Static and Dynamic Measurements in an Electrical Discharge using Mid-Infrared Dual-Comb Spectroscopy

Strong ro-vibrational transitions of molecules in the mid-infrared wavelength region, coupled with fast data acquisition of dual-comb spectroscopy (DCS), yields an effective tool for time-resolved simultaneous monitoring of the concentrations of different species involved in a gas-phase chemical reaction [1]. Here we used the broadband and high resolution properties of mid-infrared DCS, to monitor the static and dynamic concentrations of methane and the reaction products, in an electrical discharge. The mid-infrared dual comb source is based on an optical parametric oscillator (OPO) in a singly resonant cavity configuration, containing two temperature stabilized, 5 mm long, MgO: PPLN crystals. The crystals are pumped by two mode-locked Yb:fiber lasers, counter propagating and crossed polarized, emitting around 1040 nm. The two Yb:fiber lasers have a slightly different repetition rate (Δf
_rep
≈ 250 Hz) near 90 MHz [2]. The two coherent mid-infrared output combs, emitted from the OPO, are combined and sent through a discharge tube filled with methane. The discharge tube is 50 cm long, with an internal diameter of 3 mm, and is water cooled. The cathodes are at the two ends of the tube and the anode as at the center. A stabilized high-voltage (HV) power supply (25 kV, 50 mA) is used for making a DC discharge in the tube. To monitor the dynamic events in the discharge, the current of HV power supply is modulated (at 2 Hz) by an external signal generator. After the discharge tube, the two combs are focused on a single TE cooled HgCdTe photodetector (bandwidth 50 MHz). The time domain interferogram is recorded by a field-programmable gate array (FPGA) and post-processed to yield the spectrum after a Fourier transform. The voltage and current of the HV power supply, as well as the flow rate and the pressure of the sample gas are adjusted depending on the discharge operation.