For telecom purposes gratings are normally centered

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typical grating. For telecom purposes, gratings are normally centered around 1.55 μ m. The above figure is somewhat outdated and can readily be seen from present-day spectrums. Grating quality 6
has improved dramatically, with reflection spectrums having the same or nearly the same magnitude as the incoming light [17]. 1547 1548 1549 1550 0.0 0.2 0.4 0.6 0.8 1.0 Reflected Power ( μ W) Wavelength (nm) (a) 1547 1548 1549 1550 0.0 0.2 0.4 0.6 0.8 1.0 Reflected Power ( μ W) Wavelength (nm) (b) Figure 2.1.2 Illustration of a typical Bragg spectrum. Both reflection (a) and transmission (b) are shown. Etalons can be applied as ‘tunable’ devices, where the length is changed to ‘scan’ the modes of the incoming light. If the output is fed into a detector and displayed on an oscilloscope, the result will look similar to the plot in Figure 2.1.2. Fiber Bragg gratings behave in much the same way, in that any stress or strain on the core causes minute changes in the spacing of the index changes thereby causing a shift in the reflected spectrum. This feature has been used widely in the area of fiber sensing and is the basis for the operation of active fiber Bragg gratings [17]. This will be discussed in greater detail in Chapter 3.0 . 7