Absolute optical frequency scanning interferometry for picometer displacement measurement using femtosecond optical combs

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In this study, we performed the following three topics aiming to demonstrate the new technique for picometer interferometric measurements.(1)Development of highly efficient and high-power LD-pumped mode-locked solid-state lasersWith extremely high pump density of 220 kW/cm2 in Yb laser, optical-to-optical conversion efficiency of 81 % has been achieved in the CW operation regime, and 62 % in the pulse operation regime. By inserting additional optical Kerr material in the laser cavity, high stability and efficiency have been realized simultaneously. Er/Yb : glass laser mode-locked by saturable absorber mirrors based on polymer-embedded carbon nanotubes was developed. The shortest pulse width (68 fs) ever achieved in the Er/Yb:glass laser has been achieved.(2)Development of "wavelength comb"Frequency beat between a spectrally broadened Nd : glass-laser comb and a tunable CW laser was observed with a flat noise-floor and signal-to-noise ratio of better than 40 dB. Phase-locked tunable opt … More ical frequency generator has been developed. Wavelength reference laser was made by a CW Nd:YAG laser locked to a low-thermal-expansion-glass reference cavity. When one of the modes of the comb was locked to the reference laser, wavelengths of the surrounding modes were simultaneously stabilized. In this way, new scheme of "wavelength comb" has been developed.(3)Inteferometric displacement measurementFrequency-tunable CW laser was phase-locked to the Fabry-Perot cavity for displacement measurements and the stability was evaluated. Without the use of wavelength comb, the drift in the measured cavity-length for 10 nm during 1 hour was observed due to the environmental air-refractive-index change. On the other hand, by using the wavelength comb, the drift has been dramatically suppressed down to 10 pm.Finally, one of the cavity mirrors was moved by a commercial nano-positioning stage. Displacement of 12 um was successfully measured with smaller than 100-pm uncertainty. Evaluation of repeatability and nonlinearity of the stage was demonstrated. Less