Phase laser ranging
Phase-type laser ranging is to measure the phase change generated by the laser modulation signal in the process of propagating back and forth over the distance to be measured, and then calculate the distance represented by the phase delay change according to the frequency of the modulation signal, that is, to measure the phase change. The indirect method replaces the direct measurement of the laser time of flight, so as to realize the measurement of the distance. Phase laser ranging is usually suitable for close-range measurements and can achieve millimeter-level accuracy. For short-distance measurement, the phase laser ranging method is superior to other ranging methods in terms of speed, accuracy and stability. However, since the phase laser emission power cannot be too high, the measurement is still limited. In order to increase the measurement range as much as possible on the premise of ensuring the distance measurement accuracy and improve the signal-to-noise ratio of the system, an optical corner reflector is generally used as a combination
as a target. Since the frequency of the light wave itself is as high as 1110Hz or more, it is very difficult to directly measure the phase of such a high-frequency signal, so the phase laser rangefinder usually needs to modulate the laser. For the convenience of analysis, let the modulation frequency be f, the waveform be a sine wave, and the wavelength λ, assuming that the receiving position is at point A' (the actual transmitting and receiving positions are both at point A), AB=BA', AA'=2L, and the signal wave passes through The phase shift after AA' is ?, and the flight time is t, then the measured distance L is 
Among them, Ls is the half-wavelength, which is called the length of the ruler, m is the number of integral cycles in the full stroke of the light wave, and Δm is the remainder less than 2π. The principle of phase laser ranging is similar to measuring distance with a ruler. The length of the ruler is Ls, and the measured distance is equal to the integer multiple of the ruler plus the remainder less than one ruler. Under the given modulation frequency f and standard atmospheric conditions, the length Ls of the measuring ruler is a definite constant, so to obtain the measured distance L, it is only necessary to determine the proportional number Δm between the whole ruler m and the remaining length.
The most important task of phase ranging is to detect the phase difference between the main wave and the echo. The core device used is called a phase detector (also known as a phase detector), which can detect the phase difference between the two signals. Phase detectors can be divided into two categories: analog phase detectors and digital phase detectors. Analog phase detectors can be divided into two types: product type and superposition type according to the working principle. Due to the low accuracy of analog phase detectors, digital phase detectors are usually used in high-precision laser ranging systems at present.
