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                          FIGURE 3.46  Characteristic absorption curve.


                          net loss of light from one end of the fiber to the other. All these effects
                          combine to produce a characteristic absorption curve, which graphi-
                          cally expresses a wavelength-dependent loss relationship for a given
                          fiber (Fig. 3.46). The fiber loss parameter is expressed as attenuation
                          in dB/km as follows:

                                            Loss = –10 log (p /p )
                                                           2  1
                          where p  = light power output and p  = light power input.
                                 2                       1
                             Therefore, a 10-dB/km fiber would produce only 10 percent of
                          the input light at a distance of 1 km.
                             Because of their inexpensive lead silicate glass composition and
                          relatively simple processing techniques, bundles exhibit losses in the
                          500-dB/km range. This is several orders of magnitude greater than a
                          communications-grade fiber, which has a loss of 10 dB/km. The max-
                          imum practical length for a bundle is thus only about 3 m. Further,
                          the absence of coating on individual fibers and their small diameter
                          make them susceptible to breakage, especially in vibratory environ-
                          ments.
                             Also, because of fiber microflaws, it is especially important to
                          shield fibers from moisture and contaminants. A fiber exposed to
                          water will gradually erode to the point of failure. This is true of any
                          optical fiber, but is especially true of uncoated fibers in bundles.

                          3.13.3  Fiber Pairs for Remote Sensing
                          A viable solution to those design problems that exist with fiber bun-
                          dles is to use large-core glass fibers that have losses on a par with
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