Advanced optical communication By Er. Swapnl Kaware
Topic:- Optical Sources, Detectors and
Optical Fibre measurements
Er. Swapnil V. Kaware,
Absorption and Dispersion
• The phase speed c = λν, which is defined as the product of
wavelength and frequency, is reduced in comparison to speed
of light in a vacuum, c0, when the electromagnetic wave
travels through a medium with an index of refraction n > 1.
• The reduced value is c = c0 /n. We will show that the
frequency dependency of n leads to a dispersion, which can
be described using a classical model.
• It will be also shown that the imaginary part of a complex
index of refraction describes the damping of an
• Light Amplification by Stimulated Emission of
– “Stimulated emission ”antonym of “spontaneous emission”
– optical transition stimulated by the effect of electric field of
light wave on the contrary usually emission occur
spontaneously without help of electric field.
– Light amplification by stimulated emission of radiation, or
laser in short, is a device that creates and amplifies
electromagnetic radiation of specific frequency through
process of stimulated emission.
– In laser, all the light rays have the same wavelength and they
are coherent; they can travel long distances without diffusing.
• Light amplification by stimulated emission of radiation, that
is--lasers, have become an important tool in chemistry.
• Lasers are ideal light sources for spectroscopy, chemical
kinetics studies, and light scattering studies of molecular
• High-powered lasers are finding use as light sources for
photochemical synthesis, yielding products not available from
• Dye lasers are an important class of lasers because they can
be tuned to a range of wavelengths.
LED and LD
• LED is light emitting diode
• LD is laser diode
– Diode is a semiconductor device which has an
effect of rectification
– Both LED and LD are semiconductor diode with a
forward bias. Both emit light
– LED emits light by spontaneous emission
mechanism, while LD has an optical cavity which
enables multiplication of photon by stimulated
LED and LD
• LD (laser diode) works as LED if the operating
current does not exceed the threshold value.
Forward bias current
Development of Communication
• To meet with the growing need for large
capacity information exchange, optical fiber
communication system has been developed.
Data carrying capacity (bps)
Development of Optical Communication
θ i<θ c
total reflection and
Attenuation and dispersion in
• Attenuation: reduction
of light amplitude
Principle of Photodetection
• When a photon with an energy greater than the band
gap i.e. Eg is incident on the semiconductor, it is
absorbed and it generates an electron-hole pair, that is
an electron in the conduction band and a hole in the
• If the pair is created within the space charge region, the
electric field in the junction separates the charges and
drifts them to the neutral regions.
• The carrier drift generates a photocurrent in the external
circuit that provide an electrical signal.
Photodetectors (or photosensors ) are transducers that alter one of their
characteristics when light energy impinges on them. In this category,
photoresistors alter their ohmic resistance, rods and cones of the retina
neurons of the eye alter their electrochemical response, and chlorophyll in
plant leaves alters the rate of converting CO2 to O2.
Some other photodetectors alter the flow of electrical current or the
potential difference across their terminals.
Photodetectors with sufficiently fast response that provide a measurable
output for a small amount of light, are easily reproducible, and are
economical are worth investigating for applications in high-speed optical
This category includes avalanche photodiodes (APDs) and positive intrinsic
Spectral response relates the amount of current produced with wavelength,
assuming that all wavelengths are at the same level of light.
Photosensitivity is the ratio of light energy (in watts) incident on the device to
the resulting current (in amperes).
Quantum efficiency is the number of generated electron-hole pairs (Le.,
current) divided by the number of photons.
Dark current is the amount of current that flows through the photodiode in the
absence of any light (dark), when the diode is reverse-biased.
This is a source of noise when the diode is reverse-biased.
Forward-biased noise is a (current) source of noise that is related to the
shunt resistance of the device. The shunt resistance is defined as the ratio
voltage (near oV) to the amount of current generated. This is also called
shunt resistance noise.
Timing response of the photodetector is the time for the output signal to
climb from 10% to 90% of its amplitude (rise time) and to drop from 90% to
10% (fall time).
Frequency bandwidth is the frequency (or wavelength) range in which the
photodetector is sensitive.
Cutoff frequency is the highest frequency (wavelength) at which the
photodetector is sensitive.
• The APD is a semiconductor device that, when reverse-biased,
creates strong fields in the junction region.
• When a photon causes an electron-hole pair, the pair flows
through the junction.
• Because of the strong fields in the junction, the electron gains
enough energy to cause secondary electron-hole pairs, which in
turn cause more.
• Thus a multiplication (or avalanche) process takes place (hence
the name), and a substantial current is generated from few initial
• Optical wavelength detection in sensing can be generally
categorized into two types: passive detection schemes and active
• In passive schemes there are no power driven components
• A passive detection scheme refers to those that do not use any
electrical, mechanical or optical active devices in the optical part of
• Most of the passive devices are linearly wavelength dependent
devices such as bulk edge filters (Mille etal., 1992), biconical fiber
filters (Ribeiro et al., 1996), wavelength division couplers (Davis &
Kersey, 1994), gratings (Fallon et al., 1999), multimode interference
couplers (Wang &
The simplest way to measure the wavelength of light is to use a wavelength
dependent optical filter with a linear response.
This method is based on the usage of an edge filter, which has a narrow linear
response range with a steep slope or a broad band filter, which has a wide
range with less steep slope.
In both cases, the wavelength interrogator is based on intensity measurement,
i.e., the information relative to wavelength is obtained by
monitoring the intensity of the light at the detector.
For intensity based demodulators, the use of intensity referencing is necessary
because the light intensity may fluctuate with time.
This could occur not only due to a wavelength change but also due to a power
fluctuation of the light source, a disturbance in the light-guiding path or the
dependency of light source intensity on the wavelength.
Optical Fibers by, T. Okoshi,, Academic Press, San Diego, CA, 1982.
(ii). Optical Waveguide Theory by, A.W. Snyder and J. D. Love,,
Chapman & Hall, London, 1983.
(iii). Single-Mode Fiber Optics by, L. B. Jeunhomme,, Marcel Dekker,
New York, 1990.
(iv). Optical Fiber Communications by, T. Li, Ed, Academic Press, San
Diego, CA, 1985.
(v). Optical Fibers: Materials and Fabrication by, T. Izawa and S. Sudo,,
Kluwer Academic, Boston, 1987.
(vi). Fundamentals of Optical Fiber Communications D. B. Keck, in, M. K.
Barnoski, Ed., Academic Press, San Diego, CA, 1981.