Periodic-Error-Free Coherent Phase-Based Laser Ranging Over a Large Unambiguous Range: Harnessing Vernier Effect and IMD3 Suppression

Phase-based laser ranging (PBLR) using multi-tone microwave modulation offers high precision over long distances. A key challenge in extending the unambiguous range (UAR) is the emergence of third-order intermodulation distortion (IMD3) components near the desired signals in the spectrum, resulting in periodic ranging errors. Here, we report a coherent PBLR scheme that simultaneously achieves a large UAR and suppresses these errors. This is accomplished by harnessing the Vernier effect through optimizing the frequency intervals of the multi-tone microwave modulation, and mitigating IMD3 via a dual-parallel dual-drive Mach-Zehnder modulator. In addition, the system employs frequency-shift-based optical coherent low-frequency detection and an optical phase noise cancellation algorithm to ensure high precision, enhanced sensitivity and real-time operation. Experimental results demonstrate precision of 0.4 μm over a spool of fiber with a round-trip length of ∼1 km within an averaging time of 5.5 ms, successfully eliminating the periodic ranging errors prevalent in conventional PBLR systems based on Mach-Zehnder modulators. Additional tests with a tunable optical delay line and a free-space link further validate the robustness of the proposed scheme.