Frequency Comb Spectroscopy Research Articles

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.

Design and implementation of a new apparatus for astrochemistry: Kinetic measurements of the CH + OCS reaction and frequency comb spectroscopy in a cold uniform supersonic flow.

We present the development of a new astrochemical research tool, HILTRAC, the Highly Instrumented Low Temperature ReAction Chamber. The instrument is based on a pulsed form of the CRESU (Cinétique de Réaction en Écoulement Supersonique Uniforme, meaning reaction kinetics in a uniform supersonic flow) apparatus, with the aim of collecting kinetics and spectroscopic information on gas phase chemical reactions important in interstellar space or planetary atmospheres. We discuss the apparatus design and its flexibility, the implementation of pulsed laser photolysis followed by laser induced fluorescence, and the first implementation of direct infrared frequency comb spectroscopy (DFCS) coupled to the uniform supersonic flow. Achievable flow temperatures range from 32(3) to 111(9) K, characterizing a total of five Laval nozzles for use with N2 and Ar buffer gases by impact pressure measurements. These results were further validated using LIF and direct frequency comb spectroscopy measurements of the CH radical and OCS, respectively. Spectroscopic constants and linelists for OCS are reported for the 1001 band near 2890-2940cm-1 for both OC32S and OC34S, measured using DFCS. Additional peaks in the spectrum are tentatively assigned to the OCS-Ar complex. The first reaction rate coefficients for the CH + OCS reaction measured between 32(3) and 58(5) K are reported. The reaction rate coefficient at 32(3) K was measured to be 3.9(4) × 10-10cm3molecule-1s-1 and the reaction was found to exhibit no observable temperature dependence over this low temperature range.

Single-modulator, dual comb serrodyne spectroscopy.

Dual optical frequency comb spectroscopy allows for high speed, broadband measurements without any moving parts. Here, we combine differential chirp downconversion to probe large spectral bandwidths and serrodyne modulation to separate the positive and negative sidebands in a single modulator. As an initial demonstration, we apply this approach to measure a sharp cavity resonance to illustrate the system performance. We then measure methane transitions in the near-infrared and compare the resulting spectra to models based upon the current spectroscopic databases. The serrodyne method has lower hardware requirements compared to many existing approaches, and its simplicity enables a high degree of mutual coherence between the two combs. Further, this method is readily amenable to chip-scale photonic integration.

Measurement of the linewidth of a home-built vacuum ultraviolet comb by frequency comb spectroscopy on NO2.

Optical frequency comb in the vacuum ultraviolet (VUV)/extreme ultraviolet (XUV) region has attracted a great deal of attention, as it provides coherent VUV/XUV radiation source with a rather narrow bandwidth, facilitating precise spectroscopic measurements in the short wavelength regime. In this study, we report on the linewidth measurement of a home-built VUV comb centered at 148 nm using direct frequency comb spectroscopy with NO2. The measurement reveals that the upper bound of our comb linewidth is less than 28 MHz. Fitting the whole trace with different repetition rates shows that the center frequency of the excitation is 2 021.25 ± 0.24 THz (∼148.32 nm). Thus, we assigned this excitation to the transition from the 6a1 orbital (ν1'=0, ν2'=0) to the 3pσu orbital (ν1'=3, ν2'=8) in NO2. Our work demonstrates that VUV combs are potentially powerful tools for precision spectroscopic measurements in the short wavelength regime.

Laser frequency stabilization for continuous-wave laser enhanced direct frequency comb spectroscopy system

We present the observation of high-precision direct frequency comb spectroscopy excited by an optical frequency comb and a diode laser when each of them drives one step of the two-photon transition in a rubidium vapor system. We demonstrate a stable and low noise system by directly locking the frequency of the continuous-wave laser to the rubidium two-photon transition. The frequency stability of a diode laser via the two-photon transition locking technique is 8 × 10−11 for a 1 s gate time and 3 × 10−12 for 1000 s. It proves to be a potential technique for locking the diode laser with high stability. We chose a suitable optical frequency comb pulse and the frequency of the diode laser to fulfill the double-resonance condition. These techniques eliminate spectrum line overlap and would benefit spectroscopy measurements.

