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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Long distance optical frequency transfer over fiber: Predicting the frequency stability from the fiber noise

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

Published May 1, 2008• 4 citations
Publisher

Abstract

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.

DOI
10.1109/freq.2008.4623094

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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.
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 transfer an optical frequency over 251 km of optical fiber with a residual instability of 6x10(-19) at 100 s. This instability and the associated timing jitter are limited fundamentally by the noise on the optical fiber and the link length. We give a simple expression for calculating the achievable instability and jitter over a fiber link. Transfer of optical stability over this long distance requires a highly coherent optical source, provided here by a cw fiber laser locked to a high finesse optical cavity. A sufficient optical carrier signal is delivered to the remote fiber end by incorporating two-way, in-line erbium-doped fiber amplifiers to balance the 62 dB link loss.
SummaryPrecise and stable coherent optical frequency transfer over long-haul fiber link is required by diverse scientific applications, such as large-scale optical clock networks, global positioning and timing, very-long baseline interferometry, and high-sensitivity tests of fundamental physics. To meet the needs of the above applications, active compensation schemes for cancelling fiber-induced phase noise has developed considerably over the past ten years. However, optical signal severely reduces across distant fiber-optic networks, which degrades optical beat signal and feedback control.Here, we adopt active phase compensation and bidirectional optical frequency locking to achieve ultra-stable optical carrier transfer. The phase noise of the fiber link is effectively suppressed, meanwhile, the ultra-stable optical frequency signal is regenerated accurately at the remote site.Preliminary experiment over 100 km fiber link indicates that the additional frequency stability exhibits 1.9×10-16and 2.5×10-17at an averaging time of 1 s and 100 s, respectively. This approach has a capacity of extending to the parallel transfer of microwave and optical frequency, and free-space optical frequency transfer.
SummaryIn this paper, to the best of our knowledge, the first long-term stable dissemination of the optical frequency via an aerial fiber link with a length of 148 km with passive phase noise cancellation and the first characterization of phase noise PSD characteristics about optical frequency transmission over aerial fiber links is demonstrated in this paper. Different from buried fiber, the phase noise PSD of the aerial link in the free-running case is empirically with a slope of 1⁄f3, at Fourier frequency between 1Hz and 1kHz. The level of the phase noise PSD tested overhead fiber optic links is hundreds or even thousands of times that of similar buried links, but our final frequency transmission results are not much different from those of buried fiber optic cables of the same magnitude, and a transfer frequency instability of 7×10−19at 4000 s are achieved, This result means that aerial fiber links are also a viable option when building long-haul fiber optic networks, when buried fiber optic cables are not available or are too costly.
We demonstrate coherent optical frequency dissemination over a distance of 972 km by cascading two spans where the phase noise is passively compensated for. Instead of employing a phase discriminator and a phase locking loop in the conventional active phase control scheme, the passive phase noise cancellation is realized by feeding double-trip beat-note frequency to the driver of the acoustic optical modulator at the local site. This passive scheme exhibits fine robustness and reliability, making it suitable for long-distance and noisy fiber links. An optical regeneration station is used in the link for signal amplification and cascaded transmission. The phase noise cancellation and transfer instability of the 972-km link is investigated, and transfer instability of 1.1 × 10−19 at 104 s is achieved. This work provides a promising method for realizing optical frequency distribution over thousands of kilometers by using fiber links.
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.