Enhancing off-axis integrated cavity output spectroscopy (OA-ICOS) with radio frequency white noise for gas sensing

Continuous wave cavity ringdown spectroscopy incorporating with an off-axis arrangement, white noise perturbation, and optical re-injection

We present an ultra-sensitive continuous wave cavity ringdown spectroscopy (cw-CRDS) spectrometer to record high resolution spectra of reactive radicals and ions in a pulsed supersonic plasma. The spectrometer employs a home-made external cavity diode laser as the tunable light source, with its wavelength modulated by radio-frequency white noise. The ringdown cavity with a finesse of ∼105 is arranged with an off-axis alignment. The combination of the off-axis cavity and the white-noise perturbed laser yields quasi-continuum laser-cavity coupling without the need of mode matching. The cavity is further incorporated with an extra multi-pass cavity for optical re-injection of light reflected off the master cavity, which significantly increases the throughput power of the high-finesse cavity. A fast switchable semiconductor optical amplifier is used to modulate the cw laser beam to square wave pulses and to initialize timing controlled ringdown events, which are synchronized to the plasma pulses with an accuracy of ∼3 µs. The performance and potential of the cw-CRDS spectrometer are illustrated and discussed, based on the high resolution near-infrared spectroscopic detection of trace 13C13C radicals generated in a pulsed supersonic C2H2/Ar plasma with a pulse duration of ∼50 µs.

CH4/C2H6 dual gas sensing system using a single mid-infrared laser

Methane (CH₄) and ethane (C₂H₆) dual gas sensor with low system complexity and strong stability is proposed. The correction method based on absorbance spectrum is applied, and the cross-interference of C₂H₆ to CH₄ is eliminated. In the single gas concentration measurement, linear fitting is performed between the absorbance and concentration of CH₄ and C₂H₆, and the correlation coefficients of R² = 0.99959 and R² = 0.99994 are obtained respectively, which proves that the accuracy of the dual gas sensor is robust. In the dual gas concentration measurement, we carry out continuous measurement of five mixed gases and a long-term measurement of a mixture of gases, which verifies that our sensor has the fast response speed and strong stability. The minimum detectable column densities of 0.62 ppm·m for CH₄ and 0.1 ppm·m for C₂H₆ are achieved, respectively. The CH₄/C₂H₆ dual gas sensor assisted by the correction method has high sensitivity and strong robustness to cross-interference, and has great potential for application in various scenarios.

Ambient Hydrocarbon Detection with an Ultra-Low-Loss Cavity Raman Analyzer

The detection of ambient outdoor trace hydrocarbons was investigated with a multipass Raman analyzer. It relies on a multimode blue laser diode receiving optical feedback from a retroreflecting multipass optical cavity, effectively creating an external cavity diode laser within which spontaneous Raman scattering enhancement occurs. When implemented with ultra-low-loss mirrors, a more than 20-fold increase in signal-to-background ratio was obtained, enabling proximity detection of trace motor vehicle exhaust gases such as H₂, CO, NO, CH₄, C₂H₂, C₂H₄, and C₂H₆. In a 10-min-long measurement at double atmospheric pressure, the limits of detection obtained were near or below 100 ppb for most analytes.

Portable TDLAS Sensor for Online Monitoring of CO2 and H2O Using a Miniaturized Multi-Pass Cell

We designed a tunable diode laser absorption spectroscopy (TDLAS) sensor for the online monitoring of CO₂ and H₂O concentrations. It comprised a small self-design multi-pass cell, home-made laser drive circuits, and a data acquisition circuit. The optical and electrical parts and the gas circuit were integrated into a portable carrying case (height = 134 mm, length = 388 mm, and width = 290 mm). A TDLAS drive module (size: 90 mm × 45 mm) was designed to realize the function of laser current and temperature control with a temperature control accuracy of ±1.4 mK and a current control accuracy of ±0.5 μA, and signal acquisition and demodulation. The weight and power consumption of the TDLAS system were only 5 kg and 10 W, respectively. Distributed feedback lasers (2004 nm and 1392 nm) were employed to target CO₂ and H₂O absorption lines, respectively. According to Allan analysis, the detection limits of CO₂ and H₂O were 0.13 ppm and 3.7 ppm at an average time of 18 s and 35 s, respectively. The system response time was approximately 10 s. Sensor performance was verified by measuring atmospheric CO₂ and H₂O concentrations for 240 h. Experimental results were compared with those obtained using a commercial instrument LI-7500, which uses non-dispersive infrared technology. Measurements of the developed gas analyzer were in good agreement with those of the commercial instrument, and its accuracy was comparable. Therefore, the TDLAS sensor has strong application prospects in atmospheric CO₂ and H₂O concentration detection and ecological soil flux monitoring.