Intra-oscillator high harmonic source reaching 100-eV photon energy

Resonant enhancement inside an optical cavity has been a wide-spread approach to increase efficiency of nonlinear optical conversion processes while reducing the demands on the driving laser power. This concept has been particularly important for high harmonic generation XUV sources, where passive femtosecond enhancement cavities allowed significant increase in repetition rates required for applications in photoelectron spectroscopy, XUV frequency comb spectroscopy, including the recent endeavor of thorium nuclear clock development. In addition to passive cavities, it has been shown that comparable driving conditions can be achieved inside mode-locked thin-disk laser oscillators, offering a simplified single-stage alternative. This approach is less sensitive to losses thanks to the presence of gain inside the cavity and should thus allow higher conversion efficiencies through tolerating higher intensity in the gas target. Here, we show that the intra-oscillator approach can indeed surpass the much more mature technology of passive enhancement cavities in terms of XUV flux, even reaching comparable values to single-pass sources based on chirped-pulse fiber amplifier lasers. Our system operates at 17 MHz repetition rate generating photon energies between 60 eV and 100 eV. Importantly, this covers the highly attractive wavelength for the silicon industry of 13.5 nm at which our source delivers 60 nW of outcoupled average power per harmonic order.

Rapid generation of massive thermodynamic datasets using frequency comb spectroscopy.

We demonstrate massively parallel spectroscopic measurements of 12C2H2 using an optical frequency comb. This allows for the rapid and simultaneous estimation of self-broadening and self-shifting of more than 50 optical transitions between 1512 and 1538nm. The use of a temperature-controlled sealed gas cell allows us to measure both pressure- and temperature-mediated broadening and shifting. We present the results for the pressure-mediated self-broadening and self-shifting coefficients for 59 optical lines that make up the v1 + v3 combination band and a selection of hot bands. Our ability to measure the broadening of numerous transitions allows for the confirmation of prior work that shows that there is no measurable vibrational dependence across all acetylene bands, despite the strong dependence of the broadening coefficient on the rotational number. We also present an extensive measurement of the temperature dependence of the self-broadening for each of these 59 lines. This work shows the revolutionary power afforded by the frequency combs for rapid generation of large datasets related to thermodynamic variations of the key spectroscopic parameters of important gases.

Mid-infrared optical frequency comb spectroscopy using an all-silica antiresonant hollow-core fiber.

We present the first mid-infrared optical frequency comb spectrometer employing an absorption cell based on self-fabricated, all-silica antiresonant hollow-core fiber (ARHCF). The spectrometer is capable of measuring sub-mL sample volumes with 26 m interaction length and noise equivalent absorption sensitivity of 8.3 × 10-8 cm-1 Hz-1/2 per spectral element in the range of 2900 cm-1 to 3100 cm-1. Compared to a commercially available multipass cell, the ARHCF offers a similar interaction length in a 1000 times lower gas sample volume and a 2.8 dB lower transmission loss, resulting in better absorption sensitivity. The broad transmission windows of ARHCFs, in combination with a tunable optical frequency comb, make them ideal for multispecies detection, while the prospect of measuring samples in small volumes makes them a competitive technique to photoacoustic spectroscopy along with the robustness and prospect of coiling the ARHCFs open doors for miniaturization and out-of-laboratory applications.

Direct sampling of femtosecond electric-field waveforms from an optical parametric oscillator

Detecting the electric-field waveform of an optical pulse from the terahertz to the visible spectral domain provides a complete characterization of the average field waveform and holds great potential for quantum optics, time-domain (including frequency-comb) spectroscopy, high-harmonic generation, and attosecond science, to name a few. The field-resolved measurements can be performed using electro-optic sampling, where a laser pulse is characterized through an interaction with another pulse of a much shorter duration. The measured pulse train must consist of identical pulses, including their equal carrier-envelope phase (CEP). Due to the limited coverage of broadband laser gain media, creating CEP-stable pulse trains in the mid-infrared typically requires nonlinear frequency conversion, such as difference frequency generation, optical parametric amplification, or optical rectification. These techniques operate in a single-pass geometry, often limiting efficiency. In this work, we demonstrate field-resolved analysis of the pulses generated in a resonant system, an optical parametric oscillator (OPO). Due to the inherent feedback, this device exhibits a relatively high conversion efficiency at a given level of input power. By electro-optic sampling, we prove that a subharmonic OPO pumped with CEP-stable few-cycle fiber-laser pulses generates a CEP-stable mid-infrared output. The full amplitude and phase information renders dispersion control straightforward. We also confirm the existence of an exotic “flipping” state of the OPO directly in the time domain, where the electric field of consecutive pulses has the opposite sign.

