Significance With the continuous development of large equipment manufacturing technology, the increase of measurement scale and the improvement of accuracy requirements make the existing large size precision measurement technology is severely challenged. Length is the most basic and core observation in the field of mechanical geometry measurement. From space-scale measurement and positioning systems, to three dimensional coordinate measurement systems in industrial manufacturing, and even micro-nano scale observation systems, high precision distance measurement plays a fundamental and crucial role. Frequency scanning interferometry distance measurement, with its good comprehensive performance in accuracy, efficiency, field adaptability, traceability and other aspects, is especially suitable for large size absolute distance measurement tasks in the current industrial measurement environment.

Progress Since the 1990s, extensive research on the error factors of frequency scanning interferometry (FSI) ranging has been conducted, resulting in a series of key technologies. When the scanning of optical frequency is linear, the FSI distance measurement accuracy is mainly determined by the phase measurement accuracy of interference signal and the measurement accuracy of optical frequency change.

The phase extraction of FSI interference signals is generally achieved through the principle of orthogonal phase discrimination, which can achieve high accuracy, but requires high signal-to-noise ratio of interference signals. The spectrum analysis method can extract the frequency characteristics of the target under the condition of low signal-to-noise ratio. Since the femtosecond optical frequency comb is a series of frequency combs with equal optical frequency interval in the frequency domain and the optical frequency corresponding to each comb can be directly traced to the international standard, the scanning optical frequency range can be monitored by using the frequency characteristics of the optical frequency comb with high accuracy (Fig. 2). More intensive optical frequency monitoring information can be provided by the auxiliary interferometer, and the signals of the measured optical path are sampled at equal optical frequency interval. Thus, the scanning optical frequency range can be measured with high precision, and the nonlinear error of scanning frequency can be effectively corrected (Fig. 3). In addition to the above idea of realizing nonlinear correction through optical frequency monitoring, another idea is to rely on the phase-locked loop technology to actively correct the scanning speed of the tunable laser in real time (Fig. 4). In the industrial measurement environment, the optical path difference of the auxiliary interferometer as the measurement reference will change, so the accuracy of the optical path of the auxiliary interferometer needs to be strictly guaranteed. The optical path difference of the auxiliary interferometer is usually recalibrated regularly with a more accurate measurement reference, such as a gas absorption tank with hydrogen cyanide encapsulation (Fig. 6). For tasks requiring large scanning bandwidth or long distance measurement, the dispersion mismatch of the long fiber auxiliary interferometer will cause accuracy loss of several hundred microns, which needs to be compensated by efficient and fast numerical algorithms.

Industrial field environmentwill cause fluctuation of measurement optical path difference. When two lasers are simultaneously tuned upward and downward in frequency, the distance measurement errors can be compensated because the dynamic errors induced by optical path fluctuation are equal but opposite (Fig. 9). In addition, the dynamic error can be compensated by constructing a common path velocimeter to monitor the optical path fluctuations and through a single frequency laser (Fig. 10 and 11).

Conclusions and Prospects FSI distance measurement has formed a set of relatively mature theory and technology. FSI distance measurement can not only provide a high-precision, efficient and on-site traceable absolute distance measurement scheme to achieve spatial multi-path absolute distance measurement and multi-degree of freedom measurement, but also build a large-size spatial measurement and positioning system, such as the multilateral spatial coordinate positioning system and the spherical coordinate positioning system. Measurement instruments based on the principle of FSI distance measurement have been widely used in large advanced industrial equipment and scientific equipment manufacturing and monitoring process in the industrial measurement tasks. At present, the laser radar, which adopts photoelectric phase-locked loop and dual-laser optical frequency opposite scanning technology, has been widely used in industrial measurement. The absolute multiline technology, which based on the phase comparison method of auxiliary interferometer, has the highest FSI distance measurement uncertainty. In recent years, with the improving of the tunable laser and key optoelectronic devices, many new FSI distance measurement techniques, such as optical frequency resampling based on auxiliary interferometer and optical frequency comb, have been extensively and deeply studied. However there are some shortcomings in efficiency of extracting interferometry signal phase and frequency spectrum, stability of measurement datum, reliability of the dynamic error suppression, etc. It is hoped that with the continuous breakthrough of laser source and key technology, researchers can continuously improve the measurement performance index and further develop new large-size space measurement equipment, which will bring new breakthroughs for FSI distance measurement technology and its industrial measurement application.