Page 87 - Fiber Bragg Gratings

66 Chapter 3 Fabrication of Bragg Gratings

maximized [31]. The fiber should be placed in the region of the fringes
such that the propagation axis is normal to the fringe planes, since any
angular misalignment increases radiation loss from the light propagating
in the fiber (see Chapter 4) and shifts the Bragg wavelength.
If mutual counterrotation of the mirrors is incorporated in the phase-
mask interferometer, grating inscription becomes infinitely flexible with
a single phase mask. Two modifications are required if Bragg wavelength
tunability is required: the mirrors need to be rotated, and the distance
of the fiber from the phase mask must be changed. Using a translation
stage to hold the fiber in situ easily incorporates the latter. The
alignment of the interferometer is simply and quickly carried out by
using a borosilicate glass microscope coverslip to view the fluorescence
of the individual UV beams. The coverslip is moved toward or away from
the interferometer until the fluorescence from the two beams overlaps.
The fiber simply replaces the glass slide for grating inscription. Figure
3.8 shows a photograph of the fully flexible interferometer in use at BT
Laboratories.
The polarization of the UV laser beam affects the inscription of the
grating in the fiber [32-39]. To ensure that the inscribed grating has low
polarization sensitivity, the polarization of the UV laser beam should be
oriented parallel to the propagation axis of the fiber [40]. (Gratings in-
scribed with UV laser radiation polarized orthogonal to the propagation
direction show significant birefringence due to effects of induced birefrin-
gence.) This may be achieved by placing an appropriately oriented half-
wave plate before the phase mask.
Replacement of the two mirrors in Fig. 3.5 by a rectilinear UV trans-
mitting silica block [23] results in an extremely compact and stable inter-
ferometer. The diffracted UV beams enter a face of the silica block and
are totally internally reflected by adjacent sides to emerge through the
opposite face, interfering at the fiber. The beam paths are shown in Fig.
3.9.
The silica block is placed halfway between the phase mask and the
fiber. The maximum grating length, which can be written in a fiber is
related to the dimensions of the block. The length L g of the grating is a
function of the length of the side of the silica block (mirror) as