Coherent metrology, which is known for its high precision, immunity to ambient light, and traceability, represents an important class of methods with extensive applications in both scientific and industrial fields, such as space exploration, medical diagnosis, and advanced manufacturing. In particular, the flourishing developments of autonomous driving and precision fabrication have driven the need for accurate and dynamic measurements. Consequently, coherent ranging methods, such as frequency-modulated continuous wave (FMCW), have become a focus of research, and recent advancements have significantly improved their capabilities. However, conventional approaches to improving ranging resolution typically require expanding the frequency-swept range through complex laser designs or multi-source signal stitching, significantly increasing system complexity and cost.
In a new paper published in Light: Science & Applications, a team led by Professor Yidong Tan from Tsinghua University has developed a phase multiplication technique that overcomes these limitations. Their innovative approach leverages the laser feedback effect and cavity dynamics to actively generate interference harmonics that effectively enhance the ranging resolution.
The operating principle harnesses an interesting physical phenomenon: when the frequency-swept laser beam reflected from a target re-enters the laser cavity, it interferes with the intracavity light field. “The reinjected light generates a spontaneously amplified beat signal and modulates the cavity mode,” the team stated. “Through nonlinear cavity dynamics, the system produces harmonics of the interference signal without high energy thresholds.” Higher-order harmonics exhibit higher phase sensitivity, equivalent to possessing a much larger frequency-swept range and higher resolution. Unlike conventional methods that require additional optical components or complex signal processing, this approach achieves active enhancement using the laser's intrinsic nonlinear response, instead of physically broadening the frequency-swept range.
Experimental results demonstrate remarkable performance. “The μW-level feedback power generates harmonics exceeding the 10th order, and we achieve 3-fold to 13-fold phase multiplication." Using the 13th harmonic, the team reconstructed the target motion with a 0.1 mm step, while the conventional method (using the fundamental wave) failed. Repeatability tests showed a standard deviation of less than 50 μm across multiple measurements. In 3D imaging experiments, the 3rd harmonic imaging also outperformed conventional approaches, demonstrating the practical significance of this enhancement.
"Compared with NOON-state-based phase multiplication techniques, our system demonstrates a higher enhancement factor and is robust against ambient noise and vibrational interference,” they explained. Furthermore, intracavity interference eliminates the need for an external reference arm, enabling a compact design with reduced complexity. The method is broadly applicable to both FMCW and heterodyne interferometric systems across various precision measurements.
This breakthrough may accelerate the development of next-generation perception systems that combine high resolution with practical implementation advantages. Furthermore, it represents a paradigm shift in how we approach resolution enhancement in coherent ranging systems, proving that sometimes solutions can be found in fundamental physical processes. The research team anticipates that their approach will inspire new thinking in laser ranging design and open new possibilities for high-performance, cost-effective measurement systems across multiple industries.
Phase-multiplied interferometry via cavity dynamics for resolution-enhanced coherent ranging