According to the working principle and structure, photodetectors are generally divided into photodetectors and pyroelectric detectors, in which photodetectors include vacuum photoelectric devices (photomultiplier tubes, etc.) and solid photodetectors (photodiodes, photoconductive detectors, CCD, etc.) .

● Photomultiplier tube (PHOTOMULTIPLIER TUBES, PMT)

Photomultiplier tube (PMT) is a photodetector device with extremely high sensitivity, and also has the characteristics of fast response, low noise, large area cathode (photosensitive surface) and so on.

A typical photomultiplier tube, in its vacuum tube, includes a device of photoemission cathode (photocathode) and focusing electrode, electron multiplier and electron collector (anode). When light illuminates the photocathode, the photocathode excites photoelectrons into the vacuum. These photoelectrons enter the multiplying system according to the electric field of the focusing pole and are multiplied and amplified by further secondary emission; the amplified electrons are collected by the anode as a signal output (analog signal output). Because of the secondary emission multiplication system, the photomultiplier tube has extremely high sensitivity and extremely low noise in the photodetector device that can detect the radiant energy in the ultraviolet, visible and near infrared regions.

In terms of receiving incident light, the photomultiplier tube has two structures: a side window type (Side-on) and an end window type (Head-on).

The side window type photomultiplier tube receives incident light from the side of the glass shell, while the end window type photomultiplier tube receives incident light from the top of the glass shell. Under normal circumstances, the side window type photomultiplier tube is cheaper, and is widely used in spectrophotometers and general photometry. Most side-window photomultiplier tubes use an opaque photocathode (reflective photocathode) and a ring-focusing electron multiplier structure, which makes them more sensitive at lower operating voltages.

The end-window type (also called top-window type) photomultiplier tube deposits a translucent photocathode (transmissive photocathode) on the inner surface of its entrance window, making it more uniform than the side window type. The characteristics of the end-window type photomultiplier tube also include that it has a larger area of ​​light-sensitive surface (tens of square millimeters to hundreds of square centimeters of photocathode). Among the end-window type photomultiplier tubes, there are also photomultiplier tubes for large-size hemispherical light windows that can capture incident light at a wide angle for high-energy physics experiments.

Because the change of the applied voltage will cause the change of the gain of the photomultiplier tube, which has a great influence on the output. Therefore, the voltage of the working power supply to the photomultiplier tube is high, and it must have excellent stability. Zhuoli Hanguang's HVC series high-voltage stabilized power supply, the stability of which can reach ± ​​0.03% / h, is very suitable as a photomultiplier tube high-voltage power supply.

At the same time, it should be noted that due to the large gain of the photomultiplier tube, it is generally not allowed to expose to sunlight to measure visible light when applying high pressure to avoid damage. When used as a photodetector, the photomultiplier tube needs to be sealed. The photomultiplier tube package provided by Zhuoli Hanguang is packaged in strict accordance with the requirements to ensure the normal and safe use of customers.

In addition, the photomultiplier tube is greatly affected by temperature. Lowering the ambient temperature of the photomultiplier tube can reduce thermionic emission and thus reduce the dark current. Especially when using a long wave (near infrared band, commonly known as red sensitive) photomultiplier tube, the ambient temperature of the photomultiplier tube should be strictly controlled. In addition, most photomultiplier tubes are affected by magnetic fields. The magnetic field will cause the electrons to deviate from the intended orbit and cause a decrease in gain. Therefore, the working efficiency of the photomultiplier tube is affected. Therefore, the packaging of the photomultiplier tube should pay special attention to electromagnetic shielding; the photomultiplier tubes provided by Zhuo Lihanguang are effectively electromagnetically shielded.

● Photodiode (Photodiode)

The working principle of the photodiode is mainly based on the photovoltaic effect.

Photovoltaic effect is the effect of generating electromotive force on PN junction after semiconductor material absorbs light energy.

● Photoconductive Detector (Photoconductive Detector)

The photoconductive detector is a photodetector device made by utilizing the photoconductive effect of semiconductor materials.

The so-called photoconductive effect refers to a physical phenomenon that the conductivity of the irradiated material changes due to radiation.

Generally, any semiconductor material with a suitable forbidden band width has a photoelectric effect. However, manufacturing practical devices must also consider factors such as performance, process, and price. Commonly used photoconductive detector materials include CdS, CdSe, CdTe, Si, Ge, etc. in the ray and visible light bands; PbS, PbSe, InSb, Hg0.75Cd0.25Te, etc. in the near infrared band; Hg1-xCdxTe, PbxSn1-x, Te, Si doping, Ge doping, etc .; CdS, CdSe, PbS and Other materials can be made of photoconductive detectors in the form of polycrystalline thin films.

