APPLICATION

The photoconductor is attractive for its simple structure, low cost, and rugged features. For these reasons, it is quite widely used in applications such as photographic meters, flame/smoke/burglar detectors, card readers, lighting controls for street lamps, etc. Extrinsic photoconductors can extend the long-wavelength limit without using materials of very narrow energy gap, and they are used as popular infrared photodetectors. A photoconductor is also used to detect nuclear radiation. Besides being a photodetector, it has another useful application as an electrical switch controlled by light. A very short light pulse, such as that from a laser, can generate sharp voltage-pulse waveforms. This technique is useful in high-speed optoelectronics, such as sampling in analog-to-digital conversion, and generating high-voltage pulses to drive another laser. Another potential use of a photoconductor is to provide variable resistance in a circuit.

There exists a large variety of photodetectors. To choose a photodetector specific application, the first criterion is the applicable range of wavelength. Next, one has to consider speed, responsivity and cost. The applications of photodetectors are listed below.

1. Object detection: a light source coupled to a photodetector detects an object that interrupts the light beam. This pair can arrange the reflection mode or direct mode. Examples are liquid-level detection, tachometer, and range finders.

2. Data retrieval: The light-detector pair is used to read stored optical. Examples are compact-disc player and card reader.

3. Optical-fiber communication: The main advantage compared to an electrical system bandwidth. The light source can be either a laser or an LED. The chosen wavelength in this application is determined by the optical characteristics of the optical fiber. It is for minimum attenuation 1.5 m is most suitable and for minimum chromatic dispersion, 1.3 is optimal. The repeater serves to boost up the signal and clean up the noise a long-distant fiber. It consists of a detector, an amplifier, followed by a source.

4. Opto-isolator: An opto-isolator (opto-coupler) consists of a light source which converts an electrical signal to a light signal and a photodetector which converts the light signal back to the electrical signal.