This document summarizes a presentation about vertical cavity surface emitting lasers (VCSELs). It begins with an introduction to VCSELs and their history. It then discusses VCSEL structure, including distributed Bragg reflectors (DBRs) for optical and current confinement, and rate equations that describe VCSEL characteristics. Continuous wave performance is examined through current-light output curves. Applications include optical fiber data transmission, displays, and sensors. Later sections provide examples of single-mode oxide-confined VCSELs for printers and sensors, electrically pumped GaSb-based VCSELs, and all-optical flip-flop operation using polarization bistable VCSELs.

1. 1
By:
Hojjatollah Sarvari
Esmaeil Dastjerdi
Department of Physics
Shiraz University, Shiraz-Iran
March-2009
Vertical Cavity Surface Emitting Lasers
(VCSELs)

2. 2
Outline of talk
 Introduction.
 Why the Surface-Emitting Laser?
 History of VCSEL research.
 Structure:
 How Does the DBR Work?
 Current Confinement.
 Optical Confinement.
 CW Characteristics.
 Rate Equations.
 P–I Curve of a 1.3-um InGaAsP laser.
 Applications and new structures of VCSELs
 Advantages of VCSELs
 Conclusion

3. 3
Introduction
 The vertical-cavity surface-emitting laser (VCSEL) is a type of
semiconductor laser diode with laser beam emission perpendicular from the
top surface.
 The term VCSEL was coined in a publication of the Optical Society of
America in 1987.
 The cavity is along the vertical direction, with a very short length, typically
1-3 wavelengths of the emitted light.

4. 4
Why the Surface-Emitting Laser?
 There are several advantages to producing VCSELs when compared with the production
process of edge-emitting lasers.
 Edge-emitters cannot be tested until the end of the production process. If the edge-emitter
does not work, whether due to bad contacts or poor material growth quality, the production
time and the processing materials have been wasted.
 VCSELs , however, can be tested at several stages throughout the process to check for
material quality and processing issues.
 Additionally, because VCSELs emit the beam perpendicular to the active region of the laser
as opposed to parallel as with an edge emitter, tens of thousands of VCSELs can be processed
simultaneously on a three inch Gallium Arsenide wafer.
 Some of VCSEL’s applications are as follows:
– Optical fiber data transmission .
– Analog broadband signal transmission.
– Laser printers .
– computer mouse.

5. 5
Why the Surface-Emitting Laser?

6. 6
History of VCSEL research
 It is recognized that Mr Iga suggested VCSEL in 1977.
 The first VCSEL was presented in 1979 by Soda, Iga, Kitahara and
Suematsu (Soda 1979), where the 1.3 um-wavelength GaInAsP/InP
material was used for the active region.
 In 1986 a 6-mA thereshold GaAs device was made by that group.
 But devices for CW operation at room temperature were not reported until
1988 (Koyama 1988).
 And after that in 1989, Jack Jewell demonstrated an InGaAs SE laser
exhibiting 2-mA threshold.
 Today, VCSELs have replaced edge-emitting lasers in applications for
short-range fiber optic communication such as Gigabit Ethernet and Fiber
Channel.

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History of VCSEL research

8. 8
History of VCSEL research
 Following figure gives plots of the lowest threshold current versus the year
of publication for 980 nm diode lasers of any type:

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History of VCSEL research
 Following figure shows the improvement in the electric to optical power
conversion (or “wall-plug”) efficiency versus year:

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Structure
 The laser resonator consists of two distributed Bragg reflector (DBR) mirrors
parallel to the wafer surface with an active region consisting of one or more
quantum wells for the laser light generation in between.
 The reflectivity required for low threshold currents is greater than 99.9%,
Distributed Bragg Reflectors (DBRs) are needed for this reflectivity.

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How Does the DBR Work?
 The planar DBR-mirrors consist of layers with alternating high and low refractive
indices. Each layer has a thickness of a quarter of the laser wavelength in the
material, yielding intensity reflectivities above 99%.
 The DBR layers also carry the current in the device, therefore, more layers increase
the resistance of the device.
 The wavelength of VCSELs may be tuned, within the gain band of the active region,
by adjusting the thickness of the reflector layers.

