Integrated tunable narrow-linewidth wavelength filters based on TE-TM mode conversion require a high quality processing. The discussion is based on numerical simulations of typical technology-induced defects and on a comparison with experimental results. We show that uniformity of the crystal growth currently limits the linewidth to about 0.6 nm for spectral filters operating at 1.55 μm. Moreover, such a filter would require a technology with strict conditions of uniformity over several centimeters. The discussion is focused on electro-optic filters but the results can be extended to any device based on codirectional mode coupling, such as acousto-optic filters. © 2001 Society of Photo-Optical Instrumentation Engineers.