Time-resolved continuous-filtering Vernier spectroscopy of H2O and OH radical in a flame

Simultaneous measurement of OH radical, H2O concentration, and temperature in a premixed CH4/air flame using TDLAS with an improved analysis method

The OH radical concentration was measured by applying tunable diode laser absorption spectroscopy, which is an in situ optical method. An optical absorption region (P7.5ff transition at 1502.7 nm) of the OH radical was selected in the near-infrared range to measure the OH radicals quantitatively in premixed CH₄/air flames. An improved direct absorption spectroscopy (DAS) method based on wavelength division multiplexing was proposed to extract the H₂O absorption signal that interfered with the OH light absorption signal, and the integral intensity of OH* chemiluminescence was compared to the measured OH radical concentration based on the improved DAS method.

Cavity-Enhanced Frequency Comb Vernier Spectroscopy

Vernier spectroscopy is a frequency comb-based technique employing optical cavities for filtering of the comb and for enhancement of the interaction length with the sample. Depending on the ratio of the cavity free spectral range and the comb repetition rate, the cavity transmits either widely spaced individual comb lines (comb-resolved Vernier spectroscopy) or groups of comb lines, called Vernier orders (continuous-filtering Vernier spectroscopy, CF-VS). The cavity filtering enables the use of low-resolution spectrometers to resolve the individual comb lines or Vernier orders. Vernier spectroscopy has been implemented using various near- and mid-infrared comb sources for applications ranging from trace gas detection to precision spectroscopy. Here, we present the principles of the technique and provide a review of previous demonstrations of comb-resolved and continuous-filtering Vernier spectroscopy. We also demonstrate two new implementations of CF-VS: one in the mid-infrared, based on a difference frequency generation comb source, with a new and more robust detection system design, and the other in the near-infrared, based on a Ti:sapphire laser, reaching high sensitivity and the fundamental resolution limit of the technique.

Robust, fast and sensitive near-infrared continuous-filtering Vernier spectrometer

We present a new design of a robust cavity-enhanced frequency comb-based spectrometer operating under the continuous-filtering Vernier principle. The spectrometer is based on a compact femtosecond Er-doped fiber laser, a medium finesse cavity, a diffraction grating, a custom-made moving aperture, and two photodetectors. The new design removes the requirement for high-bandwidth active stabilization present in the previous implementations of the technique, and allows scan rates up to 100 Hz. We demonstrate the spectrometer performance over a wide spectral range by detecting CO₂ around 1575 nm (1.7 THz bandwidth and 6 GHz resolution) and CH₄ around 1650 nm (2.7 THz bandwidth and 13 GHz resolution). We achieve absorption sensitivity of 5 × 10^-9 cm^-1 Hz^-1/2 at 1575 nm, and 1 × 10^-7 cm^-1 Hz^-1/2 cm^-1 at 1650 nm. We discuss the influence of the scanning speed above the adiabatic limit on the amplitude of the absorption signal.

Time-resolved mid-infrared dual-comb spectroscopy

Dual-comb spectroscopy can provide broad spectral bandwidth and high spectral resolution in a short acquisition time, enabling time-resolved measurements. Specifically, spectroscopy in the mid-infrared wavelength range is of particular interest, since most of the molecules have their strongest rotational-vibrational transitions in this "fingerprint" region. Here we report time-resolved mid-infrared dual-comb spectroscopy, covering ~300 nm bandwidth around 3.3 μm with 6 GHz spectral resolution and 20 μs temporal resolution. As a demonstration, we study a CH₄/He gas mixture in an electric discharge, while the discharge is modulated between dark and glow regimes. We simultaneously monitor the production of C₂H₆ and the vibrational excitation of CH₄ molecules, observing the dynamics of both processes. This approach to broadband, high-resolution, and time-resolved mid-infrared spectroscopy provides a new tool for monitoring the kinetics of fast chemical reactions, with potential applications in various fields such as physical chemistry and plasma/combustion analysis.