Nowadays development of infrared fibers and hollow waveguides is in good progress to meet growing market demands for laser power delivery, flexible IR-imaging, and remote process-spectroscopy in the range from 1 to 18 μm.
Silica fibers possess high transmission in a broad range from 180 nm to 2.2 μm. They can be used for spectroscopy in UV-Vis and near-infrared range. As silica fibers transmission is limited the other types of materials are used to fabricate fibers transparent in the mid-infrared range, IR-fibers are fabricated by drawing from the different types of IR-glasses, while polycrystalline fibers are extruded from solid solutions of silver halides crystals and single crystalline fibers are grown by pulling from the melt crystals.
In addition to the silica fibers, there is a variety of IR-glass fibers for 1-4 μm range, which is produced from Ge-oxide, fluoride, chalcogenide and other glasses.
IR-transmitting materials for the spectral range from 2 to 18 μm are known for many years. But not all of the types can be used for the manufacturing of uniform long-length IR fibers with low optical attenuation and acceptable mechanical bending properties. Nowadays the best fibers for 1.5-6 μm are drawn from chalcogenide IR-glass based on As₂S₃-composition.
Spectroscopy analysis in MIR spectral range is the most effective for the remote in-line control of molecular compulsion in industrial and laboratory processes. IR absorption spectra contain information about the molecular content of liquid, gas or solid, which allows to identify its chemical content and, also, to trace the changes in reaction mixture composition in real time. The use of optical fibers allows transmitting radiation from the environment to the spectrometer. Fiber probes could be connected with FTIR, TDL, QCL or IR-filter spectrometers for molecular analysis of any liquids and gases composition.
Total reflectance method (ATR) is the most suitable to measure absorption spectra for molecular composition analysis of liquids, polymers, or soft surfaces. Different shapes of ATR elements are known: prism, cone, multi-bounce plate, and fiber loop. Transmission fiber probes in mid-IR range could be used for composition monitoring in gaseous.
ATR-probes can be used in any type of FT-NIR, FT-IR, and other IR-spectrometers, photometers, and IR-LED or QCL spectral sensors. ATR immersion fiber optic probes are suitable for reaction monitoring in a lab, pilot plant and for fully automated process control.