High-accuracy measurement of 240-m distance in an optical tunnel by use of a compact femtosecond laser

TLDR

The study proposes a high‑accuracy optical distance meter using a mode‑locked femtosecond laser for measuring distances in a 310‑m optical tunnel. The meter measures the phase shift of the optical beat between longitudinal modes, automatically extracts the air refractive index via two‑color fundamental and second‑harmonic pulses, and employs a wide range of beat frequencies to achieve high precision. The device achieved a 50‑µm resolution at 240 m and measured an absolute distance of 240 m with 8‑ppm accuracy.

Abstract

A high-accuracy optical distance meter with a mode-locked femtosecond laser is proposed for distance measurements in a 310-m-long optical tunnel. We measured the phase shift of the optical beat component between longitudinal modes of a mode-locked laser. A high resolution of 50 microm at 240-m distance was obtained without cyclic error correction. The group refractive index of air is automatically extracted to an accuracy of 6 parts per million (ppm) by two-color measurement with the pulses of fundamental and second-harmonic wavelengths. Finally, an absolute mechanical distance of 240 m was obtained to within 8-ppm accuracy by use of a series of beat frequencies with the advantage of a wide range of intermode frequency, together with the results of the two-color measurement.