Sources (Sec. 3.5) Mar. 27, 2018

+ Monochromatic source: Laser or LED. They can be external modulated or more conveniently direct modulated with a time varying current.

+ Diode: Laser diode and LED are diodes. Require forward biasing to emit light from active (depletion) layer at the PN junction. They are devices that convert electrons to photons.

+ Comparison:
Output power [equation] - laser (LD) [equation] where [equation] is the slope efficiency and [equation] is the threshold current.
LED [equation] where [equation] is the total quantum efficiency, [equation] is the external quantum efficiency and [equation] is the internal quantum efficiency.
Threshold current - LD has it but not LED. [equation] temperature dep.
Spectral width - LD has very narrow peaks. It can be single or multiple longitudinal modes.
LED has wide spectral width [equation] which is temp dep [equation] and has maximum freq [equation]. Note: [equation] at 300K.
Modulation - LD requires biasing current above [equation].
LED requires positive modulation current.
Modulation bandwidth - LD has [equation] where [equation] is the rising time of LD.
LED has [equation] or [equation] where [equation] is the electron-hole recombination time.
Divergence angle - LD has [equation].
LED has angle proportional to physical size.
Temp. dep. - LD has [equation], [equation] and [equation] temp. dep.
LED has [equation], [equation] and [equation] temp. dep.

+ Temp. control: Thermoelectric cooler with thermistor providing temp. feedback.
Power monitoring with built-in photodetector.
Aging drives up [equation].

+ 3 radiative process for laser: absorption, spontaneous emission and stimulated emission.
Lasing generated by electron-hole recombination in stimulated emission.

+ Semiconductor materials: Radiation freq. determined by the bandgap energy [equation]. By choosing appropriate materials, we generate certain wavelength, e.g. [equation] has [equation] (eV).

+ 3 Lasing requirements : A pumping source (current injection), positive feedback (FP cavity) and special device structure to enhance stimulated emission (heterojunctions, quantum wells).

+ Lasing conditions: Ampl condition [equation] threshold gain coef. [equation]
Phase condition [equation] resonant freq [equation] and [equation].
Example for calculations with lasing conditions

Last Modified: March 26, 2018
Copyright © < >