Ultrahigh-resolution distance meter using a frequency comb of a femtosecond mode-locked laser

Abstract

High-resolution long-distance measurements have been required in many application fields such as optical geodesy, shape measurement of parabola antennae, and sensors. Conventionally, several methods have been used for length measurements. Optical length-measuring interferometer and time-of-flight measurement of optical pulses have been used for high-resolution measurements, however they are not suitable for long distances, because they require continuous scanning of the optical path as a reference. On the other hand, phase measurement of a sinusoidally modulated cw optical wave has been used for long distance measurement because real-time measurement without a scanning reference is possible [1]. In this method, either high-resolution phase meter or high-frequency modulation is required to realize high resolution in the distance measurement. Recently, high-frequency cw modulation as 28 GHz has been reported [2], but there was an inevitable problem that caused a large cyclic error, which was independent of the distance originating from electric and optical crosstalk due to an external modulator with a complex setup. We have developed an optical distance meter based on phase measurement of an intermode beat of a frequency comb using a mode-locked femtosecond laser [3]. It does not require an external modulator, thus we can significantly suppress the cyclic error and, moreover, the setup is simple, which is important for practical application. Furthermore, the femtosecond distance meter has marked advantages over cw methods in that it is suitable for multicolor measurement and is less sensitive to sharp absorption lines in air. These qualities are important for reliability and accuracy. In the previous study, we achieved 50 um resolution at a 240 m distance using 1 GHz beat frequency. Without a modulator, it is easy to utilize high frequency by simply selecting one of the beat components in the broad and stable frequency comb. In this study, we achieved ultrahigh one-micrometer resolution in the femtosecond distance meter by using up to 10 GHz components of beat frequencies.