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Optical and Quantum Sensing and Precision Metrology II

Micro-integrated laser distribution modules for low-SWaP optical atomic clocks

Jonas Hamperl, Martin Gärtner, Nora Goossen-Schmidt, Bassem Arar, Marcel Bursy, Sriram Hariharan, Simon Kubitza, Armin Liero, Sonja Nozinic, Max Schiemangk, Sandy Szermer, Christoph Tyborski, Andreas Wicht

Published Mar 19, 2025• 1 citation
Publisher

Abstract

Quantum sensors, such as optical atomic clocks, are increasingly used in out-of-the-lab environments. This development necessitates a high degree of integration and robustness across all components of these systems. In this context, we present the status of our efforts to develop and realize compact laser distribution modules for the implementation of low-SWaP (size, weight, and power) strontium-based optical clocks. These modules are designed to control and manipulate the properties of laser light, such as frequency, phase or intensity, prior to the light’s interaction with the atoms of the clock. By integrating miniaturized free-space acousto- and electro-optic modulators into these modules, they enable applications such as laser power or fiber path length stabilization, as well as frequency-modulation spectroscopy. In this paper, we present a distribution module designed for 461 nm that was assembled using our hybrid micro-integration technology and will be integrated into a compact optical clock demonstrator. Additionally, we provide an outlook on other distribution modules currently under development.

