is no small matter where so many men are at work; but if I am to open my office doors to the outside world—well, life is too short. For instance, I discuss these things with you, but I should decline to discuss them with any man who dropped in out of the street.” “Yes, I see the difficulty, but don’t you think you might make a concession in this instance, to avoid trouble?” “It wouldn’t be avoiding trouble, it would merely be postponing it. It would form a precedent, and I would have this man or that interfering time and again. I would have to make a stand some time, perhaps when I was not so well prepared. If there is to be a fight, I want it now. We need some new machinery in, and we could do with a week’s shut-down.” Marsten shook his head. “The shut-down would be for longer than a week,” he said. “I know that. The strike will last exactly three weeks. At the end of that time there will be no Union.” “Perhaps there will also be no factory.” “You mean there will be violence? Very well. In that case the strike will last but a fortnight. You see, my boy, we are in London, and there are not only the police within a moment’s call, but, back of them, the soldiers, and back of them again the whole British Empire. Oh no, Marsten, it won’t do, you know, it won’t do. “The men are very determined, Mr. Sartwell.” “All the better. I like a determined antagonist. Then you get things settled once for all. I don’t object to a square stand-up fight, but eternal haggling and higgling and seeing deputations and arbitrations, and all that sort of thing, I cannot endure. Let us know where we are, and then get on with our work.”
- 73. “Then you havenothing to propose, Mr. Sartwell? Nothing conciliatory, I mean.” “Certainly I have. Let the men request that blatant ass Gibbons to attend to his secretarial duties and then let a deputation from our own workshops come up and see me. We’ll talk the matter over, and if they have any just grievance I will remedy it for them. What can be fairer than that?” “It’s got to be a matter of principle with the men now—that is, the inclusion of Gibbons has. It means recognizing of the Union.” “Oh, I’ll recognize the Union and take off my hat to it; that is, so far as my own employees are concerned. But I will not have an outsider, who knows nothing of this business, come up here and spout his nonsense. It’s a matter of principle with me as well as with the men.” Marsten sighed. “I’m afraid there is nothing for it then but a fight,” he said. “Perhaps not. One fool makes many. Think well, Marsten, which side you are going to be with in this fight. I left a Union, and although I was older than you are at the time, I never repented it. It kept me out of employment, but not for long, and they kept me out of it in the very business of which I am now manager. The Union is founded on principles that won’t do, you know. Any scheme that tends to give a poor workman the same wages as a good workman is all wrong.” “I don’t agree with you, Mr. Sartwell. The only hope for the workingman is in combination. Of course we make mistakes and are led away by demagogues, but some day there will be a strike led by an individual Napoleon, and then we will settle things once for all, as you said a while ago.” Sartwell laughed, and held out his hand.
- 74. “Oh, that’s yourambition is it? Well, good luck attend you, my young Napoleon. I should have chosen Wellington, if I had been you. Good-night. I am waiting for my daughter, to whom I foolishly gave permission to call for me here in a cab.” Marsten held the hand extended to him so long that the manager looked at him in astonishment. The colour had mounted from the young man’s cheeks to his brow and his eyes were on the floor. “Mr. Sartwell,” he said, with an effort, “I came tonight to speak with you about your daughter and not about the strike.” The manager dropped his hand as if it had been red-hot, and stepped back two paces. “About my daughter?” he cried, sternly. “What do you mean?” Marsten had to moisten his lips once or twice before he could reply. His released hand opened and shut nervously. “I mean,” he said, “that I am in love with her.” The manager sat down in the office chair beside his table. All the former friendliness had left his face, and his dark brows lowered over his keen eyes, into which their usual cold glitter had returned. “What folly is this?” he cried, with rising anger. “You are a boy, and from the gutter at that, for all I know. My daughter is but a child yet; she is only——” He paused. He had been about to say seventeen when it occurred to him that he had married her mother when she was but a year older. Marsten’s colour became a deeper red when the manager spoke so contemptuously of the gutter. He said slowly, and with a certain doggedness in his tone: “It is no reproach to come from the gutter—the reproach is in staying there. I have left it, and I don’t intend to return.”
