Page 112 - Fiber Bragg Gratings

3.1 Methods for fiber Bragg grating fabrication 91

Radic and Agrawal [85] reported that using an additional quarter-
wave phase shift within a grating opens up yet another gap. An extension
of this principle directly leads to the superstructure grating, which has
been extensively used in tunable semiconductor-laser design [861. A sche-
matic of the superstructure grating is shown in Fig. 3.24. The composite
grating consists of a number of subgratings of length AL (but not necessar-
ily of identical lengths), which are separated by "dead" zones of length 8L
(these lengths may be different). The superstructure grating was first
demonstrated in an optical fiber by Eggleton et al. [87], produced by a
phase mask. A more general problem of stitching errors in phase masks
has been addressed by Ouellette et al. [88]. Multiple reflections occur
within the bandwidth of a single subgrating; each reflection has a band-
width defined by the length of the grating without the gaps, i.e., A/AL =
L g - (N - 1)S.
The Fourier components of the grating shown in Fig. 3.24 basically
have a fundamental component with a uniform period A^ and a fundamen-
tal modulation envelope of period A e = SI + AL. Thus, the reflection
spectrum will have components at the sum and difference frequencies.
The new reflection wavelengths, ^Bragg an ^ ^Bragg> are calculated from
Eq. (3.1.4):

and

Figure 3.24: A schematic of a superstructure grating. This is constructed by
blanking (N — 1) sections of length Si in a long continuous grating of length L g. The
superstructure grating is a collection of cascaded Fabry-Perot interferometers.