Abstract Electro-optic frequency combs (EO combs) enable high spectral and temporal resolution in atomic, molecular, and cavity-based spectroscopy that is at the heart of many quantum sensing applications. However, producing EO combs in the visible range remains challenging due to the limited availability of key components such as narrow-linewidth lasers and electro-optic phase modulators. Rubidium (Rb) is a highly nonlinear material that supports various four-wave mixing (4WM) processes, which are utilized for spectral translation and the generation of squeezed light. Here, we show the spectral translation of a 780 nm EO comb to 420 nm (a spectral separation of 330 THz) via a 4WM process in Rb vapor. We demonstrate the capability to extend the output comb bandwidth and signal-to-noise ratio by varying input power levels. Through this coherent process, we can generate frequency-agile combs in the blue, including a 1 kHz spaced comb with over one million individual comb teeth, of relevance for spectroscopic measurement of narrow-linewidth features such as electromagnetically induced transparency. Leveraging this approach across different 4WM interactions in Rb should allow for further wavelength conversion and the generation of correlated frequency combs, paving the way for sub-shot-noise (squeezed-light) measurements.

We combine nonlinear frequency conversion and electro-optic interferometry in thin-film lithium niobate photonics to demonstrate spectroscopy of multiple alkali atomic vapor species using a telecommunications-band laser.

We demonstrate a Doppler thermometer based on direct optical frequency comb spectroscopy of an ^85Rb vapor with a chirped electro-optic frequency comb (EOFC). The direct EOFC Doppler thermometer is accurate to within its approximately 1 K statistical uncertainty. We experimentally compare direct EOFC spectroscopy with conventional Doppler spectroscopy using a single-frequency, step-scanned laser probe. Our results show that direct EOFC spectroscopy mitigates transit-induced optical pumping distortion of the atomic lineshape, which is the dominant systematic temperature shift in alkali atom Doppler thermometry. Optical Bloch equation simulations of conventional and direct EOFC Doppler spectroscopy confirm that EOFC spectroscopy can use higher optical power to reduce statistical noise without optical pumping distortion. Our results indicate that EOFC Doppler thermometry is a promising approach to realizing a primary thermometer with size and measurement rate sufficient for applications including pharmaceutical manufacturing and nuclear waste monitoring.

We demonstrate Doppler-broadening thermometry based on direct optical frequency comb spectroscopy of an 85Rb vapor with a chirped electro-optic frequency comb (EOFC). The direct EOFC Doppler-broadening thermometry measurements are accurate to within their approximately 1 K statistical uncertainty. We experimentally compare direct EOFC spectroscopy with conventional Doppler spectroscopy using a single-frequency, step-scanned laser probe. Our results show that direct EOFC spectroscopy mitigates transit-induced optical pumping distortion of the atomic lineshape, which is the dominant systematic temperature shift in alkali atom Doppler-broadening thermometry. Optical Bloch equation simulations of conventional and direct EOFC Doppler spectroscopy confirm that EOFC spectroscopy can use higher optical power to reduce statistical noise without optical pumping distortion. Our results indicate that EOFC Doppler-broadening thermometry is a promising approach to realizing a primary thermometer with size and measurement rate sufficient for applications including pharmaceutical manufacturing and nuclear waste monitoring.

Photonic integrated circuits commonly feature visible or near-infrared lasers that are vulnerable to destabilizing back-reflections and must be protected by isolators-non-reciprocal optical components enforcing one-way light propagation. Despite recent progress, high-performance isolators remain bulky off-chip components, while on-chip implementations suffer from challenging fabrication, high optical absorption or narrow optical bandwidth. Here we propose and experimentally demonstrate a magnet-free, intrinsically broadband travelling-wave isolator built from foundry-compatible components. Using radio-frequency electro-optic modulation to create synthetic motion across four parallel waveguides, we realize dynamic rotating destructive interference that continuously cancels backward-propagating light while leaving forward-propagating light unaffected. We reach ~30 dB peak isolation, maintain >24 dB isolation across a 30-nm-wavelength span with thermo-optic adjustment and show >20 dB isolation for two lasers simultaneously within 10 nm without any adjustment. The demonstration's 770-800-nm-wavelength span covers key alkali atomic transitions, enabling on-chip laser isolation for atomic spectroscopy, laser cooling and locking applications. Our isolator approach, applicable from the visible to telecom wavelength spectrum, offers a compelling practical solution, opening the way for fully integrated atomic clocks, quantum sensors, advanced telecommunications and tunable laser systems on a single chip.

