Noise Equivalent Absorption Research Articles

Hydrogen Sulfide Detection in the Midinfrared Using a 3D-Printed Resonant Gas Cell

A fast and reliable photoacoustic (PA) sensor for trace gas detection is reported. The sensor is based on a 3D-printed resonant cell in combination with a continuous wave mode-hop-free external cavity quantum cascade laser to rapidly acquire gas absorption data in the midinfrared range. The cell is designed so as to minimize the window PA background at a selected acoustic resonance. The goal is a resonant PA cell capable of detecting the traces of gases using wavelength modulation of the laser source and second harmonic detection. The versatility and enhancement of the limit of detection at sub-ppm levels are investigated by monitoring specific lines of hydrogen sulfide (H2S). The noise-equivalent absorption normalized to laser-beam power and detection bandwidth is 1.07×10-8 W cm-1 Hz-1/2 for H2S targeting the absorption line at 1247.2 cm−1. These properties make the sensor suitable for various practical sensors for water quality applications.

Miniaturized ring-down spectrometer for CubeSat-based planetary science.

A robust, miniaturized cavity ring-down spectrometer has been developed as a laboratory demonstration model for future CubeSat deployments of near- and mid-infrared spectrometers for in situ planetary science. The spectrometer is compact enough to ensure compatibility with standard CubeSat spacecraft buses, with a probed gas volume of less than 2.5 cc to ease mass, volume, and power requirements of sample gas handling subsystems. When operated at 1.39μm for water vapor isotope measurements, a noise-equivalent absorption coefficient of 3.7×10-9 cm-1 Hz-1/2 is obtained. Oxygen isotope measurements were performed to demonstrate scanning performance. The spectrometer has been designed to use only components with functional equivalents throughout the 1-5μm range to maintain flexibility across a wide array of planetary science targets. Preliminary results from a 3.27μm implementation intended for methane measurements are also presented.

Laser frequency locking and intensity normalization in wavelength modulation spectroscopy for sensitive gas sensing.

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 CH4:N2 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-1Hz-1/2.

A fiber-tip photoacoustic sensor for in situ trace gas detection.

Most trace gas detection methods developed so far largely rely on active sampling procedures, which are known to introduce different kinds of artifacts. Here, we demonstrate sampling-free in situ trace gas detection in millimeter scale volumes with fiber coupled cantilever enhanced photoacoustic spectroscopy. Our 2.4 mm diameter fiber-tip sensor is free from the wavelength modulation induced background signal (a phenomenon that is often overlooked in photoacoustic spectroscopy) and reaches a normalized noise equivalent absorption coefficient of 1.3 × 10-9 W cm-1 Hz-1/2 for acetylene detection. To validate its in situ gas detection capability, we inserted the sensor into a mini fermenter for headspace monitoring of CO2 production during yeast fermentation. Our results show that the sensor can easily follow the different stages of the CO2 production of the fermentation process in great detail.

Versatile photoacoustic spectrometer based on a mid-infrared pulsed optical parametric oscillator.

We demonstrate the usefulness of a nanosecond-pulsed single-mode mid-infrared (MIR) optical parametric oscillator (OPO) for photoacoustic (PA) spectroscopic measurements. The maximum wavelength ranges for the signal and idler are 1.4μm to 1.7μm and 2.8μm to 4.6μm, respectively, with a MIR output power of up to 500mW, making the OPO useful for different spectroscopic PA trace-gas measurements targeting the major market opportunity of environmental monitoring and breath gas analysis. We perform spectroscopic measurements of methane (CH4), nitrogen dioxide (NO2), and ammonia (NH3) in the 2.8μm to 3.7μm wavelength region. The measurements were conducted with a constant flow rate of 300mL/min, thus demonstrating the suitability of the gas sensor for real-time trace-gas measurements. The acquired spectra are compared with data from the HITRAN database, and good agreement is found, demonstrating a resolution bandwidth of 1.5 cm1. An Allan deviation analysis shows that the detection limit for methane at optimum integration time for the PA sensor is 8ppbV (nmol/mol) at 105s of integration time, corresponding to a normalized noise equivalent absorption coefficient of 2.9×10-7 W cm-1 Hz-1/2.

Trace methane gas detection by wavelength modulated off-axis integrated cavity output spectroscopy

An instrument based on wavelength modulation off-axis integrated cavity output spectroscopy (WM-OA-ICOS) was developed for detection of trace methane (CH4) gas. The wavelength modulation technique was employed to obtain the second harmonic of the CH4 absorption signal. The modulation parameters were optimized to obtain a maximum second harmonic signal. A noise-equivalent absorption sensitivity of 9.4 × 10−11 cm−1 Hz−1/2 (corresponding to a detection limit of 28.9 ppbv for CH4 at this wavelength) was achieved using 2f detection. Compared with the traditional off-axis integrated cavity output spectroscopy (OA-ICOS) technique, WM-OA-ICOS provided an 8-fold improvement in detection sensitivity. CH4 concentration measurements were also achieved by normalization of second harmonic signal to first harmonic signal (2f/1f). The dynamic range and linearity of WM-OA-ICOS for both 2f method and 2f/1f method were investigated. The result showed that the 2f/1f method exhibited better linearity than 2f method.

