Single mode optical fiber sensors
Single mode fibers are used for sensing when extreme sensitivity is required or when a well defined polarization of light is needed at a remote sensing point. Most sensors which use single mode fibers are of the intrinsic type (i.e. the action of the measurand on the light occurs within the fiber itself). The sensitivity advantage of single mode fibers arises because they permit the user to construct guided wave interferometers directly from the fiber itself so as to measure small phase changes in light transmitted through the measuring region. This is achieved by comparing the phase of a light wave which has traversed a sensing path with the phase of another light wave originating from the same source but arriving via a protected, reference path. The phase difference can be measured with a sensitivity of ∼10-6 of a wavelength  and the pathlength for the measuring interaction can be millions of wavelengths long. This leads to a possible measurement resolution for the optical path of one in 1012! Simultaneously, the absence of free space optical paths between sources and detectors eliminates slow alignment drifts which could easily occur if bulk-optical interferometers had been used. In practice, single mode fiber sensors tend to need very stable, highly coherent sources with low phase noise in order to gain full advantage of their potential sensitivity. When such sources are used, absolute calibration of phase difference is normally not possible and a range limit arises from the periodic nature of the interferometer output. These points will be explained later in this chapter. Recently, both of these problems have been avoided by using sources emitting in a broad wavelength range, with some compromise regarding the ultimate sensitivity achievable with any particular sensor. The concluding part of the chapter will be devoted to such sensors. Another important point to understand is that this type of sensor ultimately measures optical pathlength. Anything which changes the pathlength will therefore produce a signal. Since there are a multitude of effects which can affect the optical pathlength through a fiber, great care must always be taken to reduce or to compensate for these unwanted changes.
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