Solid-state Diode lasers (Semiconductor diode laser)

Solid-state Diode lasers (Semiconductor diode laser)

Laser light can also be produced in semiconductors. The most compact of all lasers is semiconductor diode laser. It is also called injection laser.

Types of semiconductor diode lasers

Broadly there are two types of semiconductor diode lasers. They are

1. Homojunction semiconductor diode lasers

2. Heterojunction semiconductor diode lasers.

Homojunction Semiconductor Laser

A diode laser which makes use of the same type of semiconductor material on both sides of the junction is known as a homojunction diode laser.

Example: Gallium Arsenide (GaAs) laser

Heterojunction Semiconductor Laser

A diode laser which makes use of different semiconductor materials on the two sides of the junction is known as heterojunction diode laser.

Example: A junction laser having GaAs on one side and GaAlAs on the other side.

SEMICONDUCTOR LASER (Homo - junction)

Definition

It is a specially fabricated p-n junction diode.

This diode emits laser light when it is forward - biased.

Principle

When the p-n junction diode is forward-biased (Fig. 5.24(a)), the electrons from n-region and holes from p-region cross the junction and recombine with each other.

During the recombination process, the light radiation (photons) is released from a certain specified direct band gap semiconductors like Ga-As. This light radiation is known as recombination radiation (Fig. 5.24(b)).

The photon emitted during recombination stimulates other electrons and holes to recombine. As a result, stimulated emission takes place and laser light is produced.

Construction

The construction of homo-junction semiconductor laser is shown in fig 5.25. The active medium is a p-n junction diode made from a single crystal of gallium arsenide. This crystal is cut in the form of a platelet having a thickness of 0.5 mm.

This platelet consists of two regions n- type and p-type.

The metal electrodes are connected to both upper (p-region) and lower (n-region) surfaces of the semiconductor diode. The forward bias voltage is applied through metal electrodes.

Now the photon emission is stimulated in a very thin layer of pn junction.

The end faces of the pn junction are well polished and parallel to each other. They act as an optical resonator through which the emitted light comes out.

Working

The energy level diagram of homojunction semiconductor laser is shown in fig 5.26.

When the pn junction is forward-biased, the electrons and holes are injected into junction region.

The region around junction contains a large number of electrons in the conduction band and holes in the valance band.

Now the electrons and holes recombine with each other. During recombination, light photons are produced.

When the forward - biased voltage is increased, more light photons are emitted. These photons trigger a chain of stimulated recombinations resulting in the emission of more light photons in phase.

These photons moving at the plane of the junction travel back and forth by reflection between two polished surfaces of the junction. Thus, the light photons grow in strength.

After gaining enough strength, laser beam of wavelength 8400 Å is emitted from the junction.

The wavelength of laser light is given by

Characteristics

• Type: Solid state semiconductor laser. 

• Active medium: A pn junction diode made from a single crystal of gallium arsenide. 

• Pumping method : Direct conversion method. 

• Power output: a few mW. 

• Nature of output: Continuous wave or pulsed output. 

• Wavelength of output: 8300 Å to 8500 Å.

Advantages

• This laser is very small in size and compact. 

• It has high efficiency. 

• The laser output can be easily increased by increasing the junction current.

• It is operated with less power than ruby and co, lasers. 

• It requires very little additional equipment. 

• It emits a continuous wave output or pulsed output.

Disadvantages

• Laser output beam has large divergence. 

• The purity and monochromacity are poor. 

• It has poor coherence and stability.

Application

• This laser is widely used in fibre optic communication. 

• It is used in laser printers and CD players. 

• It is used to heal the wounds by infrared radiation. 

• It is also used as a pain killer.

Semiconductor Laser (Hetero Junction)

A diode laser with a pn junction made up of different semiconductor materials in two regions ie., n - type and p-type is known as heterojunction semiconductor laser.

Principle

When the pn junction diode is forward biased, the electrons from n-region and the holes from p-region recombine with each other at the junction. During recombination process, light photon is released.

Example

Diode laser having a junction of GaAs and GaAlAs

Construction

Generally, this laser consists of five layers as shown in fig. 5.27. A layer of GaAs p-type (3rd layer) acts as active region. This layer is kept between two layers having wider band gap Ga Al As - p - type (2nd layer) and Ga Al As n - type (4th layer).

The bias voltage is applied through the metal electrodes fixed on top and bottom layers of hetero junction semiconductor laser.

The end faces of the junctions of 3rd and 4th layers are well polished and parallel to each other. They act as an optical resonator.