Birefringence free planar optical waveguide made by flame hydrolysis deposition (FHD) through tailoring of the overcladding

Stresses developing in a planar waveguide resulting from the different ther mal expansion coefficients of the substrate and the three glass layers (buf fer, core and cladding) were analyzed using a finite element method. It can be shown that mainly the thermal expansion of the overcladding determines the birefringence in the finished waveguide, Based on that result, recipes for an overcladding made with the flame-hydrolysis-deposition-(FHD)-process were devised. We demonstrate the absence of birefringence in a commercial waveguide layer overclad with this glass, The high doping levels required f or the cladding to have a thermal expansion coefficient sufficient for this raises concerns about the moisture sensitivity of such a glass. We examine d the depth dependent composition of the glass using WD-ESCA (wavelength di spersive electron microprobe) and show, that at the surface a layer deplete d of dopants is formed during the high temperature sintering process. This layer can serve as a protective coating to isolate the underlying, higher d oped layer from the effects of moisture. Analysis of the stresses shows tha t this does not effect the birefringence behavior of the waveguide.