In this laser semiconductors like GaAs, InP, GaN etc. are used as the active medium and population inversion is achieved by forward biasing the semiconductor.
Semiconductor Laser Construction and Working
Semiconductor Diode Laser Principle
Semiconductor lasers are basically PN junction diode. When a p-type semiconductor (has an excess of holes) is in intimate contact with n-type semiconductor (has an excess of the electron), PN junction is formed at the interface.
When the PN junction is forward biased with an external supply voltage, electrons from the n- region and hole from the p- region are forced into the junction.
These electrons and holes attract each other. When they collide, they neutralize each other and as a result, emit recombination radiation.
The electrons in n-type semiconductors are in the conduction band (i.e.higher energy) and the hole in p-type semiconductors are in the valence band (i.e lower energy).
The energy difference between the conduction band and valence bands is called the band gap or Forbidden energy gap of the given material. The energy of the photon emitted as recombination radiation is equal to the band gap of the material for a semiconductor laser.
Construction of Semiconductor Diode Laser
The semiconductor laser is made of different materials like gallium arsenide (Ga As), Indium phosphide (InP), gallium nitride (GaN), etc. The band gap of the semiconductor laser is different and hence light of different wavelengths is emitted by this laser.
For example band gap of GaAs is 1.42 eV at room temperature and laser light of a wide range of wavelengths from near-infrared region to the visible region of e.m. spectrum is emitted by a laser made of GaAs.
The band gap of InP is 1.35 eV and this material is used to produce laser light of wavelength 1.5 μm. Similarly, GaN has a band gap equal to 3.36 eV. A laser made of GaN is used to Amit blue light and Ultraviolet rays.
Working of Semiconductor Diode Laser
The active medium of the semiconductor laser is a PN junction. In this laser, mirror is not used as in other resonator or cavity for optical feedback to sustain laser oscillation.
In this case, the reflectivity due to the refractive indices of two layers of a semiconductor laser is used for optical feedback. The end faces of two types of semiconductors (i.e. P-type and n-type) are cleaved and are perfectly parallel to each other for achieving optical feedback.
If the active medium or junction is made of a single type of semiconductor material, then the semiconductor laser is also known as homojunction laser.
On the other hand, if the junction is made of different types of semiconductor materials, then the semiconductor laser is known as a heterojunction laser.
The population inversion in semiconductor diode (p-n junction diode) is achieved by forward biasing of PN junction diode.
The forward Bias Voltage causes the carrier pairs (i.e. electron in n region and hole in p region) to inject into the junction religion, where they recombine by means of stimulated emission.
The process can be explained as follows: The valence band and conduction band of a p-n junction diode are shown in the figure.
At the equilibrium, the Fermi level is inside the conduction band of n-type semiconductor and it is inside the balance band of P-type semiconductor.
When a p-n junction is forward biased, the electrons will be injected into conduction band along and side, and the number of holes are produced in the valence band along the p-side of the junction.
Thus, there will be more electrons in the conduction band than that in the valence band. Hence, population inversion is achieved.
If the forward voltage is low i.e. forward current is small, the electron jumps from the conduction band to the valence band where they recombine with hole and emit incoherent light. This is the function of the light-emitting diode(LCD).
However, if the forward voltage is high i.e. forward current is large, the electron jumps from the conduction band to the valence band, then due to the recombination of electron and holes, a photon of energy equal to the forbidden energy gap (Eg) is emitted by spontaneous emission in the junction region of GaAs semiconductor.
This Photon stimulated the laser action near the junction. The laser action takes place in the narrow region and the laser oscillation (i.e. optical feedback) is done due to the repeated reflection between the cleaved end surfaces.
The variation of a typical output of a semiconductor laser with the current flowing through the laser shown in the figure.
When the forward current is low, the population inversion built compensate losers in the system i.e. absorption exceeds the gain and hence no lasing action takes place. In this case, laser output varies linearly with current.
When the current exceeds the critical value (Ic) called threshold current, the gain exceeds the losses in the system and lasing action takes place. In this case, laser output increases dramatically with an increase in the current.
|1.||Type||Solid State Semiconductor laser.|
|2.||Active Medium||The active medium of a semiconductor laser is a p-n junction.|
|3.||Pumping Method||Direct conversion method.|
|4.||Power output||The power output from this laser is 1mW.|
|5.||Nature of output||Continuous-wave or pulsed output.|
|6.||Wavelength of output||8300 to 8500Å.|
Homo junction laser has a number of disadvantages
- A large current is needed to operate such laser and they may be damaged if this large current is made to glow continuously to the p-n junction.
- They have low efficiency.
- The radiation spread out sideways from the given region and hence loss is more than the gain or amplification.
- They are used only in the pulsed mode.
These limitations are however absent in case of hetero junction laser.
Such lasers are made by sandwiching a semiconductor of the low bandgap (like GaAs) in between two layers of different semiconductors having a high band gap.
Therefore, the inner layer forms two heterojunctions at the two interfaces between the semiconductors. The refractive index of a semiconductor having a low band gap is greater than the refractive index of a semiconductor having a high band gap.
Therefore the inner layer of semiconductor causes confinement of laser radiation inside the active medium by the phenomenon of total internal reflection.
Hence, laser oscillations are sustained in the active medium. The higher band gap or Forbidden energy gap of the surrounding semiconductor creates a potential barrier, which prevents the charge carrier to diffuse from the junction region, and hence the charge carriers remain in the junction region.
This condition increases the population inversion and hence stimulated emission. The confinement of charge carriers in the junction region decreases the threshold current considerably for the operation of the Heterojunction laser. The efficiency of these types of laser is quite high.
[ Note. Germanium and silicon semiconductors cannot be used for producing laser because, in Germanium and silicon, the recombination of electrons and holes releases energy in the form of heat.]
Advantages of semiconductor lasers
- Semiconductor laser requires low power for its operation.
- They are compact and lightweight.
- They have a long life.
- The output of the laser can be easily increased by controlling the junction current.
- The laser has a continuous wave output or pulsed output.
- This laser exhibits high efficiency.
- The arrangement is simple and compact.
Application or uses of the semiconductor laser
- They are used in optical fiber communication to provide high-frequency waves for modulating the low-frequency signal.
- They are used as a laser pointer.
- They are used for storing data on CD or DVD.
- They are used as a pumping source in a solid-state laser.
FAQ on Semiconductor Laser Diode
What is the material used in semiconductor laser?
The semiconductor laser is made of different materials like gallium arsenide (Ga As), Indium phosphide (InP), gallium nitride (GaN), etc. The band gap of the semiconductor laser is different and hence light of different wavelength is emitted by this laser.
What is semiconductor laser ?
Semiconductor lasers are basically PN junction diode. When a p-type semiconductor (has an excess of holes) is in intimate contact with n-type semiconductor (has an excess of the electron), PN junction is formed at the interface. When PN junction is forward biased with an external supply voltage, electrons from n- region and hole from the p- region are forced into the junction.These electrons and holes attract each other. When they collide, they neutralize each other and as a result, emit recombination radiation.
What are the advantages of semiconductor laser?
1. Semiconductor laser requires low power for their operation.
2. They are compact and lightweight.
3. They have a long life.
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