Compact mid-infrared dual-comb spectrometer for outdoor spectroscopy

Pulse interaction induced systematic errors in dual comb spectroscopy

Systematic errors are observed in dual comb spectroscopy when pulses from the two sources travel in a common fiber before interrogating the sample of interest. When sounding a molecular gas, these errors distort both the line shapes and retrieved concentrations. Simulations of dual comb interferograms based on a generalized nonlinear Schrodinger equation highlight two processes for these systematic errors. Self-phase modulation changes the spectral content of the field interrogating the molecular response but affects the recorded spectral baseline and absorption features differently, leading to line intensity errors. Cross-phase modulation modifies the relative inter-pulse delay, thus introducing interferogram sampling errors and creating a characteristic asymmetric distortion on spectral lines. Simulations capture the shape and amplitude of experimental errors which are around 0.1% on spectral transmittance residuals for 10 mW of total average power in 10 meters of common fiber, scaling up to above 0.6% for 20 mW and 60 m.

Quartz-enhanced multiheterodyne resonant photoacoustic spectroscopy

The extension of dual-comb spectroscopy (DCS) to all wavelengths of light along with its ability to provide ultra-large dynamic range and ultra-high spectral resolution, renders it extremely useful for a diverse array of applications in physics, chemistry, atmospheric science, space science, as well as medical applications. In this work, we report on an innovative technique of quartz-enhanced multiheterodyne resonant photoacoustic spectroscopy (QEMR-PAS), in which the beat frequency response from a dual comb is frequency down-converted into the audio frequency domain. In this way, gas molecules act as an optical-acoustic converter through the photoacoustic effect, generating heterodyne sound waves. Unlike conventional DCS, where the light wave is detected by a wavelength-dependent photoreceiver, QEMR-PAS employs a quartz tuning fork (QTF) as a high-Q sound transducer and works in conjunction with a phase-sensitive detector to extract the resonant sound component from the multiple heterodyne acoustic tones, resulting in a straightforward and low-cost hardware configuration. This novel QEMR-PAS technique enables wavelength-independent DCS detection for gas sensing, providing an unprecedented dynamic range of 63 dB, a remarkable spectral resolution of 43 MHz (or ~0.3 pm), and a prominent noise equivalent absorption of 5.99 × 10-6 cm-1·Hz-1/2.

Deep-Learning-Enabled High-Fidelity Absorbance Spectra from Distorted Dual-Comb Absorption Spectroscopy for Gas Quantification Analysis

Dual-comb absorption spectroscopy has been a promising technique in laser spectroscopy due to its intrinsic advantages over broad spectral coverage, high resolution, high acquisition speed, and frequency accuracy. However, two primary challenges, including etalon effects and complex baseline extraction, still severely hinder its implementation in recovering absorbance spectra and subsequent quantification analysis. In this paper, we propose a deep learning enabled processing framework containing etalon removal and baseline extraction modules to obtain absorbance spectra from distorted dual-comb absorption spectroscopy. The etalon removal module utilizes a typical U-net model, and the baseline extraction module consists of a modified U-net model with physical constraint and an adaptive iteratively reweighted penalized least squares method serving as refinement. The training datasets combine experimental baselines and simulated gas absorption with different concentrations, fully exploiting prior information on gas absorption features from the HITRAN database. In the simulated and experimental test, the CO2 absorbance spectrum covering 25 cm-1 shows high consistency with the HITRAN database, of which the mean absolute error is less than 1% of the maximum absorbance value, and the retrieved concentration has a relative error under 2%, outperforming traditional approaches and indicating the potential practicality of our data processing framework. Hopefully, with a larger network volume and proper datasets, this processing framework can be extended to precise quantification analysis in more comprehensive applications such as atmospheric measurement and industrial monitoring.

