Long-distance measurement applying two high-stability and synchronous wavelengths

A high-precision laser range finder based on highly stable synchronous measuring wavelengths with a large range is proposed in this paper. In this method, the coarse and fine measuring wavelengths are generated by the beat signals of lasers in different frequencies. The lasers are produced by frequency shifting one laser mode with three acousto-optic modulators working at different frequencies. To realize an appropriate coarse and fine measuring wavelengths for the required measurement range and resolution, the driving frequencies of the acousto-optic modulators are controlled by direct digital synthesizer (DDS) in high precision. At the same time, the frequency drift of the laser source and other common errors are reduced by applying the optical heterodyne effect. Consequently, the stability of measuring wavelengths is significantly improved. Furthermore, an improved digital method of phase measurement with same basic clock DDS is proposed to achieve synchronous phase measurement with a little error. Results from the experiments show that the measuring frequency stability is better than 1.75 × 10^-9 (τ =100s), and a measurement with relative uncertainty of 0.63 × 10^-6 at a distance of 100 m is achieved under laboratory conditions.