A particularly attractive application for IR fiber optics is their use in sensor systems. In general, silica-based fiber optic sensors have been studied for many years and they now appear in a myriad of applications. These include simple on-off-type sensors, chemical and radiometric sensors, and the sophisticated fiber optic gyros. Not surprisingly, there have been far fewer analogous applications for IR fibers. The reason for this is that IR fibers do not have as low a loss as silica fiber and, therefore, they are usually not employed in lengths longer than about 3 m. Additionally, many of the most sensitive fiber sensors are those based on the interferometric measurement of very small phase shifts. Normally SM fibers are used, and frequently they must also preserve polarization. Other than some special fluoride glass fibers, IR fibers are not SM and, therefore, interferometric fiber sensors using IR fibers are generally not an option. In fact, there is probably little advantage to using an SM IR fiber over a conventional SM fiber in such applications even if a good SM IR fiber did exist. The real advantage of IR fibers in sensor applications is simply their ability to transmit long-wavelength information. Otherwise, silica fiber optics would be the sensor fiber of choice.
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