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

Coherent Optical Frequency Transfer over 100 km Fiber Link Using Bidirectional Active Optics

Hanxu Wu, Xiaoming Zhang, Weinan Zhao, Yang Fu, Xinyi Chen, Sitong Jia, Wenzhe Yang, Junfeng Ge, Shengkang Zhang, Honglei Yang

Published May 15, 2023• 0 citations

Abstract

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.

DOI: 10.1109/eftf/ifcs57587.2023.10272206

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Coherent Regeneration of Optical Frequency with 1-s Instability at

10^{-17}

The remote regeneration of coherent optical frequency technique finds increasing applications in the fields of large-scale optical clock networks, precision spectroscopy, very-long baseline interferometry, and high-sensitivity tests of fundamental physics. Meanwhile, with the improvements of coherent optical frequency generation, the requirement of stable transfer is more and more demanding. Here, we demonstrate a regeneration system of coherent optical frequency based on bidirectional phase locking, in which the locking loop at the local site is for eliminating optical phase noise over the link, and the locking loop at the remote site is for locking to the incoming coherent optical frequency. Preliminary experiment over a 1.5 km fiber link indicates that the additional frequency stability can reach

\mathbf{5.9}\times \mathbf{10}^{-\mathbf{17}}

and

\mathbf{7.0} \times \mathbf{10}^{-\mathbf{19}}

at an averaging time of 1 s and 103s, respectively. This method can be further extended to remote optical frequency comparisons for multiple users.

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.

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.

We have demonstrated accurate frequency transfer via an ultra long span optical-fiber link by passive phase conjugation, which is based on frequency mixing. Combing with the phase-lock-loop, our compensation technique can eliminate the effect of phase noise that introduced during the optical-fiber transmission, so the phase of remote signal is synchronized to the phase of local reference signal. The transfer stability of our experiment with 2.4 GHz frequency signal through a 1007 km fiber link is 6.9_10-14/s and 4.1_10-16/104s.

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.

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.