The measurement of high frequency and micro-vibration plays a critical role in multiple fields, such as nondestructive testing, micro machinery, materials science, biomedicine, and aerospace. Optical measurement methods with high accuracy, non-contact, and other advantages are widely applied to avoid device interference and damage. Compared with out-of-plane vibration measurement, in-plane vibration measurement is difficult because the measured light cannot be incident parallel to the in-plane vibration direction. Speckle interferometry is the most commonly used method for in-plane displacement measurement. However, owing to the high-speed performance limitation of the charge-coupled device (CCD) in the imaging process, its measurement frequency cannot be increased. In addition, the measured vibration surface is typically rough, leading to low measurement sensitivity for traditional laser interferometry. The dynamic holographic vibration measurement method based on a photorefractive crystal has attracted increasing attention due to advantages such as wavefront matching and low-frequency cutoff. In this study, a dynamic holographic in-plane vibration measurement system based on bismuth silicate (BSO) crystal is examined, which measures high frequency and small in-plane vibration in real time. It has strong anti-interference ability and is suitable for rough surfaces.
The laser beam emitted from the source is split into a reference beam and a signal beam. The signal beam carrying in-plane vibration information passes through a scatterer driven by shear piezoelectric ceramics. Both beams interfere in a BSO crystal. According to the photorefractive properties of BSO crystal, a refractive index grating is formed on the crystal which is equivalent to the interference recording in holography. The grating automatically meets the Bragg condition, and the reference beam generates Bragg diffracted beam in real time, which is equivalent to the diffraction reproduction in holography. The diffracted beam of the reference beam passing through the grating interferes with the transmitted beam of the signal beam. Vibration information can be obtained by receiving the interference signal and demodulating it through the detector. The relationship between the intensity of the diffraction signal and the light intensity ratio of the two interference beams is measured to obtain a better interference signal. A commercial out-of-plane laser vibrometer PDV100 (Polytec, Germany) is used for comparison, and the feasibility of the proposed system for measuring in-plane micro-vibration is verified. The capability of the proposed system to measure high-frequency vibration is also verified by demodulating the loading frequency beyond the measurement upper limit of PDV100. To this end, the demodulation frequency is obtained directly by the spectrum analyzer.
In contrast to out-of-plane vibration which is straightforward to measure, this paper proposes a dynamic holographic measurement system for in-plane vibration. The signal beam is modulated by the in-plane vibration of a scatterer after passing through it. The signal and reference beams form dynamic holography in a BSO crystal. Interference between the transmitted scattered beam and the diffraction beam of the reference beam occurs, and the in-plane vibration signal is obtained by demodulation of the interference signal. Photorefractive properties of the BSO crystal enable it to record dynamic holography and diffract in real time. As a result, the automatic wavefront match of the two interference beams can be achieved, which is suitable for measuring the vibration of rough surfaces. Combined with the low-frequency cutoff of BSO crystals, higher measurement sensitivity can be obtained and it is more suitable for high-frequency vibration measurement with small amplitude. The feasibility of the proposed system is verified by the comparison with the measurement results of a commercial vibrometer. With the scatterer as the object, the submicron in-plane vibration with a frequency of 240 kHz can be measured.