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

Polarization Control for Continuous Optical Frequency Transfer

Przemysław Krehlik, Łukasz Buczek, Łukasz Śliwczyński, Krzysztof Turza

Published May 15, 2023• 0 citations
Publisher

Abstract

SummaryIn this work we present a solution for controlling the polarization state of the signal reaching the remote laser station terminating the optical frequency transfer link (or its section). The proposed controller eliminates the occasional fading of the beat note between the laser and the incoming signal, which is typically observed due to random wandering of the state of polarization of the signal traveling down an optical fiber. The efficiency of the solution is demonstrated for optical frequency transfer via an optical path consisting of real outdoor fibers, as well as fibers on spools and also polarization scramblers speeding up natural polarization fluctuations.

DOI
10.1109/eftf/ifcs57587.2023.10272062

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SummaryIn this work we present a solution for controlling the polarization state of the signal reaching the remote laser station terminating the optical frequency transfer link (or its section). The proposed controller eliminates the occasional fading of the beat note between the laser and the incoming signal, which is typically observed due to random wandering of the state of polarization of the signal traveling down an optical fiber. The efficiency of the solution is demonstrated for optical frequency transfer via an optical path consisting of real outdoor fibers, as well as fibers on spools and also polarization scramblers speeding up natural polarization fluctuations.
We demonstrate a novel polarization control system based on a gradient descent algorithm, applied to a 450-km optical frequency transfer link. The power of the out-loop beat note is retrieved by controlling the polarization state of the transferred signal, with a recovery time of 24 ms, thereby ensuring the long-term evaluation of the fiber link. As a result, data utilization is enhanced from 70% to 99% over a continuous measurement period of ∼12 h. A fractional transfer instability of 7.2 × 10−20 is achieved at an integration time of 10000 s. This work lays the foundation for the comparison of a remote optical clock system via a long-haul optical fiber link.
Polarization variations in optical fibers are complex and would severely affect the performances of polarization-sensitive signal distribution systems. Owing to advances in experimental techniques and theoretical tools, we observe the effect of polarization variations in the optical fibers and we demonstrate that polarization mode dispersion (PMD) dominates the free-running fiber noise. Ultrahigh correlation, over 99%, is found between the phase fluctuation induced by polarization variation and the temperature fluctuation impacted on the optical fibers. By means of a polarization scrambling technique, the polarization variations in the optical fiber are averaged and the phase perturbations are minimized. It provides a promising solution for suppressing the random disturbances of polarization in the fiber-based optical frequency transfer system and quantum key distribution (QKD) system.
We propose and analyze a novel all-optical fiber polarization scrambler based on the transfer (via the Kerr effect) of the intensity fluctuations of an incoherent pump beam into polarization fluctuations of a frequency-shifted signal beam, copropagating in a randomly birefringent telecom fiber. Optimal signal polarization scrambling results whenever the input signal and pump beams have nearly orthogonal states of polarization. The nonlinear polarization scrambler may operate on either cw or high-bit-rate pulsed signals.
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.
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.
In recent years, the dissemination of optical frequency signals along phase-stabilized fibers has allowed the comparison of optical clocks at unprecedented levels of accuracy. This has a strong relevance in frequency metrology and boosted the measurement capabilities also in other research fields such as radioastronomy [1], geodesy [2] and fundamental physics [3]. Fiber-based frequency dissemination relies on the transmission of a continuous-wave, narrow-linewidth laser whose frequency is accurately measured with an atomic clock at the National Metrology Institute (NMI) where it is generated. Specific techniques are used to compensate for the acoustic and seismic noise and temperature variations, which degrade the phase stability of the transmitted signal. However, in existing implementations, it is not possible to detect and compensate for the birefringence variations of the optical fiber. These can induce variations in the state of polarization (SOP) of the transmitted signal, affecting the optical beatnotes at the receiving end. This not only degrades the achievable performances in terms of phase stability of the metrological signal, but can also lead to beatnote fading, with consequent failure of the entire system. This problem currently represents one of the strongest limitations to the reliability of fiber-based frequency dissemination.