High-performance, compact optical standard

Exploration of a vapor cell optical frequency standard scheme implemented using Doppler-free spectroscopy

We implement a compact optical frequency standard scheme with laser frequency locked to the 5S1/2 (F = 2) - 6P3/2 (F' = 3) transition of the second excited state of 87Rb atoms in a 3 mm cubic glass cell, using a Doppler-free saturated absorption spectroscopy. The experimental results show the frequency stability at the level of 2.2 × 10-12 at 1 s. Furthermore, we conduct an experimental study on the effect of a repump laser on the frequency performance of the saturated absorption spectroscopy optical frequency standard, providing valuable experimental results with reference values for implementing this type of optical atomic clock.

Atomic frequency standard based on direct frequency comb spectroscopy

We have developed a high performance atomic frequency standard based on Doppler-free direct frequency comb excitation of a two-photon transition in 87Rb. We demonstrate equivalent performance compared to an identical system based on cw laser excitation of the clock transition. This approach greatly simplifies optical clock architecture and eliminates the need for cw lasers in many two-photon frequency standards. The clock transition linewidth and ac-Stark shift measured directly with the frequency comb are shown to be nearly identical to those obtained using a cw laser of equal average power. We directly count this reference via the frequency comb repetition rate, achieving instabilities of 1.9 × 10-13 at 1 s averaging down to 7.8(38) × 10-15 at 2600 s, currently limited by temperature dependent shifts.

Ultraviolet astronomical spectrograph calibration with laser frequency combs from nanophotonic lithium niobate waveguides

Astronomical precision spectroscopy underpins searches for life beyond Earth, direct observation of the expanding Universe and constraining the potential variability of physical constants on cosmological scales. Laser frequency combs can provide the required accurate and precise calibration to the astronomical spectrographs. For cosmological studies, extending the calibration with such astrocombs to the ultraviolet spectral range is desirable, however, strong material dispersion and large spectral separation from the established infrared laser oscillators have made this challenging. Here, we demonstrate astronomical spectrograph calibration with an astrocomb in the ultraviolet spectral range below 400 nm. This is accomplished via chip-integrated highly nonlinear photonics in periodically-poled, nano-fabricated lithium niobate waveguides in conjunction with a robust infrared electro-optic comb generator, as well as a chip-integrated microresonator comb. These results demonstrate a viable route towards astronomical precision spectroscopy in the ultraviolet and could contribute to unlock the full potential of next-generation ground-based and future space-based instruments.

Wafer-scale fabrication of evacuated alkali vapor cells

We describe a process for fabricating a wafer-scale array of alkali metal vapor cells with low residual gas pressure. We show that by etching long, thin channels between the cells on the Si wafer surface, the residual gas pressure in the evacuated vapor cell can be reduced to below 0.5 kPa (4 Torr) with a yield above 50%. The low residual gas pressure in these mass-producible alkali vapor cells can enable a new generation of low-cost chip-scale atomic devices such as vapor cell optical clocks, wavelength references, and Rydberg sensors.

Sub-Doppler spectroscopy of the Cs atom 6S1/2-7P1/2 transition at 459 nm in a microfabricated vapor cell

We report on the characterization of sub-Doppler resonances detected by probing the 6S1/2 - 7P1/2 transition of the Cs atom at 459 nm in a microfabricated vapor cell. The dependence of the sub-Doppler resonance (linewidth, amplitude) on some key experimental parameters, including the laser intensity and the cell temperature, is investigated. These narrow atomic resonances are of interest for high-resolution spectroscopy and instrumentation and may constitute the basis of a high-stability microcell optical standard.

