Mid-infrared optical parametric oscillators and frequency combs for molecular spectroscopy

Abstract

Nonlinear optical frequency conversion is one of the most versatile methods to generate wavelength-tunable laser light in the mid-infrared region. This spectral region is particularly important for trace gas detection and other applications of molecular spectroscopy, because it accommodates the fundamental vibrational bands of several interesting molecules. In this article, we review the progress of the most significant nonlinear optics instruments for widely tunable, high-resolution mid-infrared spectroscopy: continuous-wave optical parametric oscillators and difference frequency generators. We extend our discussion to mid-infrared optical frequency combs, which are becoming increasingly important spectroscopic tools, owing to their capability of highly sensitive and selective parallel detection of several molecular species. To illustrate the potential and limitations of mid-infrared sources based on nonlinear optics, we also review typical uses of these instruments in both applied and fundamental spectroscopy.

Mid-infrared continuous-filtering Vernier spectroscopy using a doubly resonant optical parametric oscillator

“…Advances in mid-infrared (MIR) frequency comb sources have made possible broadband high-resolution absorption spectroscopy in the molecular fingerprint region [1,2] allowing simultaneous multispecies detection [3] and precision line position measurements [4]. The coherence of comb sources enables measurements over long distances [5] and efficient coupling to multipass cells [3] and cavities [6] to enhance the interaction length with the sample.…”

Perspective: Accurate ro-vibrational calculations on small molecules

“…High resolution spectroscopy can obtain transition frequencies with exquisite precision 16 and therefore provides a stringent test of potentials. For some time now, this process has routinely been treated as an inverse problem with nuclear motion calculations used to determine spectroscopically accurate PESs using observed data.…”

Fourier transform and Vernier spectroscopy using an optical frequency comb at 3–54 μm

“…Optical frequency comb sources in the mid-infrared (MIR) wavelength range (3-12 µm) have large potential for molecular spectroscopy, since the fundamental absorption bands of many species lie in this fingerprint region [1,2]. The maximum achievable wavelength of low repetition rate (<1 GHz) direct comb sources is still limited to <3 µm, and longer wavelengths (>3 µm) can only be reached through nonlinear frequency conversion.…”

“…The figure of merit of the system is on the order of 10 −8 cm −1 Hz −1∕2 per spectral element, and multiline fitting yields minimum detectable concentrations of 10-20 ppb Hz −1∕2 for CH4, NO and CO. For the first time in the mid-infrared, we perform continuous-filtering Vernier spectroscopy using a low finesse enhancement cavity, a grating and a single detector, and measure the absorption spectrum of CH4 and H2O in ambient air at ~3.3 μm. © 2016 Optical Society of America OCIS codes : (190.4410) Nonlinear optics, (190.7110) Ultrafast nonlinear optics; (190.4970 Optical frequency comb sources in the mid-infrared (MIR) wavelength range (3-12 µm) have large potential for molecular spectroscopy, since the fundamental absorption bands of many species lie in this fingerprint region [1,2]. The maximum achievable wavelength of low repetition rate (<1 GHz) direct comb sources is still limited to <3 µm, and longer wavelengths (>3 µm) can only be reached through nonlinear frequency conversion.…”

“…The repetition rate, frep, of the pump laser is locked to an RF source referenced to a Rb clock and the carrier-envelope offset frequency is free running. The DROPO is configured in a 2.4 m long ring cavity, comprising a dielectric coated input coupler (IC), two concave gold mirrors (M1, 2) with 50 mm radius of curvature, and three plane gold mirrors (M3-5). The IC (Lohnstar, ZnSe substrate) has high reflectivity (R>99%) in the 3-6 µm range and it is anti-reflection coated for the pump wavelengths (T>85%).…”