定义：用来探测光的装置。

光电探测器是用来探测光的装置，大部分情况下是探测光功率。由于实际应用的要求变化多样，因此存在很多种适合不同用途的光电探测器： 

• 光电二极管是具有p-n结或者p-i-n结构的半导体器件（i代表本征层）（参阅PIN型光电二极管），其中光在耗尽层被吸收产生光电流。这一装置尺寸非常小，采用合适的电子装置情况下，该器件具有响应快，响应高线性以及很高的量子效率（即单个入射光子几乎可以产生一个电子），动态范围大的特点。其中非常灵敏的一类为雪崩光电二极管，有时甚至可以用于光子计数。 
• 金属-半导体-金属光电探测器包含两个肖特基接触。它比光电二极管响应更快，带宽为几百GHz。 
• 光电晶体管与光电二极管类似，来使利用了光电流的内放大。它没有光电二极管常用。 
• 光敏电阻器是利用了一些特定的半导体材料，例如，硫化镉（CdS）。它的价格比光电二极管低，那时响应慢，并且不灵敏，具有强的非线性响应。 
• 光电倍增管采用的是真空管。它同时具有很高的灵敏度（甚至可用于光子计数）和很高的响应速度。但是，它的价格很高，尺寸较大，并且需要很高的工作电压。 
• 热释电探测器利用非线性晶体（例如，LiTaO3）在吸收涂层处吸收光脉冲受热后产生的热释电压脉冲。它们通常用于测量调Q激光器中产生的毫焦耳脉冲能量。 
• 热学探测器（功率计）可以测量吸收光引起的温度升高。这种探测器非常坚固，可以测量非常高的激光功率，但是灵敏度较低，中等的线性响应，相对较小的动态范围。 
• 目前关于碳纳米管和石墨烯的光电探测器还在研发阶段，它有潜力可以提供很大的波长范围和很快的响应速度。将这一器件集成到光电芯片上的方法也在研究当中。 

光电探测器的一些重要性质 
根据实际要求，光电探测器需要满足一下要求： 

• 探测器需要在某一给定波长范围具有很高的灵敏度。有些情况下，需要灵敏度不变，或者至少在给定波长区域内不变。有时也需要在一些其它波长区域没有响应；例如，日盲探测器，只在很短的紫外光波段是灵敏的，而对于太阳光不敏感。 
• 探测器必须在某一功率范围内工作正常。限制探测器功率的最大值取决于非线性响应或者损伤问题，而最小功率则是由噪声决定。动态范围（最大探测功率与最小探测功率的比值用分贝表示）也非常重要。有些探测器在动态范围大于70dB时也具有很高的线性响应。 
• 有些情况下，不仅需要很高的响应度，还需要很高的量子效率，否则会引入额外的量子噪声。这可以用来探测光的压缩态，也会影响单光子探测器的光子探测概率。 
• 当探测激光二极管中发出的强发散光束时，需要考虑探测器的有源区大小。如果光源具有很大并且变化的光束发散角，很难在有源区全部探测到这些光。这时可以采用一个积分球来测量总功率。 
• 探测带宽可能从0 Hz或者某一有限频率处开始，而峰值频率由内部过程（例如，半导体材料中载流子的速度）或者相关的电子学器件（例如，引入一些RC时间常数）来决定。某些谐振腔探测器只工作在很窄的频率范围内，非常适合锁相探测。 
• 有些探测器（例如热释电探测器）非常适合探测脉冲，不适合探测连续光。 
• 当探测脉冲（几个光子量级）时，计时精度非常重要。有些探测器探测一个脉冲后具有一段“死区时间”，在该时间内灵敏度很低。 
• 不同类型的探测器需要不同的复杂电子学装置。当需要施加高电压或者探测非常小电压时，就会使器件尺寸变大，成本变高。 
• 尤其是有些中红外光探测器需要冷却到很低的温度。因此它在很多情况下不适用。 
• 有时需要采用一维或者二维光电探测器阵列。这时需要考虑一些其它的因素，例如，交叉干扰和读出技术。 
• 很多应用都对器件尺寸、坚固性和成本有要求。 

如上所列的各种不同类型的探测器具有很多不同的性质。在传统的应用领域，一些基本要求可以排除某些探测器类型，可供选择的探测器类型很有限。需要注意的是，在应用时也需要在一些方面进行权衡。例如，很难同时得到很高的探测带宽和高的灵敏度。 
 

Definition: devices used for the detection of light

Alternative terms: light detectors, photon detectors

More general term: light detectors

More specific terms: infrared detectors, position-sensitive detectors, photodiodes, phototransistors, metal–semiconductor–metal photodetectors, velocity-matched photodetectors, photomultipliers, pyroelectric detectors, photoconductive detectors, photoemissive detectors, solar-blind photodetectors

Opposite terms: thermal detectors

Photodetectors are devices used for the detection of light – in most cases of its optical power. More specifically, photodetectors are usually understood as photon detectors, which in some way utilize the photo-excitation of electric carriers; thermal detectors are then not included by the term, and are also not treated in this article.

Photodetectors usually deliver an electronic output signal – for example, a voltage or electric current which is proportional to the incident optical power. They are thus belonging to the area of optoelectronics.