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Structure
 In common VCSELs the upper and lower mirrors are doped as
p-type and n-type materials, forming a diode junction. In more
complex structures, the p-type and n-type regions may be
buried between the mirrors, requiring a more complex
semiconductor process to make electrical contact to the active
region, but eliminating electrical power loss in the DBR
structure.
 In laboratory investigation of VCSELs using new material
systems, the active region may be pumped by an external light
source with a shorter wavelength, usually another laser. This
allows a VCSEL to be demonstrated without the additional
problem of achieving good electrical performance; however
such devices are not practical for most applications.

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Current Confinement
 The ultimate threshold current depends on how to make the active volume
small, and how well the optical field can be confined in the cavity to
maximize the overlap with the active region.
 Typical models for current confinement are as follows:
a) Ring-Electrode type:
 This structure can limit the current flow in the vicinity of the ring electrode.
 The light output can be taken out from the center window.
 This is easy to fabricate, but the current cannot completely be confined to a small area due to diffusion.

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Current Confinement
b) Proton-Bombardment type:
 An insulating layer was used by proton (H+) irradiation to limit the current spreading toward the area.
 The process is rather simple and most commercialized devices are made by this method.
c) Buried-Heterostructure (BH) type:
 The mesa including the active region was buried with a wide-gap semiconductor to limit the current.
 The refractive index can be small in the surrounding region.
 This is one of the ideal structures in terms of current and optical confinement.
 The problem is that the necessary process is rather complicated: in particular in making a tiny 3D device.

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Current Confinement
d) Selective AlAs oxidation type:
 The AlAs layer was oxidized to make an insulator.
e) Oxidized DBR type:
 The same method is applied to oxidize DBR consisting of AlAs and GaAs.
 This is one of the volume-confinement methods and can reduce the non-radiative recombination.

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Current Confinement
 Recently the two main methods of restricting the current in a VCSEL were
characterized by two types of VCSELs: ion-implanted VCSELs and Oxide
VCSELs.
 In the early 1990s, telecommunications companies tended to favor ion-
implanted VCSELs. Ions, (often hydrogen ions, H+), were implanted into
the VCSEL structure everywhere except the aperture of the VCSEL,
destroying the lattice structure around the aperture, thus inhibiting the
current.
 In the mid to late 1990s, companies moved towards the technology of
oxide VCSELs. The current is confined in an oxide VCSEL by oxidizing
the material around the aperture of the VCSEL. A high content aluminium
layer that is grown within the VCSEL structure is the layer that is oxidized.
 Oxide VCSELs also often employ the ion implant production step. As a
result in the oxide VCSEL, the current path is confined by the ion implant
and the oxide aperture.

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Optical Confinement
a) Fabry-Perot type:
 The optical resonant field is determined by the two reflectors.
b) Gain-Guide type:
 The field simply was limited at the region where the gain exists.

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Optical Confinement
c) Buried Heterostructure (BH):
d) Selective AlAs oxidation type:
 Due to the index difference between the AlAs and the oxidized region, the optical field can
be confined as well by a kind of lens effect.

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CW Characteristics
 The operating characteristics of semiconductor lasers
are well described by a set of rate equations that govern
the interaction of photons and electrons inside the
active region.
 A rigorous derivation of the rate equation generally
starts from the Maxwell’s equations.
 The rate equations can also be written heuristically by
considering various physical phenomena through which
the number of photons, P , and the number of electrons,
N, change with time inside the active region.

