Cavity-enhanced Direct Frequency Comb Spectroscopy Research Articles

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.

Breath analysis by ultra-sensitive broadband laser spectroscopy detects SARS-CoV-2 infection

Rapid testing is essential to fighting pandemics such as coronavirus disease 2019 (COVID-19), the disease caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Exhaled human breath contains multiple volatile molecules providing powerful potential for non-invasive diagnosis of diverse medical conditions. We investigated breath detection of SARS-CoV-2 infection using cavity-enhanced direct frequency comb spectroscopy (CE-DFCS), a state-of-the-art laser spectroscopic technique capable of a real-time massive collection of broadband molecular absorption features at ro-vibrational quantum state resolution and at parts-per-trillion volume detection sensitivity. Using a total of 170 individual breath samples (83 positive and 87 negative with SARS-CoV-2 based on reverse transcription polymerase chain reaction tests), we report excellent discrimination capability for SARS-CoV-2 infection with an area under the receiver-operating-characteristics curve of 0.849(4). Our results support the development of CE-DFCS as an alternative, rapid, non-invasive test for COVID-19 and highlight its remarkable potential for optical diagnoses of diverse biological conditions and disease states.

Ultrasensitive multispecies spectroscopic breath analysis for real-time health monitoring and diagnostics

Breath analysis enables rapid, noninvasive diagnostics, as well as long-term monitoring of human health, through the identification and quantification of exhaled biomarkers. Here, we demonstrate the remarkable capabilities of mid-infrared (mid-IR) cavity-enhanced direct-frequency comb spectroscopy (CE-DFCS) applied to breath analysis. We simultaneously detect and monitor as a function of time four breath biomarkers-[Formula: see text]OH, [Formula: see text], [Formula: see text]O, and HDO-as well as illustrate the feasibility of detecting at least six more ([Formula: see text]CO, [Formula: see text], OCS, [Formula: see text], [Formula: see text], and [Formula: see text]) without modifications to the experimental apparatus. We achieve ultrahigh detection sensitivity at the parts-per-trillion level. This is made possible by the combination of the broadband spectral coverage of a frequency comb, the high spectral resolution afforded by the individual comb teeth, and the sensitivity enhancement resulting from a high-finesse cavity. Exploiting recent advances in frequency comb, optical coating, and photodetector technologies, we can access a large variety of biomarkers with strong carbon-hydrogen-bond spectral signatures in the mid-IR.

Broadband and high resolution measurements of cavity loss and dispersion

We present a method for broadband measurements of dispersion and loss of an optical cavity. We employ an optical frequency comb directly coupled into a cavity to scan the cavity modes and retrieve their shapes and positions. The measurement data is acquired using instrumental-line-shape-free Fourier transform spectrometry. This method can be developed into a powerful tool for optical loss and dispersion measurements for broadband characterization of optical elements as well as absorption and dispersion spectroscopy. Full Text: PDF ReferencesTh. Udem, R. Holzwarth, T.W. Hänsch, "Optical Frequency Metrology", Nature 416, 233–237 (2002). CrossRef R.A. McCracken, J.M. Charsley, D.T. Reid "A decade of astrocombs: recent advances in frequency combs for astronomy", Optics Express 25, 13, 15058-15078 (2017). CrossRef M.J. Thorpe, D. Balslev-Clausen, M.S. Kirchner, J. Ye, "Cavity-enhanced optical frequency comb spectroscopy: application to human breath analysis", Optics Express 16, 4, 2387-2397 (2008). CrossRef J. Domysławska et al., "Cavity ring-down spectroscopy of the oxygen B-band with absolute frequency reference to the optical frequency comb", The Journal of Chemical Physics 136, 024201 (2012). CrossRef P. Masłowski K.C. Cossel, A. Foltynowicz, J. Ye., "Cavity-Enhanced Direct Frequency Comb Spectroscopy", in G. Gagliardi and H.-P. Loock, "Cavity-Enhanced Spectroscopy and Sensing", Springer (2014). CrossRef L. Rutkowski et al., "Sensitive and broadband measurement of dispersion in a cavity using a Fourier transform spectrometer with kHz resolution", Optics Express Vol. 25, Issue 18, pp. 21711-21718 (2017). CrossRef E.D. Black, "An introduction to Pound-Drever-Hall laser frequency stabilization", American Journal of Physics 69, 79 (2001) CrossRef A. Cygan et al., "One-dimensional frequency-based spectroscopy", Optics Express. 23, 14472. 10.1364 ( 2015). CrossRef P. Masłowski et al., "Surpassing the path-limited resolution of Fourier-transform spectrometry with frequency combs", Phys. Rev. A 93, 021802(R) (2016). CrossRef L. Rutkowski, P. Masłowski, A.C. Johansson, A. Khodabakhsh, A. Foltynowicz, "Optical frequency comb Fourier transform spectroscopy with sub-nominal resolution and precision beyond the Voigt profile", Jour. of Quant. Spect. and Rad. Trans., 204, 63-73 (2018). CrossRef S. Schiller, "Spectrometry with frequency combs," Opt. Lett. 27, 766-768 (2002). CrossRef A. Cygan et al., "Absolute molecular transitions frequencies measured by three cavity-enhanced spectroscopy techniques" J. Chem. Phys. 144 214202-1-214202-11 (2016). CrossRef I.E. Gordon et al., "The HITRAN2016 Molecular Spectroscopic Database", J. Quant. Spect. Radiat. Trans. (2017). CrossRef

