Characterization of Silver Halide Fiber Optics and Hollow Silica Waveguides for Use in the Construction of a Mid-Infrared Attenuated Total Reflection Fourier Transform Infrared (ATR FT-IR) Spectroscopy Probe

Abstract

Advances in fiber optic materials have allowed for the construction of fibers and waveguides capable of transmitting infrared radiation. An investigation of the transmission characteristics associated with two commonly used types of infrared-transmitting fibers/waveguides for prospective use in a fiber/waveguide-coupled attenuated total internal reflection (ATR) probe was performed. Characterization of silver halide polycrystalline fiber optics and hollow silica waveguides was done on the basis of the transmission of infrared light using a conventional fiber optic coupling accessory and an infrared microscope. Using the fiber optic coupling accessory, the average percent transmission for three silver halide fibers was 18.1 ± 6.1% relative to a benchtop reflection accessory. The average transmission for two hollow waveguides (HWGs) using the coupling accessory was 8.0 ± 0.3%. (Uncertainties in the relative percent transmission represent the standard deviations.) Reduced transmission observed for the HWGs was attributed to the high numerical aperture of the coupling accessory. Characterization of the fibers/waveguides using a zinc selenide lens objective on an infrared microscope indicated 24.1 ± 7.2% of the initial light input into the silver halide fibers was transmitted. Percent transmission obtained for the HWGs was 98.7 ± 0.1%. Increased transmission using the HWGs resulted from the absence or minimization of insertion and scattering losses due to the hollow air core and a better-matched numerical aperture. The effect of bending on the transmission characteristics of the fibers/waveguides was also investigated. Significant deviations in the transmission of infrared light by the solid-core silver halide fibers were observed for various bending angles. Percent transmission greater than 98% was consistently observed for the HWGs at the bending angles. The combined benefits of high percent transmission, reproducible instrument responses, and increased bending tolerance indicated HWGs should be preferred in the construction of a fiber/waveguide-coupled ATR probe.

Review of diverse optical fibers used in biomedical research and clinical practice

“…[72][73][74][75][76][77][78][79][80][81] Compared to conventional telecom silica fibers, IR fibers usually are more fragile and have larger refractive indices, lower melting or softening points, and greater thermal expansion. Typically, in biomedical photonics, IR fibers are used in lengths <2 to 3 m for fiber optic sensors and optical power delivery applications.…”

“…[79][80][81] These fibers can be classified into single-crystal and polycrystalline fiber types. Although there are many varieties of halide crystals with excellent IR transmission features, only a few have been fabricated into optical fibers because most of the materials do not meet the required physical property specifications needed to make a durable fiber.…”

“…Polycrystalline silver-halide fibers with AgBr cores and AgCl claddings have exhibited excellent crystalline IR fiber properties. [78][79][80][81] There are several extrinsic absorption bands for Ag-halide fibers, for example, 3 and 6.3 μm due to residual water ions. The attenuation of Ag-halide fibers is normally <1 dB∕m in the 5-to 12-μm spectral band and 0.3 to 0.5 dB∕m at 10.6 μm and can be as low as 0.2 dB∕m at this wavelength.…”

Mid-IR evanescent-field fiber sensor with enhanced sensitivity for volatile organic compounds

“…The exact fabrication procedure, as well as the mechanical, physical and optical properties of these bers, are discussed in detail elsewhere. 29,30 Briey, a single crystal is rst grown from a hot melt of AgCl and AgBr at a predened concentration of each constituent, and then is extruded through a die, to form polycrystalline bers of a desired diameter. A Ubend with a radius of approximately 0.5 cm was prepared by bending the ber gently around a small cylinder.…”

“…These spectra show the effect of increased absorbance upon adding pSi layers on the ber, as well as peaks associated with the silver-halide ber (described elsewhere), atmospheric absorption, and absorption by the components of the FTIR spectrometer. 29 The raw measured spectrum of the pSi-coated ber shows a slight overall decrease in intensity compared to the uncoated ber, and a prominent absorption band of a coated ber at 1070 cm À1 , corresponding to a Si-O-Si stretch, pointing to oxidized pSi microparticles. [31][32][33][34] Indeed, microparticles oxidize far more rapidly than bulk Si due to a larger surface area.…”