Abstract
We developed an all-optical link system for making remote comparisons of two distant ultra-stable optical clocks. An optical carrier transfer system based on a fiber interferometer was employed to compensate the phase noise accumulated during the propagation through a fiber link. Transfer stabilities of 2 × 10(-15) at 1 second and 4 × 10(-18) at 1000 seconds were achieved in a 90-km link. An active polarization control system was additionally introduced to maintain the transmitted light in an adequate polarization, and consequently, a stable and reliable comparison was accomplished. The instabilities of the all-optical link system, including those of the erbium doped fiber amplifiers (EDFAs) which are free from phase-noise compensation, were below 2 × 10(-15) at 1 second and 7 × 10(-17) at 1000 seconds. The system was available for the direct comparison of two distant (87)Sr lattice clocks via an urban fiber link of 60 km. This technique will be essential for the measuring the reproducibility of optical frequency standards.
Highlights
Clock comparisons are essential to confirm the reproducibility of frequency standards
We developed an all-optical link system that consists of Ti:sapphire (Ti:S) frequency combs, nonlinear crystals for frequency doubling, fiber amplifiers, a 1.5 μm stable laser, an optical carrier transfer system, and an active polarization control system
An external-cavity diode laser (ECDL) operating at 1.5 μm is phase-locked to the frequency comb through its second harmonic generation (SHG) light generated by the periodically poled lithium niobate (PPLN)
Summary
Clock comparisons are essential to confirm the reproducibility of frequency standards. For the past twenty years, it has been possible to remotely calibrate the frequencies of microwave atomic clocks by using satellite links such as GPS or two-way satellite time and frequency transfer (TWSTFT). Instead, transferring a stable optical frequency over optical fibers is regarded as a strong candidate for making direct comparisons of highly stable optical clocks and, it has been studied extensively [3, 4, 5]. Optical carrier transfer techniques have been developed to observe the relative frequency stability of optical clocks and cavity stabilized lasers [11, 12, 13]. We developed an all-optical link system that consists of Ti:sapphire (Ti:S) frequency combs, nonlinear crystals for frequency doubling, fiber amplifiers, a 1.5 μm stable laser, an optical carrier transfer system, and an active polarization control system. In this paper the details of the system and its performance are described with the result of direct comparison of distant optical clocks
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