The photoconductive detectors in the visible band are rarely used for spectral detection, and are often called photoresistors. Therefore, the photodetectors in the visible band used by Zhuoli Hanguang are usually PMT and photodiodes.

The common response bands of the photoconductive detectors PbS and Hg1-xCdxTe in the infrared band are 1 ～ 3μm, 3 ～ 5μm and 8 ～ 14μm. Due to their narrow band gap, at room temperature, thermal excitation is sufficient to cause a large number of free carriers in the conduction band, which greatly reduces the sensitivity to radiation. In response to light with a longer wavelength, this situation is more pronounced for electrical conductors, where detectors in the 1 to 3 μm band can operate at room temperature (with a slight decrease in sensitivity). The detectors in the 3 ～ 5μm band are divided into three situations: 1. 'Working at room temperature, but the sensitivity is greatly reduced, and the detection degree is generally only 1 ～ 7 × 108cm · Hz / W; 2. Working at thermoelectric cooling temperature (about- 60 ℃), the detection degree is about 109 cm · Hz / W; working at 3, 77K or lower, the detection degree can reach more than 1010 cm · Hz / W. Detectors in the 8-14 μm band must be operated at low temperatures, so photoconductive devices usually need to be used under refrigeration.

The time constant of the infrared detector. The time constant of PbS detector is generally 50 ～ 500μs, and the time constant of HgCdTe detector is in the order of 10-6 ～ 10-8s. Infrared detectors sometimes need to detect very weak radiation signals, such as 10-14 W; the output electrical signal is also very small, so a special preamplifier is required.

● Pyroelectric Detector

Pyroelectric infrared detectors are made of pyroelectric crystals (ferroelectrics) with polarization phenomena. The radiation it detects must be variable; for constant infrared radiation, it must be modulated (chopped), so that the constant radiation becomes alternating radiation, thereby continuously causing the temperature change of the detector to cause the pyroelectricity to be generated and output The corresponding electrical signal.

Compared with the previous optoelectronic devices, pyroelectric detectors have the following characteristics: 1. No selectivity: the response rate is independent of the wavelength; 2. The response is slow.

● The main performance parameters of the light detector

â—† Spectral responsivity

Spectral responsivity refers to the ratio of the voltage or current output by the detector to the incident optical power at a certain wavelength.

The relationship curve of the spectral responsivity with the wavelength is the spectral response curve (absolute response curve) of the detector.

If the maximum value of the spectral response curve is normalized, the relative spectral response curve is obtained.

â—† Equivalent Noise Power (NEP)

The equivalent noise power is the minimum radiated power that the detector can detect when the signal-to-noise ratio is 1, that is, the minimum detectable power.

â—† Detection rate (D) / specific detection rate (D *)

The detection rate D is the reciprocal of NEP. The larger the D, the better the detection performance of the detector.

The specific detection rate D * is the normalized detection rate, also called detection sensitivity. Its unit is: cm · Hz1 / 2 · W-1.

â—† Time constant

The time constant represents the rate of change of the detector output signal with the incident light signal, Ï„ = 1 / (2Ï€f).

● How to choose a suitable photodetector?

In the photoelectric test system, various detectors need to be selected according to actual needs, and special attention should be paid to the following aspects:

1. The actual spectrum measurement range, this is the first thing to pay attention to when choosing a light detector;

2. The photomultiplier tube is a highly sensitive detector with a limited wavelength range (usually 900nm, some models can get more than 1000nm, but the price is usually very expensive), and the use of high-voltage power supply with high stability is required;

3. The photovoltaic detector has the characteristics of fast response and high sensitivity. Generally, a lock-in amplifier is not required when it is used. When detecting weak signals, a lock-in amplifier can be used to improve the signal-to-noise ratio;

4. The light guide type detector has a slow response, and the signal light must be modulated when used, and it needs to be matched with a lock-in amplifier for signal detection, and at the same time, pay attention to the choice of modulation frequency;

5. In the selection of detectors, it is especially necessary to pay attention to the selection of supporting preamplifiers in order to maximize the detection efficiency of the detector;

6. When choosing a TE-cooled detector, you should also pay attention to the selection of the corresponding thermostat. The detector, thermostat and preamplifier must be selected separately according to needs;

7. Infrared detectors usually require cooling and use with a lock-in amplifier.

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