Fiber Optics Receivers November 13th, 2008

Fiber optics receivers are among the three basic units of the fiber optics relay systems. Some relay systems also possess an optical regenerator, which may be essential to boost the degraded light signal (for long distances). Fiber optics receivers accept light signals from an optical fiber and decode them into electrical signals. A typical receiver contains an optical detector, a low-noise amplifier, and signal conditioning circuitry.

The optical detectors are types of photodiode semiconductors that convert the incoming optical signal into an electrical signal. Commonly used photodiodes are positive-negative (PN) junctions, positive-intrinsic-negative (PIN) photodiodes, and avalanche photodiodes (APD). In short wavelength (400 - 1100nm) fiber optics receivers, the optical detector is made of silicon. Indium gallium arsenide (InGaAs) photodiodes are used in long wavelength systems (900 - 1700nm).

The converted signals are amplified by using the low-noise amplifier to a level appropriate for further signal processing. The function of a signal conditioning circuitry is to produce the output electrical signal. The receiver’s type of modulation and its electrical output requirements determine the type of signal conditioning circuitry contained within the receiver. Typically, data outputs are transistor-transistor logic (TTL), emitter-coupled logic (ECL), video, radio frequency (RF), and complementary metal oxide semiconductor (CMOS) signals.

Fiber optics receivers use various types of connectors, such as D4, FC (Field Connector), MU, FDDI, LC (Lampert Connector), MTP (Message Transfer Part), SC (Subscription Channel), and ST (Straight Tip) connectors.

Among the performance specifications for fiber optics receivers are data rate, receiver rise time, dynamic range (a measure of receiver sensitivity), sensitivity, responsiveness (a measure of absolute sensitivity expressed in Ampere or Watt), and bandwidth (the frequency range of the transceiver).

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