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OPTICAL DETECTORS

A transducer is a device that converts input energy of one form into output energy of another. An optical detector is a transducer that converts an optical signal into an electrical signal. It does this by generating an electrical current proportional to the intensity of incident optical radiation. The relationship between the input optical radiation and the output electrical current is given by the detector responsivity. Responsivity is discussed later in this chapter.

OPTICAL DETECTOR PROPERTIES

Fiber optic communications systems require that optical detectors meet specific performance and compatibility requirements. Many of the requirements are similar to those of an optical source. Fiber optic systems require that optical detectors:

• Be compatible in size to low-loss optical fibers to allow for efficient coupling and easy packaging.
• Have a high sensitivity at the operating wavelength of the optical source.
• Have a sufficiently short response time (sufficiently wide bandwidth) to handle the system's data rate.
• Contribute low amounts of noise to the system.
• Maintain stable operation in changing environmental conditions, such as temperature.

Optical detectors that meet many of these requirements and are suitable for fiber optic systems are semiconductor photodiodes. The principal optical detectors used in fiber optic systems include semiconductor positive-intrinsic-negative (PIN) photodiodes and avalanche photodiodes (APDs).

Q.5 Describe the operation of an optical detector.
Q.6 For efficient operation, should a detector have a high or low responsivity at the operating wavelength?
Q.7 List the two principal optical detectors used in fiber optic systems.

SEMICONDUCTOR PHOTODIODES

Semiconductor photodiodes generate a current when they absorb photons (light). The amount of current generated depends on the following factors:

• The wavelengths of the incident light and the responsivity of the photodiode at those wavelengths
• The size of the photodiode active area relative to the fiber core size
• The alignment of the fiber and the photodiode

The optical fiber is coupled to semiconductor photodiodes similarly to the way optical sources are coupled to optical fibers. Fiber-to-photodiode coupling involves centering the flat fiber-end face over the photodiode active area. This is normally done directly by butt coupling the fiber up to the photodiode surface. As long as the photodiode active area is larger than that of the fiber core, fiber-to-detector coupling losses are very low. In some cases a lens may be used to couple the fiber end-face to the detector. However, this is not typically done.

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