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.