Over 3.8 W, 3.4 µm picosecond mid-infrared parametric conversion based on a simplified one-to-many scheme

In this paper, we demonstrate a simplified one-to-many scheme for efficient mid-infrared (MIR) parametric conversion. Such a scheme is based on a continuous wave (CW) single longitudinal mode master oscillator power-amplifier (MOPA) fiber system as the signal source and a picosecond pulsed MOPA fiber system, exhibiting multiple longitudinal modes, as the pump source. The signal and pump beams are combined and co-coupled into a piece of 50-mm long 5% MgO-doped PPLN crystal for the parametric conversion. As high as ∼3.82 W average power at a central idler wavelength of ∼3.4 µm is achieved when the launched pump and signal powers are ∼41.73 and ∼11.45 W, respectively. Above some threshold value, the delivered idler power shows a roll-over effect against the signal power and saturation-like effect against the pump power. Consequently, the highest conversion efficiency is observed at such a threshold pump power. To the best of our knowledge, our result represents the highest average power produced from any single-pass parametric conversion source with >3 µm idler wavelength feeding with a CW signal. Moreover, our proposed scheme can simplify the design of parametric conversion system significantly and meanwhile make the system more robust in applications. This is attributed to two main aspects. Firstly, the scheme's one-to-many feature can reduce wavelength sensitivity remarkably in the realization of quasi-phase-matching. Secondly, for moderate power requirement it does not always require a high peak power synchronized pulsed signal source; a CW one can be an alternative, thereby making the system free from complex time synchronization and the related time jitter.

High-SNR mid-infrared dual-comb spectroscopy using active phase control cooperating with CWs-dependent phase correction

Mid-infrared (MIR) dual-comb spectroscopy (DCS) is a highly effective method for molecular metrology of rovibrational transition spectra in a quick accurate manner. However, due to limited comb frequency instability, manipulating coherence between two frequency combs to accomplish high-quality spectral analysis in the MIR region is a huge challenge. Here, we developed a comb-teeth resolved MIR DCS based on active phase control cooperating with a CWs-dependent (CWD) interferogram timing correction. Firstly, four meticulously engineered actuators were individually integrated into two near-infrared (NIR) seed combs to facilitate active coherence maintenance. Subsequently, two PPLN waveguides were adopted to achieve parallel difference frequency generations (DFG), directly achieving a coherent MIR dual-comb spectrometer. To improve coherence and signal-to-noise ratio (SNR), a CWD resampled interferogram timing correction was used to optimize the merit of DCS from 7.5 × 105 to 2.5 × 106. Meanwhile, we carried out the measurement of MIR DCS on the methane hot-band absorption spectra (v3 band), which exhibited a good agreement with HITRAN by a standard deviation on recording residual of 0.76%. These experimental results confirm that this MIR DCS with CWD interferogram timing correction has significant potential to characterize the rovibrational transitions of MIR molecules.

Frequency comb with a spectral range of 0.4-5.2 µm based on a compact all-fiber laser and LiNbO3 waveguide

This Letter presents a 0.4-5.2-µm frequency comb from a compact laser. We designed an integrated fiber device for a figure-9 laser and constructed an all-fiber laser system. The spectrum of the fiber laser was scaled to the broadband region using a chirped periodically poled lithium niobate waveguide. To use this system for gas sensing, a mid-infrared comb with a spectral range of 2.5-5.2 µm and average power of 2.1 mW was divided using an optical filter. The optical part was packaged in a 305 mm × 225 mm × 62 mm box. The comb was stabilized by locking the repetition rate and carrier-envelope offset frequency of the seed source. The system provided an ultrabroadband spectral range from 0.4 to 5.2 µm, which could be applied to spectroscopy, frequency metrology, and optical synthesizers.