- 75. “Oh, ‘intend’!” criedthe manager, impatiently. “We all know what is paved with intentions. Why; you have never even spoken to the girl!” “No, but I mean to.” “Do you? Well, I shall take very good care that you do not.” “What have you against me, Mr. Sartwell?” “What is there for you? Perhaps you will kindly specify your recommendations.” “You are very hard on me, Mr. Sartwell. You know that if I came from the gutter, what education I have, I gave to myself. I have studied hard, and worked hard. Does that count for nothing? I have a good character, and I have a good situation——” “You have not. I discharge you. You will call at the office to- morrow, get your week’s money, and go.” “Oh!” “Yes, ‘oh!’ You did not think that of me, did you?” “I did not.” “Well, for once you are right. I merely wish to show you how your good situation depends on the caprice of one man. I have no intention of discharging you. I am not so much afraid of you as that. I’ll look after my daughter.” Marsten said bitterly: “Gibbons, ass as he is, is right when he says that no one is so hard on a workman as one who has risen from the ranks. You were no better off than I am, when you were my age.” Sartwell sprang to his feet, his eyes ablaze with anger.
- 76. “Pay attention, youngman,” he cried. “All the things you have done, I have done. All the things you intend to do, I have already done. I have, in a measure, educated myself, and I have worked hard night and day. I have attained a certain position, a certain responsibility, and a certain amount of money. I have had little pleasure and much toil in my life, and I am now growing old. Yet as I look back I see that there was as much luck as merit in what success I have had. I was ready when the chance came, that was all; if the chance hadn’t come, all my readiness would have done me little good. For one man who succeeds, a dozen, equally deserving, fail. “Now, why have I gone through all this? Why? For myself? Not likely. I have done it so that she may not have to be that tired drudge—a workman’s wife—so that she may begin where I leave off. That’s why. For myself, I would as soon wear a workman’s jacket as a manager’s coat. And now, having gone through all this for her sake —you talk of love! What is your love for her compared to mine? When I have done all this that she might never know what it means, shall I be fool enough, knave enough, idiot enough, to thrust her back where I began, at the beck of the first mouthing ranter who has the impudence to ask for her? No, by God, no! Now you have had your answer, get out, and don’t dare to set foot in this office until you are sent for.” Sartwell in his excitement smote the desk with his clenched fist to emphasize his sentences. Marsten shrank before his vehemence, realizing that no workman had ever seen the manager angry before, and he dreaded the resentment that would rise in Sartwell’s heart when the coldness returned. He felt that he would have been more diplomatic to have left sooner. Nevertheless, seeing that things could be no worse, he stood his ground. “I thought,” he said, “that it would be honourable in me to let you know——” “Don’t talk to me of honour. Get out.”
- 77. At that momentthe door from the private stairway opened and a young girl came in. Her father had completely forgotten his appointment with her, and both men were taken aback by her entrance. “I knocked, father,” she said, “but you did not hear me.” “In a moment, Edna. Just step into the hall for a moment,” said her father, hurriedly. “I beg of you not to leave, Miss Sartwell,” said Marsten, going to the other door and opening it. “Good-night, Mr. Sartwell.” “Good-night,” said the manager, shortly. “Good-night, Miss Sartwell.” “Good-night,” said the girl sweetly, with the suggestion of a bow. The eyes of the two men met for a moment, the obstinacy of the race in each; but the eyes of the younger man said defiantly: “I have spoken to her, you see.”
- 78. W CHAPTER II. e speakof our individuality as if such a thing really existed —as if we were actually ourselves, forgetting that we are but the sum of various qualities belonging to ancestors, most of whom are dead and gone and forgotten. The shrewd business-man in the City imagines that his keen instincts are all his own; he does not recognize the fact that those admirable attributes which enable him to form a joint-stock company helped an ancestor in the Middle Ages to loot a town, or a highwayman of a later day to relieve a fellow-subject of a full purse on an empty heath. Edna Sartwell possessed one visible, undeniable, easily recognized token of heredity: she had her father’s eyes, but softened and luminous and disturbingly beautiful—eyes to haunt a man’s dreams. They had none of the searching rapier-like incisiveness that made her father’s eyes weapons of offence and defence; but they were his, nevertheless, with a kindly womanly difference, and in that difference lived again the dead mother. “Edna,” said her father, when they were alone, “you must not come to this office again.” There was more sharpness in his tone than he was accustomed to use toward his daughter, and she looked up at him quickly. “Have I interrupted an important conference?” she asked. “What did the young man want, father?” “He wanted something I was unable to grant.” “Oh, I am so sorry! He did appear disappointed. Was it a situation?” “Something of the sort.”