Isotopic trace analysis of water vapor with multipass cavity Raman scattering

Cavity-enhanced spontaneous Raman scattering was investigated as a means of simple and inexpensive isotopic water analysis. A multimode blue laser diode equipped with a feedback-generating multipass cavity provided a 100-fold Raman enhancement at a pump linewidth of 3.5 cm^-1. Samples containing trace amounts of ¹H²H¹⁶O were probed at deuterium-hydrogen concentration ratios ranging from 157 parts-per-million (local seawater) down to 8 parts-per-million (deuterium depleted water). All measurements were performed in argon or dried air at atmospheric pressure at ¹H²H¹⁶O concentrations nearing 100 parts per billion with an uncooled camera at exposure times as short as a few minutes.

Robust, fast and sensitive near-infrared continuous-filtering Vernier spectrometer

We present a new design of a robust cavity-enhanced frequency comb-based spectrometer operating under the continuous-filtering Vernier principle. The spectrometer is based on a compact femtosecond Er-doped fiber laser, a medium finesse cavity, a diffraction grating, a custom-made moving aperture, and two photodetectors. The new design removes the requirement for high-bandwidth active stabilization present in the previous implementations of the technique, and allows scan rates up to 100 Hz. We demonstrate the spectrometer performance over a wide spectral range by detecting CO₂ around 1575 nm (1.7 THz bandwidth and 6 GHz resolution) and CH₄ around 1650 nm (2.7 THz bandwidth and 13 GHz resolution). We achieve absorption sensitivity of 5 × 10^-9 cm^-1 Hz^-1/2 at 1575 nm, and 1 × 10^-7 cm^-1 Hz^-1/2 cm^-1 at 1650 nm. We discuss the influence of the scanning speed above the adiabatic limit on the amplitude of the absorption signal.

A Dual-Laser Sensor Based on Off-Axis Integrated Cavity Output Spectroscopy and Time-Division Multiplexing Method

In this article, a compact dual-laser sensor based on an off-axis integrated-cavity output spectroscopy and time-division multiplexing method is reported. A complete dual-channel optical structure is developed and integrated on an optical cavity, which allows two distributed feedback (DFB) lasers operating at wavelengths of 1603 nm and 1651 nm to measure the concentration of CO₂ and CH₄, simultaneously. Performances of the dual-laser sensor are experimentally evaluated by using standard air (with a mixture of CO₂ and CH₄). The limit of detection (LoD) is 0.271 ppm and 1.743 ppb at a 20 s for CO₂ and CH₄, respectively, and the noise equivalent absorption sensitivities are 2.68 × 10⁻¹⁰ cm⁻¹ Hz^−1/2 and 3.88 × 10⁻¹⁰ cm⁻¹ Hz^−1/2, respectively. Together with a commercial instrument, the dual-laser sensor is used to measure CO₂ and CH₄ concentration over 120 h and verify the regular operation of the sensor for the detection of ambient air. Furthermore, a first-order exponential moving average algorithm is implemented as an effective digital filtering method to estimate the gas concentration.

Development of a Wide-Range Non-Dispersive Infrared Analyzer for the Continuous Measurement of CO2 in Indoor Environments

Carbon dioxide (CO2) is an indicator of indoor air quality. Ventilation based on the use of a CO2 indicator helps to prevent people from acquiring many diseases, especially respiratory viral infections. Therefore, the monitoring of CO2 is a pivotal issue in the control of indoor air quality. A nondispersive infrared (NDIR) analyzer with a wide range of measurements (i.e., ppmv to percentage levels) was developed for measuring carbon dioxide (CO2) in an indoor environment. The effects of optical pathlength and interfering gases were investigated. The pathlengths of the analyzer were varied at 4.8, 8, 10.4 and 16 m, and the interference gases were CO; NO2; SO2; H2O; BTEX (i.e., benzene, toluene, ethylbenzene and m-/p-xylene) and formaldehyde. The lower detection limit, selectivity and sensitivity were determined to evaluate the performance of the analyzer. It was found that different pathlengths should be used to produce linear calibration curves for CO2 from ppmv to percentage levels. As a result, a wide-range NDIR analyzer, coupled with flexible pathlengths from 4.8 to 10.4 m, was developed. In terms of interference, only H2O should be taken into account due to its high concentration in indoor air. CO should be considered in some special locations at the ppmv level. The measurement errors for ppmv and the percentage levels were 0.4 and 0.9%, respectively.