Post on 06-Mar-2021
transcript
Professor, Department of Electrical Engineering,
Laser Technology Program,
Indian Institute of Technology, Kanpur
Prof. Utpal Das
http://www.iitk.ac.in/ee/faculty/det_resume/utpal.html
Lecture 18: Introduction to Diode Lasers - I
Semiconductor Optical Communication
Components and Devices
For efficient light emission in
lasers, direct band-gap
semiconductors are required.
As carriers recombine across
the active region of the
device, the wavelength of the
light emitted is then
dependent on the difference
between the quasi Fermi
energies and therefore
proportional to this band-gap.
For efficient electrical pump lasers, carrier mobilities in the
semiconductor should also be high. In general, narrower band-gap
minimums lead to higher carrier mobilities. However, too narrow a band-
gap facilitates spontaneous thermally generated carriers. In
semiconductor lasers this phenomenon manifests itself in the form of
„dark current‟, i.e., leakage current at the junctions.
X valley
L valley
G valley
Energy
K
Egd
Egi1Egi2
Heavy HoleLight Hole
Split Band
Semiconductor Laser Materials
Direct Band Gap
)(kE
)(kE
gE phE phE
2
1
0kk
cE
vE
E
Absorption
Recombination
Emission
gE
pgph EEE
0kk
cE
vE
E
Recombination
emission
pE
h
Indirect Band Gap
Reduced Plank’s Constant
Frequency of the Photon
k
AP
ka
OP
P
k
ka
0g
E
.0
elec holek k ph
E
Direct and Indirect transition
in Semiconductors
Energy Spontaneous
Emission
LED’s
+
+
- -
Defects in Laser Materials
Crystals used in semiconductor lasers should be generally free of line
dislocations as they may produce non-radiative recombination centers.
Dislocation also creates traps that reduce electrical conductivity.
Heterostructures are often used in laser structures to increase efficiency,
where a smaller band-gap material is sandwiched between larger band-
gap materials as described in the section on semiconductor basics.
Normally the lattice constants are well matched between junctions and
the substrate. Initially lasers were fabricated from high quality lattice
matched layers from LPE (equilibrium process could not grow good
highly lattice mismatched epitaxial layers), mterial. With the advent of
non-equilibrium growth processes such as MBE and MOCVD, when the
lattice mismatch strain can be contained, it produces one of the most
efficient lasers till date. These are Strained Quantum well lasers, which
until recently had been the work horse for Fiber-Amplifiers, although they
are still used in certain older systems.
LASER: Light Amplification by Stimulated Emission of Radiation
LED LASER Diode
BAR: No lateral confinement Gain Guided Index Guided
Energy
Spontaneous
Emission
LED’s
+
+
-
-
Stimulated Emission: Light
that is monochromatic (same
wavelength) coherent (in
phase) and polarized
Other Feedback Mechanisms
Over the years, numerous optical-cavity designs have evolved to
couple out of laser diodes. The most widely used configuration is
the classic Fabry-Perot Cavity mentioned above. But other
resonant cavities have been devised for applications that require
a highly coherent beam of light with a narrow band of
frequencies. One of the simplest alternatives is to use a
Reflection Grating as an external rear mirror. An antireflection
coating on the back facet avoids excess Fresnel-reflection loss,
and tilting the grating tunes the laser‟s output frequency.
Two other important laser-diode cavity designs that use
diffraction gratings
1. Distributed-Bragg-Reflection (DBR) laser.
2. Distributed-Feedback (DFB) laser.
In the DFB laser, a grating structure placed alongside the active
layer provides back reflected (diffracted) feedback only for a
specific wavelength.
AR CoatingActive Region
Front
Mirror
Collimating
Optics
Front
„mirror‟
GRATINGExternal cavity
Active RegionGRATING
Distributed Feedback Laser
(DFB)
Front
Mirror
Active RegionRear
„mirror‟
GRATING
Distributed Bragg Reflector
(DBR)
BRAGG Grating Diode LaserAll the other wavelengths experience
higher cavity losses and cannot reach
threshold. The DBR laser applies the
same concept, but the grating lies
beyond the active layer and requires
and index-guiding layer to optically
link it to the gain region of the cavity.
Both cavity arrangements (especially
DFBs) are commercially used for fiber
optic communications in the 1.3- and
1.55-μm spectral regions. In some
devices, grating are applied to
stabilize and/or control the output
wavelength. In this fabrication step,
holographic interference patterns are
generated with lasers, obviating the
use of masks and generating high
accuracy gratings. Final step in the
fabrication process include packaging
and, in some cases, attaching fiber
“pigtails” to allow flexibility in the
direction of the laser output
Non-Edge emitting Lasers
Vertical-Cavity Surface Emitting Laser (VCSEL)
Planar-Cavity Surface
Emitting Laser (PCSEL)
Substrate
Light
Light
All of the laser-diode structures discussed so far emit radiation from
their edges. But another critically important class of laser diode
radiates from its surface. The optical cavity of these kind of laser comes
in two basic variety: Planar-Cavity Surface Emitting Laser (PCSEL) and
Vertical-Cavity Surface Emitting Laser (VCSEL). Both are well suited for
two-dimensional laser-diode array.
Angle facet vertically deflect the light
from an edge-emitting laser-diode,
transforming it into PCSEL (left).
Cross section of a VCSEL (right)
reveals a laser vertical cavity with
mirrors built of thin-film HR (High
Reflector) stacks.
PCSEL and VCSEL
A PCSEL essentially consists of an edge-emitting laser
with an optical structure to redirect the optical beam
through the surface. Therefore, these lasers are at least as
long as the conventional edge-emitting variety and have
the same elliptical far-field beam patterns. In a VCSEL,
however, the laser cavity is perpendicular to the active
layer, which yields a circular beam pattern and makes the
whole device extremely compact. A typical VCSEL cavity
spans about 6mm, creating very coherent light.
Monolithic, two-dimensional arrays of PCSELS and
VCSELS have tremendous potential for optoelectronic
applications such as massive parallel processing, high-
speed optical storage, and fiber optic network
interconnects. Their development and the development of
all modern laser diodes stand as elegant tributes to the
extraordinary capabilities of semiconductor technologies.
Review Questions
1. Which of the semiconductors Si, Ge, GaAs, and GaP are suitable for
the fabrication of diode lasers? Justify your conclusion.
2. An In0.53Ga0.47As semiconductor at 300K has parabolic conduction
and valence bands [Ec,v a k2]. The effective masses of the electrons
and holes in this material are me*=0.06mo and mhh*=0.15mo,respectively. If the electron concentration peak is 0.5kBT above the
bottom of the conduction band, then find the hole energy (eV) below
the top of the valence band for efficient photon emission, assuming
the density of states for the conduction and the valence bands to be
the same.
3. What are the different varieties of Fabry-Perrot cavity diode lasers?
4. What are the configurations for which one would be able to get
narrow linewidths for diode lasers?
5. Why is it essential that the diode laser output is emitted normal to
the surface of the semiconductor substrate? What are the
disadvantages associated with it?