Accurate spectrogram restoration algorithm for an echelle spectrometer based on adaptive parameters

The echelle spectrometer is a high-resolution spectrometer that can realize transient direct readings of a full spectrum. To improve the accuracy of the spectrogram restoration model in calibration, multiple-integral time fusion, and an improved adaptive-threshold centroid algorithm are used to overcome noise and improve the accuracy of calculating the light spot position. A seven-parameter pyramid-traversal method is proposed to optimize the parameters of the spectrogram restoration model. The deviation of the spectrogram model is significantly reduced after the parameters are optimized, and the deviation curve fluctuation becomes mild, which greatly improves the model's accuracy after curve fitting.The test results show that the accuracy of the spot position determination algorithm proposed in this paper is 0.1 pixels. In addition to this, the accuracy of the spectral restoration model is controlled within 0.3 pixels in a short-wave stage and 0.7 pixels in a long-wave stage. Compared with the traditional algorithm, the accuracy of spectrogram restoration is more than two times, and the spectral calibration time is less than 45 min.

Open-path dual-comb spectroscopy of methane and VOC emissions from an unconventional oil well development in Northern Colorado

We present results from a field study monitoring methane and volatile organic compound emissions near an unconventional oil well development in Northern Colorado from September 2019 to May 2020 using a mid-infrared dual-comb spectrometer. This instrument allowed quantification of methane, ethane, and propane in a single measurement with high time resolution and integrated path sampling. Using ethane and propane as tracer gases for methane from oil and gas activity, we observed emissions during the drilling, hydraulic fracturing, millout, and flowback phases of well development. Large emissions were seen in drilling and millout phases and emissions decreased to background levels during the flowback phase. Ethane/methane and propane/methane ratios varied widely throughout the observations.

Upconversion time-stretch infrared spectroscopy

High-speed measurement confronts the extreme speed limit when the signal becomes comparable to the noise level. In the context of broadband mid-infrared spectroscopy, state-of-the-art ultrafast Fourier-transform infrared spectrometers, in particular dual-comb spectrometers, have improved the measurement rate up to a few MSpectra s^-1, which is limited by the signal-to-noise ratio. Time-stretch infrared spectroscopy, an emerging ultrafast frequency-swept mid-infrared spectroscopy technique, has shown a record-high rate of 80 MSpectra s^-1 with an intrinsically higher signal-to-noise ratio than Fourier-transform spectroscopy by more than the square-root of the number of spectral elements. However, it can measure no more than ~30 spectral elements with a low resolution of several cm^-1. Here, we significantly increase the measurable number of spectral elements to more than 1000 by incorporating a nonlinear upconversion process. The one-to-one mapping of a broadband spectrum from the mid-infrared to the near-infrared telecommunication region enables low-loss time-stretching with a single-mode optical fiber and low-noise signal detection with a high-bandwidth photoreceiver. We demonstrate high-resolution mid-infrared spectroscopy of gas-phase methane molecules with a high resolution of 0.017 cm^-1. This unprecedentedly high-speed vibrational spectroscopy technique would satisfy various unmet needs in experimental molecular science, e.g., measuring ultrafast dynamics of irreversible phenomena, statistically analyzing a large amount of heterogeneous spectral data, or taking broadband hyperspectral images at a high frame rate.

Open-path measurement of stable water isotopologues using mid-infrared dual-comb spectroscopy

We present an open-path mid-infrared dual-comb spectroscopy (DCS) system capable of precise measurement of the stable water isotopologues H216O and HD16O. This system ran in a remote configuration at a rural test site for 3.75 months with 60% uptime and achieved a precision of < 2‰ on the normalized ratio of H216O and HD16O (δ D ) in 1000s. Here, we compare the δ D values from the DCS system to those from the National Ecological Observatory Network (NEON) isotopologue point sensor network. Over the multi-month campaign, the mean difference between the DCS δ D values and the NEON δ D values from a similar ecosystem is < 2‰ with a standard deviation of 18‰, which demonstrates the inherent accuracy of DCS measurements over a variety of atmospheric conditions. We observe time-varying diurnal profiles and seasonal trends that are mostly correlated between the sites on daily timescales. This observation motivates the development of denser ecological monitoring networks aimed at understanding regional- and synoptic-scale water transport. Precise and accurate open-path measurements using DCS provide new capabilities for such networks.

