Improved Fiber—Detector Coupling for MIR Spectroscopy Employing Shaped Silver Halide Fibers

Abstract

An improved fiber-detector coupling for mid-infrared (MIR) spectroscopy employing silver halide fibers (AgCl x Br1– x) and a mercury cadmium telluride (MCT) detector was developed. The key feature was the use of specially machined fiber pieces capable of focusing MIR radiation onto small detector elements. For this purpose new units for the machining and characterization of silver halide fibers were constructed. For fiber machining, a computer-controlled turning lathe was developed, which allowed highly precise fabrication of various fiber tips such as spherically, hyperboloidally, and paraboloidally shaped ones. The intensity distribution in space of MIR radiation emitted by the produced fibers was investigated with a dedicated MCT detector employing a flexible fiber-detector coupling. For this purpose the position of the fiber was kept constant while the light distribution in front of the fiber tip was scanned with the MCT element by moving the detector housing via stepper motors. Different fiber tip geometries were investigated for their influence on the spatial light distribution. With the use of shaped fiber tips, focusing of the IR beam was achieved, allowing the use of smaller detector elements, which in turn improves the signal-to-noise ratio in the case of MCT detection elements. Optimum conditions were found for fibers having a shape radius of 1 mm, being two times the fiber radius, whereas no significant differences between spherically and hyperboloidally as well as paraboloidally shaped fibers were observed. The developed fiber-detector coupling was furthermore combined with a fiber-optic flow-through cell for transmission measurement (optical path: 30 μm) and incorporated in a flow injection analysis system. With this experimental setup, reasonable MIR spectra of an aqueous sucrose solution could be recorded by injecting as little as 30 nL of sample containing 0.5 μg of sucrose.