image: (top) Overview of the 113-km experimental path for BDCR. (bottom) The primary apparatus at two experimental sites. The OFCs at the two terminals of the same wavelength have a slight frequency difference, forming a set of equipment for BDCR. The performance of the BDCR method is evaluated by comparing the results of two independent ranging devices at different wavelengths. USL, ultra-stable laser; HP OFC, high-power optical frequency comb; WDM, wavelength-division multiplexer; GPS, global positioning system; ADC, analog-to-digital converter; Rb FS, rubidium frequency standard.
Credit: ©Science China Press
Accurate long-distance ranging technology is crucial for various applications in the fields of science and industry. For instance, the accurate measurement of each satellite's position is the foundation for the coordinated operation of the constellation; in the field of geographical research, long-distance high-precision ranging can be used for topographic mapping and the construction of the Earth's gravity model; and when setting up a space telescope array, precisely measuring the baseline between satellites is key to achieving high-resolution imaging. However, traditional ranging methods often encounter limitations in long-distance applications due to a restricted ambiguity range or inadequate measurement precision. For example, continuous-wave laser interferometry can achieve sub-nanometer resolution, but its ambiguity range is at most half a wavelength; while laser ranging techniques based on pulsed or frequency-modulated signals have a larger ambiguity range, their precision can only reach the sub-millimeter level. Fortunately, dual-comb ranging technology can combine time-of-flight measurement with phase interferometry, thereby potentially achieving both high precision and a large ambiguity range. Previously, researchers have explored various high-precision ranging techniques based on optical frequency combs, but due to high transmission losses and noise over long-distance paths, only a few techniques have been verified over open paths, and the verification distances were all less than 10 kilometers. Therefore, achieving nanometer-level absolute ranging over ultra-long distances still faces significant challenges.
Recently, professors Jian-Wei Pan, Hai-Feng Jiang, and Qiang Zhang from the University of Science and Technology of China, in collaboration with researchers from other institutions, proposed a bistatic dual-comb ranging approach, enabling nanometer-level absolute distance measurement over a 113-kilometer path, with the precision reaching 82nm@21s. This approach is expected to provide breakthrough technical support for large-scale, high-precision space research programs such as space telescope arrays and satellite gravity measurement. This achievement has been published in the National Science Review, entitled "113 km absolute ranging with nanometer precision", with professors Qiang Zhang, Hai-Feng Jiang, and Jian-Wei Pan as the co-corresponding authors.
In this study, the research team proposed an innovative bistatic dual-comb ranging (BDCR) approach. This approach significantly extends the ambiguity range while maintaining high precision. When distances exceed 100 kilometers, the measurable distance of BDCR is 2.5 times that of the traditional monostatic dual-comb ranging at the same detection sensitivity. Thus, it supports nanometer-level absolute ranging over ultra-long distances and enables the exploration of the limits of absolute ranging technology. The research team conducted an absolute ranging experiment on a 113-kilometer path to verify this technology. In the experiment, the research team also adopted high-power optical frequency combs, large-aperture telescopes, low-noise photodetectors, and precise data acquisition and processing systems, further mitigating the limitations imposed by high atmospheric transmission loss. Through air dispersion analysis and synthetic repetition rate technique, the team successfully extended the ambiguity range, enabling it to cover a distance of over 100 kilometers. By cross-verifying with two sets of ranging systems of different wavelengths, the BDCR achieved a ranging precision of 11.5 micrometers at 1.3 ms, 681 nanometers at 1 second, and 82 nanometers at 21 seconds over a distance of 113 kilometers. This is the first time that such a high level of absolute distance measurement precision has been achieved over a path exceeding 100 kilometers. This technology is expected to enhance the angular resolution of space telescope arrays, improve the measurement capabilities of gravity satellites for the Earth's gravitational field in natural disasters, and provide breakthrough technical support for large-scale high-precision space applications such as satellite formation flying or constellation navigation.
Journal
National Science Review