laser fundamentals

1. Fundamentals of LASER
Example: Laser Diode

2. Optical emission from semiconductors
In direct bandgap semiconductors, electrons
and holes on either side of the forbidden energy
gap have the same value of crystal momentum.
Thus when electron-hole recombination occurs
the momentum of the electron remains
virtually constant and the energy released,
which corresponds to the bandgap energy Eg,
may be emitted as light.
In indirect bandgap semiconductors, the
maximum and minimum energies occur at
different values of crystal momentum and
the electron-hole recombination only
possible with the aid of a third particle, a
phonon. Hence the recombination in
indirect bandgap semiconductor is
relatively slow.
Laser Diode

3. Optical emission from p-n diodes
In an p-n diode, the normally empty electron states
in the conduction band of the p-type material and
the normally empty hole states in the valence band
of the n-type material are populated by injected
carriers (forward bias) which recombine across the
bandgap. The energy released as a photon is
approximately equal to the bandgap energy
and the optical wavelength is
This spontaneous emission of light from within the
diode structure is known as electroluminescence.


hc
hfEg
gE
24.1

Laser Diode

4. Stimulated emission in semiconductor diode
Population inversion may be obtained at a p-n junction by heavy doping (degenerative doping) of
both the p and n type material. Heavy p type doping with acceptor impurities causes a lowering of
the Fermi level or boundary between the filled and empty states into the valence band. Similarly,
degenerative n type doping causes the Fermi level to enter the conduction band of the material.
When a forward bias nearly equal to the bandgap
voltage is applied, an active region exists near
the depletion layer that contains simultaneously
degenerate populations of electron and holes (or
double degenerate), where population inversion
is occuring.
Laser Diode

5. The laser commonly takes the form of a rectangular parallelepiped. The junction is a plane within
the structure. Two of the sides perpendicular to the junction are purposely roughened so as to
reduce their reflectivity. The other two sides are made optically flat and parallel, by either
cleaving or polishing. These two surfaces form the mirrors for the laser cavity. The reflectivity of
the air-semiconductor interface is high enough that no other mirrors are needed, although
sometimes one of the reflecting surfaces may be coated to increase the reflectivity and to enhance
laser operation.
Diode laser
In operation, electrons are injected into the device from the n-type side and is frequently called
injection lasers. The semiconductor laser is a current-controlled device, rather than a voltage-
controlled device. This will affect the design of the power supply.
Laser Diode

6. Operational efficiency of the semiconductor laser
Differential external quantum efficiency, is the ratio of the increase in photon output
rate for a given increase in the number of injected electrons (the slope quantum efficiency)
Internal quantum efficiency, i=
External quantum efficiency, e =
number of photons produced in the laser cavity
number of injected electrons
total number of output photons
total number of injected electrons
Laser Diode

7. The first major division is into edge-emitting and surface-emitting devices. For edge-emitting
structures, the light emerges from the edge of the device, where the junction intersects the surface.
The light emerges in the plane of the junction. The configuration is simple and easy to fabricate.
Most diode lasers are edge-emitters. But they do suffer from the drawback that the volume of
material that can contribute to the laser emission is limited and they are difficult to package as two-
dimensional arrays.
Classification of major types of semiconductor laser structures
The edge-emitting devices may be divided into
homojunctions, single heterojunctions, and
double heterojunctions. The homojunctions are
no longer used and double heterojunctions
dominate most applications.
For surface-emitting diodes, the light emerges
from the surface of the chip rather than from
the edge. This feature is attractive because
devices could be packed densely on a
semiconductor wafer and it would be possible
to fabricate two-dimensional arrays easily.
Laser Diode

8. Homojunction Laser
The p-n junction of the basic GaAs laser described previously is called a homojunction because
only one type of semiconductor material is used in the junction with different dopants to produce
the junction itself. The index of refraction of the material depends upon the impurity used and the
doping level.
The junction region is actually lightly doped with p-type material and has the highest index of
refraction. The n-type material and the more heavily doped p-type material both have lower indices
of refraction. This produces a light pipe effect that helps to confine the laser light to the active
junction region. In the homojunction, however, this index difference is low and much light is lost.
Laser Diode

9. Single heterojunction Laser
A fraction of the gallium in the p-type layer has been replaced by aluminum. This reduces the index
of refraction of this layer and results in better confinement of the laser light to the optical cavity.
This in turn leads to lower losses, lower current requirements, reduced damage, and longer lifetime
for the diodes. Single heterostructure diode lasers are still in use, but have largely been replaced by
double heterostructure devices.
Laser Diode

10. Double Heterojunction Laser
In the double heterojunction laser, only the junction region is
composed of GaAs. Both the p and n regions are of AlGaAs. The
result is better confinement of the optical standing wave on both
sides of the optical cavity.
The band-gap discontinuities that exist in DH laser also
confined the injected carriers in the GaAs layer and made
to recombine in the active region.
This confinement greatly reduces the optical loss, but also leads
to two additional difficulties.
The optical radiation in the cavity is so well confined that the
irradiance at the diode optical surfaces may easily reach the
damage threshold, increasing the likelihood of catastrophic
failure.
The tight confinement of the beam also reduces the effective
width of the output aperture of the laser. This increases the
divergence angle in the direction perpendicular to the junction.
Laser Diode

11. Some common material systems used in fabrication of heterojunction lasers.
Each of the advances described has lowered the operating threshold of GaAs lasers.
The typical current densities necessary to achieve the lasing threshold of the various
junction types at 300° K.
Homojunction 40,000 A/cm2
Single heterojunction 10,000
Double heterojunction 1,300
Double heterojunction, large optical cavity 600
The difficulties in DH laser are overcome by a further development of a large-optical-
cavity (LOC) laser and uses regions of AlGaAs of varying composition.
Laser Diode