Sensitivity and resolution in frequency comb spectroscopy of buffer gas cooled polyatomic molecules

We discuss the use of cavity-enhanced direct frequency comb spectroscopy in the mid-infrared region with buffer gas cooling of polyatomic molecules for high-precision rovibrational absorption spectroscopy. A frequency comb coupled to an optical enhancement cavity allows us to collect high-resolution, broad-bandwidth infrared spectra of translationally and rotationally cold (10–20 K) gas-phase molecules with high absorption sensitivity and fast acquisition times. The design and performance of the combined apparatus are discussed in detail. Recorded rovibrational spectra in the CH stretching region of several organic molecules, including vinyl bromide (CH\( {}_2 \)CHBr), adamantane (C\( {}_{10} \)H\( {}_{16} \)), and diamantane (C\( {}_{14} \)H\( {}_{20} \)) demonstrate the resolution and sensitivity of this technique, as well as the intrinsic challenges faced in extending the frontier of high-resolution spectroscopy to large complex molecules.

Direct frequency comb measurement of OD + CO → DOCO kinetics.

The kinetics of the hydroxyl radical (OH) + carbon monoxide (CO) reaction, which is fundamental to both atmospheric and combustion chemistry, are complex because of the formation of the hydrocarboxyl radical (HOCO) intermediate. Despite extensive studies of this reaction, HOCO has not been observed under thermal reaction conditions. Exploiting the sensitive, broadband, and high-resolution capabilities of time-resolved cavity-enhanced direct frequency comb spectroscopy, we observed deuteroxyl radical (OD) + CO reaction kinetics and detected stabilized trans-DOCO, the deuterated analog of trans-HOCO. By simultaneously measuring the time-dependent concentrations of the trans-DOCO and OD species, we observed unambiguous low-pressure termolecular dependence of the reaction rate coefficients for N2 and CO bath gases. These results confirm the HOCO formation mechanism and quantify its yield.

Self-referenced, accurate and sensitive optical frequency comb spectroscopy with a virtually imaged phased array spectrometer

We present a cavity-enhanced direct optical frequency comb spectroscopy system with a virtually imaged phased array (VIPA) spectrometer and either a dither or a Pound-Drever-Hall (PDH) locking scheme used for stable transmission of the comb through the cavity. A self-referenced scheme for frequency axis calibration is shown along with an analysis of its accuracy. A careful comparison between both locking schemes is performed based on near-IR measurements of the carbon monoxide ν=3←0 band P branch transitions in a gas sample with known composition. The noise-equivalent absorptions (NEA) for the PDH and dither schemes are 9.9×10(-10) cm(-1) and 5.3×10(-9) cm(-1), respectively.

Broadband cavity-enhanced molecular spectra from Vernier filtering of a complete frequency comb.

We present a new approach to cavity enhanced-direct frequency comb spectroscopy where the full emission bandwidth of a titanium:sapphire laser is exploited, currently at gigahertz resolution. The technique is based on low-resolution Vernier filtering obtained with an appreciable actively stabilized mismatch between the cavity-free spectral range and the laser repetition rate, using a diffraction grating and a split-photodiode. Spectra covering 1300 cm⁻¹ (40 THz) are acquired in less than 100 ms, and a baseline noise of 1.7×10⁻⁸ cm⁻¹ is reached with a cavity finesse of only 300, providing an absorption figure of merit M=6×10⁻¹¹ cm⁻¹·Hz(-1/2).

Frequency Comb Velocity-Modulation Spectroscopy

We have demonstrated a new technique that provides massively parallel comb spectroscopy sensitive specifically to ions through the combination of cavity-enhanced direct frequency comb spectroscopy with velocity-modulation spectroscopy. Using this novel system, we have measured electronic transitions of HfF⁺ and achieved a fractional absorption sensitivity of 3×10⁻⁷ recorded over 1500 simultaneous channels spanning 150 cm⁻¹ around 800 nm with an absolute frequency accuracy of 30 MHz (0.001 cm⁻¹). A fully sampled spectrum consisting of interleaved measurements is acquired in 30 min.