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Rate Equations
gm(N) = σg (N – NT) (4)
(5)
τp
-1 = Vg αcav = Vg (αmir+ αint) (6)
 For a single-mode laser, these equations take the form:
 G = the net rate of stimulated emission
(1)
(2)
(3)

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P–I Curve of a 1.3-um InGaAsP laser
The laser performance degrades at high temperatures.
Ith(T) = Io exp(T/To) (7)

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CW Characteristics
 In the case of CW operation at a constant current I, the time derivatives in Eqs.
(1) and (2) can be set to zero.
GP – P/ τp = 0 P ( G τp – 1) = 0 (8)
 For currents such that G τp < 1, P = 0 and I/q – N/τc = 0. (9)
 The threshold is reached at a current for which G τp = 1.
G τp = GN τp (N - No) = 1 Nth = No + (GN τp)-1 (10)
Ith = q Nth/ τc = q/ τc (No + 1/( GN τp)) (11)
 For I > Ith, we have: -dN/dt = dP/dt.
And hence the photon number P increases linearly with I as:
P = (τp /q)(I – Ith) (12)

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CW Characteristics
 The emitted power Pe is related to P by the relation
Pe = (Vg αmir) ħωP (13)
Pe = ħω/q (ηint αmir)/(αmir+ αint) (I – Ith) (14)
 dPe/dI = ħω/q ηd with ηd = ηint αmir/(αmir+ αint) (15)
 ηext = (photon-emission rate)/(electron-injection rate)
ηext = (Pe/ħω)/(I/q) = q/(ħω) Pe/I (16)
ηext = ηd (1 – Ith/I) (17)
 Similar to the case of LEDs, one can define the total quantum
efficiency ( or wall-plug efficiency) as ηtot = Pe/(Vo I), where
Vo is the applied voltage. It is related to ηext as:
ηtot = ħω/(qVo) ηext ~ Eg/(qVo) ηext (18)

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VCSELApplication
• WDM Applications :
Tuning wavelength with MEMS technology
2D VCSELs Array with special wavelength for each VCSEL
• Optical Memory : VCSELs are used to read the data on compact disks and CD-
ROMs ,DVD, Near field
• Optoelectronic: Printer, Laser pointer, Mobile tools, Mouses
• Optical Information Processing : Optical processors, Parallel processing
• Optical Sensing : Longer wavelength VCSELs for sensor application, Optical fiber
sensing, Bar code readers, Telemetry
• Displays: Shorter wavelength VCSELs for displays, Array light sources, Multi-beam
search lights, High efficiency sources

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Material and Wavelength of VCSELs
• AlGaInP/AlGaAs for Red wavelength (650-680nm) VCSELs
• GaInAsP/AlGaAs for Near-IR wavelength (780-850nm) VCSELs
• AlGaInAs for wavelength 850nm VCSELs
• GaInAsN for Long-wavelength (1.3-1.55um) VCSELs
• Sb for Long-wavelength (1.3-1.55um) VCSELs
• III-V Nitride for Visible wavelength VCSELs

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Single-Mode Oxide-Confined VCSEL for Printers and Sensors
Hiromi Otoma, Akemi Murakami, Yasuaki Kuwata, Nobuaki Ueki, Naotaka Mukoyama
2006 Electronics Systemintegration Technology Conference,Germany, 1-4244-0553-1/06/$20.00 2OO6 IEEE
• p-DBR mirror consists of 17 pairs of
Al0.9Ga0.1As/Al0.12Ga0.88As
• n-DBR mirror consists of 34 pairs of
Al0.9Ga0.1As/Al0.12Ga0.88As
• P-contact: Ti/Au
• N-contact: AuGe/Au
• Passivation layer (SiNx)
• Dm < Dox
• MOCVD for Fabrication

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Single-Mode Oxide-Confined VCSEL for Printers and Sensors
Hiromi Otoma, Akemi Murakami, Yasuaki Kuwata, Nobuaki Ueki, Naotaka Mukoyama
2006 Electronics Systemintegration Technology Conference,Germany, 1-4244-0553-1/06/$20.00 2OO6 IEEE
• Dox=3.1um & Dm=3.9um
• Lasing at λ=857nm in single mode
• P=4.7mW at I=8mA & room temperature

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Single-Mode Oxide-Confined VCSEL for Printers and Sensors
Hiromi Otoma, Akemi Murakami, Yasuaki Kuwata, Nobuaki Ueki, Naotaka Mukoyama
2006 Electronics Systemintegration Technology Conference,Germany, 1-4244-0553-1/06/$20.00 2OO6 IEEE
• Wavelength shifts with temperature and injection current
• Tunable VCSELs useful as light source for Atomic Clocks
• Cut off frequency is 5GHz for I=2mA
• Lifetime of device is about 30 years for I=1mA at room temperature

