Abstract
We present gapless, high-resolution absorption and dispersion spectra obtained with quantum cascade laser frequency combs covering 55 cm-1. Using phase-sensitive dual comb design, the comb lines are gradually swept over 10 GHz, corresponding to the free spectral range of the laser devices, by applying a current modulation. We show that with interleaving the spectral point spacing is reduced by more than four orders of magnitude over the full spectral span of the frequency comb. The potential of this technique for high-precision gas sensing is illustrated by measuring the low pressure (107 hPa) absorption and dispersion spectra of methane spanning the range of 1170 cm-1 - 1225 cm-1 with a resolution of 0.001 cm-1.
Highlights
Recent developments in optical frequency combs (FCs), especially in the mid-infrared (MIR) spectral region, promise improved molecular sensing for industrial, environmental and biomedical applications [1,2]
We present gapless, high-resolution absorption and dispersion spectra obtained with quantum cascade laser frequency combs covering 55 cm−1
While there are several techniques for magnitude and phase retrieval [2], the present work relies on dual comb spectroscopy (DCS), which is emerging as a powerful technique for fast and sensitive broadband molecular spectroscopy
Summary
Recent developments in optical frequency combs (FCs), especially in the mid-infrared (MIR) spectral region, promise improved molecular sensing for industrial, environmental and biomedical applications [1,2]. Frequency combs with emission in the MIR spectral region – where many relevant molecules possess strong and characteristic fundamental ro-vibrational absorption bands [3,4,5,6] – include sources based on non-linear mixing with near-infrared femtosecond fiber lasers [7,8,9,10,11], microresonator-based approaches [12,13], interband cascade lasers [14], and quantum cascade lasers (QCLs) [15,16] The latter two are of special interest because they are compact, robust, and they provide high optical power. The spectral point spacing is reduced by more than four orders of magnitude from 9.8 GHz (FSR of the lasers) down to 300 kHz over the full 55 cm−1 (1.7 THz) span of the interrogating comb, paving the way for multi-species gas detection with QCL-FCs with sub-second time resolution
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