Gas spectroscopy through multimode self-mixing in a double-metal terahertz quantum cascade laser.

A multimode self-mixing terahertz-frequency gas absorption spectroscopy is demonstrated based on a quantum cascade laser. A double-metal device configuration is used to expand the laser's frequency tuning range, and a precision-micromachined external waveguide module is used to enhance the optical feedback. Methanol spectra are measured using two laser modes at 3.362 and 3.428THz, simultaneously, with more than eight absorption peaks resolved over a 17GHz bandwidth, which provide the noise-equivalent absorption sensitivity of 1.20×10-3 cm-1 Hz-1/2 and 2.08×10-3 cm-1 Hz-1/2, respectively. In contrast to all previous self-mixing spectroscopy, our multimode technique expands the sensing bandwidth and duty cycle significantly.

Quartz-tuning-fork enhanced photothermal spectroscopy for ultra-high sensitive trace gas detection.

A gas sensing method based on quartz-tuning-fork enhanced photothermal spectroscopy (QEPTS) is reported in this paper. Unlike usually used thermally sensitive elements, a sharply resonant quartz-tuning-fork with the capability of enhanced mechanical resonance was used to amplify the photothermal signal level. Acetylene (C2H2) detection was used to verify the QEPTS sensor performance. The measured results indicate a minimum detection limit (MDL) of 718 ppb and a normalized noise equivalent absorption coefficient (NNEA) of 7.63 × 10-9 cm-1W/√Hz. This performance demonstrates that QEPTS can be an ultra-high sensitive technique for gas detection and shows superiority when compared to usually used methods of tunable diode laser absorption spectroscopy (TDLAS) and quartz-enhanced photoacoustic spectroscopy (QEPAS). Furthermore, when compared to an optical detector, especially a costly mercury cadmium telluride (MCT) detector with cryogenic cooling used in TDLAS, a quartz-tuning-fork is much cheap and tiny. Besides, compared to the QEPAS technique, QEPTS is a non-contact measurement technique and therefore can be used for standoff and remote trace gas detection.

Cavity ring down spectroscopy of 17O enriched water vapor near 1.73 µm

The room temperature absorption spectrum of water vapour highly enriched in 17O is recorded by cavity ring down spectroscopy (CRDS) near 1.73 µm. Two series of recordings were performed with pressure values of about 0.12 and 8.5 Torr from 5693 to 5850 cm−1. The noise equivalent absorption (αmin) of the spectra is better than 10−10 cm−1 allowing for the measurements of water lines with intensity down to 10−29 cm/molecule. A total of 1132 lines were assigned to 1220 transitions of five water isotopologues (H216O, H217O, H218O, HD16O, and HD17O). Their intensities span near five orders of magnitude from 10−29 to 10−24 cm/molecule at 296 K. The assignments were performed using known experimental energy levels as well as calculated line lists based on the results of Schwenke and Partridge.Most of the new results concern the H217O isotopologue: 483 transitions were measured while only the 27 strongest were previously known. Overall 104 levels were newly determined for H217O (64 levels), H218O (16), HD17O (23) and HD16O (1) and a few additional levels were corrected compared to the literature. The obtained results are compared to the water line list in the HITRAN2016 database, to the ExoMol energy levels and to the empirical energy levels recommended by an IUPAC task group.

All-Optical Fiber Photoacoustic Gas Sensor With Double Resonant Enhancement

An all-optical fiber photoacoustic gas sensor with double resonant signal enhancement is presented. The sensor uses a fiber-tip graphene nano-mechanical resonator operating at one of its resonances as the acoustic detector and a micro-acoustic resonant tube, whose resonant frequency is matched to that of the graphene resonator, as the PA gas cell. Compared with the single resonant detection with a graphene resonator, a five-fold signal enhancement is achieved. With 138-mW pump power and 1-s lock-in time constant, a noise equivalent concentration of ~25-ppb C2H2 is achieved, corresponding to a normalized noise equivalent absorption of $1.29\times 10^{-8}$ cm−1WHz−1/2.

Lock-in white-light-interferometry-based all-optical photoacoustic spectrometer.

