title: High-accuracy measurement of 240-m distance in an optical tunnel by use of a compact femtosecond laser.
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.
authors: National Research Laboratory of Metrology, 1-1-4 Umezono, Tsukuba, Ibaraki 305-8563, Japan. mino@nrlm.go.jp
authors: K
authors: K
authors: Minoshima
authors: H
authors: H
authors: Matsumoto