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Single-Mode Oxide-Confined VCSEL for Printers and Sensors
Hiromi Otoma, Akemi Murakami, Yasuaki Kuwata, Nobuaki Ueki, Naotaka Mukoyama
2006 Electronics Systemintegration Technology Conference,Germany, 1-4244-0553-1/06/$20.00 2OO6 IEEE
• 8*4 VCSEL Array single-mode at λ=780nm for color laser printer
• SMSR > 20dB & Ith=0.32mA
• 1.2mW at 1mA for printer with 80ppm speed & 2400DPI resolution

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Electrically Pumped GaSb-based VCSEL
With Buried Tunnel Junction
A.Bachmann, T. Lim, K. Kashani-Shirazi, O. Dier, C. Lauer and M.-C. Amann
2008 Optical Society of America, CLEO/QELS 2008
• GaSb-based VCSELs are good light source for Mid-Infrared (2-4um)
• Active layer : Ga.63In.37As.03Sb.97 with 11nm thickness
• BTJ produce with n+InAsSb & p+GaSb
• Bottom DBR : 24 pair AlAsSb/GaSb with R=99.8%
• Top DBR : 4 pair Si/SiO2 with R=99.7%
• Ith=2.7 & 3.3mA for -10°C & 20°C respectively

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Electrically Pumped GaSb-based VCSEL
With Buried Tunnel Junction
A.Bachmann, T. Lim, K. Kashani-Shirazi, O. Dier, C. Lauer and M.-C. Amann
2008 Optical Society of America, CLEO/QELS 2008
• Wavelength shift due to Temperature and Current make wavelength tuning
as high as 10 nm
• For sensing CO gas at 2.325um
• Some feature of this VCSEL for sensor application are single-mode
operation, low-cost production and tuning

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All-Optical Flip-Flop Operation Using 1.55um Polarization
Bistable VCSELs
Takeo Katayama, Toshiyuki Kitazawa, Hitoshi Kawaguchi
2008 OSA / CLEO/QELS 2008 (978-1-55752-859-9/08/$25.00 ©2008 IEEE)
• Device fabricated with low-pressure MOVPE on InP substrate
• Air gap for current confinement produced with Wet-Etching Method
• VCSEL has two lasing modes with polarization 0° & 90°

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Optimal radii of photonic crystal holes within DBR mirrors
in long wavelength VCSEL
Tomasz Czyszanowski, Maciej Dems, Hugo Thienpont, Krassimir Panajotov
2007 OSA,5 February 2007 / Vol. 15, No. 3 / OPTICS EXPRESS 1301
• 4QWs AlGaInAs active region with 10nm thickness for λ=1.3um
• Top & bottom mirror with 28 & 34 pairs of AlGaAs/GaAs
• Photonic crystal produce by Etching in top DBR mirror
• Structure uses tunnel junction for better current confinement
• Advantages of using PC in DBR mirror :
Better current confinement
Increase SMSR & Single-Mode operation

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Nano electro-mechanical optoelectronic tunable VCSEL
Michael C.Y. Huang, Ye Zhou, and Connie J. Chang-Hasnain
2007 Optical Society of America, 5 February 2007 / Vol. 15, No. 3 / OPTICS EXPRESS 1222
• QWs active layer
• Bottom DBR : 34 pairs
• Top mirror : 4 pairs and HCG
(High-index-Contrast sub-wavelength Grating)
• 4 pairs DBRs for inject current
• AlO2 aperture for current confinement
• HCG with 230nm thickness cause :
- R > 99.9%
- Single-Mode with SMSR > 40dB
- Tuning wavelength on 2.5nm distance
- Small thickness instead DBRs and therefore reduce mass, increase
tuning speed and reduce tuning voltage
• HCG consists of periodic strip of Al.6Ga.4As and Air as clad

