The principle and application of laser sensor

Laser sensors are sensors that use laser technology to measure. It consists of a laser, a laser detector and a measuring circuit. Laser sensor is a new type of measuring instrument. Its advantages are that it can realize non-contact long-distance measurement, fast speed, high precision, large range, strong anti-light and electrical interference ability, etc.
Light and Lasers Lasers were one of the most significant scientific and technological achievements to emerge in the 1960s. It has developed rapidly and has been widely used in various aspects such as national defense, production, medicine and non-electrical measurement. Unlike ordinary light, a laser needs to be generated by a laser. For the working substance of the laser, under normal conditions, most atoms are in a stable low energy level E1. Under the action of external light of appropriate frequency, the atoms in the low energy level absorb the photon energy and are excited to transition to the high energy level E2. The photon energy E=E2-E1=hv, where h is Planck's constant and v is the photon frequency. Conversely, under the induction of light with frequency v, atoms at energy level E2 will transition to a lower energy level to release energy and emit light, which is called stimulated radiation. The laser first makes the atoms of the working substance abnormally in a high energy level (that is, the population inversion distribution), which can make the stimulated radiation process dominant, so that the induced light of frequency v is enhanced, and can pass through parallel mirrors The avalanche-type amplification is formed to generate powerful stimulated radiation, which is referred to as laser.

Lasers have 3 important properties:
1. High directivity (that is, high directivity, small divergence angle of the speed of light), the expansion range of the laser beam is only a few centimeters away from a few kilometers;
2. High monochromaticity, the frequency width of laser is more than 10 times smaller than that of ordinary light;
3. High brightness, the maximum temperature of several million degrees can be generated by the use of laser beam convergence.

Lasers can be divided into 4 types according to the working substance:
1. Solid-state laser: Its working substance is solid. Commonly used are ruby lasers, neodymium-doped yttrium aluminum garnet lasers (ie YAG lasers) and neodymium glass lasers. They have roughly the same structure, and are characterized by being small, robust, and high-power. Neodymium-glass lasers are currently the devices with the highest pulse output power, reaching tens of megawatts.
2. Gas laser: its working substance is gas. Now there are various gas atom, ion, metal vapor, gas molecule lasers. Commonly used are carbon dioxide lasers, helium neon lasers and carbon monoxide lasers, which are shaped like ordinary discharge tubes, and are characterized by stable output, good monochromaticity, and long life, but with low power and low conversion efficiency.
3. Liquid laser: It can be divided into chelate laser, inorganic liquid laser and organic dye laser, the most important of which is organic dye laser, its biggest feature is that the wavelength is continuously adjustable.
4. Semiconductor laser: It is a relatively young laser, and the more mature one is the GaAs laser. It is characterized by high efficiency, small size, light weight and simple structure, and is suitable for carrying on airplanes, warships, tanks and infantry. Can be made into rangefinders and sights. However, the output power is small, the directionality is poor, and it is greatly affected by the ambient temperature.

Laser Sensor Applications
Using the characteristics of high directivity, high monochromaticity and high brightness of the laser can realize non-contact long-distance measurement. Laser sensors are often used for the measurement of physical quantities such as length, distance, vibration, speed, and orientation, as well as for flaw detection and monitoring of atmospheric pollutants.
Laser length measurement:
Precise measurement of length is one of the key technologies in precision machinery manufacturing industry and optical processing industry. Modern length measurement is mostly carried out by using the interference phenomenon of light waves, and its accuracy mainly depends on the monochromaticity of light. Laser is the most ideal light source, which is 100,000 times purer than the best monochromatic light source (krypton-86 lamp) in the past. Therefore, the laser length measurement range is large and the precision is high. According to the optical principle, the relationship between the maximum measurable length L of monochromatic light, the wavelength λ and the spectral line width δ is L=λ/δ. The maximum length that can be measured with a krypton-86 lamp is 38.5 cm. For longer objects, it needs to be measured in sections, which reduces the accuracy. If a helium-neon gas laser is used, it can measure up to tens of kilometers. Generally measure the length within a few meters, and its accuracy can reach 0.1 microns.
Laser Ranging:
Its principle is the same as that of radio radar. After the laser is aimed at the target and launched, its round-trip time is measured, and then multiplied by the speed of light to obtain the round-trip distance. Because the laser has the advantages of high directivity, high monochromaticity and high power, these are very important for measuring long distances, determining the orientation of the target, improving the signal-to-noise ratio of the receiving system, and ensuring the measurement accuracy. increasingly received attention. The lidar developed on the basis of the laser rangefinder can not only measure the distance, but also measure the azimuth, speed and acceleration of the target. Radar, ranging from 500 to 2000 kilometers, the error is only a few meters. At present, ruby lasers, neodymium glass lasers, carbon dioxide lasers and gallium arsenide lasers are often used as light sources for laser rangefinders.

Laser vibration measurement:
It measures the vibration velocity of an object based on the Doppler principle. The Doppler principle means that if the wave source or the observer receiving the wave moves relative to the medium in which the wave propagates, the frequency measured by the observer depends not only on the vibration frequency emitted by the wave source but also on the motion of the wave source or observer. The magnitude and direction of the velocity. The difference between the measured frequency and the frequency of the wave source is called the Doppler shift. When the vibration direction is consistent with the direction, the Dopp frequency shift is fd=v/λ, where v is the vibration velocity and λ is the wavelength. In the laser Doppler vibration velocity measuring instrument, due to the light round trip, fd=2v/λ. This kind of vibrometer converts the vibration of the object into the corresponding Doppler frequency shift by the optical part during measurement, and converts the frequency shift into an electrical signal by the photodetector, and then sends it to the multi-channel after proper processing by the circuit part. The Doppler signal processor converts the Doppler-shifted signal into an electrical signal corresponding to the vibration velocity, which is finally recorded on a magnetic tape. This vibrometer uses a helium-neon laser with a wavelength of 6328 angstroms (┱), uses an acousto-optic modulator for optical frequency modulation, uses a quartz crystal oscillator and a power amplifier circuit as the driving source of the acousto-optic modulator, and uses a photomultiplier tube. For photoelectric detection, a frequency tracker is used to process the Doppler signal. Its advantages are that it is easy to use, does not require a fixed reference frame, does not affect the vibration of the object itself, and has a wide measurement frequency range, high precision and large dynamic range. The disadvantage is that the measurement process is greatly affected by other stray light.
Laser speed measurement:
It is also a laser velocity measurement method based on the Doppler principle. The laser Doppler flowmeter (see laser flowmeter) is used more, which can measure the wind tunnel airflow velocity, rocket fuel flow velocity, aircraft jet airflow velocity, Atmospheric wind speed and particle size and convergence speed in chemical reactions, etc.