Broadband 1-GHz mid-infrared frequency comb

Mid-infrared (MIR) spectrometers are invaluable tools for molecular fingerprinting and hyper-spectral imaging. Among the available spectroscopic approaches, GHz MIR dual-comb absorption spectrometers have the potential to simultaneously combine the high-speed, high spectral resolution, and broad optical bandwidth needed to accurately study complex, transient events in chemistry, combustion, and microscopy. However, such a spectrometer has not yet been demonstrated due to the lack of GHz MIR frequency combs with broad and full spectral coverage. Here, we introduce the first broadband MIR frequency comb laser platform at 1 GHz repetition rate that achieves spectral coverage from 3 to 13 µm. This frequency comb is based on a commercially available 1.56 µm mode-locked laser, robust all-fiber Er amplifiers and intra-pulse difference frequency generation (IP-DFG) of few-cycle pulses in χ⁽²⁾ nonlinear crystals. When used in a dual comb spectroscopy (DCS) configuration, this source will simultaneously enable measurements with μs time resolution, 1 GHz (0.03 cm^-1) spectral point spacing and a full bandwidth of >5 THz (>166 cm^-1) anywhere within the MIR atmospheric windows. This represents a unique spectroscopic resource for characterizing fast and non-repetitive events that are currently inaccessible with other sources.

Compact double-pass Echelle spectrometer employing a crossed diffraction grating

This contribution presents the design and implementation of a compact and robust Echelle-inspired cross-grating spectrometer which is arranged as a double pass setup. This allows use of the employed refractive elements for collimation of the incoming light and, after diffraction at the reflective crossed diffraction grating, for imaging the diffracted light onto the detector. The crossed diffraction grating combines the two dispersive functionalities of a classical Echelle spectrometer in a single element and is therefore formed by a superposition of two blazed linear gratings which are oriented perpendicularly. The refractive elements and the plane grating are arranged in a rigid objective group which is beneficial in terms of stability and robustness. The experimental tests prove that the designed resolving power of more than 300 is achieved for the addressed spectrum ranging from 400 nm to 1100 nm by using an entrance pinhole diameter of 105 µm. The utilization of a single mode fiber increases the resolving power to more than 1000, but leads to longer acquisition times.

Absolute frequency referencing in the long wave infrared using a quantum cascade laser frequency comb

Optical frequency combs (OFCs) based on quantum cascade lasers (QCLs) have transformed mid-infrared spectroscopy. However, QCL-OFCs have not yet been exploited to provide a broadband absolute frequency reference. We demonstrate this possibility by performing comb-calibrated spectroscopy at 7.7 µm (1305 cm^-1) using a QCL-OFC referenced to a molecular transition. We obtain 1.5·10^-10 relative frequency stability (100-s integration time) and 3·10^-9 relative frequency accuracy, comparable with state-of-the-art solutions relying on nonlinear frequency conversion. We show that QCL-OFCs can be locked with sub-Hz-level stability to a reference for hours, thus promising their use as metrological tools for the mid-infrared.

Fourier transform spectrometer based on high-repetition-rate mid-infrared supercontinuum sources for trace gas detection

We present a fast-scanning Fourier transform spectrometer (FTS) in combination with high-repetition-rate mid-infrared supercontinuum sources, covering a wavelength range of 2-10.5 µm. We demonstrate the performance of the spectrometer for trace gas detection and compare various detection methods: baseband detection with a single photodetector, baseband balanced detection, and synchronous demodulation at the repetition rate of the supercontinuum source. The FTS uses off-the-shelf optical components and provides a minimum spectral resolution of 750 MHz. It achieves a noise equivalent absorption sensitivity of ∼10^-6 cm^-1 Hz^-1/2 per spectral element, by using a 31.2 m multipass absorption cell.

