Towards free-space ultrastable optical frequency transfer

Nowadays optical frequency transfer has become an essential component for numerous quantum technology applications. There is a gradual increase in demand for such systems due to the rapid development of quantum technologies themselves, e. g. optical frequency transmission became crucial for high-precision timekeeping and communication systems relying on the frequency stability of quantum clocks that offer unparalleled accuracy, making ideal for metrology, navigation, and testing fundamental physical theories. Moreover, optical frequency transmission has already found popularity in such up-to-date topics as quantum communication and quantum cryptography.

For over 6 years, our laboratory has been developing various systems for the transmission of stabilized frequency signals. In 2017, we started with the elaboration of a phase noise compensation system in a 5-meter long optical fiber frequency transmission line and afterwards extended the line to 2.8 kilometers. Taking advantage of the result we also connected three of our laboratories with fiber optic cables with phase noise compensation systems to facilitate the process of comparing developed frequency standards. With an awareness of the fiber optic links applicability limitations (such as insufficient mobility and flexibility) we developed a 5-meter long free-space optical link with the same phase-noise compensation system in 2020 and increased its length to 17 meters with the addition of a precision pointing system in 2021.

Finally, in 2023, we introduced a 215-cm free-space optical transmission link with a flexible pointing system that enables dynamically stable tracking of moving objects: potentially drones or even satellites. The test scheme of this transfer system with defined upgrades is presented in Figure 1.

Phase noise compensation system for both fiber and free-space optical links The transmission link introduces phase noise into the signal. To compensate for corresponding frequency shifts, a laser beam used for transmission is split into two parts. The first part passes through a reference arm of an interferometer, while the second is transmitted to the receiver via an acousto-optic modulator (AOM1) and partly reflected back. The returned signal contains doubled link noise and is heterodyned with the reference beam. The resulting beat signal is used in a phase-locked loop that controls the frequency shift introduced by AOM1 and compensates the link noise.

Pointing system for free-space optical link To compensate for small high-frequency beam direction fluctuations, a precise (fast) pointing system was used, which includes a position-sensitive quadrant photodetector managing a mirror with two-coordinate galvanic control. To expand the pointing range, we implement a coarse (slow) pointing system by attaching the optical plate to an alt-azimuth telescope mount. The servo signal of the fast system is used to correct the angular velocity of the dynamic tripod rotation around two axes.

In the near future, we plan to extend the free-space link length and test the system in actual environments when pointing at a moving UAV. In addition, we plan to focus on processing the received noise signal in order to use it to obtain information about atmospheric parameters.

We strongly believe that the development of optical transmission systems is extremely useful for the expansion of quantum technologies.