Precise multispecies agricultural gas flux determined using broadband open-path dual-comb spectroscopy

Ultra-broadband coherent open-path spectroscopy for multi-gas monitoring in wastewater treatment

Wastewater treatment plants significantly contribute to greenhouse gas emissions, including nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4). Current methods to measure these emissions typically target specific molecular compounds, providing limited scope and potentially incomplete emissions profiles. Here, we show an innovative ultra-broadband coherent open-path spectroscopy (COPS) system capable of simultaneously monitoring multiple greenhouse gases. This novel approach combines Fourier transform spectroscopy with a coherent, ultra-broadband mid-infrared light source spanning 2-11.5 μm at approximately 3 W power. Positioned above an aeration tank, the COPS system selectively detected absorption signatures for CH4, CO2, N2O, ammonia (NH3), carbon monoxide (CO), and water vapor (H2O), enabling real-time, path-integrated concentration measurements with a temporal resolution of 40 s. Elevated concentrations of CH4 and CO2 were clearly identified within emission plumes traversing the beam path above the aeration tank. Additionally, CH4 emission patterns closely tracked variations in ammonium loading from incoming wastewater, whereas CO2 emissions correlated strongly with oxygen concentrations introduced during aeration. Measurements of N2O, NH3, and CO were stable and aligned closely with traditional point-based measurements from commercial gas analyzers. Our findings demonstrate that COPS offers a robust, comprehensive solution for the simultaneous real-time monitoring of multiple gases in complex and heterogeneous emission environments. This capability significantly enhances atmospheric and industrial emission assessments, potentially transforming the approach to emissions quantification and environmental management.

Modulated ringdown comb interferometry for sensing of highly complex gases

Gas samples relevant to health1-3 and the environment4-6 typically contain many molecular species that span a huge concentration dynamic range. Mid-infrared frequency comb spectroscopy with high-finesse cavity enhancement has allowed the most sensitive multispecies trace-gas detections so far2,7-13. However, the robust performance of this technique depends critically on ensuring absorption-path-length enhancement over a broad spectral coverage, which is severely limited by comb-cavity frequency mismatch if strongly absorbing compounds are present. Here we introduce modulated ringdown comb interferometry, a technique that resolves the vulnerability of comb-cavity enhancement to strong intracavity absorption or dispersion. This technique works by measuring ringdown dynamics carried by massively parallel comb lines transmitted through a length-modulated cavity, making use of both the periodicity of the field dynamics and the Doppler frequency shifts introduced from a Michelson interferometer. As a demonstration, we measure highly dispersive exhaled human breath samples and ambient air in the mid-infrared with finesse improved to 23,000 and coverage to 1,010 cm-1. Such a product of finesse and spectral coverage is orders of magnitude better than all previous demonstrations2,7-20, enabling us to simultaneously quantify 20 distinct molecular species at above 1-part-per-trillion sensitivity varying in concentrations by seven orders of magnitude. This technique unlocks next-generation sensing performance for complex and dynamic molecular compositions, with scalable improvement to both finesse and spectral coverage.

Mid-infrared supercontinuum laser source with efficient spectral enhancement in 3.7 µm

A 3-5 µm mid-infrared (MIR) laser has a wide range of applications in biological tissue ablation, remote spectral fingerprint recognition, and directional infrared countermeasures. However, the performance of conventional MIR lasers has long been hindered by restricted wavelength radiation, spectral power efficiency, and system stability. Here, a highly efficient, compact, and stable MIR light source is reported, which is directly generated from a supercontinuum (SC) laser with a long wavelength edge of 4.2 µm in a 7 µm core diameter fluorotellurite fiber. Based on the integration of a high-peak-power pump light source and a small-core-diameter nonlinear medium, efficient nonlinear frequency conversion from traditional near-infrared laser to mid-infrared laser has been achieved, resulting in a significantly enhanced MIR spectrum of 3.7 µm, exceeding the pump peak of 2 µm by more than 12 dB. The pump conversion efficiency is 50.8%, with 94.3% of the spectral power distributed above 2.4 µm and 71.4% above 3 µm. This study has opened up a feasible avenue for obtaining high-efficiency mid-infrared band lasers that meet practical application needs.

