The role of laser detection system is to change the received laser signal into an electrical signal, that is, the light information into electrical information, and through different information processing methods to obtain different information and achieve the purpose of detection. Laser detection technology is divided into two types of direct detection and outlier detection according to the detector mode, and can be classified into unit detection and multiple array detection according to the detector mode.

Direct detection technology

Direct detection is the direct conversion of the laser signal into an electrical signal. The amplitude of the electrical signal output from the photodetector is proportional to the received optical power and does not require a coherent signal, so this detection method is also known as non-coherent detection.

At present, the vast majority of LIDAR uses direct detection, such as laser fire control measurement systems, laser ranging systems, laser reconnaissance systems, atmospheric radar, etc.. This is mainly due to the following advantages of direct detection: detection technology is simple, easier to obtain the required information; detection system reliability, long-term stability is good; strong adaptability to the working environment, the ambient temperature and atmospheric pressure on the detection system is small; simple structure, small size, etc.

Minimum detectable power

The minimum detectable power of direct detection technology is related to the sensitivity of the photodetector, amplifier characteristics, filtering and signal processing methods. The minimum detectable power is the minimum power of the laser signal that can be detected by the system under the conditions of the environmental background used, the minimum requirements to meet the probability of laser detection and false alarm probability. Laser direct detection of the minimum detectable power also depends on the output signal-to-noise ratio of the photoelectric detection system. In the laser detection system, the detector output noise has detector noise, background noise and quantum noise three. If the photomultiplier tube is used to receive, the detector noise mainly from the spontaneous emission of photocathode caused by scattered particle noise, that is, dark current noise and thermal noise of the output loop. When the internal gain of the photomultiplier tube is large enough, the thermal noise can be ignored; cooling the photocathode can reduce the dark current by 1~2 orders of magnitude. If the photodiode is used to receive, the thermal noise of the detector is the main noise source. When receiving with an avalanche photodiode, the background noise is the main noise source during daytime measurements. Even under ideal conditions where both photodetector noise and background noise do not exist, quantum noise still exists due to the quantum nature of the signal radiation itself (light quanta). In addition, the signal entering the signal processor includes the white noise of the amplifier. In general, except for the single-photon detection technique used in the specific detection conditions of the human-guard laser ranging system, at present, for the direct detection technique of the pulsed laser signal, the minimum detectable power is usually within W, and the minimum can reach W.

Single photoelectron detection

Laser single photoelectron detection can reach the sensitivity limit of the most direct detection, it is mainly used for laser distance measurement of artificial satellites and the lunar surface, laser detection system can obtain the target distance information after processing the received one photoelectron signal.

Outlier detection technology

Due to the high monochromatic nature of laser light, its spectral lines are extremely narrow, so it can be used to achieve coherent detection of light by mixing two laser signals in the optical frequency band, so optical aberration detection is also called coherent detection. The principle of optical external aberration is the same as the principle of electronic external aberration. Compared with the direct detection method, the difference detection more than a local oscillation laser and beam synthesizer. The mixer is a photodetector, with the function of photoelectric conversion and optical mixing, and the same photodetector used in the direct detection method.

Two beams of light frequency similar to the laser signal and the local vibration light through the beam synthesis, in the photodetector photosensitive surface phase interference (electronics called mixing) to produce interference fringes, the rate of change depends on the signal light and the local vibration light differential frequency term, the optical frequency difference is

Δf=f-f0

Where, f is the laser signal frequency; f0 is the local oscillation laser frequency; Δf is the optical frequency difference.

Although the detector does not respond to very high optical frequency changes, but can respond well to the differential frequency changes, so it can output the optical external differential signal current that is the differential frequency current, which contains the amplitude, frequency and phase information of the optical signal. Compared with direct detection, the differential detection can detect not only the intensity of the light signal, but also the optical frequency change.

From the principle of outlier detection, it is clear that the basic condition to meet the optical outlier detection is the condition that the signal and the local oscillator light can produce coherence. For effective mixing, the local oscillation light and the signal light should be strictly parallel; and the optical frequency of the laser transmitter and the local oscillator must be stable. In addition, the longer the laser wavelength, the easier it is to meet the interference conditions. Among the technically mature lasers, the CO2 laser has the longest wavelength of 10.6um, and the stable frequency CO2 laser has a very high optical frequency stability, with short-term stability up to a total frequency shift of less than 2 kHz within a few seconds. in addition, the 10.6um wavelength is in the atmospheric window, and has a CdHgTe infrared detector with good performance at this wavelength. Therefore, the stable frequency CO2 laser is an ideal light source for coherent detection.

In addition, due to atmospheric turbulence, there is a wavefront phase difference in the incident light within the optical receiving aperture, and optical adaptive techniques are usually used to compensate for the phase difference during long-range measurements.

Ectrodifferential detection is used for speed measurement, the same principle as radio speed measurement, due to the Doppler effect, the laser frequency reflected back from the moving target has a small change relative to the irradiated laser frequency, the use of optical ectrodifferential measurement of frequency change worth the target speed, called Doppler speed measurement. Since the value of the frequency change due to velocity is proportional to the incident light frequency, which is very high (Hz), the accuracy of speed measurement using optical external aberration is very high and can measure very small moving velocities. Compared with centimeter wave radar, the Doppler effect of laser is larger by times ~ times.

Principle of outlier detection of microwave modulated laser intensity

In this technical regime, the fundamental frequency of the external aberration is a microwave frequency, not an optical frequency. Since the optical ectopia reception technique is both complex and unstable, in some laser measurement radars, the laser is used as the carrier wave and the intensity modulation of the laser is performed using microwave or high frequency signals so that the laser can be detected directly and the frequency shift of the modulation frequency can be measured by electrical ectopia to find the target motion speed.

Laser Detectors

Detectors used for LIDAR are divided into two main categories: external photoelectric effect detectors (vacuum tube type) and internal electrical effect detectors.

External photoelectric effect detectors

Commonly used photomultiplier tube, microchannel plate tube, like enhanced tube and stripe tube belong to this category. The first two are more sensitive to a single photon energy, and has a high internal gain, internal gain can reach, they are generally applied to visible laser wavelength, weak signal, response speed of high occasions. The latter two applications of laser imaging detection. The response wavelength of photoemission detectors can be extended to 1.1um, but the photoelectric conversion efficiency is extremely low compared to visible light.

Internal photoelectric effect detector

This is a PN semiconductor device, such as commonly used silicon photodiodes, PIN tubes, avalanche photodiodes (APDs), and multiple array semiconductor detectors. This type of detector has good photoelectric conversion performance from visible to near-infrared wavelengths. At present, in addition to a few single photon detection and imaging detection LIDAR, generally use the internal photoelectric effect detector, of which APD tube is used most, its internal gain can reach 50 ~ 100. In addition, for more than 1.54um human eye safety laser, more InGaAS and HgCdTe.