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Conference on Lasers and Electro-Optics

Practical performance limits on optical frequency transfer over fiber optic links

Paul A. Williams, William C. Swann, Nathan R. Newbury

Published May 1, 2008• 0 citations
Publisher

Abstract

We present theory and experiment quantifying the limitations to stable transport of optical frequencies over optical fiber. These are fundamental fiber noise, propagation delay, bidirectional propagation and system noise in the measurement interferometers.
DOI
10.1109/cleo.2008.4551239

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We present theoretical predictions and experimental measurements for the achievable phase noise, timing jitter, and frequency stability in the coherent transport of an optical frequency over a fiber-optic link. Both technical and fundamental limitations to the coherent transfer are discussed. Measurements of the coherent transfer of an optical carrier over links ranging from 38 to 251 km demonstrate good agreement with theory. With appropriate experimental design and bidirectional transfer on a single optical fiber, the frequency instability at short times can reach the fundamental limit imposed by delay-unsuppressed phase noise from the fiber link, yielding a frequency instability that scales as link length to the 3/2 power. For two-way transfer on separate outgoing and return fibers, the instability is severely limited by differential fiber noise.
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.
We theoretically and experimentally investigate relevant noise processes arising in optical fiber links, which fundamentally limit their relative stability. We derive the unsuppressed delay noise for three configurations of optical links: two-way method, Sagnac interferometry, and actively compensated link, respectively designed for frequency comparison, rotation sensing, and frequency transfer. We also consider an alternative two-way setup allowing real-time frequency comparison and demonstrate its effectiveness on a proof-of-principle experiment with a 25 km fiber spool. For these three configurations, we analyze the noise arising from uncommon fiber paths in the interferometric ensemble and design optimized interferometers. We demonstrate interferometers with very low temperature sensitivity of −2.2, −0.03, and 1 fs/K. We use one of these optimized interferometers on a long haul compensated fiber link of 540 km. We obtain a relative frequency stability of 3·10−20 after 10,000 s of integration time.
We have recently demonstrated the coherent transfer of an optical signal over a 251 km link of optical fiber by use of the standard Doppler-cancellation approach to remove the effects of the fiber-link noise. The fundamental limit to the frequency instability on the transmitted optical frequency is set by residual phase noise on the optical frequency resulting from the unavoidably imperfect Doppler cancellation of the fiber-link noise. Here we demonstrate that it is possible to quantitatively predict the phase noise and instability of the Doppler-cancelled transmitted optical frequency directly from the measured fiber-link noise. The ability to predict the frequency instability from the measured fiber noise can be a useful tool in evaluating whether a coherent fiber optic link is operating at its fundamental limit, or whether there is additional excess noise from the measurement system present in the link.
The stability of the transmitted optical frequency in a single bidirectionally operated fiber was compared with stability in a link consisting of two separate unidirectional fibers. The measurements were performed in a standard closed-loop system canceling the fiber induced phase noise. It was found that phase fluctuations in the higher frequency band (acoustic noise) are well suppressed in both configurations (single bidirectional fiber and separate unidirectional fibers). The long-term drifts however are not fully compensated in a link consisting of two separate unidirectional fibers, but still about two orders of magnitude noise reduction was observed in comparison with not compensated transfer.
Bidirectional phase-stabilized optical fiber links allow state-of-the-art optical clocks to be compared on a continental scale.However, intercontinental comparisons are still based on satellite techniques, preventing optical clocks from being compared without degradation.We performed the first optical frequency transfer experiment over submarine links, measuring levels of environmentally induced noise substantially lower than for on-land links.From these measurements, we predict the transfer stability that can be achieved over transoceanic links.We also performed the first optical frequency transfer over fibers installed in power cables, observing optical perturbations caused by the high-voltage field.Finally, we show that the low background noise of fibers on the seafloor allows applications in geophysical sensing.