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Optics Letters

Coherence transfer of subhertz-linewidth laser light via an optical fiber noise compensated by remote users

Lifei Wu, Yanyi Jiang, Chaoqun Ma, Hongfu Yu, Zhiyi Bi, Long-Sheng Ma

Published Sep 13, 2016• 19 citations
Publisher

Abstract

We present a technique for the coherence transfer of laser light through a fiber link, where the optical phase noise induced by environmental perturbation via the fiber link is compensated by remote users. When compensating the fiber noise by remote users, the time base at the remote site independent from that at the local site does not destroy the performance of the fiber output light. Using this technique, we demonstrate the transfer of subhertz-linewidth laser light through a 25-km-long, lab-based spooled fiber. After being compensated, the relative linewidth between the fiber input and output light is 1 mHz, and the relative frequency instability is 4×10−17 at 1 s averaging time and scales down to 2×10−19 at 800 s averaging time. The frequency uncertainty of the light after transferring through the fiber relative to that of the input light is 3.0×10−19. This system is suitable for the simultaneous transfer of an optical signal to a number of end users within a city.

DOI
10.1364/ol.41.004368

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We present a technique for the coherence transfer of laser light through a fiber link, where the optical phase noise induced by environmental perturbation via the fiber link is compensated by remote users. When compensating the fiber noise by remote users, the time base at the remote site independent from that at the local site does not destroy the performance of the fiber output light. Using this technique, we demonstrate the transfer of subhertz-linewidth laser light through a 25-km-long, lab-based spooled fiber. After being compensated, the relative linewidth between the fiber input and output light is 1 mHz, and the relative frequency instability is 4×10−17 at 1 s averaging time and scales down to 2×10−19 at 800 s averaging time. The frequency uncertainty of the light after transferring through the fiber relative to that of the input light is 3.0×10−19. This system is suitable for the simultaneous transfer of an optical signal to a number of end users within a city.
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.
We demonstrate optical coherence transfer of subhertz-linewidth laser light through fiber links by actively compensating random fiber phase noise induced by environmental perturbations. The relative linewidth of laser light after transferring through a 32-km urban fiber link is suppressed within 1 mHz (resolution bandwidth limited), and the absolute linewidth of the transferred laser light is less than 0.36 Hz. For an 82-km fiber link, a repeater station is constructed between a 32-km urban fiber and a 50-km spooled fiber to recover the spectral purity. A relative linewidth of 1 mHz is also demonstrated for light transferring through the 82-km cascaded fiber. Such an optical signal distribution network based on repeater stations allows optical coherence and synchronization available over spatially separated places.
We present a technique for coherence transfer of laser light through a branching fiber link, where the optical phase noise induced by environmental perturbations via the fiber link is passively compensated by remote users without the requirements of any active servo components. At each remote site, an acousto-optic modulator (AOM) is simultaneously taken as a frequency distinguisher for distinguishing its unique frequency from other sites' and as an optical actuator for compensating the phase noise coming from the optical fiber. With this configuration, we incorporate a long outside loop path consisting of a fiber-pigtailed AOM into the loop, enabling the significant reduction of the outside loop phase noise in the passive way. To further address the residual out-of-loop phase noise coming from the interferometer and the two-way optical frequency comparison setup, we design a low-noise active temperature stabilization system. Measurements with a back-to-back system show that the stability in our stabilization system is 2×10-16at 1 s, reaching 2×10-20after 10,000s. Adopting these techniques, we demonstrate transfer of a laser light through a branching fiber network with 50 km and 145 km two fiber links. After being compensated for the 145 km fiber link, the relative frequency instability is 3.4×10-15at the 1 s averaging time and scales down to 3.7×10-19at the 10,000 s averaging time. This proposed technique is suitable for the simultaneous transfer of an optical signal to a number of independent users within a local area.
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.
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 coherent transfer of an ultra-stable optical frequency at 192.8 THz over 50-km spooled fiber. Random phase noise induced by environmental disturbance through fiber is detected and suppressed by feeding a correctional signal into an acousto-optic modulator. After being compensated, the fiber-induced frequency instability is 2×10 −17 at 1-s averaging time and reaches 8×10 −20 after 16 h. The noise floor of the compensation system could be as low as 2×10 −18 at 1-s averaging time.