Significance There is no science without measurement. More accurate measurement of physical quantities is highly desired in modern science and technologies. Laser interferometric precision measurements have outstanding advantages, including traceability, nanometer or even picometer resolution, and ultralong measuring range up to several meters, kilometers, or even thousands of kilometers. It is widely used in advanced technologies and frontier research, such as IC devices, CNC machines, ultraprecision micromanufacturing, and gravitational wave detection.

However, laser interferometric precision measurements have many key problems that demand urgent solutions. The most crucial one is that the laser source requires to be independent, whereas the current domestic market cannot produce satisfactory dual-frequency lasers for heterodyne interferometry. Traditional laser sources have frequency differences lower than 3 MHz, which limits the maximum measuring speed. This severely restricts the processing efficiency of IC chips or machine tools. Furthermore, the output power of the widely used lasers is only 0.5 mW, which is low for further multidimensional measurements. More importantly, dual-frequency lasers exhibit a nonlinear error of several nanometers, thus affecting the precision of the interferometers. For example, the Agilent dual-frequency interferometer has a nonlinear error of 3 nm [Fig. 10(a)]. When it is used for collimation at the precise location of machine tools, this error is considered in the final precision evaluation. Another troublesome problem is that, to generate an interference signal with a high signal-to-noise ratio, traditional laser heterodyne interferometry requires a target mirror or highly reflective surface of the test object to reflect a sufficiently strong beam. However, in many cutting-edge science and ultraprecision applications, a target mirror cannot be placed on the test object and the measured surface is not highly reflective, such as flexible film deformable mirrors for laser fusion, thermal and gravitational deformation of space camera primary and secondary mirrors in low-temperature vacuum environments, and large-travel Abbe error calibration of machine tools and the like. Laser interferometry requires not only nanometer precision measurements but also to match the optical path, which has become a bottleneck in the field. Thus, it restricts the technological innovation and development of precise measurements.

Many scientific frontier studies, such as gravitational wave detection, lithography machine positioning, and interstellar exploration, require ultrahigh precision measurement technology. Since the advent of laser interference technology, it has been crucial in precision measurements, and the demand for accurate measurements will increase from micro-nano level to picometers, or even femtometers, in the future. Therefore, independently developing novel interferometers with better performance is required for domestic laser interferometry precision measurements. Furthermore, summarizing the characteristics and limitations of existing interferometers is crucial to guide future development in this field more rationally.

Progress Owing to the above application requirements and technical problems, we have been devoted to investigating laser and laser-feedback interference in the past several decades. We have recorded great breakthroughs in dual-frequency innovative lasers with large frequency difference and high-power maintenance and in feedback interference for nanometer measurements without target mirrors.

On one hand, a new laser source based on the principle of the Zeeman-birefringence dual-frequency has been developed and produced independently with a higher dual-frequency difference and output power (Fig. 13). The core technical parameters are comparable with or surpass similar advanced lasers worldwide. With this laser source, various interferometers for displacement, angle, linearity, and flatness measurement have been developed. Zeeman-birefringence dual-frequency laser interferometers in China are at the mass manufacturing and production level for the first time. On the other hand, to address the measurements without target mirrors or measuring a low-reflectivity target, we consider the basic principles of self-mixing interferometry and have successfully developed a laser feedback interferometer (Fig. 23). Due to the high sensitivity of the self-mixing modulation, the produced interferometer can achieve an ultrahigh gain amplification of the detected signals. Therefore, the feedback interferometer has a wide application range, including displacement and velocity measurements, vibration recovery, refractive index sensing, and biological imaging. The developed interferometers solve the key measuring problems in laser interferometry precision measurements. They have demonstrated notable performance in nanometer precision measurements; thus, they are employed in many frontier research and industrial applications.

Conclusions and Prospects Herein, we presented the latest achievements and research progress in the research team on dual-frequency laser measurement and feedback interferometry technology in the past decade. We also presented the prospect of laser interferometry in precision measurements. Based on the results obtained herein, we shall focus on innovation, seek breakthroughs in new measurement principles and methods, and continuously improve the performance of the developed interferometers, bringing breakthroughs to laser interference ultraprecision measurements and their applications.