Page 172 - Fiber Bragg Gratings

4.5 Polarization couplers: Rocking filters 149

Figure 4.9: Cross-coupled transmission for codirectional coupling. The data
shown is for coupling constants of 7r/2 with a grating length of L (curve A) and
length 2L (curve C). The bandwidth is halved for the longer grating. Also shown
is data for «L = Trand grating length L (curve B). Note that the grating overcouples
at zero detuning. The transmission spectra of LPGs are identical to that shown
above for each of the cladding modes to which the guided mode couples.

output polarization. Nondegenerate orthogonally polarized modes can
only exist in birefringent fibers and so require a periodic perturbation
equal to the beat length.
Generically, the polarization coupler behaves in a similar way to the
intermodal coupler, except that the coupling is between the two eigenpo-
larization states of the fiber rather than two different order modes. As a
result, gratings that have a uniform refractive index modulation across
the core are used rather than blazed (or tilted) gratings. Coupling between
two dissimilar order modes occurs when symmetry is broken by slanting
the grating in the direction of propagation; for coupling between the eigen-
polarization states of the same order, symmetry is broken by orienting
the grating at 45° to the polarization axes of the fiber. "Slanting" the
grating azimuthally at an angle of 45° to the birefringent axes "rocks"
the birefringence [34] of the fiber backward and forward, with a period
equal to the beat length,