The application of microwave frequency combs to the interrogation of a high-overtone bulk acoustic resonator (HBAR) is presented for high-speed measurement of resonances with sub-millisecond time resolution. Unlike conventional approaches such as a phase locked loop (PLL), this method tracks the complete frequency response, providing resonance frequencies, damping coefficients, and line shapes for multiple modes simultaneously and at a much higher speed than traditional approaches, such as vector network analysis (VNA). When applied to mass sensing using an HBAR, it is shown that both frequency shifts and increased damping induced by the application of a drop of isopropyl alcohol (IPA) can be tracked with 0.2 ms time resolution. These are the first published results using microwave frequency combs to excite and measure micromechanical resonators, and this work will enable high-speed and data-rich mass sensing with HBARs for biological, chemical, and process control applications.

We generate electro-optic (EO) frequency combs at 420 nm by translating 780 nm EO combs using a four-wave-mixing process in rubidium vapor. We translate $\approx 5\mathrm{x}10^{4}$ comb teeth across a bandwidth of ≈200 MHz.

Interpreting measured absorption spectroscopy data can require repeated simulations of the expected absorption spectrum to fit the data. In cases of high temperature or broadband spectra, the computational load of the spectral analysis can be expensive due to the large number of individual absorption transitions that contribute to each simulation. We present a Graphics Processing Unit (GPU) Accelerated Absorption Simulator (GAAS) - a fast, hardware-accelerated, line-by-line absorption simulation software for generating absorption spectra based on Voigt and Hartmann-Tran lineshape profiles. We show that GAAS produces the same output spectra as the hi gh- resolution trans mission molecular absorption database (HITRAN) Application Programming Interface (HAPI) to within 32-bits of numerical precision for spectra based on both Voigt and Hartmann-Tran profiles. We also measure the performance increase compared to HAPI and demonstrate that GAAS can reduce simulation time by up to 115x for spectra containing many (several thousand or more) absorption transitions. The software is provided as an open-source python library which is built around an OpenCL implementation of the Voigt and Hartmann-Tran lineshape functions. GAAS can be run on a variety of GPU hardware including integrated GPUs on most computers and high-performance external GPUs. It is installed as a standalone Python library, making it accessible and easy to use for many applications. GAAS will enable researchers to more efficiently analyze complex spectra, especially using advanced lineshapes, to ultimately increase the accuracy of complex spectroscopic measurements.

High-resolution electro-optic frequency combs (EO combs) consisting of thousands to millions of comb teeth across a bandwidth between 1 and 500 GHz are powerful tools for atomic, molecular, and cavity-based spectroscopy, including in the context of deployable quantum sensors. However, achieving sufficiently high signal-to-noise ratio (SNR) EO combs for use across the broad range of wavelengths required in the aforementioned applications is hindered by the corresponding unavailability of relevant components such as narrow-linewidth lasers, electro-optic phase modulators with adequate optical power handling, and low-noise optical amplifiers. Here, we address the latter two points by showing that optical injection locking of commercial Fabry-Perot (FP) laser diodes can help enable high-SNR EO combs. We injection-lock a 780 nm FP laser diode to more than 106 comb teeth at injected comb powers as low as 1 nW and produce a high-SNR replica of the EO comb. In comparison to a commercial semiconductor optical amplifier, injection locking achieves approximate to 100x greater SNR for the same input power (when <1 W) and equal SNR for >35x lower input power. Such low-power injection locking is of particular relevance in conjunction with nanophotonic spectral translation, which extends the range of wavelengths available for EO combs. We show that the usable wavelength range of a 780 nm EO comb produced by photo-induced second harmonic generation of a 1560 nm EO comb in a silicon nitride resonator is significantly increased when combined with optical injection locking. Our results demonstrate that optical injection locking provides a versatile and high-performance approach to addressing many different scenarios in which the EO comb SNR would be otherwise limited.

