Miniaturized optical frequency reference for next-generation portable optical clocks
Vincent N. Maurice, Zachary L. Newman, Susannah M. Dickerson, Morgan Rivers, James Hsiao, Phillip Greene, Mark Mescher, John E. Kitching, Matthew T. Hummon, Cort Johnson
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 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.
We demonstrated a compact 780 nm rubidium optical clock, which includes an optical frequency standard and an optical frequency comb, with an optical volume of 11.6 liters. Unlike the 778 nm rubidium atomic clocks based on two-photon transition, here, the laser frequency is stabilized to the Rb D2 transition, using modulation transfer spectroscopy. This approach effectively eliminates Doppler background and provides a high signal to noise ratio and high sensitivity. A nearly 300 MHz microwave signal, whose phase exactly tracks that of the optical frequency standard, is generated via the optical frequency comb, yielding a frequency instability of 1.91 E-13 @1 s and 5.29 E-14 @1000 s in the electronic domain. To the best of our knowledge, this is the most precise frequency stabilization result for the first-excited-state transition of alkali metal atoms to date and represents the first optical clock based on this transition. These results offer a promising approach for the development of portable optical clocks.
Space-borne optical frequency references based on spectroscopy of atomic vapors may serve as an integral part of compact optical atomic clocks, which can advance global navigation systems or can be utilized for earth observation missions as part of laser systems for cold atom gradiometers. Nanosatellites offer low launch costs, multiple deployment opportunities, and short payload development cycles, enabling rapid maturation of optical frequency references and underlying key technologies in space. Toward an in-orbit demonstration on such a platform, we have developed a CubeSat-compatible prototype of an optical frequency reference based on the D2 transition in rubidium. A frequency instability of 1.7 × 10 − 12 at 1 s averaging time is achieved. The optical module occupies a volume of 35 c m 3 , weighs 73 g, and consumes 780 mW of power.
Compactness, robustness and autonomy of optical frequency references are prerequisites for reliable operation in mobile systems, on ground as well as in space. We present a standalone plug and play optical frequency reference device based on frequency modulation spectroscopy of the D2-transition in rubidium at 780 nm. After a single button press the hand-sized laser module, based on the micro-integrated laser-optical bench described in [ J. Opt. Soc. Am. B 38 , 1885 ( 2021 ) 10.1364/JOSAB.420875 ], works fully autonomous and generates 6 mW of frequency stabilized light with a relative frequency instability of 1.4×10−12 at 1 s and below 10−11 at 105 s averaging time. We describe the design of the device, investigate the thermal characteristics affecting the output frequency and demonstrate short-term frequency stability improvement by a Bayesian optimizer varying the modulation parameters.
Optical wavelength standards, based on the optical transitions of atoms or molecules, provide ultra-high precision frequency references and are the core technology of next-generation optical clocks. Their stability far exceeds that of the microwave frequency range, laying the foundation for precision metrology and cutting-edge scientific research. The wavelength standard device based on the 5S1/2-5D3/2 hyperfine level two-photon transition of 87Rb atoms exhibits significant advantages such as no Doppler broadening, high signal-to-noise ratio (SNR), and potential for miniaturization and chip integration, showing important application potential in precision measurement fields like rubidium atomic clocks and optical communication. In this paper, an experimental setup for two-photon transition was constructed using an external cavity diode laser (ECDL), achieving stable locking of the 778 nm wavelength by modulating the laser current. The effects of the temperature and length of the atomic vapor cell on the blue fluorescence linewidth, SNR, and frequency stability were systematically studied, further optimizing the system. By using an atomic vapor cell designed at Brewster's angle, optical reflection loss was effectively reduced, compensating for the impact of shortened optical path on frequency stability in miniaturized atomic vapor cells. This provides new research directions and performance enhancement solutions for the miniaturization and chip integration of two-photon transition systems. In the improved system, a Brewster-angle glass vapor cell with a length of 30 mm was used and heated to 110°C, achieving a linewidth of 1.69 MHz full width at half maximum (FWHM) and a SNR of 20 dB. The system demonstrated a instability of 9.86×10-11 over a 1-second averaging time and a minimum instability of 5.79×10-11 over a 5.7-second averaging time. This achievement provides key technological support for the practical application of the next generation of miniaturized optical atomic clocks, with the potential for widespread use in portable metrology equipment and space science through integrated design and noise suppression optimization in the future.
Optical clocks represent the hallmark of precise timekeeping. The most precise atomic clocks are laboratory-based ones that utilize transitions in the microwave/optical domain of neutral atoms or trapped ions. These clocks define an international time scales, confirm the consistency of physical constants and stimulate the search for new physics outside the Standard Model. The other area is the regime of portable atomic clocks based on warm atomic vapor, which is small, power-efficient and deployable in the field with a variety of civilian applications, including sensing, communication and navigation. Two-photon resonance and optical transition based field deployable clocks present a viable alternative that may outperform current commercial chip scale clocks by at least a factor of 10 in terms of both long-term and short-term stability. For portable clocks, two-photon transitions in alkaline earth metals are of special interest. One of the systematic inaccuracies in the optical reference is eliminated by using counter-propagating laser beams to create Doppler-free spectroscopy. At IIT Tirupati, we are engaged in developing the next generation of atomic vapor based portable frequency standard using two-photon transition in Rubidium (Rb) for atomic sensing and quantum positioning applications. This article discusses the physics behind the two-photon transition in Rubidium atoms suitable for a clock reference. Briefly, it discusses the experimental methodology for realizing an optical clock using warm rubidium vapor.
Laboratory optical atomic clocks achieve remarkable accuracy (now counted to 18 digits or more), opening possibilities to explore fundamental physics and enable new measurements. However, their size and use of bulk components prevent them from being more widely adopted in applications that require precision timing. By leveraging silicon-chip photonics for integration and to reduce component size and complexity, we demonstrate a compact optical-clock architecture. Here a semiconductor laser is stabilized to an optical transition in a microfabricated rubidium vapor cell, and a pair of interlocked Kerr-microresonator frequency combs provide fully coherent optical division of the clock laser to generate an electronic 22 GHz clock signal with a fractional frequency instability of one part in 10^13. These results demonstrate key concepts of how to use silicon-chip devices in future portable and ultraprecise optical clocks.