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

Abstract

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 1.4 µm to 1.7 µm and 2.8 µm to 4.6 µm, respectively, and with a MIR output power of up to 500 mW. 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) nitrodioxide (NO2) and ammonia (NH3) in the 2.8 µm to 3.7 µm wavelength region. The measurements were conducted with a constant flow rate of 300 ml/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 cm 1 . An Allan deviation analysis shows that the detection limit for methane at optimum integration time for the PA sensor is 8 ppbV (nmol/mol) at 105 seconds of integration time, corresponding to a normalized noise equivalent absorption (NNEA) coefficient of 2.9×10 −7 W cm −1 Hz −1/2 .

Intrinsic Spectral Resolution Limitations of QEPAS Sensors for Fast and Broad Wavelength Tuning

“…For highly sensitive and selective PAS measurements it is desirable to have a MIR light source with high power, fast wavelength sweeping and large wavelength tunability covering the 2.8 m to 4.4 m region, where most trace gasses have strong vibrational transitions. For this a nanosecond pulsed MIR-OPO is a good candidate, since it provides good molecular selectivity, due to large wavelength tunability, relative narrow linewidth, and high optical output powers [ 34 , 35 ]. The MIR-OPO used is based on a 30 × 20 × 1 mm periodically poled lithium niobate (PPLN) nonlinear crystal with a fanned-out structure of poling period ranging from 27.4 m to 31.75 m (HC Photonics).…”

“…This leads to a degraded spectral resolution of the QEPAS sensor, when performing measurements at fast spectral tuning rates. Consequently, when performing measurements on a complex gas mixture, where cross-interference effects are present, it becomes difficult, or almost impossible, to identify individual gases and quantify their concentrations [ 34 ].…”

Multicomponent gas sensors based on photoacoustic spectroscopy technology: A review

“…Lamard L et al demonstrated a single longitudinal mode pulsed OPO laser. 69 The OPO delivered an average power output of 345 mW, with continuous tunability in the range of 2.8-4.6 μm, which achieved spectroscopic analysis of CH 4 , NO 2 , and NH 3 with the resolution of 1.5 cm −1 . And the LODs of CH 4 and NO 2 was 8 ppb and 1.6 ppm (1σ).…”

LED-Based Photoacoustic NO2 Sensor with a Sub-ppb Detection Limit

“…3 Mid-infrared laser sources have been used for PA detection of NO 2 . 4,5 Detection limit down to 0.5 ppb was reached with a quantum cascade laser emitting at the wavelength of the strong vibrational transitions of NO 2 around 6250 nm, 5 but laser sources at long wavelengths are often expensive and have limited wall-plug efficiencies in the order of a few percent. Fortunately, NO 2 absorbs strongly also in the visible wavelength range, where detection limits below 1 ppb have been demonstrated in PA measurements, with lasers as light sources.…”

“…As opposed to most other sensing techniques based on optical absorption, PA detectors have great potential toward miniaturization because the PA signal is inversely proportional to the sample volume . Mid-infrared laser sources have been used for PA detection of NO 2 . , Detection limit down to 0.5 ppb was reached with a quantum cascade laser emitting at the wavelength of the strong vibrational transitions of NO 2 around 6250 nm, but laser sources at long wavelengths are often expensive and have limited wall-plug efficiencies in the order of a few percent. Fortunately, NO 2 absorbs strongly also in the visible wavelength range, where detection limits below 1 ppb have been demonstrated in PA measurements, with lasers as light sources. − The high sensitivity is achieved using high-power lasers and resonant acoustic cells.…”