National standards of length for high-capacity optical fiber communication systems

Abstract

Frequency measurement in the optical domain has a number of applications other than the length standard, among which the most important field in a social context is optical fiber communications. In the 1990s, wavelength-division multiplexing began to increase communication capacity, and many specialists anticipate that precise optical frequency management is required in the near future. It was therefore necessary to add a stabilized laser in the 1.5-m optical communication band as a frequency standard to the CIPM's recommendation, and to develop the technology to measure laser frequency in the 1.5-m band. In Japan, where the 633-nm iodine-stabilized He-Ne laser served as the national measurement standard, the 1.5-m stabilized lasers needed to be traceable to the 633-nm national measurement standard.It was in 1999 that the first breakthrough came in overcoming the difficulties in conventional optical frequency measurement using the frequency chain. German and American groups achieved absolute measurement of laser frequency using the "optical frequency comb" with a mode-locked laser, [5][6] which led to enormous technical innovations in this field. The optical frequency comb was a great success and enabled laser frequency to be measured to the accuracy of the cesium atomic frequency standard (11 to 16 digits depending on the mean time or oscillator types). The linkage of optical frequency with microwave frequency by means of the optical frequency comb IntroductionBefore the invention of the optical frequency comb, frequency measurement in the optical domain was extremely difficult. Requiring a number of microwave oscillators, special frequency multipliers/mixers, and wavelength or frequencystabilized lasers (hereinafter referred to as "stabilized lasers") as measuring devices, the "frequency chain," [1][2] which linked with frequency in the optical domain after repeatedly multiplying and mixing in sequence, was based on 9 192 631 770 Hz, the microwave frequencies generated from the cesium atomic frequency standard. The extremely extensive measuring equipment required not only development but also enormous costs and human resources. In addition, this equipment could only measure the frequency of a single laser, which meant that lasers with different wavelengths required different frequency chains to be constructed.Under these circumstances, the International Committee for Weights and Measures (CIPM) prepared a list of frequencystabilized lasers based on the laser frequencies measured with frequency chains and recommended that they be used as wavelength standards (length standards) for the purpose of a practical metric realization.[3] Frequency-stabilized lasers developed in national metrology institutes of different nations are gathered to validate their equivalence for the international comparison of frequencies (hereafter referred to as "international comparison"). The lasers compared internationally serve as the basis for the reference laser for length measurement in a nation. The most internationally compare...