Optical frequency comb spectroscopy

Optical frequency combs offer enormous potential in the detection and control of atoms and molecules by combining their vast spectral coverage with the extremely high spectral resolution of each individual comb component. Sensitive and multiplexed trace gas detection via cavity-enhanced direct frequency comb spectroscopy has been demonstrated for various molecules and applications; however, previous demonstrations have been confined to the visible and near-infrared wavelength range. Future spectroscopic capabilities are created by developing comb sources and spectrometers for the deep ultraviolet and mid-infrared spectral regions. Here we present a broadband high resolution mid-infrared frequency comb-based Fourier transform spectrometer operating in the important molecular fingerprint spectral region of 2100-3600 cm(-1) (2.8-4.8 microm). The spectrometer, employing a multipass cell, allows simultaneous acquisition of broadband, high resolution spectra (down to 0.0035 cm(-1) of many molecular species at concentrations in the part-per-billion range in less than 1 min acquisition time. The system enables precise measurements of concentration even in gas mixtures that exhibit continuous absorption bands. The current sensitivity, 2 x 10(-8) cm(-1) Hz-1/2 per spectral element, is expected to improve by two orders of magnitude with an external enhancement cavity. We have demonstrated this sensitivity increase by combining cavity-enhanced frequency comb spectroscopy with a scanning Fourier transform spectrometer in the near-infrared region and achieving a sensitivity of 4.7 x 10(-10) cm(-1) Hz(-1/2). A cavity-enhanced mid-infrared comb spectrometer will provide a near real-time, high sensitivity, high resolution, precisely frequency calibrated, broad bandwidth system for many applications.

Analysis of trace impurities in semiconductor gas via cavity-enhanced direct frequency comb spectroscopy

Cavity-enhanced direct frequency comb spectroscopy (CE-DFCS) has demonstrated powerful potential for trace gas detection based on its unique combination of high bandwidth, rapid data acquisition, high sensitivity, and high resolution, which is unavailable with conventional systems. However, previous demonstrations have been limited to proof-of-principle experiments or studies of fundamental laboratory science. Here we present the development of CE-DFCS towards an industrial application -- measuring impurities in arsine, an important process gas used in III-V semiconductor compound manufacturing. A strongly absorbing background gas with an extremely complex, congested, and broadband spectrum renders trace detection exceptionally difficult, but the capabilities of CE-DFCS overcome this challenge and make it possible to identify and quantify multiple spectral lines associated with water impurities. Further, frequency combs allow easy access to new spectral regions via efficient nonlinear optical processes. Here, we demonstrate detection of multiple potential impurities across 1.75-1.95 um (5710-5130 cm-1) with single-channel detection sensitivities of ~1 x 10-7 cm-1 Hz-1/2 in nitrogen and identify water doped in arsine with a sensitivity of ~1 x 10-6 cm-1 Hz-1/2.

Cavity-Enhanced Direct Frequency Comb Spectroscopy: Technology and Applications

Cavity-enhanced direct frequency comb spectroscopy combines broad bandwidth, high spectral resolution, and ultrahigh detection sensitivity in one experimental platform based on an optical frequency comb efficiently coupled to a high-finesse cavity. The effective interaction length between light and matter is increased by the cavity, massively enhancing the sensitivity for measurement of optical losses. Individual comb components act as independent detection channels across a broad spectral window, providing rapid parallel processing. In this review we discuss the principles, the technology, and the first applications that demonstrate the enormous potential of this spectroscopic method. In particular, we describe various frequency comb sources, techniques for efficient coupling between comb and cavity, and detection schemes that utilize the technique's high-resolution, wide-bandwidth, and fast data-acquisition capabilities. We discuss a range of applications, including breath analysis for medical diagnosis, trace-impurity detection in specialty gases, and characterization of a supersonic jet of cold molecules.

Tomography of a supersonically cooled molecular jet using cavity-enhanced direct frequency comb spectroscopy

Cavity-enhanced direct frequency comb spectroscopy is used to obtain simultaneously high resolution and broad spectral-bandwidth measurements of a supersonically cooled jet of acetylene molecules. We demonstrate a complete spatial mapping of density, velocity, and internal state distributions of a cold molecular jet via tomographic reconstructions. These measurements reveal interesting and never-before-observed features of the pulsed jet expansion, including anomalous C 2H 2 absorption lineshapes and anomalous spatial variations of the rotational temperature distribution. The demonstrated capabilities establish this technique as a powerful new tool for studies of cold molecules.

Cavity-enhanced direct frequency comb spectroscopy

Cavity-enhanced direct frequency comb spectroscopy combines broad spectral bandwidth, high spectral resolution, precise frequency calibration, and ultrahigh detection sensitivity, all in one experimental platform based on an optical frequency comb interacting with a high-finesse optical cavity. Precise control of the optical frequency comb allows highly efficient, coherent coupling of individual comb components with corresponding resonant modes of the high-finesse cavity. The long cavity lifetime dramatically enhances the effective interaction between the light field and intracavity matter, increasing the sensitivity for measurement of optical losses by a factor that is on the order of the cavity finesse. The use of low-dispersion mirrors permits almost the entire spectral bandwidth of the frequency comb to be employed for detection, covering a range of ~10% of the actual optical frequency. The light transmitted from the cavity is spectrally resolved to provide a multitude of detection channels with spectral resolutions ranging from a several gigahertz to hundreds of kilohertz. In this review we will discuss the principle of cavity-enhanced direct frequency comb spectroscopy and the various implementations of such systems. In particular, we discuss several types of UV, optical, and IR frequency comb sources and optical cavity designs that can be used for specific spectroscopic applications. We present several cavity-comb coupling methods to take advantage of the broad spectral bandwidth and narrow spectral components of a frequency comb. Finally, we present a series of experimental measurements on trace gas detections, human breath analysis, and characterization of cold molecular beams.