4-3 Frequency Transfer Using Optical Fibers

Three methods can be considered for frequency transfer via optical fi bers: 1. RF transfer, 2. Optical frequency transfer, and 3. Optical frequency comb transfer [8]‒[18]. In method 1, the light modulated by an RF signal is transmitted. In method 2, the light itself is transmitted, by which the most precise results are currently achieved. An optical frequency comb is a frequency ruler in optical region, and is widely used for optical frequency measurement [19]. In method 3, these optical frequency combs are transmitted. With the gaps between the teeth of the comb, optical frequency comb allows for dissemination of both the light itself and RF signal to users. This is considered diffi cult because not only the physical length of the optical fi ber but also the wavelength dispersion must be constantly stabilized. NICT fi rst started development on the RF transfer system from method 1, and has achieved some signifi cant results [20]‒[22]. Next, the optical frequency transfer system from method 2 is under development aimed at highly precise transfer and direct comparison of optical frequency standards. In this paper, we introduce the outlines and results of these various methods.