DOI
10.1117/12.3038738

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Quantum sensors, such as optical atomic clocks, are increasingly used in out-of-the-lab environments. This development necessitates a high degree of integration and robustness across all components of these systems. In this context, we present the status of our efforts to develop and realize compact laser distribution modules for the implementation of low-SWaP (size, weight, and power) strontium-based optical clocks. These modules are designed to control and manipulate the properties of laser light, such as frequency, phase or intensity, prior to the light’s interaction with the atoms of the clock. By integrating miniaturized free-space acousto- and electro-optic modulators into these modules, they enable applications such as laser power or fiber path length stabilization, as well as frequency-modulation spectroscopy. In this paper, we present a distribution module designed for 461 nm that was assembled using our hybrid micro-integration technology and will be integrated into a compact optical clock demonstrator. Additionally, we provide an outlook on other distribution modules currently under development.
We present our latest development of photonic modules for quantum technology applications and show first results of their electro-optical performance. Based on the ECDL-MOPA concept, our laser modules combine a high output power with a narrow linewidth. Our compact light control units enable efficient beam conditioning through switching and modulation, utilizing miniaturized acousto-optic and electro-optic modulators. These modules serve various quantum technology applications, e.g. quantum sensing, timing and precision metrology. In addition, our hybrid micro-integration technology allows us to build our modules as compact and robust as it is needed for applications in space.
Optical atomic clocks with precisions and accuracies in the 10−^-−1^118^88 level [1] are now the most advanced man-made timekeeping devices. They outperform the microwave cesium atomic clocks that realize the SI definition of the second. Scaling down the size of optical atomic clocks may open the door to a range of industrial and space applications. In this thesis, the design and preliminary results of a compact strontium cooling system are presented. In the first cooling stage, the high power 461 nm laser with 300 mW output features a modular design, while smaller laser sources for demonstrating a strontium magneto-optical trap have also been investigated. An innovative design that couples a spectroscopy cell directly into the scientific chamber reduces the overall size and power consumption of the system. Additionally, using strontium oxide as a source of strontium atoms suitable for optical clocks has achieved initial success. For the first time, a single-beam MOT configuration is applied to strontium. In this novel apparatus, the blue MOT and red MOT broadband cooling phases are able to trap 5x106^66 and 1000 atoms, respectively. This work shows promising progress towards developing a functional miniaturized strontium optical lattice clock.
Abstract The primary technology for high accuracy position, navigation and timing (PNT) capability worldwide is currently provided by global navigation satellite systems (GNSS). The vulnerability of GNSS to intentional interference, clock component failure or extreme space weather is increasingly pointing to the need for alternative independent navigation and timing solutions to maintain critical national infrastructures, defence, security, commerce and logistics. The availability of low size, weight and power (SWaP) portable atomic clock technology with resilience and extended holdover times for use both in space and terrestrially is now increasingly urgent. In this paper, we report on progress in the development of compact optical clocks that offer the capability to out-perform existing commercial microwave atomic clock systems. We discuss designs for a compact transportable cold trapped strontium ion optical clock with both a low SWaP design and high accuracy that can address extended holdover for future navigation and timing requirements. We describe how key sub-systems of an optical atomic clock, namely the single ion trap physics package and multi-wavelength dual-axis cubic cavity clock control unit have been redesigned to reduce SWaP. The optical cavity is needed to frequency stabilise the lasers for laser cooling and state preparation as well as the clock laser for interrogating the ion’s clock transition. We describe the results of finite element analysis (FEA) to model the electrical and thermal behaviour of a compact ion trap design. We also consider the design space for small cubic optical cavities, investigating the possible limits for achieving a minimal SWaP for the system.
The free-fall atomic clock has shown considerable potential for long-term stability and mobility, yet reports on its design and implementation remain limited. Herein, we introduce a simple integrated optical system for free-fall cold Rb atomic clocks. A distributed feedback (DFB), frequency-doubled fiber laser, and a 780 nm DFB laser diode are utilized as the cooling and repumping light source, respectively. Both lasers feature a broad mode-hop-free tuning range and reduced sensitivity to acoustic noise. Benefiting from a high signal-to-noise ratio of modulation transfer spectroscopy (MTS), the linewidth of the fiber laser is measured to be 42 kHz. The power and frequency management of the cooling beams is implemented by two 110-MHz acousto-optic modulators (AOMs), enabling a frequency detuning as large as −92 MHz. The power-frequency management module is integrated into a single side bench of 316 mm × 270 mm × 52 mm. The power fluctuation of the probe beam is measured to be 0.8% (RMS) over 24 hours measurement. Using an ultra-high vacuum system, the functionality of the optical system is preliminarily verified by the time-of-flight (TOF) signal of the cold atom cloud.
Optical frequency standards, or lasers stabilized to atomic or molecular transitions, are widely used in length metrology and laser ranging, provide a backbone for optical communications and lie at the heart of next-generation optical atomic clocks. Here we demonstrate a compact, low-power optical frequency reference based on the Doppler-free, two-photon transition in rubidium-87 at 778 nm implemented on a micro-optics breadboard. Our optical reference achieves a fractional frequency instability of 2.9×10−12/ τ for averaging times τ less than 103 s, has a volume of ≈35 cm3 and operates on ≈450 mW of electrical power. The advanced optical integration presented here demonstrates a key step towards the development of compact optical clocks and the broad dissemination of SI-traceable wavelength references.
Optical atomic clocks provide exceptionally accurate and precise signals for timekeeping and precision measurements, but they require high-power, free-space laser configurations that limit scalability. We introduce and explore a scalable infrastructure for strontium (Sr) optical-lattice clocks that incorporates co-design of atomic-beam slowing and a magneto-optical trap (MOT) from an effusion source, generation of complex, three-dimensional free-space laser configurations with a photonic integrated circuit (PIC) and metasurface (MS) optics, and laser stabilization to a frequency-comb supercontinuum generated with integrated nonlinear photonics. With these elements, we realize MOTs of all stable strontium isotopes (84^{84}84Sr, 86^{86}86Sr, 87^{87}87Sr, 88^{88}88Sr) with populations commensurate with natural abundances, demonstrating precise beam control and robustness. Access to laser-cooled alkaline-earth atoms with scalable integrated photonics enables system engineering for optical clocks, quantum sensing, and quantum information, and our experiments demonstrate extensible technologies that advance toward a Sr optical clock largely free of bulk optics.