Page 99 - Fiber Bragg Gratings

78 Chapter 3 Fabrication of Bragg Gratings

grating in a fiber [55,56]. The latter scheme requires the higher-order
mask to be M X the length of the photo-reduced grating, where M is
the demagnification factor, posing a problem for the production of fiber
gratings that are longer than a few millimeters. The projection scheme
enables the production of gratings with a single 20 ns pulse from a KrF
excimer laser at a wavelength of 248 nm.
One- to six-micron period gratings have been produced by projection
of an amplitude mask with multilayer stacked high-reflectivity dielectric
stripes as the pattern (5 to 120 /um wide stripes). An image demagnifica-
tion of 1:10 was used with 0.3NA optics to produce 4-mm from long grat-
ings of 6th, llth, and 12th order. Reflectivities as high as —70% were noted
for the lower-order gratings with correspondingly lower reflectivities of
8 and 2% for the llth and 12th orders, respectively. An advantage of the
projection system is that the fluence at the fiber is increased by the
demagnification, reducing the power density at the mask plate [56]. Also
noted was the imprinting of a physical grating on the surface of the fiber
cladding, penetrating some 2 /urn into the cladding. A threshold for the
2
production of the grating in the core at —0.8 J/cm was observed, while
2
1.4 J/cm was required for optimal production of the grating. It appears
that the physical damage grating in the cladding produces a phase grating
in the core due to heavy surface modification, causing light to be scattered
out of the core. Gratings formed by physical damage, known as Type II,
will be discussed in Section 3.2.
A similar technique of photoreduction can also be applied for the
projection of a phase mask and is shown schematically in Fig. 3.17. Projec-
tion of the phase mask rather than the amplitude mask overcomes the
problem of the Rayleigh-limited resolution for the 0.3NA UV transmitting
lens used in a photoreducer. The limit with the amplitude mask is 0.6
//-m. It therefore cannot be used for the production of first-order gratings
[55] with periods of —0.5 /mi. Disadvantages of the projection scheme for
use with both the amplitude and phase mask are the requirement of large-
scale high-quality UV-grade spherical optics, and the use of large-area
amplitude and phase masks. The additional cost and complexity of the
projection system may well offset the cost advantage of using coarser
features.
A 10.66-yLtm period phase mask has been photoreduced by xlO to
generate first-order gratings in a Ge-doped fiber at a Bragg wavelength
of 1530 nm. It was reported that the production of damage type II gratings
was more reproducible using the phase-mask projection scheme [55].