- 79. “And why couldn’tyou give it to him? Wasn’t he worthy?” “No, no. No, no!” “He seemed to me to have such a good face—honest and straightforward.” “Good gracious! child, what do you know about faces? Do not interfere in business matters; you don’t understand them. Don’t chatter, chatter, chatter. One woman who does that is enough in a family—all a man can stand.” The daughter became silent; the father pigeonholed some papers, took them out again, rearranged them, and placed them back. He was regaining control over himself. He glanced at his daughter, and saw tears in her eyes. “There, there, Edna,” he said. “It is all right. I’m a little worried to- night, that’s all. I’m afraid there’s going to be trouble with the men. It is a difficult situation, and I have to deal with it alone. A strike seems inevitable, and one never can tell where it will end.” “And is he one of the strikers? It seems impossible.” A look of annoyance swept over her father’s face. “He? Why the——Edna, you return to a subject with all the persistency of a woman. Yes. He will doubtless go on strike to- morrow with all the rest of the fools. He is a workman, if you want to know; and furthermore, he is going on strike when he doesn’t believe in it—going merely because the others go. He admitted it to me shortly before you came in. So you see how much you are able to read in a man’s face.” “I shouldn’t have thought it,” said the girl, with a sigh. “Perhaps if you had given him what he wanted he would not go on strike.” “Oh, now you are making him out worse than even I think him. I don’t imagine he is bribable, you know.”
- 80. “Would that bebribery?” “Suspiciously like it; but he can strike or not as he wishes—one more or less doesn’t matter to me. I hope, if they go, they will go in a body; a few remaining would only complicate things. Now that you understand all about the situation, are you satisfied? It isn’t every woman I would discuss it with, you know, so you ought to be flattered.” Sartwell was his own man once more, and he was mentally resolving not to be thrown off the centre again. “Yes, father, and thank you,” said the girl. “The cab is waiting,” she added, more to let him know that so far as she was concerned the discussion was ended, than to impart the information conveyed in her words. “Let it wait. That’s what cabs are for. The cabby usually likes it better than hurrying. Sit down a moment, Edna; I’ll be ready presently.” The girl sat down beside her father’s table. Usually Mr. Sartwell preferred his desk to his table, for the desk was tall where a man stands when he writes. This desk had three compartments, with a lid to each. These were always locked, and Sartwell’s clerks had keys to two of them. The third was supposed to contain the manager’s most private papers, as no one but himself ever saw the inside of it. The lid locked automatically when it was shut, and the small key that opened it dangled at Sartwell’s watch-chain. Edna watched her father as he unlocked one after another of the compartments and apparently rearranged his papers. There was always about his actions a certain well-defined purpose, but the girl could not help noticing that now he appeared irresolute and wavering. He seemed to be marking time rather than making progress with any definite work. She wondered if the coming strike was worrying him more than he had been willing to admit. She wished to help, but knew that nothing would be more acceptable to
- 81. him than simplyleaving him alone. She also knew that when her father said he would be ready to go home with her at a certain hour he usually was ready when that hour came. Why, then, did he delay his departure? At last Sartwell closed down the lid of one desk and locked it as if he were shutting in his wavering purpose, then he placed the key from his watch-guard in the third lock and threw back the cover. An electric light dangling by a cord from the ceiling, threw down into the desk rays reflected by a circular opal shade that covered the lamp. The manager gazed for a few moments into the desk, then turning to his daughter, said: “Edna, you startled me when you came in tonight.” “I am very sorry, father. Didn’t you expect me?” “Yes, but not at that moment, as it happened. You are growing very like your mother, my girl.” There was a pause, Edna not knowing what to say. Her father seldom spoke of his dead wife, and Edna could not remember her mother. “Somehow I did not realize until to-night—that you were growing up. You have always been my baby to me. Then—suddenly—you came in. Edna, she was only four years older than you when she died. You see, my dear, although I grow older, she always remains young—but I sometimes think that the young man who was her husband is dead too, for there is not much likeness to him in me.” Sartwell had been drumming lightly with his fingers on the desk top as he spoke; now he reached up and turned off the electric light as if its brilliancy troubled him. The lamp in the centre of the room was sufficient, and it left him in the shadow. “I suppose there comes a time in the life of every father, when he learns, with something of a shock, that the little girl who has been playing about his knee is a young woman. It is like when a man
- 82. hears himself alludedto as old for the first time. I well remember how it made me catch my breath when I first heard myself spoken of as an old man.” “But you are not old,” cried the girl, with a little indignant half sob in her voice. She wished to go to her father and put her arms around his neck, but she felt intuitively that he desired her to stay where she was until he finished what he had to say. “I am getting on in that direction. None of us grows younger, but the dead. I suppose a daughter is as blind to her father’s growing old, as he to her advancing womanhood. But we won’t talk of my age. We are welcoming the coming, rather than speeding the going, to-night. You and I, Edna, must realize that we, in a measure, begin life on a new line with each other. We are both grown-up people. When your mother was a little older than you are, I had her portrait painted. She laughed at me and called me extravagant. You see, we were really very poor, and she thought, poor girl, that a portrait of herself was not exactly a necessity. I have thought since that it was the one necessary thing I ever bought. I had it copied, when I got richer, by a noted painter, who did it more as a favour to me than for the money, for painters do not care to copy other men’s work. Curiously enough he made a more striking likeness of her than the original was. Come here, my girl.” Edna sprang to her father’s side and rested her hand lightly on his shoulder. Sartwell turned on the electric light. At the bottom of the desk lay a large portrait of a most beautiful woman. The light shone down on the face, and the fine eyes looked smilingly up at them. “That was your mother, Edna,” said the father, almost in a whisper, speaking with difficulty. The girl was crying softly, trying not to let her father know it. Her hand stole from the shoulder next her to the other, his hand caressed her fair hair. “Poor father!” she said, trying to speak bravely.
