In July 2009, the national standard tool for measuring length in Japan changed from an iodine-stabilized helium-neon (He-Ne) laser to a femtosecond optical frequency comb (FOFC). Owing to the outstanding frequency stability of the FOFC, in the near-future General Conference of Weights and Measures (Conférence générale des poids et measures: CGPM), the FOFC is expected as a new standard tool for the International System of Units (SI) of the meter. Because the traceability of the meter is the infrastructure for both scientific and industrial uses, how to practically perform a distant metrology that is directly linked to an FOFC length standard tool is the most urgent challenge. The basic idea of applying an FOFC''s temporal coherence characteristic to an absolute length measurement is simple. Motivated by the analogy between monochromatic light source and phase-related multi-wavelength light source, in this paper, a novel interferometric technique is proposed to measure arbitrary length. In the case of a monochromatic light source, using the interference, measuring length based on the wavelength of light is a general method. In the case of an FOFC, using the interference, we propose a method of measuring length based on, instead of the wavelength of light, the repetition interval between the pulse and the pulse from an FOFC.
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