About stress modeling:
One bottleneck to the widespread application of planar waveguide circuits is polarization dependent loss (PDL). An important originating source of PDL is stress, in the waveguide layers of a circuit, built-up either during manufacture or device operation. This stress causes an anisotropic change of the refractive index of the waveguide material. Due to this anisotropy, the planar waveguide circuit is likely to exhibit PDL. Stress-induced PDL is currently a significant yield killer for many different planar waveguide technologies. In order to minimize PDL it is paramount that design engineers have a good quantitative understanding of stress distributions, in planar waveguides, and their impact on the optical properties of components. The stress module of OlympIOs is the first commercially available modeling tool that integrates stress and optical modeling capabilities.

Stress module benefits:
The current version of the stress module deals with two different cases of stress origin. The first calculates stress build up during cooling, to room temperature at the end of the manufacturing process, as the result of different expansion coefficients between waveguide layers and substrate. The second calculates the stress induced when operating a thermally activated device (such as a VOA or optical switch to exploit the thermo-optic effect). The polarization dependent refractive index perturbation can be added as an overlay to the background refractive index distribution. Subsequently, all sorts of optical simulations can be performed using various optical simulation modules available within OlympIOs. Thanks to the extensive parameterization and optimization capabilities available within the platform, optimum solutions with low birefringence can be conveniently located.

Features:

• Calculates stress induced by process temperature
• Calculates stress induced by heater electrodes
• Integrates stress with optical modeling.
• Adaptive (non-equidistant) grids

Generic simulation features:

• Extensive parameterization capabilities
• Vary runs
• Material library

Benchmark study:
Analysis of a structure previously discussed by Kilian et al. [1]: A typical silica on silicon substrate waveguide structure with, in this case, core and upper cladding layers deposited using the flame hydrolysis method (FHD).
When cooling, to room temperature, the glass layer is compressed by the (thick) silicon substrate, which has a higher coefficient of expansion. The consequence would be a non-zero TE/TM spectral shift for any interferometric component (AWG or Mach-Zehnder based WDM) fabricated using this technology. Adjusting the doping levels of the cladding to increase its thermal expansion coefficient can minimize the stresses involved, arriving at net zero birefringence [1].

Figure 2 shows simulated net birefringence as a function of the cladding thermal expansion coefficient (tce); confirming that net zero birefringence can be achieved with a top cladding expansion coefficient of about 3.7 10-6 1/K. This is only slightly higher than the optimum value of 3.5 10-6 1/K predicted in [1]. Note that Kilian et al took into account only the average refractive index change in the core region, whereas the calculations presented here are more rigorous taking into account the full refractive index perturbation.

1. A. Kilian et al., "Birefringence free planar optical waveguide made by flame hydrolysis deposition (FHD) through tailoring of the overcladding", Journal of Lightwave Technology, Vol. 18, no. 2, pp. 193-198, 2000.