Unveiling the origin of precision limits in distance measurements by frequency-comb-based spectral interferometry

Abstract

Over the last two decades, frequency combs have brought a breakthrough in length metrology with traceability to length standards. In particular, frequency-comb-based spectral interferometry is regarded as a promising technology for next-generation length standards. However, nanometer-level precision inherent in laser displacement interferometer is primarily required to achieve that. Here, we report the fundamental precision limits of a frequency-comb-based spectral interferometry for distance measurements. In our theoretical model, two parameters, the intensity noise and the frequency noise, can be major factors affecting measurement precision. The measurement precision was experimentally confirmed as 0.67 nm at an averaging time of 25 μs. The measurement sensitivity was found to be 4.5∙10-12m/Hz1/2, close to the quantum-limited sensitivity. The numerically predicted measurement sensitivity and precision are in good agreement with the experimental results. Hence, intrinsic noise sources affecting measurement precision typically consist of intensity noise and frequency noise. As a practical example of observing precise physical phenomena, we demonstrated measurements of acoustic-wave-induced vibration and laser eavesdropping. Our study of ultra-precision distance measurements and an analysis of the origin of measurement precision will be an important step toward the practical realization of upcoming length standards.