基于飞秒激光模间拍频法的大尺寸测距方法

Large-scale absolute distance measurement using inter-mode beat of a femtosecond laser

本文基于飞秒激光模间拍频法实现多波长相位式绝对距离测量,通过改变光频梳重复频率合成波长扩大测距量程,并采用监测臂和双快门切换系统补偿和消除由电路产生的相位差单向漂移和大幅抖动.实验中以20倍重复频率的拍频进行测量,在30 min内相位测量的标准偏差为0.022?;与双频激光干涉仪比对1125 mm行程内位移测量结果,测距精度优于50μm;实验验证了合成波长方法扩大量程方案的可行性,获得的测距重复性优于3μm,该系统理论上可扩展量程至7.5 km.

Large-scale absolute distance measurement system with high accuracy plays a significant role in science and engineering applications. In many fields such as aerospace technology, large-scale manufacture, geodetic survey and civil engineering, absolute distance measurement systems with a range of up to kilometers and accuracy of better than several micrometers are generally required. Traditional laser ranging methods such as the time-of-flight method and the interferometry method are difficult to achieve both large scale and high accuracy. With the development of femtosecond optical frequency comb technology, several ranging methods with larger range and higher accuracy are developed. In the frequency domain, the optical frequency comb has a large number of stable mode lines, or the longitudinal modes, at regular intervals, which generates the inter-mode beat signal. In this study, based on the inter-mode beat of a femtosecond laser, an absolute distance measurement system using multi-wavelength interferometric method is demonstrated. It has a simple experimental setup with high accuracy but in a limited range of 2.5 m due to the 2π-period of phase detection.To achieve a large-scale measurement system, the measurement range of the experimental system is extended by using the synthetic wavelength generated by tuning the repetition frequency of the laser. With a repetition frequency change of0.2 MHz, a synthetic wavelength of up to 1.5 km is realized, thus the measurement range of the experimental setup can be extended to 0.75 km. Besides the reference and measurement path beams, a monitor path beam and two alternately opened mechanical shutters are used to measure and compensate for the phase drift due to the unbalanced drift of the electronic circuit. By using this method, the standard deviation of the phase measurement results in 30 min is 0.022？in the experiment, and the phase drift can be compensated for very well. The measurement results from the experimental system are compared with the results from a commercial heterodyne interferometer, and the comparison between results shows a precision of better than 50 μm in a displacement of 1125 mm. In the experiment, the repeatability of absolute distance measurement using the range extending method is better than 3 μm, thus the range of the distance measurement system can be theoretically extended up to 7.5 km. In conclusion, we demonstrate that a large-scale absolute distance measurement system using inter-mode beat of a femtosecond laser, has a range of up to 7.5 km, an accuracy of better than 50 μm and a repeatability of better than 3 μm. The accuracy of the experimental system can be further improved by using photodetectors with higher bandwidth so that a higher inter-mode beat and a shorter wavelength can be used.