Page 82 - Fiber Bragg Gratings

3.1 Methods for fiber Bragg grating fabrication 61

used as a mask for etching into the silica plate using CHF 3:Ar RIE. The
final depth of 262 nm, for use at a UV wavelength of 244 nm, is achieved
by a two-stage etch. A scanning electron microscope photograph of a phase-
mask plate is shown in Fig. 3.4. Generally, the phase mask is fabricated
in small fields, which are then stitched together to form a long grating.
Common problems with phase masks processed by e-beams have to do
with inaccurate stitching of the fields. The positioning accuracy of the e-
beam and variation of the silica mask plate height cause phase steps to
occur between fields, and the resolution of the photoresist causes random
variations in the individual periods of the grating. Techniques have been
developed to minimize these errors [46,20]; however, the random variation
in the absolute positioning of the e-beam is a fundamental limitation.
Typically, the writing of long phase-masks by e-beam needs constant
referencing and correcting owing to small temperature variations during
the exposure period, which may last several hours. These problems are
of greater importance as the length of the grating increases. Phase masks
as long as 120 mm have been reported [221, although the quality of the
fabricated Bragg grating has not been reported in detail. Stitching is
not an issue when the alternative technique of holographic phase-mask
fabrication is used. This technique is, in principle a superior method for
phase-mask production. However, long phase masks have been difficult

Figure 3.4: SEM photograph of a high-quality phase mask used for grating
inscription of ~1060-nm Bragg gratings. (Courtesy Ian Lealman, BT Labs)