semiconductor laser basic principles

2. • 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. 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. Semiconductor Laser
Homojunction
Diode Laser
Heterojunction
Diode Laser
Double
Heterojunction
Diode Laser
Single
Heterojunction
Diode Laser

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

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

8. 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. metal contact
P end
N end
Laser Beam
Ga As
GaAIAs

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