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

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Abstract
Stresses developing in a planar waveguide resulting from the different thermal expansion coefficients of the substrate and the three glass layers (buffer, 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-deposit ion (FHD)-process were devised. We demonstrate the absence of birefringence in a commercial waveguide layer overclad with this glass. The high doping levels required for the cladding to have a thermal expansion coefficient sufficient for this raises concerns about the moisture sensitivity of such a glass. We examined the depth dependent composition of the glass using WD-ESCA (wavelength dispersive electron microprobe) and show, that at the surface a layer depleted of dopants is formed during the high temperature sintering process, This layer can serve as a protective coating to isolate the underlying, higher doped layer from the effects of moisture. Analysis of the stresses shows that this does not effect the birefringence behavior of the waveguide.