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arXiv:1108.2775 · Optics Express

All-optical link for direct comparison of distant optical clocks

Miho Fujieda, Motohiro Kumagai, Shigeo Nagano, Atsushi Yamaguchi, Hidekazu Hachisu, Tetsuya Ido

Published Aug 11, 2011• 93 citations
arXivHTMLPDF

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.

DOI
10.1364/oe.19.016498

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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.
We have investigated the performance of an optical carrier transfer system for a remote comparison of optical clocks. In the frequency transfer system, technical problems and fundamental principles limit the coherent transfer on a long-haul optical fiber. They contribute to noise processes that come from the propagation delay in the fiber, the design of an interferometer, burst length fluctuations of the fiber, asymmetrical propagation characteristics, and so on. The feasibility of a full optical link with high stability is discussed. The phase detection with a high degree of precision and cancellation of the fiber-induced noise on the round-trip light are achieved with a well-known standard technique using an all-fiber-based interferometer. We report the characteristics of these noise processes with results on a spooled fiber. An Allan deviation of the compensated link which is extended to 90 km reaches 5 × 10-18 close to the theoretical limit imposed by the delay-unsuppressed fiber noise at averaging time of 103s.
Comparison of ultra-stable frequency references located in geographically distributed areas require a transfer technique, which offers stability of 10−17at 1 s over tens of kilometer. Such remote frequency transfer will allow comparing the optical frequency references of same or different atomic species (through a frequency comb bridge) for test of relativistic geodesy [1] or time variation of fundamental constants [2], and will contribute towards the database of frequency ratio measurements-an essential requirement for the re-definition of the SI second based on optical clocks [3]. It will also facilitate in creating an optical to microwave frequency link that can be used by various SI standards laboratory requiring stable microwave sources. In this paper, I discuss the modulated optical carrier wave based ultra-stable frequency reference transfer technique, and present a study of a laser interferometer based active fiber noise cancellation system and its applications.
SUMMARY We have investigated the performance of an optical carrier transfer system for remote comparison of optical clocks. In the frequency transfer system, technical problems and fundamental principles limit coherent transfer on a long‐haul optical fiber. They contribute to noise processes that come from the propagation delay in the fiber, the design of the interferometer, burst length fluctuations of the fiber, asymmetrical propagation characteristics, and so on. The feasibility of a full optical link with high stability is discussed. Phase detection with a high degree of precision and cancelation of the fiber‐induced noise on the round‐trip light are achieved with a well‐known standard technique using an all‐fiber‐based interferometer. We report the characteristics of these noise processes with results on a spooled fiber. The Allan deviation of the compensated link which is extended to 90 km reaches 5 × 10 −18 , close to the theoretical limit imposed by the delay‐unsuppressed fiber noise at an averaging time of 103 s.
In this paper, we present a high-precision optical frequency noise detection and comparison technique using a two-way transfer method over a 260 km field fiber link. This method allows for the comparison of optical frequencies between remote optical references without the need for data transfer through communication. We extend a previously established two-way comparison technique to obtain all data at the local site. Two optical carrier signals are injected into the bidirectional fiber from both ends, and one carrier is reflected back from the remote end. This enables the phase comparison of the two carrier signals at a single site without the need to transmit experimental data. The common-mode frequency noise induced by the bidirectional fiber link is detected and effectively suppressed without the need for sophisticated active fiber noise control. Our demonstration system, which uses a 260 km field fiber link and a common laser source, achieves a fractional instability of 2.5×10-17 at 1 s averaging time and scales down to 3.5×10-21 at 8000 s. This scheme offers the distinct advantage of completing the comparison at a single site, eliminating the need for remote data transfer via communication. This method is expected to enhance reliability for high-precision frequency comparisons between remote optical clocks and advanced atomic clocks.
We demonstrate a 30-km-long optical fiber link for frequency comparison between two strontium optical lattice clocks being developed at Riken and the University of Tokyo. We use a transfer laser at 1397 nm which is twice the wavelength of the clock transition of strontium clocks. The link stability is estimated to be 1×1017for an averaging time τ = 1 s, which is in good agreement with the theoretical limit calculated from the fiber noise spectrum. We discuss a remote clock comparison with stability of 10 (τ/s) by synchronously operating the two distant clocks.
We demonstrate a 30-km-long optical fiber link for frequency comparison between two strontium optical lattice clocks being developed at RIKEN and the University of Tokyo. We use a transfer laser at 1397 nm, which is twice the wavelength of the clock transition of strontium clocks. The link stability is estimated to be 1 × 10−17 for an averaging time of τ = 1 s, which is in good agreement with the theoretical limit calculated from the fiber noise spectrum. We discuss a remote clock comparison with a stability of 1 × 10−17(τ/s)−1/2 by synchronously operating two distant clocks.