Page 317 - Fiber Bragg Gratings

294 Chapter 6 Fiber Grating Band-pass Filters

Two types of polarization rocking filters can be fabricated. Birefrin-
gent polarization maintaining fiber can be used with a given predeter-
mined beat length [101], or nonbirefringent fibers, which are deliberately
made birefringent, may also be used [102]. In the first case, Hill et al. [101]
showed that a filter with only 85 periods had a polarization conversion
efficiency of 89% at a wavelength of 1292 nm in a FW bandwidth of ~8
nm. The filter length was reported to be 0.87 m, fabricated in an Andrew
4
Corp. fiber with a birefringence B = 1.27 X 10~ . The transmission spec-
trum of this filter is shown in Fig. 6.50. The narrow bandwidth is a direct
result of the high birefringence of the fiber. Apodized filters have also
been demonstrated using the same principle, but with a grating coupling
constant which is weighted as a function of length [103]. For the filter to
function properly, care needs to be taken, since the input polarization has
to remain stable.
In the second type of filter, a nonbirefringent fiber is wrapped on a
cylinder to impart a known birefringence [104] (see Chapter 3). Given
that the birefringence can be controlled, the beat length and therefore
the rocking period is known. This methods allows the fabrication of very

Figure 6.50: Transmission spectrum of an 85-step, polarization-rocking filter
fabricated in a polarization maintaining fiber (from: Hill K. O., Bilodeau R, Malo
B., and Johnson D. C., "Birefringent photosensitivity in monomode optical fibre:
application to external writing of rocking filters," Electron. Lett. 27(17), 1548,
1991. © IEE 1991. Ref. [101]).