Frequency Comb-based Fourier Transform Spectrometer (FTS)
FTS is a well-established technique for measuring broadband absorption spectra. It essentially functions as a Michelson interferometer. When a frequency comb, comprising a train of pulses separated by 1/frep (where frep represents the comb repetition rate), is used as the light source, the FTS produces a series of bursts separated by c/frep (here c is the speed of light).
We use a fast-scanning FTS, in a so-called “tilt-compensated” configuration. At the output of the FTS, we measure two out-of-phase interferograms using an auto-balanced detector. By employing the sub-nominal sampling approach [as first reported in Maslowski et al., Phys. Rev. A 93 (2016) and Rutkowski et al., JQSRT 04 (2018)], by precisely matching the nominal resolution of the FTS to the comb repetition rate, the comb lines are accurately sampled. This enables the measurement of high-resolution molecular spectra without observable distortions by the instrumental line shape, even when the absorption line widths are narrower than the nominal resolution of the spectrometer. FTS can measure complete vibrational bands of several plasma-generated molecular species simultaneously, which make it ideal for the identification of the molecular composition of plasmas. The measurement time, however, takes a few seconds, due to the mechanical movements involved in the FTS, which limits its applicability to steady-state measurements.
Cross-dispersive Virtually Imaged Phased Array (VIPA) Spectrometers.
A VIPA spectrometer employs a cross-dispersive detection system, in which a VIPA etalon vertically disperses the comb spectrum, and a grating further disperses it horizontally. The resulting 2D dispersion image is recorded using a detector array. VIPA detection enables faster measurements, primarily limited by the camera’s collection rate.
Our innovative approach to VIPA spectroscopy involves using an air-spaced VIPA etalon, which exhibits minimal sensitivity to temperature fluctuations, unlike commonly used solid VIPA etalons typically made of CaF2 substrate. The air-spaced etalon allows resolution of low repetition rate combs without requiring an optical filter cavity. This is crucial for measuring narrow linewidth absorption profiles in short measurement time in low-pressure reactive molecular plasmas. With solid VIPA etalons, optical filter cavities are necessary to surpass the VIPA resolution limit and resolve the comb modes. This results in a ‘new’ comb with larger spacing between modes, which will necessitate longer measurement times to sample narrow linewidth absorption profiles at various repetition rates of the comb.
Cavity ringdown spectroscopy ( in the linear and saturated absorption regime)
Cavity Ringdown Spectroscopy (CRDS) is a very sensitive absorption spectroscopic technique. Its exceptional detection sensitivity is achieved through the use of optical enhancing cavities, which effectively extend the apparent absorption pathlength from a mere few centimeters to several kilometers, thanks to highly reflective mirrors.
While most CRDS demonstrations (mostly with continuous wave lasers) aim to avoid optical saturation by operating in the linear absorption regime, sensitivity enhancements have been realized through the introduction of Saturated Absorption Cavity Ringdown Spectroscopy (Sat-CRDS, or the so-called SCAR) [Giusfredi et al., Phys Rev Lett 104 (2010)]. Sat-CRDS allows for the extraction of both gas absorption and empty cavity decay constants from a single ringdown signal. This is possible because the degree of sample saturation varies during the ringdown event, leading to distinct non-exponential ringdown behavior. At the outset of the ringdown event, the sample is highly saturated (dominating empty cavity losses), but as light intensity decreases, the sample becomes less saturated and start to influence the decay in the presence of absorbing species.
We investigated the working limits and saturation dynamics of the new Sat-CRDS approach. As a model system, we have chosen CH4 absorption in argon within the mid-infrared region. Furthermore, we have introduced an innovative approach to address the challenge of cross-sensitivity.
Our proof-of-principle study overcomes cross-sensitivity by leveraging the unique optical saturation characteristics of different gas mixture components. Through intentional optical saturation to control the absorption contribution of a selected species, we have enabled the simultaneous and quantitative detection of two interfering species without the need for spectral scanning. This introduces a novel concept for “two species–one wavelength detection” (2S1W).
Selected Publications:
[1] Sadiek I, Mikkonen T, Vainio M, Toivonen J, Foltynowicz A. Optical frequency comb photoacoustic spectroscopy. Phys. Chem. Chem. Phys. 2018;20:27849-55. (Hot PCCP article).
[2] Sadiek I, Friedrichs G. Two species – one wavelength detection based on selective optical saturation spectroscopy. Sci. Rep. 2023;13: 17098/1-9.
[3] Sadiek I, Shi Q, Wallace DWR, Friedrichs G. Quantitative mid-Infrared cavity ringdown detection of methyl iodide for monitoring applications. Anal. Chem. 2017;89:8445-52.