Tunable dual-comb spectrometer for mid-infrared trace gas analysis from 3 to 4.7 µm

Abstract

Dual-frequency comb spectroscopy has emerged as a disruptive technique for measuring wide-spanning spectra with high resolution, yielding a particularly powerful technique for sensitive multi-component gas analysis. We present a spectrometer based on two electro-optical combs with subsequent conversion to the mid-infrared via tunable difference frequency generation, operating in the range from 3 to 4.7 µm. The repetition rate of the combs can be tuned from 250 to 500 MHz. For 500 MHz, the number of detected comb modes is 440 with a signal-to-noise ratio exceeding 105 in 1 s. The conversion preserves the coherence of the combs within 3 s measurement time. Concentration measurements of 5 ppm methane at 3.3 µm, 100 ppm nitrous oxide at 3.9 µm and a mixture of 15 ppm carbon monoxide and 5% carbon dioxide at 4.5 µm are demonstrated with a noise-equivalent absorption coefficient of 6.4(3) x 10−6 cm−1 Hz−1/2.

Rapid Quantitative Analysis of IR Absorption Spectra for Trace Gas Detection by Artificial Neural Networks Trained with Synthetic Data

“…The high computation times of the classical evaluation result from the absence of reference spectra (where no absorbing gas is present), as complex baseline contributions to the spectra had to be covered in the fit, whereas the ANN is trained robustly against these contributions. This renders the recurrent gas exchange of the sample gases with nitrogen as in [ 23 , 33 ] unnecessary, hence reducing the experimental effort needed to acquire spectra. Additionally, no initial approximations on the gas concentrations are needed to reliably evaluate the spectra.…”

“…The spectrometer is based on electro-optic modulation of a common continuous wave laser at 1550 nm, subsequent spectral broadening in dispersion-compensating fibers, and conversion into the mid-infrared by difference frequency generation with a tunable pump source. A detailed description of the spectrometer can be found in [ 23 ]. The central wavelength of the spectrometer is set to 4.57 µm with a spectral coverage of 150 GHz (5 cm −1 ) and a mode spacing of 500 MHz (0.017 cm −1 ), which defines the effective frequency resolution.…”

“…The resulting mixture spectrum is then superimposed with a random baseline, whose parameters are randomly chosen with a uniform distribution between the values given in Table 2 , to construct a synthetic spectrum, which is inserted into the network after adding noise. The synthetic noise follows a normal distribution with a sigma value of 1.45 × 10 −2 which is derived from the noise equivalent absorbance of the spectrometer for 0.1 s [ 23 ]. At each 10th iteration within an epoch, where an epoch is defined by an iteration over 10 7 datasets, the training is stopped and the model is evaluated with validation data, that are synthesized the same way as the training data but are never seen by the model while training.…”

“…The proposed synthetic training was evaluated on measurements taken with a mid-infrared dual frequency comb spectrometer, suited for the analysis of multi-component gas mixtures [ 23 , 24 ]. For the application demonstration, the spectrometer system was set up for the detection of CO and N 2 O in a wavelength band around 4.57 µm, which shows overlapping absorption profiles for both gases.…”

Compact mid-infrared dual-comb spectrometer over 3–4 μm via intra-pulse difference frequency generation in LiNbO3 waveguides

“…In particular, frequency comb spectroscopy in the mid-infrared (MIR, 3-5 µm) region of the atmospheric window has made significant progress in various applications, such as greenhouse gas monitoring, atmospheric monitoring, green agriculture and breath analysis [1][2][3][4] . Generally, these MIR frequency combs can be obtained via χ (2) and χ (3) nonlinear processes in lithium niobate (LN) [5][6][7][8][9] , which is a widely used optical material that can provide a wide transparency window, high nonlinear coefficient and electro-optical effect for integrated chip-based laser devices [10] . In comparison with bulk crystals, periodically poled LN (PPLN) waveguides with strong light confinement ensure high-efficiency nonlinear frequency conversion over a long interaction path.…”