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Optical Sources in Fiber Optic Communication - Lesson Summary

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The key points from this module are:
The basic structure and the gain medium of a laser diode are also the same as of LED.
The pn junction diode is typically formed by doping the semiconductor substrate with p-type and
n-type

A modification that is most popular is a hetero-junction, fabricated in a semiconductor structure. Heterojunction
consists of a double heterostructure. Double heterostructure consists of p-type and n-type doping.

The density of states, p(k) is defined as the number of states occupied by electrons with wavenumbers (momentum) between k and k+dk.

Internal Quantum Efficiency is a measure of the fraction of recombinations of electron-hole pairs and it leads to photon emission.

There are three possible reasons for the loss of photons:
Fresnel reflection
Total internal reflection
Absorption

When measuring the bandwidth of an LED, the bandwidth of the Photodiode (measurement system) should be larger than the expected bandwidth of the LED.

The far-field spatial distribution of intensity is typically referred to as the emission pattern of the LED. The typical emission of LED follows a Lambertian profile.

The advantages of LEDs are:
Compactness
Low cost
The ability to modulate directly by modulating the forward drive current.

The key limitation of LEDs is:
Large spectral widths.

The basic component of an electronic oscillator is an amplifier.
In order to release the laser, the photons generated as a consequence of spontaneous emission are fed back into the gain medium to cause stimulated emission.
The frequency separation between two adjacent longitudinal modes is called the free spectral range (FSR) of the cavity.

There are three simultaneous phenomena in a laser that impose constraints on the allowed frequencies.

The gain spectrum
The cavity
The gain/loss condition