Optical frequency combs (OFCs) bridge the optical and microwave domains through their repetition rate. While stable repetition rates serve metrology, many applications require dynamic tuning. OFCs are typically not directly actuated but instead controlled indirectly through modifications of the cavity properties housing the OFC. On-chip physical resonator actuation, however, requires complex heterogeneous integration of piezoelectric, electro-optic, or thermal components, which can be challenging for foundry-based mass-fabrication. Here, we present an all-optical alternative to directly modulate an integrated microcomb’s repetition rate using Kerr-induced synchronization (KIS) with a modulated reference laser capturing one comb tooth, without actuating or perturbing the microring resonator. The repetition rate, determined by the main and reference pump frequencies, responds linearly to the reference frequency modulation according to optical frequency division. This enables arbitrary waveform coherent transfer to the OFC repetition rate using only a foundry-fabricated, passive silicon nitride microring resonator. This simplified approach will facilitate tunable microwave synthesis, spectroscopy, and ranging applications.

While Rydberg atoms have shown tremendous potential to serve as accurate and sensitive detectors of microwaves and millimeter waves, their response is generally limited to a single, narrow frequency band around a chosen transition. As a result, their potential to serve as agile and wideband electromagnetic receivers has not been fully realized. Here we demonstrate the use of a mid-infrared, frequency agile optical frequency comb as the coupling laser for three-photon Rydberg atom electrometry. This approach allows for rapid switching between as many as seven individual Rydberg states, allowing for multichannel detection across a frequency range from 1 GHz to 40 GHz. The generality and flexibility of this method for wideband multiplexing is anticipated to have transformative effects in the field of Rydberg electrometry, paving the way for advanced information coding, arbitrary signal detection, and the simultaneous detection of ultra-broadband radiofrequency radiation.

We present a new, to the best of our knowledge, approach for self-heterodyne optical frequency comb (OFC) spectroscopy in which a single Mach-Zehnder modulator is utilized to generate both an optical frequency comb and a frequency-shifted local oscillator. This method allows for coherent, time-domain averaging to be performed without the need for feedback mechanisms or software corrections. As an initial demonstration, we have measured acetylene rovibrational transition frequencies with coherently averaged comb spectra. Spectra averaged for less than 10 s yielded standard fit uncertainties for transition frequencies of less than 150 kHz, with deviations from literature values under 1.1 MHz, limited by pressure uncertainty. This approach has important implications for high-resolution spectroscopy, trace gas detection, and on-chip integration of electro-optic frequency combs.

Optical frequency combs have exhibited tremendous potential for rapid, highly sensitive measurements of atomic and molecular systems. However, there remains a strong need for optical frequency combs to be produced outside of the convenient near-infrared spectral region. Recently we have demonstrated two separate approaches for the spectral translation of electro-optic frequency combs, both of which rely upon optical parametric oscillation (OPO). The first utilizes silicon nitride microrings to perform nanophotonic OPO using $\chi^{(3)}$ nonlinearity [1]. On the other hand, the second approach exploits $\chi^{(2)}$ nonlinearity in a singly resonant, continuous wave OPO to produce high power mid-infrared frequency combs [2].

We injection lock Fabry-Perot lasers to electro-optic frequency combs at a comb power ≤1 n W that is > 1 OOx lower than in semiconductor optical amplification. We use this approach to extend the usability of nanophotonic spectral translation.

We present accurate transition intensities for the R(0) to R(10) manifolds of 12CH4 in the 2v3-band tetradecad region based on independent cavity ring-down spectroscopy and Fourier Transform spectroscopy measurements from three laboratories. Two sets of experiments involved multi-spectrum fits to low-pressure spectra acquired at Doppler-limited conditions which greatly reduced complications from effects caused by collisional broadening and line mixing. In a third experiment, cavity ring-down spectroscopy measurements of lineshapes made at elevated pressures were used constrain residual collisional broadening effects in the low-pressure spectra, thereby improving measurement precision. Intensities are reported for manifolds and specified multiplets comprising blended transitions, as well as for individual transitions. Summation of the manifold and multiplet intensities agree at the permille level, whereas highly blended individual transitions agree at the several permille level. For measurements of methane, the intensities reported here will enable more accurate spectroscopic determinations of amount-of-substance, and will support progress in remote sensing missions and future refinements of its other spectroscopic parameters.

