Absolute distance measurement (ADM) with high precision is required for various fields of precision engineering, which has long been implemented by means of time-of-flight measurement of a pulsed laser, intensity or frequency modulation of a continuous-wave laser, and cross-correlation of pseudo-random micro-wave signals. Recently, in response to increasing demands on the measurement precision and range beyond conventional limits, femtosecond pulse lasers began to draw attention as a new light source that permits realizing various advanced ADM principles such as synthetic radiofrequency wavelength generation, Fourier-transform-based dispersive analysis and multi-wavelength interferometry. In this talk, we present the state-of-the-art measurement principles and performance demonstrated by exploiting the unique temporal and spectral characteristics of femtosecond laser pulses for high-precision ADM applications.
We measure absolute distances by performing multi-wavelength interferometry (MWI) using four different wavelengths generated simultaneously from the frequency comb of a femtosecond laser. The measurement precision is estimated to be less than 63 nm in peak-to-valley over a distance of 1 m as compared to an incremental HeNe laser interferometer. We also evaluate the operational stability and robustness of the interferometer hardware system over a time period of 12 hours. Finally, it is concluded that the proposed frequency-comb-referenced multi-wavelength interferometry is capable of providing fast, precise and high stable absolute distance measurements, being well suited for industrial precisionengineering applications and near-future space missions.
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.