Fiber Optic Receivers

Fiber optic receivers are instruments that convert light into electrical signals. They contain a photodiode semiconductor, signal conditioning circuitry, and an amplifier. Fiber optic receivers use three types of photodiodes: positive-negative (PN) junctions, positive-intrinsic-negative (PIN) photodiodes, and avalanche photodiodes (APD). PIN photodiodes have a large, neutrally-doped region between the p-doped and n-doped regions. APDs are PIN photodiodes that operate with high reverse-bias voltages. In short wavelength fiber optic receivers (400 nm to 1100 nm), the photodiode is made of silicon (Si). In long wavelength systems (900 nm to 1700 nm), the photodiode is made of indium gallium arsenide (InGaAs). With low-impedance amplifiers, bandwidth and receiver noise decrease with resistance. With trans-impedance amplifiers, the bandwidth of the receiver is affected by the gain of the amplifier. Typically, fiber optic receivers include a removable adaptor for connections to other devices. Data outputs include transistor-transistor logic (TTL), emitter-coupled logic (ECL), video, radio frequency (RF), and complementary metal oxide semiconductor (CMOS) signals. 

Fiber optic receivers can use single-mode and/or multi-mode optical cable. Single-mode cable allows only one mode to propagate and features very small core diameters of approximately 8 µm. Single-mode cable permits signal transmissions at extremely high bandwidths and allows very long transmission distances. By contrast, multi-mode cable supports the propagation of multiple modes and features core diameters ranging from 50 to 100 µm. Multi-mode cable has a graded or stepped refractive index and allows the use of inexpensive light emitting diode (LED) light sources. With multi-mode cables, connector alignment and coupling is less critical than with single-mode fibers. Because of dispersion, however, multi-mode cable provides reduced transmission distances and transmission bandwidths. 

Fiber optic receivers use many types of connectors. D4 and FC connectors are durable, zirconia-ceramic ferrules with a keyed body for repeatability. FC connectors are used with single-mode fibers in telephone systems and high-speed communication links. MU connectors are 1.25 mm ferrules designed for high-speed data communications, voice networks, telecommunications, and dense wave division multiplexing (DWDM). FDDI connectors, 2.5 mm ferrules with a fixed shroud, are used in fiber distributed data interface (FDDI) applications. ESCON connectors, 2.55 mm ferrules with an adjustable shroud, are used in enterprise system connection (ESCON) applications. LC connectors are high-precision, zirconia-ceramic ferrules that feature an RJ-45 push-pull housing and latching. MT-RJ connectors hold two fibers with a ferrule that is smaller than the one used in MTP connectors, devices that are threaded and well-suited for high-density applications. SC and ST connectors are easy-to-assemble and feature a bayonet mounting system. SMA connectors include a low-cost, multi-mode coupling that is suitable for military applications. Loop back connectors are used to test transceiver systems. Biconic connectors have precision-tapered ends for low insertion loss. 

Performance specifications for fiber optic receivers include data rate, receiver rise time, dynamic range, sensitivity, responsiveness, and bandwidth. Data rate, the number of bits transmitted per second, measures the speed of the transceiver. Receiver rise time, the time required for a signal to change from 10% to 90% of full power, measures the speed of the receiver. Dynamic range, a measure of receiver sensitivity, is the power range for which receivers function properly. Sensitivity is the weakest optical signal that can be received. Typically, sensitivity depends on the noise floor of the transceiver front end. Responsiveness, a measure of absolute sensitivity expressed in A/W, is the ratio of radiant energy in watts (W) to the resulting photocurrent in amperes (A). Bandwidth is the frequency range of the transceiver.

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Part Numbers for Fiber Optic Receivers