I. Basic Information on light detectors

When light strikes special types of materials, a voltage may be generated, a change in electrical resistance may occur, or electrons may be ejected from the material surface. As long as the light is present, the condition continues. It ceases when the light is turned off. Any of the above conditions may be used to change the flow of current or the voltage in an external circuit and thus may be used to monitor the presence of the light and to measure its intensity.

A. Role of an optical detector

Many photonics applications require the use of optical detectors to measure optical power or energy. In laser-based fiber optic communication, a detector is employed in the receiver. In laser materials processing, a detector monitors the laser output to ensure reproducible conditions. In applications involving interferometry, detectors are used to measure the position and motion of interference fringes. In most applications of light, one uses an optical detector to measure the output of the laser or other light source. Thus, good optical detectors for measuring optical power and energy are essential in most applications of photonics technology.

Optical detectors respond to the power in the optical beam, which is proportional to the square of the electric field associated with the light wave. Optical detectors therefore are called "square-law detectors." This is in contrast to the case of microwave detectors, which can measure the electric field intensity directly. All the optical detectors that we will describe have square-law responses.

Detection and measurement of optical and infrared radiation is a well-established area of technology. This technology has been applied to photonics applications. Detectors particularly suitable for use with lasers have been developed. Some detectors are packaged in the format of power or energy meters. Such a device is a complete system for measuring the output of a specific class of lasers, and includes a detector, housing, amplification if necessary, and a readout device.