1. Optical sources Types of optical sources
Characteristics of optical sources
How light emits from semiconductors
Carrier confinement by heterostructure
Merits and demerits of LED
LED structures

2. Types of optical sources
There are three main types of optical light source are available:
Wide band sources (incandescent lamps)
Incoherent sources (LEDs)
Coherent sources (LASERs)

3. Characteristics of light sources
To couple large amounts of power, size and configuration of sources should be compatible with an optical fiber
Must accurately track the electrical signal to minimize distortion and noise
Source should emit light at wavelengths where the fiber has low loss, low dispersion, and detectors are efficient
Modulation bandwidth should be wide
It should have sufficient optical power
Output spectrum should be narrow (Narrow linewidth)
Optical power should be unaffected by changes in ambient conditions
Source should be comparatively cheap and highly reliable

4. Light emission in semiconductors
pn-junction under zero bias
p-type
n-type
Holes
Electrons Eg
Barrier potential CB
pn-junction under forward bias CB
Electrons
Radiation
Holes
n-type
p-type
Wavelength of emitted light nm

5. Light emission in semiconductorsCB CB Eg Eg VB VB
Direct band gap
Indirect band gap K

6. Emission Characteristics of various semiconductors
1110
1850

7. Heterojunction semiconductor light sources
P-type
P-type
n-type
n-type
GaAlAs
GaAlAs
GaAs
1.7 eV
1.7 eV
GaAs
Electrons
1.42 eV
Holes
Single heterostructure (SH)

8. Heterojunction semiconductor light sources
n-GaAs
N-type AlGaAs
P-type AlGaAs
Electrons
1.72 eV
1.42 eV
1.72 eV
Holes
n=3.45
n=3.59
n=3.45
Double heterostructure (DH)

9. The light emitting diodes
Drawbacks of LEDs:
Lower optical power (microwatts)
Lower modulation bandwidth
Harmonic distortion

10. The light emitting diodes
Merits of LEDS:
Simpler fabrication
Low cost
Reliability
Generally less temperature dependence
Simpler drive circuitry
Linearity

11. Surface emitting LED Fiber Epoxy resin Contact N Double heterojunction
N-type substrate N
Double heterojunction p P
SiO2
Gold heatsink

12. Edge emitting LED
Contact
SiO2 N n p
P-type substrate
Gold heatsink

13. Comparison of SLED and ELED
LED Type
Maximum modulation frequency (MHz)
Output power (mW)
Fiber coupled power (mW)
Surface emitting 60
<4
<0.2
Edge emitting
200
<7
<1.0

14. LASER light sources What is LASER? Emission processes
How laser oscillates

15. The LASERs
Light Amplification by Stimulated Emission of Radiation (LASER)
Type of lasers
Solid state lasers
Semiconductor lasers
Gas lasers
Dye lasers

16. Absorption and Emission of Radiation
Initial state
Final state
Absorption
Spontaneous emission
Stimulated emission

17. Basic construction of Laser
Photon multiplication
Mirror with 100% reflective
Mirror with partially reflective
Amplified light !!
Gain medium
Initial state
Laser pump
Energy to create non equilibrium state