Tomography of a supersonically cooled molecular jet using cavity-enhanced direct frequency comb spectroscopy

Mid-Infrared Tunable Laser-Based Broadband Fingerprint Absorption Spectroscopy for Trace Gas Sensing: A Review

The vast majority of gaseous chemical substances exhibit fundamental rovibrational absorption bands in the mid-infrared spectral region (2.5–25 μm), and the absorption of light by these fundamental bands provides a nearly universal means for their detection. A main feature of optical techniques is the non-intrusive in situ detection of trace gases. We reviewed primarily mid-infrared tunable laser-based broadband absorption spectroscopy for trace gas detection, focusing on 2008–2018. The scope of this paper is to discuss recent developments of system configuration, tunable lasers, detectors, broadband spectroscopic techniques, and their applications for sensitive, selective, and quantitative trace gas detection.

Laser induced strong-field ionization gas jet tomography

We introduce a novel in-situ strong field ionization tomography approach for characterizing the spatial density distribution of gas jets. We show that for typical intensities in high harmonic generation experiments, the strong field ionization mechanism used in our approach provides an improvement in the resolution close to factor of 2 (resolving about 8 times smaller voxel volume), when compared to linear/single-photon imaging modalities. We find, that while the depth of scan in linear tomography is limited by resolution loss due to the divergence of the driving laser beam, in the proposed approach the depth of focus is localized due to the inherent physical nature of strong-field interaction and discuss implications of these findings. We explore key aspects of the proposed method and compare it with commonly used single- and multi-photon imaging mechanisms. The proposed method will be particularly useful for strong field and attosecond science experiments.

Mid-infrared virtually imaged phased array spectrometer for rapid and broadband trace gas detection

We present and characterize a two-dimensional (2D) imaging spectrometer based on a virtually imaged phased array (VIPA) disperser for rapid, high-resolution molecular detection using mid-infrared (MIR) frequency combs at 3.1 and 3.8 μm. We demonstrate detection of CH4 at 3.1 μm with >3750 resolution elements spanning >80 nm with ~600 MHz resolution in a <10 μs acquisition time. In addition to broadband detection, we also demonstrate rapid, time-resolved single-image detection by capturing dynamic concentration changes of CH4 at a rate of ~375 frames per second. Changes in absorption above the noise floor of 5×10(-4) are readily detected on the millisecond time scale, leading to important future applications such as real-time monitoring of trace gas concentrations and detection of reactive intermediates.

Optical fiber strain sensor using fiber resonator based on frequency comb Vernier spectroscopy

A novel (to our best knowledge) optical fiber strain sensor using a fiber ring resonator based on frequency comb Vernier spectroscopy is proposed and demonstrated. A passively mode-locked optical fiber laser is employed to generate a phased-locked frequency comb. Strain applied to the optical fiber of the fiber ring resonator can be measured with the transmission spectrum. A good linearity is obtained between displacement and the inverse of wavelength spacing with an R(2) of 0.9989, and high sensitivities better than 40 pm/με within the range of 0 to 10 με are achieved. The sensitivity can be proportionally improved by increasing the length of the optical fiber ring resonator.

Precision spectroscopy of Rb atoms using single comb-line selected from fiber optical frequency comb

We demonstrated the selection of a single comb-line from an optical frequency comb (OFC) of a mode-locked femtosecond fiber laser with a 250 MHz pulse repetition rate, and applied for precision spectroscopy of Rb atoms at 1529 nm. The single comb-line was selected from the fiber-OFC with a 1.5 GHz mode-spacing using spectral-mode-filtering and femtosecond laser injection-locking. When the repetition rate of the mode-locked femtosecond fiber laser was scanned over the range of 382.6 Hz at 250 MHz, we observed the double-resonance optical pumping spectra of the 5S(1/2)-5P(3/2)-4D(3/2) transition of Rb atoms using the selected comb-line of an OFC scanned over the range of 300 MHz at 196,037,213.8 MHz.

Cavity-enhanced direct frequency comb spectroscopy: technology and applications

Cavity-enhanced direct frequency comb spectroscopy combines broad bandwidth, high spectral resolution, and ultrahigh detection sensitivity in one experimental platform based on an optical frequency comb efficiently coupled to a high-finesse cavity. The effective interaction length between light and matter is increased by the cavity, massively enhancing the sensitivity for measurement of optical losses. Individual comb components act as independent detection channels across a broad spectral window, providing rapid parallel processing. In this review we discuss the principles, the technology, and the first applications that demonstrate the enormous potential of this spectroscopic method. In particular, we describe various frequency comb sources, techniques for efficient coupling between comb and cavity, and detection schemes that utilize the technique's high-resolution, wide-bandwidth, and fast data-acquisition capabilities. We discuss a range of applications, including breath analysis for medical diagnosis, trace-impurity detection in specialty gases, and characterization of a supersonic jet of cold molecules.

Absolute density-profile tomography of molecular beams using multiphoton ionization

We describe an approach for the absolute density measurement of rotationally symmetric molecular beams via multiphoton ionization. This simple single-projection tomographic technique requires only knowledge of the spatial intensity profile and ionization characteristics of the focused laser beam that probes the pulsed molecular jet. Multiphoton ionization (MPI) of a xenon beam allowed tomographic reconstruction of a two-dimensional density profile with a peak density of (4.2+/-0.4)x10(18) m(-3), which was compared with the theoretical predictions of the sudden freeze model. An analytic solution to the Abel transform is derived for Gaussian projected density profiles which greatly simplifies the reconstruction of the absolute radial density. MPI is sufficiently general that this technique can be readily applied to atomic beams with a broad range of chemistries.

Cavity ringdown spectroscopy in a hollow Bragg waveguide: electromagnetic theory and modeling

Cavity ringdown spectroscopy (CRDS) is a gas sensing technique in which an optical cavity is formed by two or more highly reflective mirrors. Herein we present an overview and historical perspective of CRDS implementations that seek to reduce or eliminate some of the disadvantages of conventional CRDS by modifications to the ringdown cavity (RDC). The hollow waveguide (HWG) CRDS concept that we introduce in this paper reduces some of the disadvantages of conventional CRDS by utilizing a hollow waveguide as the RDC. We develop the basic mathematical theory and model for the HWG-CRDS concept and provide an initial in-depth study of the Bragg waveguide for CRDS applications. We also discuss various aspects of design and performance characteristics of HWG-CRDS, including waveguide attenuation losses with and without gases in the waveguide core, transverse and longitudinal mode propagation behavior, and methods and analysis for the HWG-CRDS excitation.