Enhanced spectral resolution in mid-infrared dual-comb spectroscopy via synchronous offset frequency tuning

Mid-infrared dual-comb spectroscopy offers significant advantages by combining the high sensitivity of mid-infrared spectroscopy with the high spectral resolution and rapid acquisition of the dual-comb method. However, its effective resolution, constrained by the inherent comb line spacing, hinders its ability to resolve narrow absorption features, common in critical applications such as sub-Doppler spectroscopy, low-pressure gas analysis, and construction of the atmospheric profile. To address this challenge, we present a synchronous offset frequency tuning method for the mid-infrared dual-comb system to improve effective resolution far beyond comb line spacing. In our system, the mid-infrared dual-comb source is generated from a near-infrared dual-comb source and a continuous-wave pump laser via difference frequency generation in a single periodically poled lithium niobate bulk. By adjusting the phase-lock frequency of the pump laser to one of the near-infrared combs, we synchronously tune the offset frequencies of both mid-infrared combs without changing the near-infrared dual-comb source. We demonstrated that this method enabled the high resolution of overlapped spectral lines of ethane around 3000 cm-1, achieving a uniform spectral sampling interval of 10 MHz in the interleaved spectrum and a 25-fold enhancement in effective resolution. This approach allows for sub-MHz spectral resolution in mid-infrared dual-comb spectroscopy without any modifications to the data acquisition system, offering possibilities for high-precision spectral analysis.

Miniature optical fiber photoacoustic spectroscopy gas sensor based on a 3D micro-printed planar-spiral spring optomechanical resonator

Photoacoustic spectroscopy (PAS) gas sensors based on optomechanical resonators (OMRs) have garnered significant attention for ultrasensitive trace-gas detection. However, a major challenge lies in balancing small size with high performance when developing ultrasensitive miniaturized optomechanical resonant PAS (OMR-PAS) gas sensors for space-constrained applications. Here, we present a miniature optical fiber PAS gas sensor based on a planar-spiral spring OMR (PSS-OMR) that is in situ 3D micro-printed on the end-face of a fiber-optic ferrule. Experimental results demonstrate that mechanical vibrational resonance can enhance the sensor's acoustic sensitivity by over two orders of magnitude. Together with a 1.4 μL non-resonant photoacoustic cell, it can detect C2H2 gas concentration at the 45-ppb level, and its response is very fast approximating 0.2 seconds. This optical fiber OMR-PAS gas sensor holds great promise for the detection or monitoring of rapidly varying trace gas in many applications ranging from production process control to industrial environmental surveillance.

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.

Greenhouse gas monitoring using an IPDA lidar based on a dual-comb spectrometer

We present the development of a multi-spectral, integrated-path differential absorption (IPDA) lidar based on a dual-comb spectrometer for greenhouse gas monitoring. The system uses the lidar returns from topographic targets and does not require retroreflectors. The two frequency combs are generated by electro-optic modulation of a single continuous-wave laser diode. One of the combs is pulsed, amplified, and transmitted into the atmosphere, while the other acts as a local oscillator for coherent detection. We discuss the physical principles of the measurement, outline a performance model including speckle effects, and detail the fiber-based lidar architecture and signal processing. A maximum likelihood algorithm is used to estimate simultaneously the gas concentration and the central frequency of the comb, allowing the system to work without frequency locking. H2O (at 1544 nm) and CO2 (at 1572 nm) concentrations are monitored with a precision of 3% and 5%, respectively, using a non-cooperative target at 700 m. In addition, the measured water vapor concentrations are in excellent agreement with in-situ measurements obtained from nearby weather stations. To our knowledge, this is the first complete experimental demonstration and performance assessment of greenhouse gas monitoring with a dual-comb spectrometer using lidar echoes from topographic targets.

