Tunable planar polymer Bragg gratings having exceptionally low polarization sensitivity

We report fabrication of planar polymer Bragg gratings that can be thermally tuned 20 nm with very low polarization sensitivity, (lambda(TE) - lambda(TM)) < +/-0.05 nm, throughout the tuning range. Polarization sensitivity in a planar film is caused by an anisotropic effective refractive index in the film typically resulting from strain. Strain in polymer Bragg gratings is caused by the photopolymerization and the thermal annealing processes utilized for the grating fabrication. For the case of polymer Bragg gratings, we show for the first time that fabrication-induced strain can be nearly eliminated by laser etching parallel trenches into the polymer layers on each side of the grating. The improvement in polarization sensitivity depends strongly on the trench separation, the thermal annealing process, and the grating tuning method.