Misc LED Stuff and Lasers

Misc LED Stuff
The frequency response of an LED (or any diode) is limited at lower frequencies by the junction capacitance, and at higher frequencies by the carrier lifetime. I believe the idea is that the carriers don't have time to recombine after traveling through the pn junction.

The wavelength of light emitted from an LED corresponds to the energy difference between the quasi fermi levels in the active layer. Higher energy = higher frequency = shorter wavelength.

The higher energy (shorter wavelength) light emitted from an edge emitting LED is absorbed back into the active layer, giving the edge emitting LED a smaller FWHM (Full Width At Half Maximum), which represents the range of light emitted.


General Laser Operating Principles
Laser stands for Light Amplification by Stimulated Emission of Radiation.

The basic idea behind the laser is similar to that of the LED: double injection occurs within a thin active layer which causes spontaneous emission. In fact, at lower biases, a laser operates in LED mode. At higher biases, different things happen however.

Stimulated emission is when incident light "stimulates" an electron and causes it to descend to the valence band, emitting a photon in the process. Due to some complex processes that occur when the light stimulates the electron, the emitted light is of the same wavelength and phase as the incident light. Thus, coherent light is produced.

These are the two sources of photon generation in a laser: spontaneous emission and stimulated emission.

There are many sources of absorption in a laser; the same as in an LED or any other device.

The lasing threshold is the level of excitation at which light generated by stimulated emission equals light absorption. Below the lasing threshold dP/dI is low; above the threshold it is high.

The active layer of a laser is capped at each end with mirror facets (in some laser diodes), which cause the light to bounce back and forth and grow in intensity. The layers directly beside the active layer (cladding layers) are set such that their refractive indices bounce light back into the active layer.