Page 316 - Fiber Bragg Gratings

6.9 Polarization rocking band-pass filter 293

have not really been exploited. It is envisaged that supervisory operations
of fiber amplifiers is another possible application in which the filter that
attenuates the spectrum can allow the monitoring of the performance and
control of the amplifier characteristics.

6.9 Polarization rocking band-pass filter

Another type of band-pass filter is the polarization-converting filter. Much
of the earlier work focused on the study of photoinduced polarization
effects and fabrication of internally written filters [89-95]. The polariza-
tion sensitivity of fiber was recognized early in the development of photo-
sensitive phenomena [96]. Birefringence can be photoinduced in Ge-doped
fibers [97] because of preferential bleaching of defects [89]. Coupling be-
tween orthogonal polarizations was shown in elliptical core fibers due to
dynamic changes in the birefringence [98]. A novel counterpropagating
polarization coupling filter has also been reported, using internally writ-
ten gratings [99], It was shown that stress birefringence can be reduced
in elliptical cored fibers illuminated with UV radiation [100]. Thus, in
principle, the control of the birefringence by external means allows the
fabrication of polarization coupling devices at any wavelength. In this
section, we consider externally written filters, which can be fabricated
for operation at any desired wavelength.
The theory of coupling between polarization can be found in Chapter
4. The band-pass filter functions by rotating the polarization of the input
mode by N7J-/2. In order to discriminate between the two states, a polariz-
ing component has to be used. A schematic of the filter is shown in Fig.
6.49.

Figure 6.49: The polarization rocking "drop" filter.