Comb spectroscopy of CO2 produced from microbial metabolism.

We have developed a direct frequency comb spectroscopy instrument, which we have tested on Saccharomyces cerevisiae (baker's yeast) by measuring its CO2 output and production rate as we varied the environmental conditions, including the amount and type of feed sugar, the temperature, and the amount of yeast. By feeding isotopically-enhanced sugar to the yeast, we demonstrate the capability of our device to differentiate between two isotopologues of CO2, with a concentration measurement precision of 260 ppm for 12C16O2 and 175 ppm for 13C16O2. We also demonstrate the ability of our spectrometer to measure the proportion of carbon in the feed sugar converted to CO2, and estimate the amount incorporated into the yeast biomass.

Near-Infrared Off-Axis Cavity-Enhanced Optical Frequency Comb Spectroscopy for CO2/CO Dual-Gas Detection Assisted by Machine Learning.

Cavity-enhanced direct frequency comb spectroscopy (CE-DFCS) is widely used as a highly sensitive gas sensing technology in various gas detection fields. For the on-axis coupling incidence scheme, the detection accuracy and stability are seriously affected by the cavity-mode noise, and therefore, stable operation inevitably requires external electronic mode-locking and sweeping devices, substantially increasing system complexity. To address this issue, we propose off-axis cavity-enhanced optical frequency comb spectroscopy from both theoretical and experimental aspects, which is applied to the detection of single- and dual-gas of carbon monoxide (CO) and carbon dioxide (CO2) in the near-infrared. An erbium-doped fiber frequency comb with a repetition frequency of ∼41.709 MHz is coupled into a resonant cavity with a length of ∼360 mm in an off-axis manner, exciting numerous high-order modes to effectively suppress cavity-mode noise. The performance of multiple machine learning models is compared for the inversion of a single/dual gas concentration. A few absorbance spectra are collected to build a sample data set, which is then utilized for model training and learning. The results demonstrate that the Particle Swarm Optimization Support Vector Machine (PSO-SVM) model achieves the highest predictive accuracy for gas concentration and is ultimately applied to the detection system. Based on Allan deviation, the detection limit for CO in single-gas detection can reach 8.247 parts per million by volume (ppmv) by averaging 87 spectra. Meanwhile, for simultaneous CO2/CO measurement with highly overlapping absorbance spectra, the LoD can be reduced to 13.196 and 4.658 ppmv, respectively. The proposed optical gas sensing technique indicates the potential for the development of a field-deployable and intelligent sensor system capable of simultaneous detection of multiple gases.

Optical Microcavities Empowered Biochemical Sensing: Status and Prospects

Optical microcavities are compact structures that confine resonant photons in microscale dimensions for long periods of time, greatly enhancing light–matter interactions. Plentiful and profound physical mechanisms within these microcavities or functional microcavities have been extensively explored, including mode shift/splitting/broadening, lasing and gain enhancements, surface plasmon resonance, fluorescence resonance energy transferring, optical frequency comb spectroscopy, optomechanical interaction, and exceptional point. The versatility in design and the diverse range of materials, particularly composites involving metals and 2-dimensional materials, have paved a way for innovative approaches and improved performance in biochemical sensing applications. Leveraging the advantages ranging from miniaturization, high sensitivity, rapid response, and inherent stability, optical microcavity-based biochemical sensors have emerged to address the growing and increasingly complex demands of biochemical detection. This review commences with an exploration of fundamental mechanisms and structures and then delves into typical applications in recent advancements, covering the detection of biomacromolecules, cells, solid particles, liquid ions, and gas molecules. This review also culminates with a forward-looking perspective, highlighting future development trends and crucial research directions.