Ultra-broadband spectroscopy using a 2-11.5 µm IDFG-based supercontinuum source

Supercontinuum sources based on intrapulse difference frequency generation (IDFG) from mode-locked lasers open new opportunities in mid-infrared gas spectroscopy. These sources provide high power and ultra-broadband spectral coverage in the molecular fingerprint region with very low relative intensity noise. Here, we demonstrate the performance of such a light source in combination with a multipass cell and a custom-built Fourier transform spectrometer (FTS) for multispecies trace gas detection. The light source provides a low-noise, ultra-broad spectrum from 2-11.5 µm with ∼3 W output power, outperforming existing mid-infrared supercontinuum sources in terms of noise, spectral coverage, and output power. This translates to an excellent match for spectroscopic applications, establishing (sub-)ppb sensitivity for molecular hydrocarbons (e.g., CH4, C2H4), oxides (e.g., SO2, NOx), and small organic molecules (e.g., acetone, ethyl acetate) over the spectral range of the supercontinuum source with a measurement time varying from seconds to minutes. We demonstrate a practical application by measuring the off-gas composition of a bioreactor containing an acidic ammonia-oxidizing culture with the simultaneous detection of multiple nitrogen oxides (NO, NO2, N2O, etc.). As the different species absorb various parts of the spectrum, these results highlight the functionality of this spectroscopic system for biological and environmental applications.

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.

Validation of open-path dual-comb spectroscopy against an O2 background

Dual-comb spectroscopy measures greenhouse gas concentrations over kilometers of open air with high precision. However, the accuracy of these outdoor spectra is challenging to disentangle from the absorption model and the fluctuating, heterogenous concentrations over these paths. Relative to greenhouse gases, O₂ concentrations are well-known and evenly mixed throughout the atmosphere. Assuming a constant O₂ background, we can use O₂ concentration measurements to evaluate the consistency of open-path dual-comb spectroscopy with laboratory-derived absorption models. To this end, we construct a dual-comb spectrometer spanning 1240 nm to 1700nm, which measures O₂ absorption features in addition to CO₂ and CH₄. O₂ concentration measurements across a 560 m round-trip outdoor path reach 0.1% precision in 10 minutes. Over seven days of shifting meteorology and spectrometer conditions, the measured O₂ has -0.07% mean bias, and 90% of the measurements are within 0.4% of the expected hemisphere-average concentration. The excursions of up to 0.4% seem to track outdoor temperature and humidity, suggesting that accuracy may be limited by the O₂ absorption model or by water interference. This simultaneous O₂, CO₂, and CH₄ spectrometer will be useful for measuring accurate CO₂ mole fractions over vertical or many-kilometer open-air paths, where the air density varies.

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.

Dual-comb quartz-enhanced photoacoustic spectroscopy

Photoacoustic spectroscopy (PAS) using two optical combs is a new-born technique, offering appealing features, including broad optical bandwidths, high resolutions, fast acquisition speeds, and wavelength-independent photoacoustic detection, for chemical sensing. However, its further application to, e.g., trace detection, is jeopardized due to the fundamentally and technically limited sensitivity and specificity. Here, we take a different route to comb-enabled PAS with acoustically enhanced sensitivity and nonlinear spectral hole-burning defined resolution. We demonstrate dual-comb quartz-enhanced PAS with two near-infrared electro-optic combs and a quartz tuning fork. Comb-line-resolved multiplexed spectra are acquired for acetylene with a single-pass detection limit at the parts-per-billion level. The technique is further extended to the mid-infrared (for methane), enabling improved sensitivity. More importantly, we measure nonlinear dual-comb photoacoustic spectra for the 12C2H2 ν1 + ν3 band P(17) transition with sub-Doppler pressure-broadening dominated homogeneous linewidths (e.g., 45.8 MHz), hence opening up new opportunities for Doppler-free photoacoustic gas sensing.

