Real-time absolute distance measurement by multi-wavelength interferometry synchronously multi-channel phase-locked to frequency comb and analysis for the potential non-ambiguity range

Optical frequency combs of the femtosecond laser have been widely used in time-frequency technology and precision spectrum measurement. The absolute ranging technology derived from time-frequency characteristics of the optical frequency comb is expected to become the incomparable means of length metrology and distance measurement in the future due to its traceability to time-frequency standard and capability of large scale and high precision. This paper proposes a real-time absolute ranging method with multi-wavelength interferometry referenced to optical frequency comb, which enables multiple continuous-wave lasers to be synchronously calibrated to selected modes of the frequency comb by means of optical phase-locked loop. With synchronous phase measurement and calculation with excess fraction algorithm, absolute distance measurement by multi-wavelength interferometry is ultimately fulfilled. The proposed measurement method can not only retain high resolution and high accuracy of traditional laser interferometry, but also can be traced to a time-frequency reference, which is of metrological significance to high-precision length and distance measurement, especially to the definition of "meter" for physical reproduction. Measured results for ranging experiments have proved that the non-ambiguity range of the four-wavelength interferometer reaches 44.6 mm, and fluctuations of air refractive index cause the non-ambiguity range change with the order of nanometers. Through theoretical analysis, it is pointed out that the non-ambiguity range of the multi-wavelength interferometer in the actual measurement environment is restricted by the calculated error of air refractive index, especially the estimation accuracy and fluctuation degree of the refractive index relationship between wavelengths. And in a good laboratory conditions, the non-ambiguity range of real-time absolute ranging by frequency-comb-calibrated multi-wavelength interferometry can reach several meters or even tens of meters. At the same time, a 2-meter linear displacement comparison has been carried out, the P.V. value of the residual errors for linear fitting is 36.1 nm, and such residual errors match the magnitude of uncertainty of air refractive index calculated by empirical formula, which prove that the multi-wavelength interferometry can perform meter-level absolute ranging. The proposed research can be directly applied to precision manufacturing of large-scale semiconductors up to several meters, and is beneficial to promoting the accuracy of laser ranging for space mission.