Optical System Principle
The core of the Sunnyinnovation laser vibrometer is a high-precision laser interferometer and a signal processor. The polarized light (with a frequency of F0) emitted by the laser in the high-precision laser interferometer is split into two beams by a beam splitter. One beam serves as the measuring light, and the other serves as the reference light. The reference light undergoes a certain frequency shift (F) through an acousto-optic modulator. The measuring light is focused onto the surface of the measured object. The vibration of the object causes a Doppler frequency shift (f = 2v/λ) of the measuring light. The system collects the reflected light and combines it with the reference light on the sensor. In this way, the two beams interfere on the surface of the sensor. The frequency of the interference signal is F0 + F + f, which carries the vibration information of the measured object. The signal processor then converts the frequency signal into the velocity and displacement signals of the object's vibration.
Doppler effect
During the propagation of a wave, its frequency changes with the relative motion between the wave source and the observer. When light shines on the surface of a moving object, for a light detector, the motion of the object causes a change in the frequency of the light scattered from its surface. This frequency change value is related to the velocity and direction of the object's motion, the wavelength, and the direction of the incident light. If the values of the latter parameters are known, the velocity of the moving object can be obtained by measuring the frequency change of the scattered light.
Principle of Optical Heterodyne Interference
Optical heterodyne interference occurs when two coherent light beams with different frequencies are simultaneously projected onto the photosensitive surface of a photodetector and interfere with each other. Then, through the square-law effect of photoelectric conversion, the frequency difference between them is obtained. This frequency difference is the required Doppler frequency shift. The remaining frequency information that is close to or higher than the optical wave frequency is filtered out because it exceeds the frequency response range of the light detector.
Sunnyinnovation Laser Doppler Vibrometer Series
