1. [1] H. Schnatz, B. Lipphardt, J. Helmcke, F. Riehle and G. Zinner: First phase-coherent frequency measurement of visible radiation, Phys. Rev. Lett., 76 (1), 18-21 (1996).
2. [2] Y. Miki, A. Onae, T. Kurosawa, Y. Akimoto and E. Sakuma: Frequency chain to 3.39-μm CH4-stabilized He-Ne-laser using Josephson point-contact as harmonic mixer, Jpn. J. Appl. Phys. Part 1, 33, 1655-1658 (1994).
3. [3] T. J. Quinn: Practical realization of the definition of the metre, including recommended radiations of other optical frequency standards (2001), Metrologia, 40 (2), 103-133 (2003).
4. [4] J. Ishikawa: Portable national length standards designed and constructed using commercially available parts, Synthesiology, 2 (4), 276-287 (2009) (in Japanese) [Synthesiology English edition, 2 (4), 246-257 (2010)].
5. [5] T. Udem, J. Reichert, R. Holzwarth and T. W. Hänsch: Absolute optical frequency measurement of the cesium D1 line with a mode-locked laser, Phys. Rev. Lett., 82, 3568-3571 (1999).