Coherent optical frequency transfer via 972-km fiber link

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 10^(4)s is achieved.This work provides a promising method for realizing optical frequency distribution over thousands of kilometers by using fiber links.

Robust Transfer of Optical Frequency over 500km Fiber Link with Instability of 10^(−21)

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.

Cascaded optical frequency transfer over 500-km fiber link using regenerative amplifier

We demonstrate a 300-km+200-km cascaded coherent phase transfer via fiber link.The transfer is divided into a 300-km span and a 200-km span with independent phase locking loops,aiming to extend the phase control bandwidth of the whole link.The phase noise and transfer instability of the cascaded transmission are investigated and compared with those in the case of a single-span 500-km transfer.We achieve the transfer instabilities of 1.8×10^-14 at 1 s,8.9×10^-20 at 104 s for the 300-km+200-km cascaded transmission,and 2.7×10^-14 at 1 s for the 500-km single-span transfer.

Novel Polarization Control Approach to Long-Term Fiber-Optic Frequency Transfer

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.

Coherent Optical Frequency Transfer via a 490 km Noisy Fiber Link

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 rad^(2)·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 phaselocked 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 2^(12)πrad.Finally,a noise suppression intensity of approximately 40 d B at1 Hz is obtained,with fractional frequency instability of 1.1×10^(-14 )at 1 s averaging time,and 3.7×10^(-19) at10000 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.

Optical frequency transfer link with remote site compensation

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.