The present invention provides a method of manufacturing a polarization-insensitive waveguide structure. The method comprises: depositing a buffer layer on a substrate; depositing a core layer on the buffer layer and etching the core layer so as to form a waveguide core; and depositing a silica-based cladding layer over the core by means of plasma-enhanced chemical vapor deposition (PECVD) in the absence of nitrogen or nitrogen-containing gases so as to complete the waveguide structure, wherein the cladding layer is deposited in a manner which substantially prevents polarization sensitivity in the waveguide structure. The cladding layer can be deposited with an intrinsic stress which cancels out any thermal stresses. The stress can be controlled by controlling the PECVD deposition conditions, such as power and ion bombardment.