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A novel CH 4 sensor based on wavelength modulation spectroscopy with a multipass cell was developed for the soil respiration measurement of CH 4 . A home-made double-enhanced Herriot-type multipass cell with an effective absorption length of 73.926 m and a fiber-coupled distributed feedback diode laser emission at 1653.74 nm were used to design the sensor. The double enhancement of the effective optical pathlength of the multipass cell, absorption line locking, laser intensity normalization, and temperature control of the multipass cell were used to improve cell performance and achieve a minimum detection limit of 10 ppbv and a measurement precision of 6.4 ppbv. Finally, the potential of the developed CH 4 sensor for ecological applications was verified by measuring the soil respiration of CH 4 and monitoring of CH 4 in the atmosphere over a long period.

We report an open-path and anti-pollution multi-pass cell based tunable diode laser absorption spectroscopy (TDLAS) sensor, which was designed for online measurement of atmospheric H 2 O and CO 2 fluxes. It is mainly composed of two plano-convex mirrors coated on a convex surface, which makes it different from traditional multi-pass cells. This design does not allow a direct contact between the coating layer of the lens and air, thereby realizing the anti-pollution effect of the coating layer. Two DFB lasers operating at 1392 nm and 2004 nm were employed to target H 2 O and CO 2 absorption lines, respectively. Allan analysis of variance indicated that detection limits of H 2 O and CO 2 were 5.98 ppm and 0.68 ppm, respectively, at an average time of 0.1 s. The sensor performance was demonstrated by measuring CO 2 and H 2 O flux emissions at Jiangdu Agricultural Monitoring Station in Jiangsu Province. The results were compared with those obtained using the commercial instrument LI-7500, which is based on non-dispersive infrared technology. The developed gas analysis instrument exhibited good consistency with commercial instruments, and its accuracy was comparable; thus, it has strong application prospects for flux measurements in any ecosystem.

The performance of a micro-electro-mechanical system (MEMS) modulator-based dual-channel mid-infrared laser heterodyne radiometer (MIR-LHR) was demonstrated in ground-based solar occultation mode for the first time. A MEMS mirror was employed as an alternative modulator to the traditional mechanical chopper, which makes the system more stable and compact. Two inter-band cascade lasers (ICL) centered at 3.53 µm and 3.93 µm, were employed as local oscillators (LO) to probe absorption lines of methane (CH4), water vapor (H 2 O) and nitrous oxide (N 2 O). The system stability greater than 1000 s was evaluated by Allan variance. The experimental MIR-LHR spectra (acquired at Hefei, China, on February 24th 2022) of two channels were compared and were in good agreement with simulation spectra from atmospheric transmission modeling. The mixing ratio of CH 4 , H 2 O and N 2 O were determined to be ∼1.906 ppm, 3069 ppm and ∼338 ppb, respectively. The reported MEMS modulator-based dual-channel MIR-LHR in this manuscript has great potential to be a portable and high spectral resolution instrument for remote sensing of multi-component gases in the atmospheric column.

Injecting radio frequency (RF) white noise to the current driving of the laser can broaden the laser emission linewidth and efficiently suppress cavity-mode noise in off-axis integrated cavity output spectroscopy (OA-ICOS). The effect of the injected RF noise level on the cavity-mode noise and the deformation of the absorption line shape in off-axis integrated cavity output spectroscopy (OA-ICOS) with a distributed feedback laser (DFB) at 1.65 µm were investigated. We measured methane at different concentrations between 0.1 ppmv and 2 ppmv associated with a -20 dBm RF noise injection. A linear spectral response of the intensity of the cavity output spectra with the CH 4 concentration was observed. A threefold improvement in the detection limit was achieved compared to unperturbed OA-ICOS. The response time of the improved OA-ICOS system is about 30 s and the minimum detectable concentration (MDC) of CH 4 is 7.6 ppbv, which corresponds to a minimum detectable fractional absorption scaled to the path length of 7.3 × 10 -10 cm -1 . The noise equivalent absorption sensitivity of the system is 5.51 × 10 -9 cm -1 Hz -1/2 .

In this Letter, a new method is proposed to further improve the detection sensitivity of the gas sensor based on off-axis integrated cavity spectroscopy (OA-ICOS) using a distributed feedback laser operating in the near-infrared at 1653 nm that relies on implementation of wavelength modulation and radio frequency (RF) white noise perturbation to diode laser current. More than sixfold improvement in detection limit is achieved for an RF noised wavelength-modulated OA-ICOS compared to the unperturbed conventional OA-ICOS approach.

