Page 93 - Fiber Bragg Gratings

72 Chapter3 Fabrication of Bragg Gratings

will be discussed in Chapter 5. The length and the quality of the phase
mask limit the scanning technique. With the best e-beam facility, the
absolute positional accuracy is around 5 nm. This positional error sets
the limit on the stitching of the fields. Although the error is random and
the effects are averaged out over the length of the mask [44], the stitching
errors are manifest in the transfer characteristics of the grating [45]. This
causes multiple reflections and structure within the reflection envelope
determined by the field size, while the reflection bandwidth is inversely
dependent on the overall length of the grating. Techniques have be applied
to reduce the effects of stitching errors in phase masks by altering the
field size of each subgrating processed by the e-beam. By overlaying N
e-beam exposures with different field sizes, each with l/N of the dose, the
total dose required to imprint the grating pattern in the photoresist on
the phase mask is maintained while averaging out the periodic nature of
the stitching errors. Developing the resist dramatically reduces the effects
of the stitch errors, which appear as multiple out-of-band reflections. This
has been successfully demonstrated [46], and the effects on the reflection
spectrum are shown in Fig. 3.13.
Another technique, albeit used less successfully, monotonically in-
creased the field sizes for a 14-mm long grating, from 100 /urn to 200 /urn
in steps of 1.055 /um. Although many of the features were eliminated,
field sizes around 200 /am produced a cluster in the reflection spectrum
[46], since the fractional change in the field-size remains small.
Stitching errors or undesirable chirp in a phase mask are replicated
in a fiber grating. It is possible to use the technique of "UV trimming" [47]
to adjust the local refractive index in the fiber to correct the transmission
spectrum. Scanning a UV beam across a phase mask while also moving
the fiber, enables the chirp in the phase mask to be compensated for
[48,49]. By adjusting the velocity of the fiber relative to the scanning UV
beam at different positions along the phase mask, the induced refractive
index change can be changed, altering the local Bragg wavelength. If the
phase mask has an unintended chirp, the fiber grating can be "trimmed."
This technique has been applied to reduce the chirp of a grating written
by using a 100-mm long phase mask that had undesired chirp situated
close to the middle of the mask. This was found by monitoring the growth
of the reflection of a grating. The velocity is adjusted in steps (5 sec to
22 nm/sec) with the help of a piezoelectric stage, while the UV beam is
scanned at a velocity of 250 /am/sec during fabrication of a second grating.
The induced wavelength shift is directly related to the velocity of the