Reflection photoelastic tomography for the detection of axial stress distribution in planar optical waveguides

One of the key challenges on silica-based planar optical waveguides technology is to overcome waveguide intrinsic birefringence and related polarization-dependent response, affecting transmission performances. Birefringence in SiO2 waveguides mainly originates from anisotropic and asymmetric distributions of stress, arising both from the deposition process and from the difference in the thermal expansion coefficients of substrate and waveguide layers. Recently, the need of evaluate stress distributions dependence on material and structural parameters, for a more efficient preliminary waveguide design, has been put into evidence [1]. Up to now, only spatially averaged residual stress values in optical waveguides have been experimentally evaluated. Photoelastic transmission tomography has already proven adequate to reconstruct local axial stress distributions in optical fibers [2]. In this paper a novel scheme of photoelastic tomography is described, to be applied to planar waveguides. It exploits the reflection of the laser probe beam at the waveguide-substrate interface to reconstruct stress distributions in waveguide cross sections. By this technique, spurious or induced local stress variations in integrated optics components can also be brought into evidence. A scheme of the polarimetric reflection tomographic set-up is shown in Fig. 1a, where a laser probe at ? = 670 nm has been chosen to examine SiO2-doped waveguides on Si substrate, the latter being strongly absorptive. The waveguide sample is kept vertically, inside a cylindrical glass cell filled up with an air/SiO2 index matching liquid, so as to avoid spurious reflections and to leave the Snell law at the waveguide/substrate interface to determine the angles ?i and ?r of the beams entering and exiting the cell. It can be demonstrated that the proposed polarimetric set-up can advantageously handle unwanted changes in the State-of-Polarization of the probe, caused by Fresnel reflection at the SiO2/Si interface; besides, for sake of reconstruction and application of tomographic algorithms, it can be shown that reflecting tomography can be considered as a transmission tomography of a mirrored waveguide structure.