Compact light control unit for use in a low-SWaP strontium optical atomic clock
Fig. 1. Micro-integrated light control unit featuring miniaturized acousto-optic and electro-optic modulators for amplitude and phase modulation.
Fig. 2. (a) Measured AOM efficiency as a function of applied RF power at 78 MHz. The efficiency was calculated from the ratio of diffracted 1st-order power and incident optical power, including losses from polarization optics. (b) Experimentally determined modulation depth as a function of RF voltage amplitude for EOM operation at 24 MHz.
Quantum sensors, such as optical atomic clocks, are increasingly deployed in metrology applications beyond the laboratory environment. Their unparalleled stability and accuracy are unlocking new possibilities in navigation, timekeeping, geophysics, and fundamental research. Applications range from synchronizing data networks and providing reference signals for global satellite navigation systems to implementations in relativistic geodesy. Expanding from terrestrial applications into space, high-performance optical clocks are envisioned as platforms for groundbreaking experiments in fundamental physics, including tests of general relativity and advanced searches for dark matter.
To leverage their precision in challenging environments – whether in the field or in space – transportable optical clock systems have to meet stringent size, weight, and power (SWaP) requirements across all components. At FBH, we address the miniaturization of the required laser systems through hybrid micro-integration of compact, ultra-narrow linewidth diode laser modules and additional light control units (LCUs). These modular LCUs are equipped with ultra-compact free-space acousto-optic and electro-optic modulators. They enable power switching, frequency shifting, and amplitude or phase modulation of optical signals to condition the light before it interacts with the atomic system of the optical clock.
Fig. 1 shows our first micro-integrated LCU, designed to operate at a wavelength of 461 nm for a transportable strontium beam clock. With a volume of just 0.5 L and a mass of 1 kg, the module features a miniature acousto-optic modulator (AOM) for amplitude modulation and an electro-optic modulator (EOM) for phase modulation of an optical signal provided by an external laser. Both the optical input and the two optical output ports are realized via polarization-maintaining single-mode fibers. Due to their resonant design, the modulators can be driven with RF power requirements on the order of a few tens of milliwatt. As shown in Fig. 2a, the AOM, driven at a frequency of 78 MHz, achieves over 80 % first-order diffraction efficiency at approximately 16 dBm (40 mW) of RF power, accounting for losses from polarization filtering. A comparable RF power level is also required to drive the EOM at 24 MHz to achieve a 1-radian phase shift, as shown in Fig. 2b.
Following assembly and electro-optical characterization at FBH, the LCU was successfully integrated into the laser subsystem of the optical atomic clock by our project partner Menlo Systems GmbH. The complete clock system is housed within an 18U 19-inch rack. In addition to two lasers operating at wavelengths of 461 nm and 689 nm, it includes a compact reference cavity, an optical frequency comb with dedicated control electronics, and the physics package featuring a compact in-vacuum optics assembly. System-level performance of the optical atomic clock is currently under evaluation at Humboldt-Universität zu Berlin.
This work was supported by VDI Technologiezentrum GmbH with funds provided by the Federal Ministry of Research, Technology and Space (BMFTR) within the funding program “Quantum technologies – from basic research to market” under grant number 13N15724.
Publication
[1] J. Hamperl, M. Gärtner, N. Goossen-Schmidt, B. Arar, M. Bursy, S. Hariharan, A. Liero, S. Nozinic, M. Schiemangk, S. Szermer, C. Tyborski, A. Wicht, “Micro-integrated laser distribution modules for low-SWaP optical atomic clocks”, SPIE Proceedings of Quantum West 2025.