Linear group delay spectral interferometry for full-range precision absolute length metrology

The optical frequency comb serves as a powerful tool for distance measurement by integrating numerous stable optical modes into interferometric measurements,enabling unprecedented absolute measurement precision.Nonetheless,due to the periodicity of its pulse train,the comb suffers from measurement dead zones and ambiguities,thereby impeding its practical applications.Here,we present a linear group delay spectral interferometer for achieving precise full-range distance measurements.By employing a carefully designed linear group delay(LGD)device for phase modulation of the comb modes,interference can occur and be easily measured at any position.Our approach effectively eliminates the dead zones and ambiguities in comb-based ranging,without the need for cumbersome auxiliary scanning reference devices or reliance on complex high-repetition-rate combs or high-resolution spectrometers.We conducted length metrology experiments using a mode-locked comb referenced to a rubidium clock,achieving a large nonambiguity range up to 0.3 m,covering the entire measurement period.The maximum deviation compared to a laser interferometer was less than 1.5μm,and the minimum Allan deviation during long-term measurements reached 5.47 nm at a 500 s averaging time.The approach ensures high accuracy while maintaining a simple structure,without relying on complex external devices,thereby propelling the practical implementation of comb-based length metrology.