Tunable off-resonant Rydberg microwave frequency comb spectroscopy based on metawaveguide coupled Rydberg atoms

Tunable off-resonant Rydberg microwave frequency comb spectroscopy based on metawaveguide coupled Rydberg atoms

GPU-enabled real-time optical frequency comb spectroscopy and a photonic readout.

We describe a GPU-enabled approach for real-time optical frequency comb spectroscopy in which data is recorded, Fourier transformed, normalized, and fit at data rates up to 2.2 GB/s. As an initial demonstration we have applied this approach to rapidly interrogate the motion of an optomechanical accelerometer through the use of an electro-optic frequency comb. We note that this approach is readily amenable to both self-heterodyne and dual-comb spectrometers for molecular spectroscopy as well as a photonic readout where the approach's agility, speed, and simplicity are expected to enable future improvements and applications.

High-resolution frequency-comb spectroscopy with electro-optic sampling and instantaneous octave-wide coverage across mid-IR to THz at a video rate

Ultrabroadband electro-optic sampling using few-optical-cycle probing pulses is a sensitive technique to detect electric field amplitudes with a high dynamic range and up to near-infrared optical frequencies. By combining this method with dual-frequency-comb spectroscopy and using a new class of ultrafast lasers, we perform high-resolution, 80 MHz/0.0027 cm−1 (10 MHz/0.0003 cm−1 with spectral interleaving), spectroscopic measurements in the frequency range 1.5–45 THz (6.6–200 µm), excluding the strongly absorbing Reststrahlen band of lattice resonances at 4.5–9 THz, with an instantaneous spectral coverage exceeding an octave (e.g., 9–22 μm). As a driving source, we use a pair of mutually coherent combs from Kerr-lens mode-locked solid-state Cr:ZnS (2.35 μm) lasers. One of the combs is frequency downconverted via intrapulse difference frequency generation to produce a longwave “sensing” comb, while the second comb is frequency doubled to produce a near-IR “probe” comb for electro-optic sampling (EOS). The low intensity and phase noise of our dual-comb system allow for capturing a large amount of spectral information (200 000 comb-mode-resolved spectral lines spaced by 80 MHz) in the mid-IR portion of the spectrum at a video rate of 69 Hz, with the signal-to-noise ratio limited by the shot noise of the near-IR EOS balanced detection system. Our dual-comb spectroscopy measurements with low-pressure gaseous ethanol, isoprene, and dimethyl sulfide reveal Doppler-limited spectroscopic signatures that have never been explored before.

Ultrafast Yb:YAG laser oscillator with gigahertz repetition rate.

We present a SESAM modelocked Yb:YAG solid-state laser providing low-noise narrowband pulses with a pulse duration of 606 fs at a 1.09-GHz repetition rate, delivering up to 2.5 W of average output power. This laser provides access to a new parameter space that could previously not be reached by solid-state lasers and, to the best of our knowledge, is the first modelocked solid-state Yb:YAG laser in the gigahertz regime. This is achieved by introducing a single additional intracavity element, specifically a nonlinear birefringent YVO4 crystal, for soliton formation, polarization selection, and cavity intensity clamping. The isotropic pump absorption in Yb:YAG allows for stable and low-noise operation with multimode fiber pumping. This laser is ideally suited as a seed source for many commercial high-power Yb-doped amplification systems operating at a center wavelength around 1.03 µm. The laser exhibits a high power per comb line of 5.0 mW which also makes it interesting for applications in frequency comb spectroscopy, especially if it is used to pump an optical parametric oscillator. We measure a relative intensity noise (RIN) of 0.03%, integrated from 1 Hz to 10 MHz. Furthermore, we show that the laser timing jitter for noise frequencies >2 kHz is fully explained by a power-dependent shift in the center wavelength of 0.38 nm/W due to the quasi-three-level laser gain material. The narrow gain bandwidth of Yb:YAG reduces this contribution to noise in comparison to other SESAM modelocked Yb-doped lasers.

