A Robust Optical Sensor for Remote Multi-Species Detection Combining Frequency-Division Multiplexing and Normalized Wavelength Modulation Spectroscopy

Frequency division multiplexing and wavelength stabilized 2f/1f wavelength modulation spectroscopy for simultaneous trace CH4 and CO2 detection

A near-infrared (NIR) dual-gas sensor has been developed for simultaneous detection of atmospheric trace methane (CH4) and carbon dioxide (CO2). To realize high sensitivity and high precision, wavelength modulation spectroscopy with 2f/1f (WMS-2f/1f) detection method was adopted for eliminating laser light intensity fluctuation, and laser wavelength locking strategy based on a self-developed proportion integration differentiation (PID) algorithm was used for suppressing laser wavelength shifting effect. Two fiber-coupled DFB diode lasers with central wavelengths near 1653.7 nm and 1579.6 nm are applied for simultaneously measuring CH4 and CO2 spectra, respectively, and frequency division multiplexing (FDM) technique is employed to resolve the potential crosstalk effect. Real-time measurement of ambient atmospheric trace CH4 and CO2 was performed to demonstrate the long-term stability of the sensor system. Allan deviation analysis indicates that detection sensitivity of 0.1 ppm for CH4 and 2.27 ppm for CO2 was achieved with a 1 s average time, which can be further improved to 18 ppb and 0.3 ppm with the optimal integration time of 462 s and 392 s, respectively.

Concentration Retrieval in a Calibration-Free Wavelength Modulation Spectroscopy System Using Particle Swarm Optimization Algorithm

This paper develops a concentration retrieval technique based on the particle swarm optimization (PSO) algorithm, which is used for a calibration-free wavelength modulation spectroscopy system. As compared with the commonly used technique based on the Levenberg-Marquardt (LM) algorithm, the PSO-based method is less dependent on the pre-characterization of the laser tuning parameters. We analyzed the key parameters affecting the performance of the PSO-based technique and determined their optimal parameter values through testing. Furthermore, we conducted a comparative analysis of the efficacy of two techniques in detecting C₂H₂ concentration. The results showed that the PSO-based concentration retrieval technique is about 63 times faster than the LM-based one in achieving the same accuracy. Within 5 s, the PSO-based technique can produce findings that are generally consistent with the values anticipated.

A Width Measurement Method of Line Shape Based on Second Harmonic Peak and Modulation Amplitude

The line width of different line shapes is a very important parameter in absorption spectroscopy sensing techniques. Based on the high sensitivity and low noise properties of wavelength modulation spectroscopy, we report a novel line width measurement method. After theoretically proving the relationship between line width, modulation amplitude and the amplitude of the second harmonic at the center frequency, the absorption lines of CH₄ near 6046.96 cm^-1 and CO₂ 4989.97 cm^-1 were chosen for simulation, and the relative errors of the line width between our method and theoretical data were kept at about 1%. A distributed feedback laser diode operating near 1653 nm with three different concentrations of CH₄ was used for experimental validation, and the results were consistent with the numerical simulation. Additionally, since only the peaks of second harmonic need to be measured, the advantages of wavelength modulation can be utilized while reducing the difficulty of data acquisition.

A Laser-Based Multipass Absorption Sensor for Sub-ppm Detection of Methane, Acetylene and Ammonia

A compact, sensitive laser-based absorption sensor for multispecies monitoring of methane (CH₄), acetylene (C₂H₂) and ammonia (NH₃) was developed using a compact multipass gas cell. The gas cell is 8.8 cm long and has an effective optical path length of 3.0 m with a sampling volume of 75 mL. The sensor is composed of three fiber-coupled distributed feedback lasers operating near 1512 nm, 1532 nm and 1654 nm, an InGaAs photodetector and a custom-designed software for data acquisition, signal processing and display. The lasers were scanned over the target absorption features at 1 Hz. First-harmonic-normalized wavelength modulation spectroscopy (f = 3 kHz) with the second harmonic detection (WMS-2f/1f) is employed to eliminate the unwanted power fluctuations of the transmitted laser caused by aerosol/particles scattering, absorption and beam-steering. The multispecies sensor has excellent linear responses (R² > 0.997) within the gas concentration range of 1–1000 ppm and shows a detection limit of 0.32 ppm for CH₄, 0.16 ppm for C₂H₂ and 0.23 ppm for NH₃ at 1 s response time. The Allan–Werle deviation analysis verifies the long-term stability of the sensor, indicating a minimal detection limit of 20–34 ppb were achieved after 60–148 s integration time. Flow test of the portable multispecies sensor is also demonstrated in this work.