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

Ultra-stable Optical Frequency Transfer via 609 km Communication Fiber Link

Xiang Zhang, Qi Zang, Xue Deng, Dan Wang, Qian Zhou, Meng Fan Wu, Tao Liu, Ruifang Dong

Published Apr 24, 2022• 0 citations
Publisher

Abstract

SummaryIn this paper, we demonstrated coherent optical frequency transfer via 609 km communication fiber link. The phase noise per-unit-length of this fiber links at 1Hz and 10Hz are 175 rad2/(Hz*km) and 5 rad2/(Hz*km), respectively. After being compensated, optical frequency transfer achieves a fractional frequency instability of 2.7×10−15at an integration time of 1 s and 2.2×10−19at 16000 s in terms of Mod-ADEV.

DOI
10.1109/eftf/ifcs54560.2022.9850456

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SummaryIn this paper, we demonstrated coherent optical frequency transfer via 609 km communication fiber link. The phase noise per-unit-length of this fiber links at 1Hz and 10Hz are 175 rad2/(Hzkm) and 5 rad2/(Hzkm), respectively. After being compensated, optical frequency transfer achieves a fractional frequency instability of 2.7×10−15at an integration time of 1 s and 2.2×10−19at 16000 s in terms of Mod-ADEV.
We demonstrate the coherent transfer of an ultrastable optical frequency reference over a 490 km noisy field fiber link. The fiber-induced phase noise power spectrum density per-unit-length at 1 Hz offset frequency can reach up to 510 rad2⋅Hz−1⋅km−1, which is much higher than the fiber noise observed in previous reports. This extreme level of phase noise is mainly due to the fiber link laying underground along the highway. Appropriate phase-locked loop parameters are chosen to complete the active compensation of fiber noise by measuring the intensity fluctuation of additional phase noise and designing a homemade digital frequency division phase discriminator with a large phase detection range of 212 π rad. Finally, a noise suppression intensity of approximately 40 dB at 1 Hz is obtained, with fractional frequency instability of 1.1 × 10−14 at 1 s averaging time, and 3.7 × 10−19 at 10000 s. The transfer system will be used for remote atomic clock comparisons and optical frequency distribution over a long-distance communication network established in China.
The phase coherence of an ultrastable optical frequency reference is fully maintained over actively stabilized fiber networks of lengths exceeding 30 km. For a 7-km link installed in an urban environment, the transfer instability is 6 x 10{-18} at 1 s. The excess phase noise of 0.15 rad, integrated from 8 mHz to 25 MHz, yields a total timing jitter of 0.085 fs. A 32-km link achieves similar performance. Using frequency combs at each end of the coherent-transfer fiber link, a heterodyne beat between two independent ultrastable lasers, separated by 3.5 km and 163 THz, achieves a 1-Hz linewidth.
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.
Along with the rapid development of the optical clocks, optical frequency transfer via communication fibers has shown much higher precision than the conventional methods. In this paper, we report the progress on the optical frequency transfer via communication fibers at NTSC. Over a 71-km spooled fiber, the optical frequency transfer with a residual instability (Allan deviation) of 5.4E-15 at 1s and 5.6E-18 at 104s is demonstrated. Furthermore, for a measurement bandwidth of 100 Hz, the instability is 1.2E-15 at one second and 1.4E-18 at 104s. This transfer instability is fundamentally limited by the fiber length. The Allan deviation curves fall as τ-1for short times, which is consistent with the fact that the transferred signal is dominated by white phase noise. This experimental achievement provides the foundation for the upcoming optical transfer experiment over a 300-km urban communication fiber 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.