Semiconductor laser

semiconductor laser basic principles

1. Semiconductor Lasers

2. Introduction Semiconductor diode (the first  laser  diode) was demonstrated in 1962 by two US groups led by Robert N. Hall at the General Electric research center and by Marshall Nathan at the IBM T.J. Watson Research Center The semiconductor laser is made in mass quantities from wafers of gallium arsenide or similar crystals.

3. Principle Of Semiconductor Laser When the P-N Junction diode is Forward Biased (i.e) the P end of the diode is connected to the positive terminal of the battery and the N end is connected to the negative terminal of the battery. The poles and electrons diffuse through the junction and combine with each other; meanwhile light radiations or photons are radiated. This is called Recombination Radiation These emitted photons stimulate Other electrons & holes to recombine which Results in stimulated emission required for Lasing Action.

4. Classification Of Semiconductor Laser Semiconductor Laser Homojunction Diode Laser Heterojunction Diode Laser Double Heterojunction Diode Laser Single Heterojunction Diode Laser

5. Homojunction Semiconductor Laser Homojunction diode lasers are those in which P end and N end of the diode are made of the same semiconductor material. Example : Ga As laser They use Direct Band Gap Semi- conductor material. P-N Junction act as the active medium. The crystal is cut at a thickness of 0.5 mm Applied voltage is given through metal contacts on both surfaces of the diode. Pulse beam of laser of 8400 Å is produced

6. Working FORWARD BIASED DIODE LASER metal contact Ga –As material on both ends P end N end Laser beam +

7. Energy Level Diagram : Homojunction FREE ELECTRONS Conduction Band energy barrier stimulated emission Band gap (Eg) Energy laser (8400 Å ) FREE HOLES Valence Band

8. Heterojunction Semiconductor Laser Heterojunction Semiconductor lasers are those in which P end is made of one type of semiconductor material and the N end is made of another type of semiconductor material Example : GaAIAs diode laser Use Direct Band Gap Semiconductor Consist of five layers namely GaAs – p type GaAIAs – p type GaAs – p type (Active Medium) GaAIAs – n type GaAs – n type The end faces of the third layer is highly polished and perfectly paralell to each other to reflect the laser beam ; one end is partially polished to release the continious beam.

9. Working metal contact P end N end Laser Beam Ga As GaAIAs

10. Energy Level Diagram : Heterojunction

11. Characteristics Of Semiconductor Laser Most SC lasers operate in 0.8 – 0.9 µm or 1 – 1.7 µm spectral region Wavelength of emission determined by the band gap Different SC materials used for different spectral regions 0.8 – 0.9 µm : Based on Gallium Arsenide 1 – 1.7 µm : Based on Indium Phosphide (InP) Pumping method : Direct Conversion High power lasers usually (1 mV )

12. Comparision HOMOJUNCTION DIODE LASER P and N regions are made of the same diode material Active medium : Single crystal of PN Diode Pulse beam Wavelength : 8300Å-8500Å Example : GaAs,InP. HETEROJUNCTION DIODE LASER P and N regions are made of different diode material Active Medium : Third layer of p type material among the five layers Continuous beam Wavelength : 8400 Å Example : GaAs/GaAIAs, InP/InAIP .

13. Advantages They are light weighted and portable. Battery supported ; easily replaceable Capability of direct modulation into Gigahertz region Small size and low cost Capability of Monolithic integration with electronic circuitry Compatibility with optical fibres

14. Disadvantages Due to relatively low power production, these lasers not suited to typical laser applications The temperature affects greatly the output of the laser Beam divergence is much greater as compared to all other lasers Cooling system required in some cases

15. Applications Fiber optic communications Barcode readers Laser pointers Disc reader Laser printing & scanning Directional lighting sources