Optical clocks at sea

Deployed optical clocks will improve positioning for navigational autonomy1, provide remote time standards for geophysical monitoring2 and distributed coherent sensing3, allow time synchronization of remote quantum networks4,5 and provide operational redundancy for national time standards. Although laboratory optical clocks now reach fractional inaccuracies below 10-18 (refs. 6,7), transportable versions of these high-performing clocks8,9 have limited utility because of their size, environmental sensitivity and cost10. Here we report the development of optical clocks with the requisite combination of size, performance and environmental insensitivity for operation on mobile platforms. The 35 l clock combines a molecular iodine spectrometer, fibre frequency comb and control electronics. Three of these clocks operated continuously aboard a naval ship in the Pacific Ocean for 20 days while accruing timing errors below 300 ps per day. The clocks have comparable performance to active hydrogen masers in one-tenth the volume. Operating high-performance clocks at sea has been historically challenging and continues to be critical for navigation. This demonstration marks a significant technological advancement that heralds the arrival of future optical timekeeping networks.

Two-photon rubidium clock detecting 776 nm fluorescence

The optical atomic clock based on the 5S1/2 → 5D5/2 two-photon transition in rubidium is a candidate for a next generation, manufacturable, portable clock that fits in a small size, weight, and power (SWaP) envelope. Here, we report the first two-photon rubidium clock stabilized by detecting 776 nm fluorescence. We also demonstrate the use of a multi-pixel photon counter as a low voltage substitute to a photomultiplier tube in the feedback loop to the clock laser.

Dual-interrogation method for suppressing light shift in Rb 778 nm two-photon transition optical frequency standard

In this study, a dual-interrogation (DI) method was used to suppress the light shift in the Rb 778 nm 5S1/2→5D5/2 two-photon transition optical frequency standard (2hν-OFS). The approach used an auxiliary system to calibrate the light shift of the primary system in real time to mitigate the absolute light shift and suppress the sensitivity of the system to the light power. Results show that after using the DI method, the absolute light shift and light-power sensitivity of the system were reduced by a factor of 10. The proposed method will improve the accuracy of the Rb 778 nm 2hν-OFS and increase the mid- and long-term stability. The method can also be applied to other vapor-cell atomic frequency standards that experience light shifts.

Measurement of the 5S1/2 to 5D5/2 two-photon clock transition frequency of rubidium-85 in high vacuum

We present a scheme to precisely resolve the unperturbed line shape of an optical rubidium clock transition in a high vacuum, by which we avoided the systematic errors of "collision shift" and "modulation shift." The spectral resolution resolved by this scheme is significantly improved such that we can use "Zeeman broadening" to inspect the stray magnetic field, through which we were able to compensate the magnetic field inside the Rb cells to be below 10-3 Gauss. We thus update the absolute frequency of the clock transition and propose a standard operation procedure (SOP) for the clock self-calibration.

Fabrication and characterization of iodine photonic microcells for sub-Doppler spectroscopy applications

We report on the development of all-fiber stand-alone iodine-filled photonic microcells demonstrating record absorption contrast at room temperature. The microcell's fiber is made of inhibited coupling guiding hollow-core photonic crystal fibers. The fiber-core loading with iodine was undertaken at 10^-1-10^-2mbar vapor pressure using what, to the best of our knowledge, is a novel gas-manifold based on metallic vacuum parts with ceramic coated inner surfaces for corrosion resistance. The fiber is then sealed on the tips and mounted on FC/APC connectors for better integration with standard fiber components. The stand-alone microcells display Doppler lines with contrasts up to 73% in the 633 nm wavelength range, and an off-resonance insertion loss between 3 to 4 dB. Sub-Doppler spectroscopy based on saturable absorption has been carried out to resolve the hyperfine structure of the P(33)6-3 lines at room temperature with a full-width at half maximum of 24 MHz on the b4 component with the help of lock-in amplification. Also, we demonstrate distinguishable hyperfine components on the R(39)6-3 line at room temperature without any recourse to signal-to-noise ratio amplification techniques.

