Receiver sensitivity

Receivers are integral part of a long distance fiber optic communication system.  A receiver includes photodetectors such as Avalanche photodiode, positive-intrinsic-negative semiconductor photodiode etc., demodulators and couplers. Receiver sensitivity is more for an optical receiver when it achieves the same performance with less optical power incident on it. The three important factors which influences receiver sensitivity are bit-error rate (BER), minimum received power and quantum limit of photodetection. Bit-error rate is defined as the probability of incorrect identification of a bit by the decision circuit of the receiver. Minimum received power is a cut-off value below which receiver operation ceases. Use of avalanche photodiode improves receiver sensitivity. But excess noise factor may degrade receiver sensitivity. Quantum limit of photodetection in almost all practical optical receivers is more than 20 dB or exceeds 1000 photons.

Non scattering losses in optical fibers

Non-scattering losses occurs in optical fibers due to nonlinear effects. The transfer of optical power from one mode to another mode or same mode either in forward direction or backward direction at different frequency is termed as nonlinear scattering. There exist two types of nonlinear scattering. They are Stimulated Brillouin Scattering (SBS) and Stimulated Raman Scattering (SRS).  SBS occurs in systems which operate above 3 mW. SBS becomes very significant when interaction length is long and line width of signal is narrow. SBS can be minimized by broadening the line width. This is achieved by the usage of RF modulation on the laser injection current, by employing an external phase modulator or a self pulsating laser. High frequency optical photons are generated in Stimulated Raman scattering. SRS is a cause of concern in WDM systems. Stimulated Raman Scattering increases exponentially with increase in power. SRS can occur in both forward and backward direction.

Reach through avalanche photodiode (RAPD)

Avalanche photodiodes are required in fiber optic communication at the receiving end. Photodiode detects the light signal and converts it into electrical form. There exist many forms of avalanche photodiodes. Reach through avalanche photodiode (RAPD) is a promising candidate in this category. Consider a positive-intrinsic-negative semiconductor photodiode operating in reverse biased mode. When the applied reverse voltage exceeds threshold value, photoelectrons generated as a result of its exposure to light, gets accelerated through the junction, collides with other atoms to produce secondary electron-hole pairs. Carrier concentration increases exponentially with the electric field intensity. This phenomenon is known as impact ionization or avalanche effect. The reach through avalanche photodiode consists of a high resistivity p-type material deposited on an epitaxial layer with an extremely high order of impurity concentration. Commonly used doping atoms to achieve this are Boron and Phosphorous. Operation of RAPD is always in fully depleted mode.