Intensity enhancement in off-axis integrated cavity output spectroscopy

Re-injection off-axis integrated cavity output spectroscopy for the simultaneous detection of N2O, H2O and CO with a mid-infrared QCL laser

Off-axis integrated cavity output spectroscopy (OA-ICOS) has attracted much interest because it potentially allows highly sensitive field measurements with robust optical alignment. In this paper, a novel instrument that employs a high-finesse optical cavity as an absorption cell has been developed for sensitive measurements of multi-component gases N2O, H2O and CO in the atmosphere based on a mid-infrared quantum cascade laser (QCL) and OA-ICOS. In order to improve the energy utilization and increase the signal-to-noise ratio (SNR) of the signal, a new type of optical path structure of the laser re-injection method is adopted. Furthermore, the system performance can be effectively improved by using a new intervention method of injecting radio frequency (RF) white noise into a laser driver to suppress cavity mode noise and combining the wavelength modulation method (WMS). We compared the sensitivity of the second harmonic signal demodulation between the re-injection method and the standard OA-ICOS, and the SNR increased by 2.68 times compared to the latter. Analysis of the spectral measurements with Allan variance indicates that within an integration time of 1 s, the measurement accuracy of N2O, H2O, and CO is 6.71 ppb, 13.945 ppm, and 1.81 ppb, respectively, and within an integration time of 820 s, the measurement accuracy of N2O, H2O, and CO can be further improved to 1.26 ppb, 2.089 ppm, and 172 ppt, respectively. Our approach represents an underlying analytical method that provides guidelines for monitoring of representative gases in the atmosphere, industrial processes, emergency safety, etc.

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.

Multi-laser sensor for simultaneous multi-gas measurements using off-axis cavity-enhanced absorption spectroscopy with an opposite two-way configuration

An opposite two-way off-axis cavity-enhanced absorption spectroscopy-based multi-gas sensor is reported. More than two lasers can be employed in the sensor for simultaneous detection of different gas species. An approximately two times improvement in magnitude of the 2f signal and the signal-to-noise ratio is achieved because the concave spherical mirrors outside each end of the cavity and the narrow bandpass filters before the detectors can act as re-injection mirrors to re-inject the light into the cavity in the scheme. The performance of the sensor is demonstrated by simultaneous measurement of CO₂ and CH₄ in the atmosphere. This Letter highlights a new, to the best of our knowledge, strategy for simultaneous multi-gas measurement in a single integrated cavity by employing as many as four lasers.

Measurement of CO2 by Wavelength Modulated Reinjection Off-Axis Integrated Cavity Output Spectroscopy at 2 μm

A high sensitivity wavelength modulated reinjection off-axis integrated cavity output spectroscopy (WM-RE-OA-ICOS) experimental setup was built at a 2 μm band. On the basis of an off-axis integrated output spectroscopy (OA-ICOS), combined with an optical reinjection (RE) approach to improve signal intensity, and wavelength modulation spectroscopy (WMS) to improve the signal-to-noise ratio (SNR) of the system, the experimental study of trace CO2 with high sensitivity was carried out using the setup. The performance was compared and evaluated, and the results show that: Compared with the OA-ICOS, the wavelength modulated reinjection OA-ICOS enhanced the signal intensity by 6.3 times, and the SNR increased 7.2 times from 179 to 1288. The Allan variance results showed that the detection limit of the system is 0.35 ppm when the average system time is 230 s. The setup was used to measure the indoor CO2 concentration for a long time (22 h), and the measured results were in line with the actual concentration change. The proposed method shows good performance enhancement for the OA-ICOS system in trace gas measurements.

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.

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

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 CH₄ 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 CH₄ 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^-1Hz^-1/2.

High-sensitivity off-axis integrated cavity output spectroscopy implementing wavelength modulation and white noise perturbation

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.