Page 227 - Fiber Bragg Gratings

204 Chapter 5 Apodization of Fiber Gratings

shown in Fig. 5.6. A clear reduction of—20 dB in the side lobes is apparent
over the unapodized grating of the same strength, with a reflectivity of
90% and a FWHM bandwidth of 0.24 nm. At ±0.4 nm away from the
peak of the reflection, the reflection is less than -40 dB relative to the
peak. The fiber used for fabricating the grating was standard Corning
SMF-28, which was hydrogen loaded prior to processing. The refractive
2
index modulation profile chosen for this grating was a cos function, which
was closely reproduced in the resultant grating. This type of a filter is
difficult to fabricate using any other technology and demonstrates the
immense signal discrimination available with properly fabricated fiber
gratings.
While the process of double exposure can produce excellent results
as already demonstrated, it requires a careful study of each and every
type of fiber to be used for the fabrication of apodized gratings. The final
result depends not only on the photosensitivity and composition of the

Figure 5.6: The reflection spectrum of an unapodized grating and a cos 2
fringe envelope profile apodised grating written by the double exposure method.
The grating length is 10 mm (from: Malo B., Theriault S., Johnson D. C., Bilodeau
R, Albert J., and Hill K. O., "Apodised in-fibre Bragg grating reflectors photoim-
printed using a phase mask," Electron. Lett. 31(3), 223-225, 1995.) © IEE 1995.