Mid-infrared virtually imaged phased array spectrometer with optical frequency comb: fast thermometry over large dynamic range

We demonstrate a broadband mid-infrared frequency comb spectrometer utilizing virtually imaged phased array (VIPA) with the potential of quantitative, fast thermometry over large dynamic range. A modified Boltzmann plot method for attaining quantitative spectral interpretation in the dispersive VIPA system is proposed and realized, despite the broadband spectral overlapping and spatially variant instrumental broadening. The developed method allows accurate interpretation of the comb spectra even without reaching single-comb-tooth resolution, and can be readily extended to other measurements with overlapped spectra. The method is validated with measurements of the v 3 band of methane, and a single-shot temperature accuracy of within 10 K is experimentally demonstrated over the range of 300–600 K at a rate of 200 Hz. The present method takes full benefit of the unique advantages of the high-resolution, broadband VIPA-based frequency comb spectroscopy and has the potential to offer fast and precise thermometry. As far as we know, this is the first study to use a mid-infrared VIPA for temperature measurement with frequency comb spectroscopy.

Analysis of Gas Mixtures with Broadband Dual Frequency Comb Spectroscopy and Unsupervised Learning Neural Network

Broadband mid‐infrared spectroscopy not only offers supreme sensitivity for the massively parallel detection of trace gases but also presents many challenges. Herein, a new platform combining the advantages of a mid‐infrared dual‐comb spectrometer based on two difference‐frequency generation combs pumped by femtosecond Er‐doped fiber comb oscillators and an unsupervised deep learning neural network consisting of information extraction and information mapping blocks is presented. The scarce data problem, the uncertainties of apparatus, and manual operations intrinsic to multicomponent gas mixture analysis are overcome by coupling an unsupervised leaning approach with a model‐agnostic, physics‐informed data augmentation strategy using simulated data from spectral databases. The system provides reliable simultaneous identification of gas species, concentration retrieval, as well as ambient pressure prediction and eliminates the negative impacts on the measurement, such as model error, baseline fluctuation, and unknown absorbers. Parallel optical detection of 31 different mixtures of 5 gas species over a 2900–3100 cm−1 spectral range with a sub part‐per‐billion sensitivity is demonstrated showing the potential in various applications such as atmospheric monitoring, diagnostics with breath biomarkers, and capturing rapid chemical reaction kinetics.

Removing biases in dual frequency comb spectroscopy due to digitizer nonlinearity.

Operation of any dual-comb spectrometer requires digitization of the interference signal before further processing. Nonlinearities in the analog-to-digital conversion can alter the apparent gas concentration by multiple percent, limiting both precision and accuracy of this technique. This work describes both the measurement of digitizer nonlinearity and the development of a model that quantitatively describes observed concentration bias over a range of conditions. We present hardware methods to suppress digitizer-induced bias of concentration retrievals below 0.1%.

In-line synthesis of multi-octave phase-stable infrared light.

Parametric downconversion driven by modern, high-power sources of 10-fs-scale near-infrared pulses, in particular intrapulse difference-frequency generation (IPDFG), affords combinations of properties desirable for molecular vibrational spectroscopy in the mid-infrared range: broad spectral coverage, high brilliance, and spatial and temporal coherence. Yet, unifying these in a robust and compact radiation source has remained a key challenge. Here, we address this need by employing IPDFG in a multi-crystal in-line geometry, driven by the 100-W-level, 10.6-fs pulses of a 10.6-MHz-repetition-rate, nonlinearly post-compressed Yb:YAG thin-disk oscillator. Polarization tailoring of the driving pulses using a bichromatic waveplate is followed by a sequence of two crystals, LiIO3 and LiGaS2, resulting in the simultaneous coverage of the 800-cm-1-to-3000-cm-1 spectral range (at -30-dB intensity) with 130 mW of average power. We demonstrate that optical-phase coherence is maintained in this in-line geometry, in theory and experiment, the latter employing ultra-broadband electro-optic sampling. These results pave the way toward coherent spectroscopy schemes like field-resolved and frequency-comb spectroscopy, as well as nonlinear, ultrafast spectroscopy and optical-waveform synthesis across the entire infrared molecular fingerprint region.