Fiber-Coupled Substrate-Integrated Hollow Waveguides: An Innovative Approach to Mid-infrared Remote Gas Sensors.

In this study, an innovative approach based on fiberoptically coupled substrate-integrated hollow waveguide (iHWG) gas cells for the analysis of low sample volumes suitable for remote broad- and narrow-band mid-infrared (MIR; 2.5-20 µm) sensing applications is reported. The feasibility of remotely addressing iHWG gas cells, configured in a double-pass geometry via a reflector, by direct coupling to a 7-around-1 mid-infrared fiber bundle is demonstrated, facilitating low-level hydrocarbon gas analysis. For comparison studies, two iHWGs with substrate dimensions of 50 × 50 × 12 mm (L × W × H) and geometric channel lengths of 138 and 58.5 mm, serving as miniature light-guiding gas cells, were fiber-coupled to a Fourier transform infrared spectrometer enabling broadband MIR sensing. In addition to the fundamental feasibility of this concept, the achievable sensitivity toward several gaseous hydrocarbons and the reproducibility of assembling the fiber-iHWG interface were investigated.

Substrate-integrated hollow waveguides: a new level of integration in mid-infrared gas sensing.

A new generation of hollow waveguide (HWG) gas cells of unprecedented compact dimensions facilitating low sample volumes suitable for broad- and narrow-band mid-infrared (MIR; 2.5-20 µm) sensing applications is reported: the substrate-integrated hollow waveguide (iHWG). iHWGs are layered structures providing light guiding channels integrated into a solid-state substrate material, which are competitive if not superior in performance to conventional leaky-mode fiber optic silica HWGs having similar optical pathlengths. In particular, the provided flexibility in device and optical design and the wide variety of manufacturing strategies, substrate materials, access to the optical channel, and optical coating options highlight the advantages of iHWGs in terms of robustness, compactness, and cost-effectiveness. Finally, the unmatched modularity of this novel waveguide approach facilitates tailoring iHWGs to almost any kind of gas sensor technology providing adaptability to the specific demands of a wide range of sensing scenarios. Device fabrication is demonstrated for the example of a yin-yang-shaped gold-coated iHWG fabricated within an aluminum substrate with a footprint of only 75 mm × 50 mm × 12 mm (L × W × H), yet providing a nominal optical absorption path length of more than 22 cm. The analytical utility of this device for advanced MIR gas sensing applications is demonstrated for the gaseous constituents butane, carbon dioxide, cyclopropane, isobutylene, and methane.

Camera for coherent diffractive imaging and holography with a soft-x-ray free-electron laser.

We describe a camera to record coherent scattering patterns with a soft-x-ray free-electron laser (FEL). The camera consists of a laterally graded multilayer mirror, which reflects the diffraction pattern onto a CCD detector. The mirror acts as a bandpass filter for both the wavelength and the angle, which isolates the desired scattering pattern from nonsample scattering or incoherent emission from the sample. The mirror also solves the particular problem of the extreme intensity of the FEL pulses, which are focused to greater than 10(14) W/cm2. The strong undiffracted pulse passes through a hole in the mirror and propagates onto a beam dump at a distance behind the instrument rather than interacting with a beam stop placed near the CCD. The camera concept is extendable for the full range of the fundamental wavelength of the free electron laser in Hamburg (FLASH) FEL (i.e., between 6 and 60 nm) and into the water window. We have fabricated and tested various multilayer mirrors for wavelengths of 32, 16, 13.5, and 4.5 nm. At the shorter wavelengths mirror roughness must be minimized to reduce scattering from the mirror. We have recorded over 30,000 diffraction patterns at the FLASH FEL with no observable mirror damage or degradation of performance.