- 83. “How lonely youmust have been. I seem to—to understand things —that I didn’t before—as if I had suddenly grown old.” They looked at the picture for some time together in silence, then she said: “Why did you never show me the portrait before?” “Well, my dear, it was here and not at the house, and when you were a small girl, you did not come to the office, you know. Then, you see, your stepmother had the responsibility of bringing you up— and—and—somehow I thought it wouldn’t be giving her a fair chance. The world is rather hard on stepmothers.” He hurriedly closed the desk. “Come, come,” he cried, brusquely, “this won’t do, you know, Edna. But this is what I want to say. I want you to remember—to understand rather—that you and I are, as it were, alone in the world; there is a bond between us in that, as well as in the fact that we are father and daughter. I want you always to feel that I am your best friend, and there must never come any misunderstanding between us.” “There never could, father,” said the girl, solemnly. “That’s right, that’s right. Now if anything should happen to trouble you, I want you to come to me and tell me all about it. I wish there to be complete confidence between us. If anything perplexes you, tell me; if it is trivial I want to know, and if it is serious I want to know. Sometimes an apparently trivial problem is really a serious one, and vice-versa; and remember, it is almost as important to classify your problem as to solve it. That’s where I can help you; for even if I could not disentangle the skein, I could perhaps show you that it was not worth unravelling.” The girl regarded her father earnestly while he spoke, and then, as if to show that woman’s intuition will touch the spot around which a man’s reason is elaborately circling, she startled him by saying:
- 84. “Father, something hashappened concerning me, that has made you anxious on my account. What is it? I think I should know. Has my step-mother been saying——” “No no, my child, your step-mother has been saying nothing about you. And if she had I would not—that is, I would have given it my best attention, and would have no hesitation in letting you know what it was. You mustn’t jump at conclusions; perhaps I am talking with unnecessary seriousness; all I wish to impress upon you is that although I am seemingly absorbed in business, you are much more important to me than anything else—that, in fact, since your mother died, you are the only person who has been of real importance to me, and so if you want anything, let me know—a new frock, for instance, of exceptional expensiveness. I think you will find that where your happiness is concerned, I shall not allow any prejudices of mine to stand in the way.” The girl looked up at her father with a smile. “I don’t think my happiness will be endangered for lack of a new gown,” she said. “Well, dress is very important, Edna, we mustn’t forget that; though I merely instanced dress for fear you would take me too seriously. And now, my girl, let us get home. This is our last conference in this office, you know, and there has somehow entered into it the solemnity that pertains to all things done for the last time. Now if you are ready, I am.” “Not quite, father. You see, I like this office—I always did,—and now—after to-night—it will always seem sacred to me. All this talk has been about an insignificant person and her clothes—but what impresses me, father, is how much alone you have been nearly all your life. I never realized that before. Now after this you must talk over your business with me; I may not be able to help, at first, but later on, who can tell? Then it will flatter me by making me think our compact is not one-sided. Is it a bargain, father?”
- 85. Welcome to ourwebsite – the ideal destination for book lovers and knowledge seekers. With a mission to inspire endlessly, we offer a vast collection of books, ranging from classic literary works to specialized publications, self-development books, and children's literature. Each book is a new journey of discovery, expanding knowledge and enriching the soul of the reade Our website is not just a platform for buying books, but a bridge connecting readers to the timeless values of culture and wisdom. With an elegant, user-friendly interface and an intelligent search system, we are committed to providing a quick and convenient shopping experience. Additionally, our special promotions and home delivery services ensure that you save time and fully enjoy the joy of reading. Let us accompany you on the journey of exploring knowledge and personal growth! ebookultra.com