How to Select Fiber Optic Receivers

Image Credit: Avago Technologies, Digi-Key, Newark

Fiber optic receivers convert light signals into electrical signals for use by equipment such as computer networks. These electro-optical devices consist of an optical detector, a low-noise amplifier, and signal conditioning circuitry. After the optical detector converts the incoming optical signal into an electrical signal, the amplifier increases it to a level suitable for additional signal processing. The modulation type and the electrical output requirements determine what other circuitry is required.

How Fiber Optic Receivers Work

Fiber optic receivers use positive-negative junctions (PN), positive-intrinsic negative (PIN) photodiodes, or avalanche photodiodes (APD) as optical detectors. The incoming light signal is sent by a fiber optic transmitter (or transceiver) and travels along single-mode or multi-mode optical cable, depending on device capabilities. A data demodulator converts the light signal back into its original electrical form.  In more complex fiber optic systems, wavelength division multiplexing (WDM) components are also used.

Figure 1 – Fiber Optic Receiver Block Diagram – Image Credit: Integrated Publishing

Semiconductors and Photodiodes

The GlobalSpec SpecSearch database allows industrial buyers to select products by semiconductor type and photodiode type. Two types of semiconductors are used in fiber optic receivers.

• Silicon semiconductors are used in short-wavelength receivers with a range of 400 nm to 1100 nm.
• Indium gallium arsenide semiconductors are used in long-wavelength receivers with a range of 900 nm to 1700 nm.

As described above, fiber optic receivers use three different types of photodiodes.

• P-N junctions are formed at the boundary of a P-type and N-type semiconductor, typically in a single crystal via doping.
• PIN photodiodes have a large, neutrally-doped intrinsic region sandwiched between P-doped and N-doped semiconducting regions.
• APDs are specialized PIN photodiodes that operate with high reverse bias voltages.

Amplifiers and Connectors

Fiber optic receivers use either low-impedance or transimpedance amplifiers.

• With low-impedance devices, bandwidth and receiver noise decrease with resistance.
• With trans-impedance devices, 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. Choices include D4, MTP, MT-RJ, MU, and SC

Receiver Performance

When using GlobalSpec to source products, buyers should specify these parameters for fiber optic receiver performance.

• Data rate is the number of bits transmitted per second, and is an expression of speed.
• Receiver rise time is also an expression of speed, but indicates the time required for a signal to change from a specified 10% to 90% power.
• Sensitivity indicates the weakest optical signal that the device can receive.
• Dynamic range is related to sensitivity, but indicates the power range over which the device operates.
• Responsivity is the ratio of radiant energy in watts (W) to the resulting photocurrent in amperes (A).

Resources

FiberOptics.Info - Parts of a Fiber Optic Link

Optical Detectors and Fiber Optic Receivers

• Fiber Optic Technology (.pdf)
  . Conventional electrical data signals are converted into a modulated light beam, introduced into the fiber and transported via a very small diameter glass or plastic fiber to a receiver that converts the light back into electrical signals. ./308604d0-31f0-4458-bbc3-10bae84d77d1 Fiber Optic...
• Overview of Fiber Optic Technology
  . For example, 100Base-FX uses two optical fibers and allows full duplex point-to-point communications up to 2000 feet. Gigabit Ethernet (1000 Mbps) options also are available using twisted pair and fiber optic media. (For more information on Ethernet physical layer options see the Ethernet Basics article...
• Fiber-Optic Connections
  Three major components in any fiber-optic system are the transmitter, receiver, and fiber cable. Transmitters are often referred to as launchers and are typically either injection laser diodes (ILDs) or light-emitting diodes (LEDs). The total power of any launcher never completely couples...