An all-optical photoacoustic spectroscopy based on lock-in white-light interferometry is proposed for trace gas detection. The cavity length of the fiber-optic Fabry-Perot cantilever microphone is demodulated by a high-speed white-light interferometer, whose spectral sampling is synchronously triggered by a phased locked signal. To improve the signal-to-noise ratio, the demodulated digital photoacoustic signal is further processed by a specially designed virtual lock-in amplifier. The designed photoacoustic spectrometer has been tested for trace acetylene (C2H2) detection in the near-infrared region. The normalized noise equivalent absorption coefficient for C2H2 is achieved to be 1.1×10-9 cm-1 W Hz-1/2.

Intensity enhancement in off-axis integrated cavity output spectroscopy.

In the field of laser-based absorption spectroscopy, off-axis integrated cavity output spectroscopy is considered to be a sensitive and robust method, employing a simple optical design. However, one of the major drawbacks of non-mode-matched cavities combined with highly reflective mirrors (>99.98%) is its low output intensity. Here, we systematically investigate the increase in cavity output intensity, using a third re-injection mirror before the absorption cavity. The presented design not only enables high transmission power but also retains a long effective path length. To investigate the intensity enhancement, we used a CO2 absorption line in the near-IR wavelength region at 6240.10 cm-1. In agreement with our simulation model, we achieved an intensity enhancement factor of 38. We achieved a noise equivalent absorption sensitivity to 1.6×10-8 cm-1 Hz-1/2, which is no longer limited by the detectivity of the detector.

Absolute density measurement of nitrogen dioxide with cavity-enhanced laser-induced fluorescence**Project supported by the National Natural Science Foundation of China (Grant Nos. 11504112, 91536218, and 11604100).

The absolute number density of nitrogen dioxide (NO2) seeded in argon is measured with cavity-enhanced laser-induced fluorescence (CELIF) through using a pulsed laser beam for the first time. The cavity ring down (CRD) signal is acquired simultaneously and used for normalizing the LIF signal and determining the relationship between the measured CELIF signal and the NO2 number density. The minimum detectable NO2 density down to (3.6 ± 0.1) × 108 cm−3 is measured in 60 s of acquisition time by the CELIF method. The minimum absorption coefficient is measured to be (2.0 ± 0.1) × 10−10 cm−1, corresponding to a noise equivalent absorption sensitivity of (2.2 ± 0.1) × 10−9 cm−1·Hz−1/2. The experimental system demonstrated here can be further improved in its sensitivity and used for environmental monitoring of outdoor NO2 pollution.

A novel photoacoustic spectroscopy gas sensor using a low cost polyvinylidene fluoride film

A proof-of-concept gas sensor based on innovative photoacoustic spectrophone was introduced for purpose of developing a low cost and simple configuration photoacoustic spectroscopy gas sensor and demonstrated, in which the photoacoustic signal was detected with a low cost polyvinylidene fluoride (PVDF) film being used as transducer, instead of using conventional microphone or quartz tuning fork. In such a PVDF-based photoacoustic spectroscopy (PVDF-PAS) approach, the PVDF film plays a role both as a cantilever used in cantilever-enhanced photoacoustic spectroscopy and as a piezoelectric quartz tuning fork used in quartz-enhanced photo-acoustic spectroscopy. Feasibility of the introduced PVDF-PAS for trace gas sensing was demonstrated by measuring H2O vapor with a minimum detection limit of 40 ppmv using a lock-in time constant of 30 ms, this corresponding to a normalized noise equivalent absorption coefficient (NNEA) of 2.4 × 10−7 cm-1W/Hz1/2. The use of polyvinylidene fluoride film as transducer for photoacoustic signal sensing offers good flexibility, water resistance, chemical stability and low cost for applications under harsh or/and specific environmental conditions.

Dual-comb spectroscopy using plasmon-enhanced-waveguide dispersion-compensated quantum cascade lasers

In this Letter, we report on sub-millisecond response time mid-infrared dual-comb spectroscopy using a balanced asymmetric (dispersive) dual-comb setup with a matched pair of plasmon-enhanced-waveguide dispersion-compensated quantum cascade lasers. The system performance is demonstrated by measuring spectra of Bromomethane (CH3Br) and Freon 134a (CH2FCF3) at approximately 7.8μm. A purely computational phase and timing-correction procedure is used to validate the coherence of the quantum cascade lasers frequency combs and to enable coherent averaging over the time scales investigated. The system achieves a noise-equivalent absorption better than 1×10-3 Hz-1/2, with a resolution of 9.8GHz (0.326 cm-1) and an optical bandwidth of 1THz (32 cm-1), with an average optical power of more than 1mW per spectral element.

