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European Frequency and Time Forum

Progress of ultra-stable frequency dissemination and synchronization in free space

Jing Miao, Bo Wang, Changjun Gao, Yu Bai, Xiaoming Zhu, L. J. Wang

Published Jun 1, 2014• 0 citations
Publisher

Abstract

We demonstrate a new ultra-stable frequency dissemination scheme on L band over 100m atmosphere. By compensating phase noise actively and a series of frequency conversions, the phase fluctuation is well compensated. Two links disseminate different frequency signals phase locked to a common reference simultaneously, and recover the disseminated frequency respectively at remote site. The stability of dissemination with phase compensation in free space is 3×10-13/s and 4×10-17/day. The relationship between dissemination stability and propagating distance is also discussed.

DOI
10.1109/eftf.2014.7331496

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Abstract: we report the first demonstration of a long-distance ultra stable frequency dissemination in the microwave range. A 9.15 GHz signal is transferred through a 86-km urban optical link with a fractional frequency instability of 1.3×10-15 at 1 s integration time and below 10-18 at one day. The optical link phase noise compensation is performed with a round-trip method. To achieve such a result we implement light polarisation scrambling and dispersion compensation. This link outperforms all the previous radiofrequency links and compares well with recently demonstrated full optical 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.
Optical frequency comb technology has been revolutionizing optical fields of ultra-precision metrology over two decades, connecting the SI unit standards of various optical clocks with utmost accuracy and stability. With an increasing demand for free-space communication and time/frequency transfer applications, the frequency comb is stretching its versatility even the outside of laboratory. Still, the environmental disturbances of free space degrade the excellent phase noise of the optical frequencies, hindering the reliable utilization of frequency comb at any desirable location. This research performs comb-to-comb synchronization over a 1.3-km Free-Space Optical link (FSO) with frequency stability of 3.80×10-15 at 0.1 s averaging time throughout the entire comb spectrum. Phase-locked loop compensates the Doppler shift of optical frequencies propagating atmospheric free-space channel, providing optical references for synchronizing distant frequency comb.
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 transfer the frequency of an ultra-stable laser over a cascaded optical link comprising two compensated links of 150 km and a repeater station. Each link passes through two important nodes of the telecommunication network and includes 114 km of Internet fiber simultaneously carrying data traffic, through a dense wavelength division multiplexing scheme. The metrological signal is inserted in and extracted from the communication network using bidirectional optical add-drop multiplexers. The repeater station is working independently without any remote control. The phase noise on the two links is compensated with the usual round-trip technique. The 300-km multiplexed cascaded link shows an Allan deviation of 3×10-15at one second and 7×10-20at 20 hours. This work paves the way to a wide dissemination of ultra stable optical clock signals between distant laboratories via the Internet network.