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?