Comparison of Robot Ear Accomplished by Laser (REAL) and Laser doppler vibrometers (LDV)
A typical scheme of LDV is shown in the literature.[1] LDV is based on the principle of laser interference, which requires the laser (He-Ne) with good monochromaticity and phase stability. The detection scheme of LDV is optical heterodyne detection, which requires the reference light with an MHz-level frequency shift produced through an acoustic-optic modulator (AOM). As a result, LDV, composed of high-performance phase-stable lasers, interference optics, AOMs, high-power drivers and complex high-frequency modulation and demodulation circuits, is much more delicate and expensive than REAL, which needs only a stable laser pointer, optical lenses and signal amplification circuits.
Analysis on LDV with rough surfaces
For the LDV, the reference laser and signal laser must be temporally and spatially coherent across the detecting surface. In many application scenarios, the signal laser is reflected from optically rough surfaces or passes through optically translucent media leading to the destruction of the coherent wavefront. This is known as speckle,[2] which significantly damages the quality of the LDV signal. Noise contribution from speckles is analyzed,[3] and it is demonstrated that the speckle is indeed the largest noise across different frequency bands.[4-6]
Here we analyse the influence of the LDV signal caused by the rough surface such as masks or throat surfaces. According to the principle of LDV, the total electric field entering the LDV detector is