Page 100 - Fiber Bragg Gratings

3.1 Methods for fiber Bragg grating fabrication 79

Figure 3.17: The projection system used to photoinscribe gratings by the
photoreduction of the phase-mask (from Rizvi N H, Gower M C, Godall F C, Arthur
G, and Herman P, "Excimer laser writing of submicrometre period fiber Bragg
gratings using a phase-shifting mask projection," Electron Lett. 31(11), 901-902,
1995, © IEE [55]).

Observation of higher-order interactions is possible with a first-order
phase mask. This is simply governed by Eq. (3.1.4). A grating written for
a Bragg wavelength A Bmg? in first order will also operate at wavelengths
A = \BragglN. Additionally, if the grating imprinted in the fiber has a
nonsinusoidal amplitude profile, for example, by effects of saturation of
the refractive index modulation or physical damage (e.g., square wave
modulation amplitude), then the grating will have Fourier frequency com-
ponents at multiples of the first order, as A = Ag/m (m — 1,2,3 . . .). These
will in turn affect the efficiency of the reflections at shorter wavelengths,
but function as first-order gratings for the Bragg wavelengths matching
each of the spatial harmonic frequencies.
High-intensity UV printing through a phase mask results in multiple-
order reflections, not least by the interference of the zero-order beam