SEMICONDUCTOR DIODE LASER
There are two types of semiconductor lasers:
- 1.Homo-junction Semiconductor Laser
- 2.Hetero-junction Semiconductor Laser
1.Homo-junction Semiconductor Laser
Definition
If the p-n junction is formed in a single crystalline material, then it is called a homo-junction laser.
Example: Gallium Arsenide (GaAs)
It is a specially fabricated p-n junction device. It emits laser light when it is forward biased. It is a specially fabricated p-n junction device. It emits laser light when it is forward biased.
Principle
The electrons in the n-region recombine with the holes in the p-region when it is forward biased. During recombination they emit radiations in the form of light. This photon induces other charges, hence stimulated emission takes place which leads to laser light.
Construction
Figure shows the basic construction of the semiconductor laser. The active medium is a single crystal of gallium arsenide. It is cut in the form of a platelet having a thickness of 0.5 mm. The platelet have two region p region and n region. The thickness of the p-n junction layer is very thin, so that the emitted radiations have less divergence. The end faces of the junction diode are polished well and kept parallel. They act as optical resonator through which the emitted light is coming out.
Working
The p-n junction is forward biased. The electrons and holes are injected into the junction region.
The region around the junction contains large amount of electrons within the conduction band and large amount of holes in the valence band. If population inversion is achieved the electrons and the holes recombine each other and produces radiation in the form of light.
When the forward bias is increased, more and more photons are produced and it stimulates the recombination process in the release of photons in phase. The photons moving at the plane of the junction travels back and forth to grow its strength. After gaining enough strength, it gives out the laser beam of wavelength 8400Å.
The wavelength of laser light is given by
Advantages
- Small and compact
- High efficiency
- Output increases with current
- Low power operation
- Minimal additional equipment
- Continuous or pulsed output
Disadvantages
- Large beam divergence
- Poor monochromaticity
- Poor coherence and stability
Applications
- Fiber optic communication
- Laser diodes
- Medical applications (IR therapy, pain relief)
- Printers
2.Hetero-junction Semiconductor Laser
Definition
If the p-n junction is formed using different semiconductor materials, it is called a hetero-junction laser.
Example: GaAs and GaAlAs
Principle
The electron in conduction band combines with a hole in the valence band and hence the recombination of electron and hole produces energy in the form of light. This photon on turn may produce another electron in the conduction band to valence band and thereby stimulate the emission of another photon.
Construction
It consists of five layers. A layer of Ga-As .P-type (3rd layer) will act as the active region. This layer is sandwiched between the two layer having wider band gap.
GaAlAs-p-type (2nd layer) and GaAlAs n-type (4th layer).Electric current applied to the crystal through the electrode which is fixed on the top and bottom layer. The end faces of the junctions of 3rd and 4thlayer are well polished and parallel to each other. They act as the optical resonator.
Working
When the p-n junction is forward biased to the charge carriers are produced in the wide band gap layers ( 2nd and 4th layer). These charge carriers are injected the active region (3rd layer).The charge carriers are continuously injected from 2nd and 4 th layer to 3ed layer until the population inversion is achieved.
At this state, some of the injected charge carriers recombine and produces spontaneously emitted photons. They stimulate the injected charge carriers to emit photon. As a result more number of stimulated emissions arises and this large number of photons is produced. These photons are reflected back and forth at the junction and hence an intense, coherent beam of laser emerges out from the p-n junctions of active region ie, between 3rd and 4th layer. The wavelength of the emitted radiation is nearly 8000Å.
Advantages
- Continuous wave output
- High power output
Disadvantages
- Difficult fabrication
- High cost
Applications
- Optical communication
- Computers (CD-ROMs)
