Page 334 - Fiber Bragg Gratings

Chapter 7

Chirped Fiber Bragg

Gratings

"Chirp" is the high-pitched varying sound emitted by certain birds and
bats. Gratings that have a nonuniform period along their length are
therefore known as chirped. Chirp in gratings may take many different
forms. The period may vary symmetrically, either increasing or decreasing
in period around a pitch in the middle of a grating. The chirp may be
linear, i.e., the period varies linearly with length of the grating [1], may
be quadratic [2], or may even have jumps in the period [3], A grating
could also have a period that varies randomly along its length [4], over
and above a general trend; for example, uniform, linear chirp. Chirp
maybe imparted in several ways: by exposure to UV beams of nonuniform
intensity of the fringe pattern, varying the refractive index along the

length of a uniform period grating [5], altering the coupling constant K ac
of the grating as a function of position [6], incorporating a chirp in the
inscribed grating [7], fabricating gratings in a tapered fiber [81, applying
of nonuniform strain [9] etc., many of which have been covered in Chapter
3. All these gratings have special characteristics, which are like signatures
and may be recognized as special features of the type of grating. Chirped
gratings have many applications. In particular, the linearly chirped grat-
ing has found a special place in optics: as a dispersion-correcting and
compensating device. This application has also triggered the fabrication
of ultralong, broad-bandwidth gratings of high quality, for high-bit-rate
transmission in excess of 40 Gb/sec over 100 km [10] or more [11] and in
WDM transmission [12]. Some of the other applications include chirped

311