Abstract
We employed a single-mode, widely tunable (~300 cm−1) external-cavity quantum cascade laser operating around 8 µm for broadband direct absorption spectroscopy and wavelength modulation spectroscopy where a modulation frequency of 50 kHz was employed with high modulation amplitudes of up to 10 GHz. Using a compact multipass cell, we measured the entire molecular absorption band of acetone at ~7.4 µm with a spectral resolution of ~1 cm−1. In addition, to demonstrate the high modulation dynamic range of the laser, we performed direct absorption (DAS) and second harmonic wavelength modulation spectroscopy (WMS-2f) of the Q-branch peak of acetone molecular absorption band (HWHM ~10 GHz) near 1365 cm−1. With WMS-2f, a minimum detection limit of 15 ppbv in less than 10 s is achieved, which yields a noise equivalent absorption sensitivity of 1.9 × 10−8 cm−1 Hz−1/2.
Highlights
In laser-based absorption spectroscopy, ultrasensitive absorption measurements are of particular interest for various applications such as environmental monitoring [1,2] and medical breath analysis [3,4,5,6].One way to achieve highly sensitive measurements is to measure the absorption of the molecular species in the mid-infrared wavelength region, where most of them have their strongest ro-vibrational molecular transitions
The measured absorption spectrum is compared to the calculated acetone spectrum using the PNNL 2014 database and the residual is shown in the lower panel of 8Figure
The residual is shown in the pressure, path length of m), measured in ms and averaged for five times compared to simulated absorption spectrum from the PNNL 2014 database in red
Summary
In laser-based absorption spectroscopy, ultrasensitive absorption measurements are of particular interest for various applications such as environmental monitoring [1,2] and medical breath analysis [3,4,5,6]. EC-QCLs because these sources can be rapidly tuned via large modulation currents [30] In this regard, the laser frequency is modulated using a rapid current modulation to maximum achievable modulation amplitude, while it is scanned slowly via a piezo actuator (PZT) across the absorption transition of the molecule. DFB-QCLs can achieve rather wide tuning ranges up to 20 cm−1 using integrated heaters [31,32]; the dynamic of this tuning is not fast enough (usually limited to a few kHz) to be used for modulating the wavelength of the laser for WMS. We target the peak in the Q-branch absorption band of acetone (Half width Half Maximum (HWHM) ~10 GHz) around 1365 cm−1 with a high modulation amplitude to provide a frequency excursion of around 10 GHz. Scanned broadband DAS and narrowband DAS signals are compared to the data from the Pacific Northwest. 2nd harmonic (WMS-2f) as the 1st harmonic (WMS-1f) is affected by the linear background [42,43]
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