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

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)

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

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

Light emission in semiconductors CB CB Eg Eg VB VB Direct band gap Indirect band gap K

Emission Characteristics of various semiconductors 1110 1850

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)

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)

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

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

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

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

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

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

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

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

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