The polarization dependence of optical wavelength filters is eliminated by using waveguide directional couplers. Material birefringence is used to compensate for the waveguide (electromagnetic) birefringence which is the original cause of the polarization dependence. Material birefringence is introduced...http://www.google.com/patents/US5515461?utm_source=gb-gplus-sharePatent US5515461 - Polarization-independent optical wavelength filter for channel dropping applications
