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An ultra-stable microwave transfer technique for the ACES and future space missions

Naomi Altman, Sascha Schediwy, John J. McFerran, Michael Edmund Tobar, Andre Nicholas Luiten

Published May 1, 2014• 0 citations
OpenAlex

Abstract

We present an actuation technique for suppressing uncorrelated noise between two optical frequencies to obtain ultra-stable microwave frequency dissemination over optical fibre. This technique will enable dual optical and microwave frequency transfer between the University of Western Australia and Yarragadee Satellite Laser Ranging station, which has application for the Atomic Clock Ensemble in Space (ACES) mission and other precision navigation and timing technologies. We build on the technique presented in [1], which demonstrates microwave transfer stability of 10−14 at 1s integration time by stabilising the group delay of an two correlated optical signals. A second actuator has been developed to suppress the remaining uncorrelated phase fluctuations between two optical frequencies by implementing optical-wavelength-scale action to vary the microwave phase. The actuator has been designed with the aid of a simulation that predicts actuation bandwidth and loss for a range of actuator parameters.

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We present an actuation technique for suppressing uncorrelated noise between two optical frequencies to obtain ultra-stable microwave frequency dissemination over optical fibre. This technique will enable dual optical and microwave frequency transfer between the University of Western Australia and Yarragadee Satellite Laser Ranging station, which has application for the Atomic Clock Ensemble in Space (ACES) mission and other precision navigation and timing technologies. We build on the technique presented in [1], which demonstrates microwave transfer stability of 10−14 at 1s integration time by stabilising the group delay of an two correlated optical signals. A second actuator has been developed to suppress the remaining uncorrelated phase fluctuations between two optical frequencies by implementing optical-wavelength-scale action to vary the microwave phase. The actuator has been designed with the aid of a simulation that predicts actuation bandwidth and loss for a range of actuator parameters.
Western Australia is becoming the premier southern hemisphere hub for high-precision ground-space timing missions. The University of Western Australia (UWA) is currently building a state-of-the-art optical atomic clock, and has been selected as the only southern hemisphere microwave-frequency ground station for the Atomic Clock Ensemble in Space (ACES) mission. We propose to construct a 60 km section of optical fibre to complete a link between UWA and the Yarragadee Satellite Laser Ranging facility and thereby enabling an optical link for the ACES and Space-Time Explorer and QUantum Equivalence Principle Space Test (STE-QUEST) missions.
We present a stabilized microwave-frequency transfer technique that is based on optical phase sensing and optical phase actuation. This technique shares several attributes with optical-frequency transfer and, therefore, exhibits several advantages over other microwave-frequency transfer techniques. We demonstrated the stabilized transfer of an 8000 MHz microwave-frequency signal over a 166 km metropolitan optical fiber network, achieving a fractional frequency stability of 6.8×10−14 Hz/Hz at 1 s integration and 5.0×10−16 Hz/Hz at 1.6×104 s. This technique is being considered for use on the Square Kilometre Array SKA1-mid radio telescope.
Comparison of microwave frequency standards by satellite techniques is currently limited in instability at the 10-15level at one day [1]. However, the best optical frequency standards have already demonstrated an instability of ~ 5 × 10-17at 2000 s and an estimated uncertainty of 9 × 10-18[2]. The predicted improvement in optical frequency standards motivates the need for a new transfer method capable of performing below these predicted levels of instability. The transmission of cw optical frequencies through optical fibre networks is the most promising alternative to satellite methods when it comes to comparing remote optical frequency standards. Intra-lab comparison of optical frequency standards can already be carried out with the use of femtosecond optical frequency combs, which can easily bridge the frequency gap between the different standards under test. However, a particularly more challenging task is to compare different standards at remote European laboratories. The comparison of remote standards, particularly like-for-like systems (such as the Yb+clocks at NPL and PTB), will be important in determining the relative uncertainty and reproducibility of these clocks. For this remote comparison method to operate below the instability level of the optical standards, a highly stable phase-coherent local oscillator is needed so as not to degrade the stability of the atomic reference being compared.
Comparison of ultra-stable frequency references located in geographically distributed areas require a transfer technique, which offers stability of 10−17at 1 s over tens of kilometer. Such remote frequency transfer will allow comparing the optical frequency references of same or different atomic species (through a frequency comb bridge) for test of relativistic geodesy [1] or time variation of fundamental constants [2], and will contribute towards the database of frequency ratio measurements-an essential requirement for the re-definition of the SI second based on optical clocks [3]. It will also facilitate in creating an optical to microwave frequency link that can be used by various SI standards laboratory requiring stable microwave sources. In this paper, I discuss the modulated optical carrier wave based ultra-stable frequency reference transfer technique, and present a study of a laser interferometer based active fiber noise cancellation system and its applications.
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.