[FONT='Constantia','serif']INTRODICTION TO OPTICAL RECIVING DEVICES[/FONT][FONT='Arial','sans-serif'][/FONT]
Optical receiving devices can be classified as either photon effect receivers or thermal effect recoveries . For communication purpose or other systems where slow response is insufficient , only photon effect types are of value. Photon effect receivers can be sub classified into device types . The types of devices are :-
1- Photo emissive –type detectors:-
The photo emissive effect , also called the photoelectric effect , was discovered by Hertz in 1887 and explained by Einstein in 1905. Einstein propounded the existence of quanta ,each having a discrete amount of energy equal to (hf),where (h) is Plank`s constant and (f) is the radiation frequency . The photoelectric effect in the simplest sense , occurs when photons fall upon a photo emissive surface and cause electrons to be ejected from the surface.
2- Photoconductive –type detectors:-
Willougby Smith in 1873 , first discovered photoconductivity by experimenting with selenium. The photoconductivity processes involve the absorption of energy from photons, the excitation of charge carriers from a nonconducting ground state to a higher – energy state where they are free to contribute to the electric conductivity , and the return of charge carrier from the conducting state to the ground state .Essentially , photoconductivity causes a decrease in electrical resistance , which increases the current through the device.
3- Photovoltaic –effect detectors:-
In this type , the action of photons produces a voltage that can be detected without need for bais supply or a load resistor .This effect occurs as the result of the absorption of ionizing radiation in the vicinity of a potential barrier , such as in p-n junction .With use of bais , we have photodiode.
4- Photoelectromagnatic effect detectors (PEM):-
This effect is basically a diffusion effect in a magnetic field. To obtain this effect the semiconductor is immersed in a magnetic field . In general , the response is inferior to other effects. However , in a few cases the PEM response may be greater.
5- Quantum Amplifier:-
The quantum amplifier essentially can be considered a preamplifier before detection . It is not a detector , itself ; however ,it is a photon effect device .Input photons of the carrier frequency are amplified within the device , so that the output power of the device shows a signal power gain at optical; , ultraviolet or infrared frequency .This device is based upon stimulated emission of radiation .
6- Photo parametric detectors :-
Parametric mode operation of the photodiode as a detecting element has been achieved .The parametric operation is at the microwave or radio frequency and is not an optical parametric amplifier. The photodiode detects the incoming optical energy and via pumping action ,such as in a microwave parametric amplifier , amplifier the demodulated signal so that the modulated output is much greater in power than if the diode was not pumped .Post-detection power gain of 25db have been reported.
(LASER COMMUNICATION SYSTEMS )
WILLIAM K. PRATT
University of Southern California