A 3D-Printed Microresonator Based on the Quartz-Enhanced Photoacoustic Spectroscopy Sensor for Methane Detection

A 3D-printed microresonator based on a quartz tuning fork has been developed. The sensor performance is evaluated by detection of methane with a fiber-coupled distributed feedback diode laser. In combination with wavelength modulation spectroscopy, a minimum detectable concentration of 15.6 ppb by volume is achieved at atmospheric pressure with an 8-mW average optical excitation power and a lock-in amplifier time constant of 1 s, which corresponds to a 1σ normalized noise equivalent absorption coefficient of 5·10–9 cm–1·W/Hz1/2. An Allan variance analysis shows that the system has long-term stability and can serve as the basis for a portable gas analyzer. Approaches to improve the current sensor performance are also discussed. This design greatly reduces the time of manufacturing the microresonator.

High sensitivity cavity ring down spectroscopy of the ν1 + 4ν3 band of NO2 near 1.34 µm

The very weak ν1+4ν3 Β-type absorption band of nitrogen dioxide (14N16O2) is newly detected by high sensitivity continuous wave-cavity ring down spectroscopy between 7376 and 7550 cm−1. The noise equivalent absorption of the recordings is αmin ≈ 1 × 10−10 cm−1. The ν1+4ν3 band is the highest energy B-type absorption band of NO2 detected so far by absorption within the ground electronic state. More than 800 lines are assigned with rotational quantum numbers N and Ka up to 50 and 7, respectively, what corresponds to 1117 spin-rotation-vibration transitions. No resonance perturbation of the spectrum was observed. The fitted set of the effective Hamiltonian parameters reproduces the observed line positions with an rms of 3.7 × 10−3 cm−1. A selected set of measured line intensities are used to determine the effective dipole moment parameters including two Herman-Wallis type parameters. The rms deviation of the intensity fit is 15.2%.

Adjacent-resonance etalon cancellation in ring-down spectroscopy.

Adjacent-resonance etalon cancellation provides a means of significantly reducing both systematic and random errors introduced in cavity ring-down spectroscopy by unwanted etalons. By stretching the ring-down cavity symmetrically about its center point and collecting two data sets at cavity lengths separated by λ/2, two fringing components offset in phase by 180deg are obtained. When these two data sets are averaged, oscillations in effective mirror reflectivities due to fringing are dramatically reduced. The technique is demonstrated in a 12cm monolithic ring-down spectrometer. Long-term time constant measurements show a decrease in noise-equivalent absorption and an increase in maximum effective averaging time due to a reduction in noise coupled in by etalons; trace water spectra demonstrate how removing the systematic fringing components eases absorption peak identification.

Sensitive Spectroscopy of Acetone Using a Widely Tunable External-Cavity Quantum Cascade Laser.

We employed a single-mode, widely tunable (~300 cm−1) external-cavity quantum cascade laser operating around 8 µm for broadband direct absorption spectroscopy and wavelength modulation spectroscopy where a modulation frequency of 50 kHz was employed with high modulation amplitudes of up to 10 GHz. Using a compact multipass cell, we measured the entire molecular absorption band of acetone at ~7.4 µm with a spectral resolution of ~1 cm−1. In addition, to demonstrate the high modulation dynamic range of the laser, we performed direct absorption (DAS) and second harmonic wavelength modulation spectroscopy (WMS-2f) of the Q-branch peak of acetone molecular absorption band (HWHM ~10 GHz) near 1365 cm−1. With WMS-2f, a minimum detection limit of 15 ppbv in less than 10 s is achieved, which yields a noise equivalent absorption sensitivity of 1.9 × 10−8 cm−1 Hz−1/2.

The absorption spectrum of 13CH4 at 80 K and 296 K near 1.73 µm

The absorption spectrum of 13CH4 is recorded in the center of the tetradecad near 1.73 μm (5695–5850 cm−1) at 296 K and 80 K by direct absorption spectroscopy (DAS). The achieved noise equivalent absorption of the spectra is αmin ≈ 5 × 10−8 cm−1 leading to line intensity detectivity threshold on the order of 5 × 10−26 and 5 × 10−27 cm/molecule at 296 K and 80 K, respectively. As a result, two empirical line lists are constructed including about 3350 and 2070 lines at room temperature and liquid nitrogen temperature, respectively. For comparison, the HITRAN database provides in the region only the strongest 13CH4 lines (about 440 lines in total). From the intensity ratios of the lines measured at the two temperatures, 1529 empirical values of the lower state energy level (Eemp) were derived. Their accuracy is reflected by the clear propensity of the corresponding empirical values of the lower state rotational quantum number, Jemp, to be close to integer values. They provide accurate temperature dependence for 91% and 99% of the total 13CH4 absorption in the region at 296 K and 80 K, respectively.The comparison to the TheoReTS ab initio line list shows significant deviations for line positions. The obtained experimental data will be valuable to tune ab initio line positions to empirical values in future versions of the theoretical line lists.