Page 296 - Fiber Bragg Gratings

6.6 The polarising beam splitter band-pass filter 273

overcoupling, the device is wavelength sensitive, with a cyclic coupling
response shown in Fig. 6.37. It has excellent polarization isolation over
a narrow band of wavelengths but may be designed to operate at any
desired wavelength by adjusting the parameters at the time of fabrication.
Figure 6.38 shows a schematic of the polarization splitting coupler
in operation with Bragg gratings in ports 3 and 4. When operated at the
correct wavelength, the polarization splitter will cross-couple only one of
the two orthogonal polarization states in the input port 1, while the other
propagates unaffected. If gratings are placed at the output ports of such a
coupler, light at the Bragg wavelength is reflected and both polarizations
are coupled back to the input port 1. If a quarter-wave plate is placed just
before a grating and oriented such that at the output of the wave plate it is
left-circularly polarized, on reflection from the grating it becomes right-
circularly polarized. Traversing the wave plate once more the linearly polar-
ized output of the wave plate is orthogonal to the incoming polarization. At
the coupler, this state of polarization remains uncoupled and is routed to
port 2 of the coupler. The same applies to the orthogonal polarization state,
which also couples to port 2. Thus, as in the Michelson arrangement, the
device operates as a polarization-independent band-pass filter [72]. Each

Figure 6.37: The transmission characteristics of the two orthogonal polariza-
tion states [72] at the output of the polarizing beam splitter. The extinction is
well over 30 dB.