282 l S l L Fiber Grating Band-Pass Filters 50:50 Output Output Wavelength Wavelength Figure 6.54 The CGSLI is shown with the complementary outputs of the interferometer, which are now aperiodic. The transfer function is identical to the Michelson interferometer with identical chirped gratings in each arm but with one reversed in direction. See Section 6.3.1. 6.12 GIRES­TOURNOIS FILTERS The basic principle of the Gires­Tournois interferometer (GTI) is based on a Fabry­Perot (FP) etalon. The main difference is that the two mirrors of the GTI have very different reflectivity, which allows the finesse and dispersion to be tai- lored. Figure 6.55 shows the GTI. The grating at the input has a low reflectivity, r 1 , at a fixed wavelength, l B . The rear grating with a reflectivity of r 2 is usually 100% and displaced from the input grating by a distance DL. The 100% reflectivity ensures an all-pass filter, whereas the input reflector determines the dispersion. The FP interferometer has a periodic reflection and transmission spectra as seen in Section 6.2. Figure 6.56 shows the reflection spectrum of a GTI with an input grating 0.2 mm long and a rear grating length of 0.8 mm. With a L Tx Core r 1 r 2 Input / output Figure 6.55 In fiber GTI fabricated with two gratings of reflectives, r 1 (low) and r 2 ($100%) separated by a distance DL. The gap determines the pass-bands of the filter and the device works in reflection with the dispersion being a function of wavelength.