568 Poling of Glasses and Optical Fibers silica glass, the question must be asked as to whether or not it is possible to make a fiber or planar waveguide device with such a material competitive with crystalline devices. 12.8 POLED FILMS AND WAVEGUIDES 12.8.1 Materials and Systems Planar waveguides in glasses allow for the integration of several functions such as high port count splitting and interferometry in diverse configurations. Active control of such devices is of great interest, particularly if associated with the low propagation, and fiber coupling losses are possible in glass waveguides (see, for example, reference [224]). The combination of poling with planar wave- guides makes natural use of the guided propagation in the thin induced nonlin- ear layer. Possible applications include electro-optic control of waveguides and interferometers as well as frequency conversion (e.g., SHG) in lithographically fabricated periodic QPM structures. Some of the techniques used for the manufacture of planar waveguides, such as plasma-enhanced chemical vapor deposition (PECVD), make use of synthetic materials of extremely high purity. In such systems, the contamination levels are