Precision Length Metrology using Fiber-Based Frequency Combs

Abstract

We developed a distance measurement technique with sub-wavelength accuracy using 821st-harmonic of the intermode-beats of a fiber-based frequency comb. The developed method can provide a coherent link to an integrated laser interferometer for absolute-long-distance measurements.OCIS Codes: (120.3940) Metrology; (120.4800) Optical standards and testing; (000.0000) Optical combs IntroductionOptical frequency comb technology opened up new fields not only in frequency metrology[1] but in length metrology. Frequency-traceable length measurement using combs provides direct realization of the definition of meter, remote calibration using a GPS technology, and precise measurements of wide range of lengths by taking advantage of high dynamic range in frequency measurements. Recent development of optical frequency combs using mode-locked fiber lasers and all-fiber-based stabilization and delivery techniques is particularly beneficial to practical length metrology because of its compactness, robustness, long-term stability, and capability of remote measurements. Particularly in absolute long-distance measurements, above mentioned advantages of fiber-based frequency combs are inevitable for practical high dynamic-range and high accuracy measurements. In this paper, we report our recent achievements on long absolute-distance measurements using fiber-based frequency combs.High-accuracy absolute long-distance measurements are essential for application to a wide range of fields such as industrial sensing and positioning in space [2]. Conventionally, phase measurements of a sinusoidally modulated CW optical wave have been carried out for long-distance measurements [3,4]. Since this method requires an external modulator, it generates rather large cyclic error that originates from electrical and optical crosstalk. We have developed an optical distance meter based on phase measurement of an intermode beat of a frequency comb using a mode-locked femtosecond fiber laser (femtosecond-comb distance meter, FCDM) [5,6]. FCDM offers several advantages such as small cyclic errors, simple setup, it is insensitive to sharp absorption lines in air, and is suitable for multicolor measurements for environmental correction. In this method, using 10-GHz frequency components of the intermode beats of the comb, we have shown micrometer level accuracy in distance measurements over 200-m by comparing to a length-measuring laser interferometer which is our length reference standard [7].On the other hand, nano/picometer-level accuracy can be achieved in displacement measurements performed using a length-measuring laser interferometer. However, because of the ambiguity in the number of optical fringes, an interferometer can provide only displacement but not absolute distance for long distance measurements. If an additional technique is available for measurement of absolute distances with sub-wavelength accuracy, a combination of the two methods can be used for long-distance measurements with nano/picometer-level accuracy over km distance. Such ultimate d...