Page 81 - Fiber Bragg Gratings

60 Chapter 3 Fabrication of Bragg Gratings

and more cost-effective simply to use a phase mask at normal incidence.
If necessary, an antireflection coating may be applied to the back facet of
the phase-mask plate to reduce reflections, which can cause the quality
of the interference fringes to be degraded [25].
The depth d of the etched sections of the grating is a function of the
UV wavelength, but the period is dependent only on the Bragg wavelength
and the effective index of the mode. However, in the case of UV writing
of gratings, it is necessary to ensure that the intensity of the transmitted
zero-order beam is minimized and, ideally, blocked from arriving at the
fiber.
To minimize the zeroth order from a UV beam normally incident on
a phase mask, the smallest etch depth d of the relief grating in silica is,

Equation (3.1.5) assumes monochromatic radiation with no divergence;
however, for a practical nonmonochromatic source, the m — 0 order cannot
be eliminated. In practice, the zeroth order can be reduced to a level of
a percent or so. At a wavelength of 244 nm, d «* 262 nm. The phase-mask
zero order can be nulled only at a single wavelength. Changing the laser
source wavelength will require a different phase mask, unless the zero
order is physically blocked. For efficient diffraction onto the first orders,
it is necessary for the relief grating to have a mark-space ratio of 1:1, or
for the corrugations in the phase mask to be purely sinusoidal.

Fabrication of the phase mask

The phase mask is normally fabricated by one of two methods: by exposure
of a photoresist overcoated, silica maskplate to an electron beam to form
the pattern [19,20], or by holographic exposure [21]. With the e-beam
facility, a silica wafer, which has a bilevel resist comprising a 450-nm
layer of AZI400-27 is hard baked at 190°C for 30 minutes, followed by a
200-nm layer of silicon-containing negative resist (SNR) baked at 85°C,
also for 30 minutes. Charge dispersal during the e-beam exposure is
effected by a evaporating a thin layer of aluminum. After exposure to
delineate the pattern, the Al coating is removed in an alkaline solution
and the SNR spray developed in MIBK for 35 sec, then rinsed in a 50:50
ratio of MIBK+IPA solution for 5 sec, followed by 15 sec in IPA. The
developed pattern is transferred to the AZI400-27 layer by reactive ion
etching (RIE) at 10 m torr in oxygen and 50 W RIE. The resist is then