Collinear opto-optical loss modulation for carrier-envelope offset stabilization of a fiber frequency comb

Opto-optical loss modulation (OOM) for stabilization of the carrier-envelope offset (CEO) frequency of a femtosecond all-fiber laser is performed using a collinear geometry. Amplitude-modulated 1064 nm light is fiber coupled into an end-pumped semiconductor saturable absorber mirror (SESAM)-mode-locked all-polarization-maintaining erbium fiber femtosecond laser, where it optically modulates the loss of the SESAM resulting in modulation of the CEO frequency. A noise rejection bandwidth of 150 kHz is achieved when OOM and optical gain modulation are combined in a hybrid analog/digital loop. Collinear OOM provides a simple, all-fiber, high-bandwidth method for improving the CEO frequency stability of SESAM mode-locked fiber lasers.

Optimized Deep Neural Network and Its Application in Fine Sowing of Crops

Winter wheat is one of the most important food products. Increasing food demand and limited land resources have forced the development of agricultural production to be more refined and efficient. The most important part of agricultural production is sowing. With the promotion of precision agriculture, precision seeding has become the main component of modern agricultural seeding technology system, and the adoption of precision seeding technology is an important means of large-scale production and cost saving and efficiency enhancement. However, the current sowing technology and sowing equipment cannot meet the requirements of wheat sowing accuracy. In this context, a differential perturbation particle swarm optimization (DPPSO) algorithm is proposed by embedding differential perturbation into particle swarm optimization, which shows fast convergence speed and good global performance. After that the DPPSO is used to optimize the convolutional neural network (CNN) to build an optimized CNN (DPPSO-CNN) model and applied to the field of crops fine sowing. Finally, the experimental results show that the proposed method not only has a faster convergence rate but also achieves better wheat seeding performance. The research of this paper an effectively improves the accuracy and uniformity of wheat seeding and lay a foundation for improving wheat yield per unit area and promotes the intelligent development of agriculture in the future.

High-resolution dual comb spectroscopy using a free-running, bidirectional ring titanium sapphire laser

We report the first measurement of resolved molecular absorption lines with dual-comb spectroscopy using a Kerr-lens mode-locked bidirectional Ti:sapphire ring laser cavity. A 3 nm broad spectrum has been recorded in 5.3 ms with a spectral resolution of ≈ 1 GHz (0.05 cm-1) corresponding to a relative spectral resolution of 2.5 × 10^-6. The measurement of spectrally resolved molecular absorption lines have been demonstrated on the oxygen A-band at 394 THz (760 nm, 13 000 cm^-1) and was obtained with two free-running 100 fs Ti:sapphire trains of pulses without the need for active phase stabilization protocol nor real-time or post-processing correction. This work demonstrates that the bidirectional laser configuration enables a sufficient level of absolute and mutual coherence for dual-comb spectroscopy of resolved molecular absorption lines. Considering the high versatility of Ti:sapphire emission spectral range (from 600 to 1100 nm) with high-peak powers, the here reported results pave the way for Dual-Comb spectroscopy in the UV range at mW average output power using a standalone set-up, in the aim to extend its applicability for atmospheric remote-sensing.

Autonomous Differential Absorption Laser Device for Remote Sensing of Atmospheric Greenhouse Gases

A ground-based, integrated path, differential absorption (IPDA) light detection device capable of measuring multiple greenhouse gas (GHG) species in the atmosphere is presented. The device was developed to monitor greenhouse gas concentrations in small-scale areas with high emission activities. It is equipped with two low optical power tunable diode lasers in the near-infrared spectral range for the atmospheric detection of carbon dioxide, methane, and water vapors (CO2, CH4 and H2O). The device was tested with measurements of background concentrations of CO2 and CH4 in the atmosphere (Crete, Greece). Accuracies in the measurement retrievals of CO2 and CH4 were estimated at 5 ppm (1.2%) and 50 ppb (2.6%), respectively. A method that exploits the intensity of the recorded H2O absorption line in combination with weather measurements (water vapor pressure, temperature, and atmospheric pressure) to calculate the GHG concentrations is proposed. The method eliminates the requirement for measuring the range of the laser beam propagation. Accuracy in the measurement of CH4 using the H2O absorption line is estimated at 90 ppb (4.8%). The values calculated by the proposed method are in agreement with those obtained from the differential absorption LiDAR equation (DIAL).