semiconductor diode laser.pptx

laser

1. Semiconductor Diode Laser The semiconductor laser is very small in size and appearance. It is similar to a transistor and has the operation like LED but the output beam has the characteristics of laser light. The material which often used in semiconductor laser is the gallium Arsenide, therefore semiconductor laser is sometimes known as Gallium Arsenide Laser. It is also called Injection Laser.

2. Components Active medium : PN Junction Diode Pumping Source : Forward Bias Voltage applied (Direct Conversion) Optical Resonator : The polished ends of the diode

3. C la ss i f i c a t i o n O f Se m ic o ndu c t o r Laser Semiconductor Laser Homojunction Semiconductor Laser Heterojunction Semiconductor Laser Double Heterojunction Semiconductor Laser Single Heterojunction Semiconductor Laser

4. Homojunction laser It is simply a laser diode where the active medium is a semiconductor similar to that found in a light- emitting diode. The most common and practical type of laser diode is formed from a p-n junction and powered by injected electric current

5. Heterojunction laser A hetero junction is the interface that occurs between two laser or regions of dissimilar crystalline semiconductors. These semiconducting materials have unequal band gaps as opposed to a homo junction.

6. H o m o j un c ti o n Se m ic o ndu c t o r La s e r 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 la s er They use Direct Band Gap Semi- conductor material. P-N Junction act s 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. metal contacts on both surfaces of the diode. Pulse beam of laser of 8400 Å is produced Fig: Diagram of ojunction S

7. SEMICONDUCTOR LASER In semiconductor diode lasers Conduction band play the role of exited level. Valence band play the role of ground level Population inversion requires the presence of large concentration of holes in the valence band. A simple way to achieve population inversion is to make a semiconductor in the form of a PN junction diode from heavily doped P and N type semiconductors

8. P umping Direct conversion method is used. PRINCIPLE P -T YPE CB P-N JUNCTION BAND STRUCTURE HOL E S JUNCT I ON VB NO VOLTAGE N -T YPE HOLE –ELECTRON RECOMBINATION ELECTRONS VO L T AGE APP L I E D EMITED LIGHT ENE R GY LEVEL IN SEMICONDUTOR LASER

9. P umping Direct Conversion When PN junction diode is forward biased, the electrons from ‘n’ region and h o l e s from ‘p’ region re c ombine with each other at the junction . During the recombination process light radiations (photons) is released from certain specified direct band gap semiconductors like Ga-As. This radiation is called recombination radiation and the corresponding energy is called activation energy.

10. Construction of Ga - As LASER The gallium Arsenide laser is designed in such a way that a piece of N-type Gallium Arsenide material is taken and a layer of natural gallium aluminum arsenide material is pasted, The third layer of p-type gallium arsenide material is pasted over that. The two ends of length wise are fully polished in order to amplify the light by cross reflection. Here one ends is partially polished from where we get the laser beam.

11. C ONSTUCTION METALLIC LAYER G a As CONTACT LAYER Ga A l As BARRIER LAYER L AS E R BE AM G a As C ON T A C T L A YER G a A l As BARRIER LAYER G a As CONTACT LAYER + - GAL L IUM ARSENI D E L ASER

12. Working of Ga – As LASER When the forward bias is applied to the metallic layer through contact points. The electric field is produced. This electric field causes the electrons to move from lower band of energy towards high band of energy level. Population inversion take place at the higher band of energy level and when the electrons falls back at the lower energy band, it emits light, through the polished end of the laser. Cross reflection of the light take place which multiplies strength of laser beam. At the end strong beam of laser comes out through the partially polished end.

13. Pictorial View ⚫ No biasing Biasi n g

14. When p-n junction diode is forward biased, then there will be injection of electrons into the conduction band along n-side and production of more holes in valence band along p-side of the junction. Thus, there will be more number of electrons in conduction band comparable to valence band, so population inversion is achieved. Therefore, when the electrons and holes are injected into the junction region from opposite sides with forward biasing, then population inversion is achieved between levels near the bottom of the conduction band and empty levels near the top of the valence band. A chievement of population inversion

15. Achievement of laser When electrons recombine with the holes in junction region, then there will be release of energy in the form of photons. This release of energy in the form of photons happen only in special types of semiconductors like Gallium Arsenide (Ga As). Otherwise in semiconductors like silicon and germanium, whenever holes and electrons recombine, energy is released in the form of heat, thus Si and Ge can not be used for the production of laser. The spontaneously emitted photon during recombination in the junction region of Ga As will trigger laser action near the junction diode. The photons emitted have a wavelength from 8200 Å to 9000 Å in the infrared region.

16. Heter o j un c t i o n Se m ic o ndu c t o r La s e r 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 : GaAlAs diode laser Use Direct Band gap Semiconductor Consist of five layers namely GaAs – p type GaAlAs – 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 parallel to each other to reflect the laser beam; one end is partially polished to release the continuous beam

17. E n e r g y Le v e l Dia g r a m : Heter o j un c ti o n Fig : Energy level Diagram of Heterojunction Semiconductor Laser

18. Characteristics Type : It is a solid state semiconductor laser. Active medium : A PN junction diode made from single crystal of gallium arsenide is used as an active medium. Pumping met h od : The di r e c t conversi o n method is used for pumping action Power output: The power output from this laser is 1mW. Natu r e o f outp u t : The nature o f ou t put is continuous wave or pulsed output.

19. Advantages of semiconductor laser It is very small in dimension. The arrangement is simple and compact. It exhibits high efficiency . Economical No need of mirrors for optical resonator Low Power consumption

20. Disadvantages of semiconductor laser The output is usually in the form of wide beam. It is greatly dependent on temperature. The temperature affects greatly the output of the laser. The lasing medium of semiconductor lasers is too short and rectangular so the output beam profile has an unusual shape. The cooling system required The purity & monochromaticity are poorer than other type of laser.

21. Applic a tions Semiconductor diode lasers used in CD and DVD players. Used for transmitting digital data Fiber optic transceivers are manufactured using alternating layers of various III-V and II-VI compound semiconductors to form lasing hetero structures. Used in laser printers & in laser diodes. Used for interface with fiber optic cables used in communication