Accurate evaluation of self-heterodyne laser linewidth measurements using Wiener filters

Self-heterodyne beat note measurements are widely used for the experimental characterization of the frequency noise power spectral density (FN-PSD) and the spectral linewidth of lasers. The measured data, however, must be corrected for the transfer function of the experimental setup in a post-processing routine. The standard approach disregards the detector noise and thereby induces reconstruction artifacts in the reconstructed FN-PSD. We introduce an improved post-processing routine based on a parametric Wiener filter that is free from reconstruction artifacts, provided a good estimate of the signal-to-noise ratio is supplied. Building on this potentially exact reconstruction, we develop a new method for intrinsic laser linewidth estimation that is aimed at deliberate suppression of unphysical reconstruction artifacts. Our method yields excellent results even in the presence of strong detector noise, where the intrinsic linewidth plateau is not even visible using the standard method. The approach is demonstrated for simulated time series from a stochastic laser model including 1/f-type noise.

Cs microcell optical reference with frequency stability in the low 10-13 range at 1 s

We describe a high-performance optical frequency reference based on dual-frequency sub-Doppler spectroscopy (DFSDS) using a Cs vapor microfabricated cell and an external-cavity diode laser at 895 nm. Measured against a reference optical signal extracted from a cavity-stabilized laser, the microcell-stabilized laser demonstrates an instability of 3 × 10^-13 at 1 s, in agreement with a phase noise of +40 dBrad²/Hz at 1-Hz offset frequency, and below 5 × 10^-14 at 10² s. The laser short-term stability limit is in good agreement with the intermodulation effect from the laser frequency noise. These results suggest that DFSDS is a valuable approach for the development of ultra-stable microcell-based optical standards.

Laser frequency stabilization in the 10 -14 range via optimized modulation transfer spectroscopy on the 87Rb D2 line

We present a high-performance laser frequency stabilization method using modulation transfer spectroscopy (MTS) on the rubidium ⁸⁷D₂ transition line. A substantial improvement of the laser frequency stability was achieved by searching for the optimal diameter and intensity settings of the probe and pump beam. The frequency instability measured from the beat frequency of two locked external cavity diode lasers (ECDLs) reached a short-term stability of 4.5×10^-14/τ and did not exceed 2 × 10^-12 until 10⁵ s, which is the best performance reported thus far with a D₂ transition. The long-term stability is limited by the offset fluctuations of the baseline induced by the residual amplitude modulation (RAM), which can be further improved by reducing the current temperature variation of about 0.2 K by means of temperature stabilization or through a further reduction of the RAM.

Micro-fabricated components for cold atom sensors

Laser cooled atoms have proven transformative for precision metrology, playing a pivotal role in state-of-the-art clocks and interferometers and having the potential to provide a step-change in our modern technological capabilities. To successfully explore their full potential, laser cooling platforms must be translated from the laboratory environment and into portable, compact quantum sensors for deployment in practical applications. This transition requires the amalgamation of a wide range of components and expertise if an unambiguously chip-scale cold atom sensor is to be realized. We present recent developments in cold-atom sensor miniaturization, focusing on key components that enable laser cooling on the chip-scale. The design, fabrication, and impact of the components on sensor scalability and performance will be discussed with an outlook to the next generation of chip-scale cold atom devices.

Compact plug and play optical frequency reference device based on Doppler-free spectroscopy of rubidium vapor

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. B38, 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.

Measurement of Optical Rubidium Clock Frequency Spanning 65 Days

Optical clocks are emerging as next-generation timekeeping devices with technological and scientific use cases. Simplified atomic sources such as vapor cells may offer a straightforward path to field use, but suffer from long-term frequency drifts and environmental sensitivities. Here, we measure a laboratory optical clock based on warm rubidium atoms and find low levels of drift on the month-long timescale. We observe and quantify helium contamination inside the glass vapor cell by gradually removing the helium via a vacuum apparatus. We quantify a drift rate of 4×10-15/day, a 10 day Allan deviation less than 5×10-15, and an absolute frequency of the Rb-87 two-photon clock transition of 385,284,566,371,190(1970) Hz. These results support the premise that optical vapor cell clocks will be able to meet future technology needs in navigation and communications as sensors of time and frequency.