Laser diodes

LASER DIODES

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. 4.23 (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 direct band gap semiconductors like GaAs. This light radiation is known as recombination radiation (fig. 4.23 (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 laser diode is shown in fig 4.24.

• 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 lower (n-region) surfaces of the lava (p-region) and 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 laser diode is shown in fig 4.25.

• 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 valence 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

E_g = hv = hc/λ

λ = hc / E_g

where E_g → band gap energy in joule

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 CO₂ 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.

Applications of Laser Diode

• Used in fibre optic communication.

• Used in various measuring devices such as range finders, bar-code readers.

• Used in printing industry both as light sources for scanning images and for resolution printing plate manufacturing.

• Infrared and red laser diodes are common in CD players, CD-ROM and DVD technology. Violet lasers are used in HD-DVD and Blue-ray technology.

• High power laser diodes are used in industrial applications such as heat treating, cladding, seam welding and for pumping other lasers.

• Used in laser medicine especially, dentistry.