Photodetectors | previous | next | feedback |
You can buy photodetectors from:
- A.L.S. GmbH offers ultrafast MSN photodetectors for a wide range of wavelength with a bandwidth of up to 35 GHz.
Ask RP Photonics for advice on photodetection, e.g. concerning details of photodiodes, electronic preamplifiers, etc.
Definition: devices used for the detection of light
Photodetectors are devices used for the detection of light – in most cases of optical powers. As the requirements for applications vary a lot concerning wavelength, maximum optical power, dynamic range, linearity, quantum efficiency, bandwidth, size, robustness and cost, there are many types of photodetectors which may be appropriate in a particular case:
- Photodiodes are semiconductor devices with a p-n junction or p-i-n structure (i = intrinsic material) (→ p-i-n photodiodes), where light is absorbed in a depletion region and generates a photocurrent. Such devices can be very compact, fast, highly linear, and exhibit a high quantum efficiency (i.e., generate nearly one electron per incident photon) and a high dynamic range, provided that they are operated in combination with suitable electronics. A particularly sensitive type is that of avalanche photodiodes, which are sometimes used even for photon counting.
- Metal-semiconductor-metal (MSM) photodetectors contain two Schottky contacts instead of a p-n junction. They are potentially faster than photodiodes, with bandwidths up to hundreds of GHz.
- Phototransistors are similar to photodiodes, but exploit internal amplification of the photocurrent. They are less frequently used than photodiodes.
- Photoresistors are also based on certain semiconductors, e.g. cadmium sulfide (CdS). They are cheaper than photodiodes, but they are rather slow, are not very sensitive, and exhibit a strongly nonlinear response.
- Photomultipliers are based on vacuum tubes. They can exhibit the combination of an extremely high sensitivity (even for photon counting) with a high speed. However, they are expensive, bulky, and need a high operating voltage.
- Pyroelectric photodetectors exploit a pyroelectric voltage generated in a nonlinear crystal (e.g. LiTaO3) when heated by absorption of light on an absorbing coating on the crystal; often used for measurement of microjoule pulse energies from Q-switched lasers
- Thermal detectors (powermeters) measure a temperature rise caused by the absorption of light. Such detectors can be very robust and be used for the measurement of very high laser powers, but exhibit a small sensitivity, moderate linearity, and relatively small dynamic range.
Very good noise performance can be achieved with photodiodes and photomultipliers. A fundamental limit to this is usually given by shot noise, resulting from the random occurrence of photon absorption events. However, there are sources of so-called nonclassical light, for which the photodetection noise can be below the shot noise level.
Optical power measurements with photodetectors can be difficult for light sources with very high and/or non-constant beam divergence. In such a case, an integrating sphere may be used.
See also: photodiodes, p-i-n photodiodes, avalanche photodiodes, metal-semiconductor-metal photodetectors, velocity-matched photodetectors, photomultipliers, powermeters, photon counting, noise specifications
