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A multi-wavelength interferometer utilizing the frequency comb of a femtosecond laser as the wavelength ruler is tested for its capability of ultra-precision positioning for machine axis control. The interferometer uses four different wavelengths phase-locked to the frequency comb and then determines the absolute position through a multi-channel scheme of detecting interference phases in parallel so as to enable fast, precise and stable measurements continuously over a few meters of axis-travel. Test results show that the proposed interferometer proves itself as a potential candidate of absolute-type position transducer needed for next-generation ultra-precision machine axis control, demonstrating linear errors of less than 61.9 nm in peak-to-valley over a 1-meter travel with an update rate of 100 Hz when compared to an incremental-type He-Ne laser interferometer.

Downloaded From: http://opticalengineering.spiedigitallibrary.org/ on 05/15/2015 Terms of Use: http://spiedl.org/termsAbsolute distance measurement with extension of nonambiguity range using the frequency comb of a femtosecond laser Abstract. We revisit the method of synthetic wavelength interferometry (SWI) for absolute measurement of long distances using the radio-frequency harmonics of the pulse repetition rate of a mode-locked femtosecond laser. Our intention here is to extend the nonambiguity range (NAR) of the SWI method using a coarse virtual wavelength synthesized by shifting the pulse repetition rate. The proposed concept of NAR extension is experimentally verified by measuring a ∼13-m distance with repeatability of 9.5 μm (root-mean-square). The measurement precision is estimated to be 31.2 μm in comparison with an incremental He-Ne laser interferometer. This extended SWI method is found to be well suited for long-distance measurements demanded in the fields of large-scale precision engineering, geodetic survey, and future space missions.

We present a progress review on the advance of distance measurements made at KAIST by making use of mode-locked lasers as the light source to meet ever-growing industrial demands on the measurement precision and functionality. Diverse principles exploited for the progress are described in this review with focus on four attributes: first, the optical spectrum of a mode-locked laser, distinctively called the frequency comb, permits multi-wavelength interferometry to be realized for absolute distance measurement up to several meters without losing the nanometer precision of well-established laser-based phase-measuring displacement measurement. Second, the frequency comb enables spectrally resolved interferometry for absolute distance measurement to be conducted with a nanometer resolution by Fourier transform analysis of the dispersive interference data captured using a spectrometer. Third, the mode-locked laser in the time domain appears as a train of ultrashort pulses, of which the time-of-flight is measured with a picosecond resolution by control of the pulse repetition rate with reference to the radio-frequency atomic clock. Fourth, the pulse-to-pulse cross-correlation occurring in the optical frequency domain is down-converted to the radio-frequency domain to achieve femtosecond pulse timing precision by means of dual-comb interference. All these principles based on unique spectral and temporal characteristics of ultrashort mode-locked lasers are anticipated to make contributions to the advance of nanotechnology particularly in manufacturing and metrology.

A prototype laser distance interferometer is demonstrated by incorporating the frequency comb of a femtosecond laser for mass-production of optoelectronic devices such as flat panel displays and solar cell devices. This comb-referenced interferometer uses four different wavelengths simultaneously to enable absolute distance measurement with the capability of comprehensive evaluation of the measurement stability and uncertainty. The measurement result reveals that the stability reaches 3.4 nm for a 3.8 m distance at 1.0 s averaging, which further reduces to 0.57 nm at 100 s averaging with a fractional stability of 1.5 × 10−10. The uncertainty is estimated to be in a 10−8 level when distance is measured in air due to the inevitable ambiguity in estimating the refractive index, but it can be enhanced to a 10−10 level in vacuum.