 Dev.: 0.06 dB Insertion Loss (dB) Insertion Loss (dB) Fig. 6.1-32 Insertion loss performance for FDDI connectors. Reprinted from Ref. [24], p. 46, courtesy of Molex Fiber Optics, Inc. breakthrough when Meltz et al. [34] invented a new reflected at each refractive index crest and maximum holographic method with which gratings of an (almost) reflection occurs when each partial reflection is in phase arbitrary periodicity can be written into the fiber. This with its neighbors, Fig. 6.1-34. opened up the possibility for making gratings at wave- Let's look at two specific applications of these Bragg lengths important for tele- and data communications. gratings. The first one is dispersion compensation. As Further improvements have led to the fabrication of mentioned in Section 6.1.2.3, the refractive index is gratings using phase masks [35], a method that allows a function of wavelength in the glass, SO a light pulse will gratings to be manufactured while the fiber is being always broaden in time as it propagates through the drawn. fiber. This broadening becomes a big problem in high An optical fiber grating reflects wavelengths at and bandwidth time-multiplexed systems because the close to the Bragg wavelength (AB = 2neff A). A typical pulses, if left alone, will eventually merge together and reflection spectrum is shown in Fig. 6.1-33. The equations destroy any hope of detecting individual pulses. By used to calculate the spectrum in Fig. 6.1-33 are found in using an FBG in which the periodicity varies linearly [26]. Reflection takes place because light is partially along the fiber, the grating is said to be chirped, one can Reflectivity Spectra KL = 2.03 KL = 0.81 1549.8 1549.82 1549.84 1549.86 1549.88 1549.9 Wavelength [nm] Fig. 6.1-33 Reflectivity spectra for two different values of the induced refractive index change in the core. Optical fiber, cable and connectors CHAPTER 6.1 Optical fiber z-direction of propagation Fig. 6.1-34 Buildup of back-reflected light in an optical fiber grating. delay (in time) differ