Reflection intensity optical fiber sensors for the mid-infrared

Two kinds of reflection intensity sensor made of chalcogenide glass fiber for the mid-IR region are demonstrated. One is a double-fiber reflection sensor based on two tied fibers with a gold-coated hollow metal waveguide connected to the far end of the fibers. The other is a single-fiber reflection sensor based on contact couplers. These reflectance sensors were coupled to a Fourier-transform IR spectrometer by a unique accessory based on nonimaging concentrators. This setup was built to measure absorption spectra of a polymer coating of an aluminum can and a sheet of drafting paper. A theoretical model treating the ratio between the signal from the target and the background is introduced. This model was helpful in deriving the sensitivity characteristics of the sensors from experimental absorption peak heights. Hence, the absorption peaks heights that we obtained using a single-fiber reflection sensor with a symmetric coupler were nearly 50% relative to those obtained with a double-fiber reflection sensor.