精密長さ計測のための光コムによる干渉計測

摘要

超短パルス発生法として広く知られるモード同期レーザーを，光周波数の精密なものさし，すなわち「光周波数コム」として用いる報告がなされてから，早くも10年が経とうとしている1）．それは，超高速と超精密という，従来直交して進展してきた技術分野が究極において融合するというドラマチックな出来事であり，その後の両者の技術に革命的な変革をもたらした．Optical comb technology has been successfully used in frequency metrology, but the potential of the technology in length metrology has not been fully explored. In this article, advantages of comb technology in length metrology is discussed. Several interferometric techniques using optical comb in previous works are categorized. As examples, two previous works by the author's group are reviewed. One is a picometer displacement interferometry. In a frequency scanning interferometer, using an optical frequency synthesizer based on an optical comb as the light source and a reference cavity to compensate environmental instability, long-range precise displacement measurement such as 10 /ira with picometer uncertainty was achieved. The second example is a direct measurement of group refractive index of air. Using interferometer between the adjacent pulses in the mode-locked laser, air-refractive-index was measured without complicated setup using a vacuum reference. Precise interferometric signal was obtained by the change of the pulse repetition frequency. Optical comb technology will be widely used in interferometry.