Applications of Laser Sensors

  • Aug 01, 2022
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Laser sensors are commonly 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. All of them are characterized by the high directionality, high monochromaticity and high brightness of laser light to achieve contactless long-distance measurement.

Laser length measurement.

Precision length measurement is one of the key technologies in the precision machinery manufacturing industry and the optical processing industry. Modern length measurement is mostly carried out by using the interference phenomenon of light waves, which depends most on the goodness of the monochromatic nature of light. Laser is the most ideal light source, it is 100,000 times more pure than the best monochromatic light source (Krypton - 86 lamp). Therefore, the laser length measurement range is large and accurate. The relationship between the maximum measurable length L of monochromatic light and the wavelength λ and spectral width δ is L = λ/δ. The maximum length measurable with a krypton-86 lamp is 38.5 cm, which reduces the accuracy for longer objects that need to be measured in sections. If a helium-neon gas laser is used, the maximum measurement can be tens of kilometers. Lengths within a few meters are generally measured with an accuracy of up to 0.1 microns.

Laser distance measurement.

It is based on the same principle as radio radar, where a laser is fired at a target, its round-trip time is measured, and then multiplied by the speed of light to obtain the round-trip distance. Since laser has the advantages of high directionality, high monochromaticity and high power, which are critical 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, laser rangefinders are increasingly valued. Lidar developed on the basis of laser rangefinder can not only measure the distance, but also the target orientation, transport speed and acceleration, etc. It has been successfully used in the ranging and tracking of artificial satellites, for example, the lidar using ruby laser, ranging from 500 to 2000 km, with an error of only a few meters. At present, ruby lasers, neodymium glass lasers, carbon dioxide lasers and gallium arsenide lasers are often used as the light source of laser rangefinders.

Laser vibration measurement.

It is based on the Doppler principle to measure the vibration speed of an object. The Doppler principle implies that if the wave source or the observer receiving the wave moves relative to the medium propagating the wave, the frequency measured by the observer depends not only on the frequency of the vibration emitted by the wave source, but also on the magnitude and direction of the speed of motion of the wave source or the observer. The difference between the measured frequency and the frequency of the wave source is called the Doppler shift. This type of vibrometer uses a helium-neon laser with a wavelength of 6328Å, an acousto-optic modulator for optical frequency modulation, a quartz crystal oscillator plus a power amplifier circuit as the drive source of the acousto-optic modulator, a photomultiplier tube for photoelectric detection, and a frequency tracker to process the Doppler signal. Its advantages are easy to use, no need for a fixed reference system, does not affect the vibration of the object itself, a wide range of measurement frequencies, high accuracy, and a large dynamic range. The disadvantage is that the measurement process can be affected by other stray light.

Laser velocimetry

It is also a laser velocimetry method based on the Doppler principle, which is more commonly used in laser Doppler flowmeter, which can measure the wind tunnel airflow speed, rocket fuel flow rate, aircraft jet flow rate, atmospheric wind speed and the size and convergence speed of particles in chemical reactions, etc.

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