To compensate for the refractive index errors in optical carrier-based microwave interferometry (OCMI), a ranging system is designed to measure the single-arm optical path of OCMI. A high-speed photodetector and a downconversion method are used to acquire the microwave envelope of the interference signal. A Hilbert transformation is used to realize phase detection. Simulation shows the linear relationship between the phase and optical length in a period. Adjusting the microwave frequency can resolve the phase ambiguity. The experimental results show that when the maximum microwave modulation frequency is set to 1.5 GHz, the standard deviation of the measured data can be limited to the level of 10 − 5 .

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High-precision spatial ranging plays a significant role in both scientific research and industrial practice. However, it is difficult for existing equipment to achieve high speed, high precision, and long distance simultaneously. Inspired by the concept of optical carrier-based microwave interferometry (OCMI), this paper reports a method of high-precision spatial distance measurement. A microwave-modulated broadband optical signal is sent to the interferometer whose measuring arm is an optical echo receiving system in free space. By scanning the microwave frequency, the measured distance can be resolved from the interferogram. Since the processing of the interference spectrum is performed in the microwave domain, this method is insensitive to the types of optical waveguides and states of optical polarizations. The experimental results show that the root mean square error (RMSE) of ten repeated measurements at 0.5 m is 0.016 µm, the RMSE is 0.023 µm within a 1 m distance, which can effectively represent the length measuring capability of the proposed system.

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Aimed at high-precision distance measurement for noncooperative targets in free space, a spatial distance measurement method is proposed. Based on the concept of optical carrier-based microwave interferometry, this method extracts distance information from the radiofrequency domain. The interference model of broadband light beams is established, and the optical interference can be eliminated by using a broadband light source. A spatial optical system with a Cassegrain telescope as the main body is designed to effectively receive the backscattered signal without cooperative targets. A free-space distance measurement system is built to verify the feasibility of the proposed method, and the results agree well with the set distances. Long-distance measurements with a resolution of 0.033 µm can be achieved, and the errors of the ranging experiments are within 0.1 µm. The proposed method has the advantages of fast processing speed, high measurement accuracy, and high immunity to disturbances as well as the potential for measurement of other physical quantities.