Mid-Infrared Frequency Comb for Doppler-Free Spectroscopy

One of the most important methods to identify the chemical composition of an unknown sample as well as to the excact the structure and function of its components is to measure very precisely how light of different wavelengths is being absorbed and emitted by the sample, i.e. to perform a so-called spectroscopic experiment. Such measurements have strong implications in particular for the analysis of unknown gas samples, for instance when monitoring environmental pollutants in air or for the early detection of diseases in human breath samples. However, the wavelength signals obtained when interrogating the gas sample are usually blurred. The reason for this blurring is called Doppler-effect: due to the different velocities of the individual molecules, which move especially fast when they are in gas-phase, each molecule absorbs light at a slightly different wavelength.
So-called Doppler-free saturation spectroscopy provides the possibility to get rid of this blurring and obtain high-resolution transition spectra that represent the “fingerprints” of these molecules: By saturating the absorption using counterpropagating beams of laser light, it is possible to cancel this blurring and thus measure the underlying narrow line structures. Most of these important transitions occur in the mid-infrared spectral region, i.e. at wavelengths from 2-20 μm. Nevertheless, to the best of our knowledge, no optical source in this wavelength range has been demonstrated yet with sufficient power to saturate these transitions at room temperature while providing sufficient spectral bandwidth to detect multiple transitions simultaneously. Hence, high-resolution measurements in this wavelength range have relied on cryogenically cooling the sample to eliminate the Doppler broadening, thereby loosing important information about energy states occupied at higher temperatures.
Here, we propose a system capable of performing broadband high-resolution Doppler-free saturation spectroscopy at room temperature based on a high-power mid-infrared frequency comb. Optical frequency combs are light sources which combine two main key features: each comb tooth is spectrally very narrow, thus providing a resolution that can potentially reach <10 kHz, while the large number (>10’000) of lines span a broad spectral bandwidth. We propose a source operating in the wavelength range of 3-5 um based on an optical parametric oscillator pumped by an amplified 150-MHz Ytterbium:fiber laser. The system will deliver unprecedented power per comb line (100 μW), i.e. an order of magnitude more than currently existing setups. To reach enough power per line saturate the absorption, we will recycle the comb in a so-called enhancement cavity. For the latter, we will make use of a novel technology developed in Austria: so-called crystalline mirrors, which show record-low loss performance in the mid-IR and will allow us to set up a new generation of enhancement cavities for comb-line resolved spectroscopy.