Types of Photodetectors

As the requirements for applications (see below) vary considerably, 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 gigahertz.
• Phototransistors are similar to photodiodes, but exploit internal amplification of the photocurrent. They are less frequently used than photodiodes.
• Photoconductive detectors are also based on certain semiconductors, e.g. cadmium sulfide (CdS). They are cheaper than photodiodes, but they are fairly slow, are not very sensitive, and exhibit a nonlinear response. On the other hand, they can respond to long-wavelength infrared light.
• Phototubes are vacuum tubes or gas-filled tubes where the photoelectric effect is exploited (→ photoemissive detectors).
• Photomultipliers are a special kind of phototubes, exploiting electron multiplication processes for obtaining a much increased responsivity. They can also have a high speed and large active area. Some of them are based on multichannel plates; they can be substantially more compact than traditional photomultipliers.
• Research is performed on novel photodetectors based on carbon nanotubes (CNT) or graphene, which can offer a very broad wavelength range and a very fast response. Ways for integrating such devices into optoelectronic chips are explored.

These devices are all based on the internal or external photoelectric effect; photoemissive detectors belong to the latter category.

Various kinds of photodetectors can be integrated into devices like power meters and optical power monitors. Others can be made in the form of large two-dimensional arrays, e.g. for imaging applications. They may be called focal plane arrays. For example, there are CCD and CMOS sensors which are used mainly in cameras.

See also the article on terahertz detectors.

Important Properties of Photodetectors

Depending on the application, a photodetector has to fulfill various requirements:

• It must be sensitive in a certain spectral region (range of optical wavelengths). In some cases, the responsivity should be constant or at least well defined within some wavelength range. It can also be important to have zero response in some other wavelength range; an example are solar-blind photodetectors, being sensitive only to short-wavelength ultraviolet light but not to visible sun light.
• The responsivity tells how much electrical signal is obtained per unit optical power. It depends on the optical wavelength.
• In some cases, not only a high responsivity, but also a high quantum efficiency is important, as otherwise additional quantum noise is introduced. This applies e.g. to the detection of squeezed states of light, and also affects the photon detection probability of photon counting detectors.
• The detector must be suitable for some range of optical powers. The maximum detected power can be limited e.g. by damage issues or by a nonlinear response, whereas the minimum power is normally determined by noise. The magnitude of the dynamic range (typically specified as the ratio of maximum and minimum detectable power, e.g. in decibels) is often most important. Some detectors (e.g. photodiodes) can exhibit high linearity over a dynamic range of more than 70 dB.
• The active area of a detector can be important e.g. when working with strongly divergent beams from laser diodes. A high enough uniformity of the responsivity may be important. For light sources with very high and/or non-constant beam divergence, it is hardly possible to get all the light onto the active area; an integrating sphere may then be used (with appropriate calibration) for measuring the total power.
• The detection bandwidth may begin at 0 Hz or some finite frequency (for AC-coupled detectors), and ends at some maximum frequency which may be limited by internal processes (e.g. the speed of electric carriers in a semiconductor material) or by the involved electronics (e.g. introducing some RC time constants). Some resonant detectors operate only in a narrow frequency range, and can be suitable e.g. for lock-in detection.
• Some detectors (such as pyroelectric detectors) are suitable only for detecting pulses, not for continuous-wave light.
• For detecting light pulses (possibly on a few-photon level), the timing precision may be of interest. Some detectors have a certain “dead time” after the detection of a pulse, where they are not sensitive.
• Different types of detectors require more or less complex electronics. Penalties in terms of size and cost may result e.g. from the requirement of applying a high voltage or detecting extremely small voltages.
• Particularly some mid-infrared detectors need to be cooled to fairly low temperatures. This makes their use under various circumstances impractical.
• For some applications, one-dimensional or two-dimensional photodetector arrays are needed – most often in the form of photodiode arrays. For detector arrays, some different aspects come into play, such as cross-pixel interference, read-out techniques and detector uniformity.
• Finally, the size, robustness and cost are essential for many applications.

Different detector types, as listed above, differ very much in many of these properties. In typical application scenarios, some requirements totally rule out the use of certain detector types, and quickly lead to a fairly limited choice. Note also that there are some typical trade-offs. For example, it is frequently difficult to combine a high detection bandwidth with a high sensitivity.

Applications of Photodetectors

Photodetectors have a very wide range of applications. Some examples:

• In radiometry and photometry, they can be used for measuring properties like optical power, luminous flux, optical intensity and irradiance, in conjunction with additional means also for properties like the radiance.
• They are used to measure optical powers e.g. in spectrometers, light barriers, optical data storage devices, autocorrelators, beam profilers, fluorescence microscopes, interferometers and various types of optical sensors.
• Particularly sensitive photodetectors are required for laser rangefinders, LIDAR, quantum optics experiments and night vision devices.
• Particularly fast photodetectors are used for optical fiber communications, optical frequency metrology and for the characterization of pulsed lasers or laser noise.
• Mostly two-dimensional arrays containing many identical photodetectors are used as focal plane arrays, mostly for imaging applications. For example, most cameras contain such devices as image sensors.

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