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

Ultra-Stable Laser-Based Two-Way Optical Frequency Comparison on a 150 km Fiber Link

Zijie Zhou, Ziang Qiu, Liang Hu, Guiling Wu, Jianping Chen

Published May 12, 2025• 0 citations
Publisher

Abstract

SummaryWe report on the realization of the high-precision two-way optical frequency comparison (TWC), utilizing two ultra-stable lasers over a 150 km spooled fiber link. These two ultra-stable lasers are achieved by stabilizing the narrow linewidth lasers to a Fabry-Perot (FP) cavity using the Pound-Drever-Hall (PDH) technique. To enhance the signal-to-noise ratio (SNR) of the optical signal and broaden the locking bandwidth, an additional electro-optic modulator (EOM) is incorporated into the locking process. According to the experimental results, the TWC system demonstrate a fractional frequency instability of 5.89×10-16at 1 s, and it decrease to 4.97×10-19at 10,000 s.

DOI
10.1109/eftf/ifcs64367.2025.11194608

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SummaryWe report on the realization of the high-precision two-way optical frequency comparison (TWC), utilizing two ultra-stable lasers over a 150 km spooled fiber link. These two ultra-stable lasers are achieved by stabilizing the narrow linewidth lasers to a Fabry-Perot (FP) cavity using the Pound-Drever-Hall (PDH) technique. To enhance the signal-to-noise ratio (SNR) of the optical signal and broaden the locking bandwidth, an additional electro-optic modulator (EOM) is incorporated into the locking process. According to the experimental results, the TWC system demonstrate a fractional frequency instability of 5.89×10-16at 1 s, and it decrease to 4.97×10-19at 10,000 s.
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.
The transfer of ultra-stable frequencies between distant laboratories is required by many applications in time and frequency metrology, fundamental physics, particle accelerators and astrophysics. This thesis describes the development of ultra-stable optical links over distances from 86 km to 300 km. The long-term objective is to connect the main European National Metrology Institutes with optical links extending up to 1000 km. The optical link is based on the transfer of the optical phase of an ultra-stable laser emitting light of wavelength 1542 nm through an optical fiber of the telecommunication network. For that purpose, ultra-stable cavity-stabilized lasers and fiber-stabilized lasers have been developed. The fractional instability of cavity-stabilized laser is approximately 10-15 at 1s integration time. The fiber-stabilized lasers are robust, compact, simple, tunable and their frequency noise is comparable to that of the cavity-stabilized laser for Fourier frequencies higher than a few tens of Hertz. Optical links were first demonstrated on 86 km and 172 km using dedicated fibers connecting the laboratories LNE-SYRTE and LPL. Fractional frequency instability in the range of or below 10-19 at 104 s integration time has been obtained. For larger distances, the ultra-stable laser signal is transferred on the Internet network simultaneously with the data traffic but on a dedicated frequency channel, using wavelength division multiplexing. Multiplexed links were demonstrated over distances up to 300 km, with fractional frequency instabilities at the same level or better than with dedicated fibers.
High-stability optical frequency comparison over fiber link enables the establishment of ultrastable optical clock networks, having the potential to promote a series of applications, including metrology, geodesy, and astronomy. In this article, we theoretically analyze and experimentally demonstrate a time-delayed local two-way (TD-LTW) optical frequency comparison scheme with improved comparison stability, showing that the fractional instability of optical frequency comparison over a 50-km transfer link can be reduced from1.30×10−151.30\times 10^{-15}1.30×10−15to5.25×10−165.25\times 10^{-16}5.25×10−16at the 1 s integration time with an improvement factor of 2.48. Additionally, we also for the first time model and experimentally verify the effect of the inhomogeneous phase noise along the fiber link on the system performance. We believe that the theory and technique proposed here will be helpful in developing the high-stability optical clock networks over large-area fiber links.
By using two-way frequency transfer, we demonstrate ultra-high resolution comparison of optical frequencies over a telecommunication fiber link of 100 km operating simultaneously digital data transfer. We first propose and experiment a bi-directional scheme using a single fiber. We show that the relative stability at 1 s integration time is 7 10^18 and scales down to 5 10^21. The same level of performance is reached when an optical link is implemented with an active compensation of the fiber noise. We also implement a real-time two-way frequency comparison over a uni-directional telecommunication network using a pair of parallel fibers. The relative frequency stability is 10^15 at 1 s integration time and reaches 2 10^17 at 40 000 s. The fractional uncertainty of the frequency comparisons was evaluated for the best case to 2 10^20. These results open the way to accurate and high resolution frequency comparison of optical clocks over intercontinental fiber networks.
We present the experimental demonstration of a local two-way optical frequency comparison over a 43-km-long urban fiber network without any requirement for measurement synchronization. We combined the local two-way scheme with a regular active noise compensation scheme that was implemented on another parallel fiber leading to a highly reliable and robust frequency transfer. This hybrid scheme allowed us to investigate the major limiting factors of the local two-way comparison. We analyzed the contributions of the interferometers at both local and remote locations to the phase noise of the local two-way signal. Using the ability of this setup to be injected by either a single laser or two independent lasers, we measured the contributions of the demodulated laser instabilities to the long-term instability. We show that a fractional frequency instability level of 10 −20 at 10,000 s can be obtained using this simple setup after propagation over a distance of 43 km in an urban area.