Frequency comb-based multidimensional coherent spectroscopy bridges the gap between fundamental science and cutting-edge technology

Optical multidimensional coherent spectroscopy (MDCS) has become a powerful and routine technique for studying optical properties of a wide range of materials. However, current implementations of MDCS have spectral resolution and acquisition speed limitations. In this Perspective, I describe how frequency comb technology can be used to overcome the limitations and also show the recent progress that has been made in this field.

Intrinsic amplitude-noise suppression in fiber lasers mode-locked with nonlinear amplifying loop mirrors

In this Letter, we investigate steady states of fiber lasers mode-locked with a nonlinear amplifying loop mirror that have an inherent amplitude-noise-suppression mechanism. Due to the interaction of the sinusoidal transmission function with the fluctuating intracavity pulse amplitude, we show that under specific preconditions, this mechanism may lead to a detectable difference in relative intensity noise at the reflected and transmitted output port of the laser. We present systematic intensity noise measurements with a nonlinear fiber-based system that replicates a single roundtrip in the laser cavity. The experimental results and simulations clearly show a reduction of the intracavity amplitude fluctuations up to 4 dB for certain steady states.

Broadband Time-Resolved Absorption and Dispersion Spectroscopy of Methane and Ethane in a Plasma Using a Mid-Infrared Dual-Comb Spectrometer

Conventional mechanical Fourier Transform Spectrometers (FTS) can simultaneously measure absorption and dispersion spectra of gas-phase samples. However, they usually need very long measurement times to achieve time-resolved spectra with a good spectral and temporal resolution. Here, we present a mid-infrared dual-comb-based FTS in an asymmetric configuration, providing broadband absorption and dispersion spectra with a spectral resolution of 5 GHz (0.18 nm at a wavelength of 3333 nm), a temporal resolution of 20 μs, a total wavelength coverage over 300 cm-1 and a total measurement time of ~70 s. We used the dual-comb spectrometer to monitor the reaction dynamics of methane and ethane in an electrical plasma discharge. We observed ethane/methane formation as a recombination reaction of hydrocarbon radicals in the discharge in various static and dynamic conditions. The results demonstrate a new analytical approach for measuring fast molecular absorption and dispersion changes and monitoring the fast dynamics of chemical reactions over a broad wavelength range, which can be interesting for chemical kinetic research, particularly for the combustion and plasma analysis community.

VECSEL-based virtually imaged phased array spectrometer for rapid gas phase detection in the mid-infrared

We present a novel, to the best of our knowledge, system for high-resolution, time-resolved spectroscopy in the mid-wave infrared based on a modelocked vertical external cavity surface emitting laser (VECSEL) frequency comb coupled to a virtually imaged phased array (VIPA) spectrometer. The GHz level repetition rate of VECSEL-based systems coupled to VIPA spectrometers enables comb tooth resolved spectra without the use of additional filter cavities often required to increase comb tooth spacing. We demonstrate absorption spectroscopy on a methane (CH₄) gas mixture at 2900cm^-1 (3.4 µm) with over 35cm^-1 spectral bandwidth in a single image. Rapid time-resolved measurements were made using a 300 µs exposure time with an acquisition rate limited to 125 Hz by the available camera. High-resolution absolute frequency measurements were performed by scanning the repetition rate of the VECSEL frequency comb.

QCL-Based Dual-Comb Spectrometer for Multi-Species Measurements at High Temperatures and High Pressures

Rapid multi-species sensing is an overarching goal in time-resolved studies of chemical kinetics. Most current laser sources cannot achieve this goal due to their narrow spectral coverage and/or slow wavelength scanning. In this work, a novel mid-IR dual-comb spectrometer is utilized for chemical kinetic investigations. The spectrometer is based on two quantum cascade laser frequency combs and provides rapid (4 µs) measurements over a wide spectral range (~1175-1235 cm-1). Here, the spectrometer was applied to make time-resolved absorption measurements of methane, acetone, propene, and propyne at high temperatures (>1000 K) and high pressures (>5 bar) in a shock tube. Such a spectrometer will be of high value in chemical kinetic studies of future fuels.