Abstract Our primary objective is to mitigate the adverse effects of temperature fluctuations on the optical frequency transmission system by reducing the length of the interferometer. Following optimization, the phase-temperature coefficient of the optical system is reduced to approximately 1.35 fs/K. By applying a sophisticated temperature control to the remained “out-of-loop” optics fiber, the noise floor of the system has been effectively lowered to 10−21 level. Based on this performance-enhanced transfer system, we demonstrate coherent transmission of optical frequency through 500-km spooled fiber link. After being actively compensated, the transfer instability of 4.5 × 10−16 at the averaging time of 1 s and 5.6 × 10−21 at 10000 s is demonstrated. The frequency uncertainty of received light at remote site relative to that of the origin light at local site is achieved to be 1.15 × 10−19. This enhanced system configuration is particularly well suited for future long-distance frequency transmission and comparison of the most advanced optical clock signals.
Abstract Our primary objective is to mitigate the adverse effects of temperature fluctuations on the optical frequency transmission system by reducing the length of the interferometer. Following optimization, the phase-temperature coefficient of the optical system is reduced to approximately 1.35 fs/K. By applying a sophisticated temperature control to the remained “out-of-loop” optics fiber, the noise floor of the system has been effectively lowered to 10−21 level. Based on this performance-enhanced transfer system, we demonstrate coherent transmission of optical frequency through 500-km spooled fiber link. After being actively compensated, the transfer instability of 4.5 × 10−16 at the averaging time of 1 s and 5.6 × 10−21 at 10000 s is demonstrated. The frequency uncertainty of received light at remote site relative to that of the origin light at local site is achieved to be 1.15 × 10−19. This enhanced system configuration is particularly well suited for future long-distance frequency transmission and comparison of the most advanced optical clock signals.
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.
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.
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.
We report a technique for coherence transfer of laser light through a fiber link, where the optical phase noise induced by environmental perturbations via the fiber link is compensated by remote users with passive phase noise correction, rather than at the local site as is conventional. Neither phase discrimination nor active phase tracking is required due to the open-loop design, mitigating some technical problems such as the limited compensation speed and the finite compensation precision as conventional active phase noise cancellation. We theoretically analyze and experimentally demonstrate that the delay-limited residual fiber phase noise after phase noise compensation is a factor of seven higher than the conventional techniques. Using this technique, we demonstrate the transfer laser light through a 145-km-long, lab-based spooled fiber. After being compensated, a relative frequency instability is 1.9 × 10-15at the integration time of 1 s and scales down to the level of 10-18-range at 10,000 s. The frequency uncertainty of the light after transferring through the fiber relative to that of the input light is (-0.36 ± 2.6) × 10-18. As the transmitted optical signal remains unaltered until it reaches the remote sites, it can be transmitted simultaneously to multiple remote sites on an arbitrarily complex fiber network, paving a way to develop a multi-node optical frequency dissemination system with post automatic phase noise correction for a number of end-users.
In this paper, we present a remote time-base-free technique for a coherent optical frequency transfer system via fiber.At the remote site, the thermal noise of the optical components is corrected along with the link phase noise caused by environmental effects.In this system, a 1 × 2 acousto-optic modulator (AOM) is applied at the remote site, with the first light being used to eliminate the noise of the remote time base and interface with remote users while the zeroth light is used to establish an active noise canceling loop.With this technique, a 10 MHz commercial oscillator, used as a time base at the remote site, does not contribute to the noise of the transferred signal.An experimental system is constructed using a 150 km fiber spool to validate the proposed technique.After compensation, the overlapping Allan deviation of the transfer link is 7.42 × 10 -15 at 1 s integration time and scales down to 1.07 × 10 -18 at 10,000 s integration time.The uncertainty of the transmitted optical frequency is on the order of a few 10 -19 .This significantly reduces the time-base requirements and costs for multi-user applications without compromising transfer accuracy.Meanwhile, these results show great potential for transferring ultra-stable optical frequency signals to remote sites, especially for point-to-multi-users.
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.