A novel method for laser frequency locking and intensity normalization in wavelength modulation spectroscopy (WMS)-based gas sensor system is reported. The center spacing between two second harmonic peaks demodulated from the rising and falling edges of a scanning triangular wave (for wavelength scan) is employed as a frequency locking reference. Amplitude of the directly acquired sine signal (for wavelength modulation) in the spectral region far away from the absorption feature is employed as an intensity normalization reference. A 50 ppm CH 4 :N 2 sample sealed in a multi-pass cell at 1 atm was employed as the target analyte for demonstration. The frequency locking significantly improves measurement accuracy, and the introduced intensity normalization method realized a ~3 times SNR improvement as compared to the commonly used 1f normalization method under frequency locking conditions. A minimum measurement precision of ~2.5 ppbv was achieved with a normalized noise equivalent absorption coefficient of 1.8 × 10 -9 cm -1 Hz -1/2 .

We overview our recent progress in the developments and applications of light emitting diode-based incoherent broadband cavity enhanced absorption spectroscopy (LED-IBBCEAS) techniques for real-time optical sensing chemically reactive atmospheric species (HONO, NO 3 , NO 2 ) in intensive campaigns and in atmospheric simulation chamber. New application of optical monitoring of NO 3 concentration-time profile for study of the NO 3 -initiated oxidation process of isoprene in a smog chamber is reported.

A novel spectrophone sensor prototype consisting of a T-shaped acoustic microresonator (T-mR) in off-beam quartz-enhanced photoacoustic spectroscopy (T-mR QEPAS) is introduced for the first time. Its performance was evaluated and optimized through an acoustic model and experimental investigation via detection of water vapor in the atmosphere. The present work shows that the use of T-mR in QEPAS based sensor can improve the detection sensitivity by a factor of up to ~30, compared with that using only a bare QTF. This value is as high as that obtained in a conventional "on-beam" QEPAS, while keeping the advantages of "off-beam" QEPAS configuration: it is no longer necessary to couple excitation light beam through the narrow gap between the QTF prongs. In addition, the T-mR is really suitable for mass production with high precision.

We applied for the first time, to our knowledge, broadband off-beam quartz-enhanced photoacoustic spectroscopy (BB-OB-QEPAS) to trace NO2 detection using a broadband blue laser diode centered at 450 nm. A detection limit of 18 ppbv (parts in 10(9) by volume) for NO2 in N2 at atmospheric pressure was achieved with an average laser power of 7 mW at a 1 s integration time, which corresponds to a 1 σ normalized noise equivalent absorption coefficient of 4.1×10(-9)  cm(-1) W=Hz(1=2). An Allan variance analysis was performed to investigate the long-term stability of the BB-OB-QEPAS-based NO2 sensor.

A wide tuning technique for mid-IR difference-frequency generation (DFG) with uniform grating periodically poled LiNbO(3) (PPLN) is presented. Based on the dispersion property of the PPLN, the quasi-phase matching (QPM) band for the pump can evolve to two separate bands, and the spacing between them can be increased with the decrease of the crystal temperature. Two such separate QPM bands can be used for increasing the idler tuning range when the crystal temperature is set to adapt the pump tuning. With the technique, an idler tuning range of 690nm is experimentally achieved with fiber laser fundamental lights.

Kalman adaptive filtering was applied for the first time, to our knowledge, to the real-time simultaneous determination of water isotopic ratios using laser absorption spectroscopy at 2.73 microm. Measurements of the oxygen and hydrogen isotopologue ratios delta(18)O, delta(17)O, and delta(2)H in water showed a 1-sigma precision of 0.72 per thousand for delta(18)O, 0.48 per thousand for delta(17)O, and 0.84 per thousand for delta(2)H, while sampling the output of the tuned Kalman filter at 1 s time intervals. Using a standard running average technique, averaging over approximately 30 s is required to obtain the same level of precision.

We report, for what is the first time to our knowledge, widely tunable mid-IR light generation over a range of greater than 1000 nm in the 4 microm region by employing a single quasi-phase-matched periodically poled niobate waveguide at room temperature. The waveguide we used was fabricated by annealed proton exchange based on periodically poled lithium niobate. A peak conversion efficiency of 10%/W and a linewidth as small as 37 MHz were achieved. The developed mid-IR light generator may find wide applications in trace gas detection of multiple atmospheric species and high-resolution spectroscopy.