An outstanding challenge for deployable quantum technologies is high-resolution laser spectroscopy at the specific wavelengths of ultranarrow transitions in atomic and solid-state quantum systems. Here we demonstrate a highly flexible approach to high-resolution spectroscopy for quantum technologies across a broad range of wavelengths, through the synergistic combination of fine-tooth electro-optic frequency combs and efficient Kerr nonlinear nanophotonics. We show that such fine-tooth combs, which provide simultaneous high spectral and temporal resolution in atomic spectroscopy, undergo coherent spectral translation with essentially no efficiency loss through third-order optical parametric oscillation (OPO) in a silicon-nitride microring. This enables nearly a million comb pump teeth, separated by a 1 kHz spacing, to be translated onto signal and idler beams that can be located across a broad range of wavelengths in the visible and short near-infrared. The generated wavelengths are subject to OPO phase and frequency-matching conditions that are highly controllable through nanophotonic dispersion engineering, and in the current implementation span between 589 and 1,150 nm, with both the electro-optic comb generation process and its spectral translation not introducing appreciable broadening to the pump laser linewidth. We further demonstrate the application of this approach to quantum systems by performing sub-Doppler spectroscopy of the hyperfine transitions of Cs atomic vapour with our electro-optically driven Kerr nonlinear light source. The generality, robustness and agility of our approach, as well as its compatibility with photonic integration, are expected to lead to its widespread applications in areas such as quantum sensing, telecommunications and atomic clocks. A nonlinear nanophotonic resonator is used to spectrally translate an electro-optic frequency comb to a controllable set of wavelengths between 600 nm and 1,050 nm, with comb properties that are advantageous for high-resolution spectroscopy preserved.

Here we show that an electro-optic frequency comb can be used as the pump laser in a nanophotonic optical parametric oscillator, allowing for efficient and accurate spectral translation through much of the visible and near-infrared.

Sensing platforms based upon photonic integrated circuits have shown considerable promise, however they require corresponding advancements in integrated optical readout technologies.Here, we present an on-chip spectrometer that leverages an integrated thin-film lithium niobate modulator to produce a frequency-agile electro-optic frequency comb for interrogating chipscale temperature and acceleration sensors.The chirped comb process allows for ultralow radiofrequency drive voltages, which are as much as seven orders of magnitude less than the lowest found in the literature and are generated using a chip-scale, microcontroller-driven direct digital synthesizer.The on-chip comb spectrometer is able to simultaneously interrogate both the on-chip temperature sensor and an off-chip, microfabricated optomechanical accelerometer with cutting-edge sensitivities of ≈ 5 µK•Hz -1/2 and ≈ 130 µm•s -2 •Hz -1/2 , respectively.This platform is compatible with a broad range of existing photonic integrated circuit technologies, where its combination of frequency agility and ultralow radiofrequency power requirements are expected to have applications in fields such as quantum science and optical computing.Photonic integrated circuit (PIC) technologies hold tremendous potential for low cost, high accuracy field-deployable sensing.However, unlocking these capabilities requires chip-scale integration of not only the sensors but also the optical readout.Chip-scale optical frequency combs are well suited to these photonic readout demands due to their capability for high speed, multiplexed measurements without the need for any moving parts, [1] thus allowing for transduction of the photonic sensor to a digital output.In particular, electro-optic frequency combs can not only be integrated, but also are capable of having sufficient frequency agility to achieve the high resolution required to probe atomic transitions as well as optical (and optomechanical) cavity-based sensors, where a measurement of the cavity motion is required to read out the sensor.[2,3] These type of measurements typically require narrow comb tooth spacings at the MHz level and comb spans at the GHz level, leading to a sensitive and high dynamic range readout.Recent advancements in thin-film lithium niobate (TFLN) technology have enabled the development of compact on-chip electro-optic modulators (EOMs) with > 100 GHz modulation bandwidth [4-7] and half-wave voltages, Vπ, which outperform traditional bulk lithium niobate EOMs.[8] When electro-optic frequency comb spectroscopy is integrated with on-chip EOMs, it becomes a scalable platform for multiplexed solid-state on-chip spectroscopy [9-17].However, these previous integrated electro-optic comb generation approaches have relied upon high radiofrequency drive powers between 0.3 W to 4 W (25 dBm to 36 dBm) and wide comb tooth spacings (generally in the GHz) which restrict their application for sensing applications.

We show that the use of an optical frequency comb probe leads to dramatically improved bandwidth (as high as 12+/-1 MHz) for the detection of modulated radio frequencies in Rydberg atom-based electrometry.