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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Optical Frequency Comb Photoacoustic Spectroscopy
摘要： Photoacoustic spectroscopy (PAS) based on continuous wave (cw) lasers provides high absorption sensitivity in small sample volume [1, 2] but it is usually restricted to single species detection because of the limited tunability of cw lasers. Broadband PAS has been demonstrated using cantilever-enhanced detectors in combination with incoherent [3] or supercontinuum [4] light sources modulated by conventional Fourier transform spectrometers (FTS), however, the spectral resolution was limited to a few cm-1. Here we report the first demonstration of optical frequency comb photoacoustic spectroscopy (OFC-PAS), which combines the wide spectral coverage and high resolution of frequency combs with the small sample volume of photoacoustic detection [5]. The OFC-PAS setup is based on a doubly-resonant optical parametric oscillator pumped by a femtosecond Tm:fiber laser with a repetition rate of 125 MHz. The intensity of the signal output centered at 3.3 μm is modulated at ~500 Hz using an FTS. One output of the FTS (4.8 mW of signal power) is directed to a 10-cm-long cantilever-enhanced photoacoustic cell with 8 mL volume (Gasera, PA201), while the other is measured with a photodetector and used for normalization of the PA spectrum. Figure 1(a) shows in red the room temperature OFC-PAS spectrum of the C-H stretch band of 100 ppm of CH4 in N2 at 1000 mbar and 1 GHz resolution (left axis) recorded in 200 s (no averaging). The blue curve shows the simulated absorption coefficient, (cid:68), based on the parameters from the HITRAN database (right axis). A zoom-in around the Q-branch region at 1000 mbar and 400 mbar is shown in Figs 1(b) and 1(c), demonstrating the high spectral resolution, no instrumental lineshape distortion, and good agreement with the simulation. The limit of detection (LOD), evaluated from the signal to noise ratio of the methane line at 3058 cm-1 at 1000 mbar, is 0.8 ppm in 200 s for power spectral density of 42 μW/cm-1. While this LOD is comparable to those obtained with other broadband PAS methods [3, 4], the resolution of OFC-PAS is more than two orders of magnitude better. The normalized noise equivalent absorption at 1000 mbar is 8 × 10-10 W cm-1 Hz -1/2, comparable to values reported with PAS based on cw lasers [2]. OFC-PAS thus extends the capability of optical sensors for high-resolution multicomponent trace gas analysis in small sample volumes.
关键词： high-resolution spectroscopy,optical frequency comb,trace gas analysis,Photoacoustic spectroscopy
更新于2025-09-12 10:27:22
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Sensitivity Enhancement of a Cantilever-Enhanced Photo-Acoustic Spectroscopic Sensor by an Optical Build-Up Cavity
摘要： Laser photo-acoustic spectroscopy (LPAS) is a highly sensitive and selective method for trace gas analysis. One of the most advanced LPAS techniques is cantilever-enhanced photo-acoustic spectroscopy (CEPAS), which can reach down to low-ppt and sub-ppt level trace gas detection sensitivities with high power lasers [1,2]. The selection of wavelengths and tunability of high power narrow linewidth lasers is, however, limited. They are also rarely suitable for field deployable analysers, which would require a small and robust form factor. An alternative approach, potentially overcoming these weaknesses, is to build up the optical power of a standard distributed feedback diode laser (DFB) or a quantum cascade laser in an external optical cavity [3,4]. Here, we show the first demonstration of enhancing the sensitivity of a CEPAS sensor with an external optical build-up cavity, a technique which we call the cavity-enhanced CEPAS (CE-CEPAS). We achieve an unprecedented normalised noise equivalent absorption (NNEA) value of 1.75×10-12 W cm-1 Hz -1/2. In our work, the NNEA results in 75 ppt noise equivalent concentration for C2H2 with a 10 s integration time in the 1530 nm wavelength range. Compared to standard CEPAS, the detection limit is better by a factor of 100, which corresponds to the power build-up factor (BUF) of our cavity. With operation in the near-infrared region, we benefit from the highly reliable and inexpensive components of the telecommunications industry, while achieving a detection sensitivity that is comparable to the state-of-the-art results obtained in the mid-infrared region, where the molecular absorption lines are typically two orders of magnitude stronger.
关键词： CEPAS,cantilever-enhanced photo-acoustic spectroscopy,CE-CEPAS,optical build-up cavity,trace gas analysis,cavity-enhanced CEPAS
更新于2025-09-11 14:15:04