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• Laser characteristics without tuning voltage :
Ith=1.3mA and P0=.5mW at 2.3Ith
At 1.3Ith λ=861nm with SMSR=40dB
Nano electro-mechanical optoelectronic tunable VCSEL
Michael C.Y. Huang, Ye Zhou, and Connie J. Chang-Hasnain
2007 Optical Society of America, 5 February 2007 / Vol. 15, No. 3 / OPTICS EXPRESS 1222

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Nano electro-mechanical optoelectronic tunable VCSEL
Michael C.Y. Huang, Ye Zhou, and Connie J. Chang-Hasnain
2007 Optical Society of America, 5 February 2007 / Vol. 15, No. 3 / OPTICS EXPRESS 1222
• Laser characteristics with tuning voltage
• For tuning λ a reverse bias voltage applied across HCG & P-DBR

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• Variation of Air Gap with applied voltage and wavelength with Air Gap
• This tunable laser useful for WDM applications, optical filters and
detectors
Nano electro-mechanical optoelectronic tunable VCSEL
Michael C.Y. Huang, Ye Zhou, and Connie J. Chang-Hasnain
2007 Optical Society of America, 5 February 2007 / Vol. 15, No. 3 / OPTICS EXPRESS 1222

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Advantages of VCSEL
• Easier to test on wafer
• Wavelength is “tunable”
• Low current needed due to small active volume
• Circular cross-section that can be easily coupled
• The VCSEL is cheaper to manufacture in quantity
• Efficiency and speed of data transfer is improved for fiber optic
communications
• Array layouts Could be treated as the 2D arrays for optical signal processor,
interconnects, and image processing and to do the calculation

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Conclusion
• The active layer is configured sometimes as
a single quantum well (SQW) and usually as
a multiple quantum well (MQW) to improve
threshold current.
• The semiconductor layers are manufactured
using epitaxial growth on a substrate that is
transparent to in the emission wavelength.
• Example: a 980 nm VCSEL uses InGaAs as
the active layer to provide 980 nm emission
along with a GaAs crystal substrate that is
transparent at 980 nm.

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Conclusion
• The height of the cavity is small (several
microns) and only permits one longitudinal
mode, however there may be one or more
lateral modes.
• A VCSEL is capable of delivering more
power than a conventional laser diode up to a
few watts with a matrix VCSEL which a
matrix emitter is a broad area surface emitting
laser with applications in optical interconnect
and optical computing.

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References
• [1] “Vertical Cavity Surface Emitting Laser Devices” by H.Li & K.Iga,Springer
• [2] “Single-Mode Oxide-Confined VCSEL for Printers and Sensors” Hiromi Otoma, Akemi Murakami,
Yasuaki Kuwata, Nobuaki Ueki, Naotaka Mukoyama,2006 Electronics Systemintegration Technology Conference,Germany, 1-
4244-0553-1/06/$20.00 2OO6 IEEE
• [3] “Electrically Pumped GaSb-based VCSEL With Buried Tunnel Junction”
A.Bachmann, T. Lim, K. Kashani-Shirazi, O. Dier, C. Lauer and M.-C. Amann, 2008 Optical
Society of America, CLEO/QELS 2008
• [4] “All-Optical Flip-Flop Operation Using 1.55 m Polarization Bistable VCSELs”
Takeo Katayama, Toshiyuki Kitazawa, Hitoshi Kawaguchi
2008 OSA / CLEO/QELS 2008 (978-1-55752-859-9/08/$25.00 ©2008 IEEE)
• [5] “Optimal radii of photonic crystal holes within DBR mirrors in long wavelength VCSEL”
Tomasz Czyszanowski, Maciej Dems, Hugo Thienpont, Krassimir Panajotov
2007 OSA,5 February 2007 / Vol. 15, No. 3 / OPTICS EXPRESS 1301
• [6] “Nano electro-mechanical optoelectronic tunable VCSEL”, Michael C.Y. Huang, Ye Zhou, and
Connie J. Chang-Hasnain, 2007 Optical Society of America, 5 February 2007 / Vol. 15, No. 3 /
OPTICS EXPRESS 1222

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Thanks for your attention