A common type is the laser rangefinder, which determines the distance to a target by recording and processing the time it takes for a light pulse to travel from emission to reception. Laser sensors must measure the transmission time with extreme precision because the speed of light is so fast.
Main functions of laser sensors
Laser sensors utilize the high directionality, high monochromaticity, and high brightness of lasers to achieve non-contact, long-distance measurements. They are commonly used for measuring physical quantities such as length, distance, vibration, velocity, and orientation, and can also be used for flaw detection and monitoring of atmospheric pollutants.
1. Laser length measurement
Precision length measurement is a key technology in precision machinery manufacturing and optical processing industries. Modern length measurement mostly utilizes the interference phenomenon of light waves, and its accuracy depends primarily on the monochromaticity of the light. Lasers are the most ideal light source, being 100,000 times purer than the best monochromatic light source before (krypton-86 lamp). Therefore, laser length measurement offers a large range and high accuracy. According to optical principles, the maximum measurable length L of monochromatic light is related to its wavelength λ and spectral linewidth δ by the formula L = λ/δ. The maximum measurable length using a krypton-86 lamp is 38.5 cm; for longer objects, segmented measurement is necessary, reducing accuracy. However, using a helium-neon gas laser, the maximum measurable length can reach tens of kilometers. For general length measurements within a few meters, accuracy can reach 0.1 micrometers.
2. Laser ranging
Its principle is the same as that of radio radar. After emitting a laser beam at the target, the round-trip time is measured, and then multiplied by the speed of light to obtain the round-trip distance. Because lasers have advantages such as high directionality, high monochromaticity, and high power, these are crucial for measuring long distances, determining target location, improving the signal-to-noise ratio of the receiving system, and ensuring measurement accuracy. Therefore, laser rangefinders are receiving increasing attention. Based on laser rangefinders, lidar can not only measure distance but also target location, velocity, and acceleration. It has been successfully used for satellite ranging and tracking. For example, lidar using ruby lasers has a ranging range of 500–2000 kilometers with an error of only a few meters. Recently, Zhenshangyou's R&D center developed the LDM series of ranging sensors, which can achieve micron-level accuracy within a measurement range of several kilometers. Ruby lasers, neodymium glass lasers, carbon dioxide lasers, and gallium arsenide lasers are commonly used as light sources for laser rangefinders.
3. Laser vibration measurement
It measures the vibration velocity of an object based on the Doppler principle. The Doppler principle states that if the wave source or the observer receiving the wave is moving relative to the medium through which the wave propagates, then the frequency measured by the observer depends not only on the vibration frequency emitted by the wave source but also on the magnitude and direction of the velocity of the wave source or the observer. The difference between the measured frequency and the frequency of the wave source is called the Doppler frequency shift. When the vibration direction is the same as the wavelength, the Doppler frequency shift fd = v/λ, where v is the vibration velocity and λ is the wavelength. In a laser Doppler vibration velocity meter, due to the round-trip time of light, fd = 2v/λ. During measurement, this type of vibration meter uses an optical component to convert the object's vibration into a corresponding Doppler frequency shift, which is then converted into an electrical signal by a photodetector. After appropriate processing by the circuitry, the signal is sent to a Doppler signal processor to convert the Doppler frequency shift signal into an electrical signal corresponding to the vibration velocity, and finally recorded on magnetic tape. This vibration meter uses a 6328 angstrom (Å) helium-neon laser, employs an acousto-optic modulator for optical frequency modulation, uses a quartz crystal oscillator with a power amplifier circuit as the driving source for the acousto-optic modulator, uses a photomultiplier tube for photoelectric detection, and uses a frequency tracker to process the Doppler signal. Its advantages include ease of use, no need for a fixed reference frame, no interference with the object's own vibration, and a wide measurement frequency range, high accuracy, and large dynamic range. Its disadvantage is that the measurement process is significantly affected by stray light.
4. Laser velocimetry
It is also a laser velocimetry method based on the Gibbs-Doppler principle. The most commonly used type is the laser Doppler velocimeter (see laser flow meter). It can measure the airflow velocity in wind tunnels, the flow velocity of rocket fuel, the flow velocity of jet air from aircraft, atmospheric wind speed, and the size and convergence velocity of particles in chemical reactions.
