Measuring millimeter-scale distances in a laser self-mixing velocimeter with low-speed wavelength modulation

A method is proposed to measure short distances using a laser self-mixing velocimeter with low-speed wavelength modulation (2 kHz). The two power spectra obtained in the frequency domain around the rising edge and the falling edge frequencies are employed for estimating the distance and the velocity of a target. The results show that a low-speed wavelength modulation is helpful for the measurement because there are not enough fringes at such distances. The noise caused by the low-speed wavelength modulation and short distances can be distinguished by power spectrum analysis. The distance measurement attains a relative standard deviation of around 10−3 in the range from 1 mm to 23 mm, and a relative standard deviation of around 10−4 is obtained in the velocity measurement.