It is well known that the thermal expansion coefficient of the cladding layer can significantly influence the stress induced
birefringence in arrayed waveguide gratings. For the first time, the so-called cladding layer is divided to two parts, i.e.
upper cladding and interlayer. The effects of the thermal expansion coefficient, the Young's modulus and the Poisson's
ratio of the interlayer, the upper cladding layer and the buffer layer on thermal stresses in buried channel silica-on-silicon
optical waveguide cores are studied by use of finite element simulation. The results show that the interlayer between
waveguide cores plays the most important role in determining thermal stresses of waveguide cores. The influences of the
upper cladding layer and the buffer layer are small, though the former affects slightly larger than the later. By adjusting
the thermal expansion coefficient of the interlayer instead of the cladding layer, it is faster to minimize the stress induced
birefringence, and the thermal stresses around waveguide cores are almost symmetry. It is also shown that the thermal
stress effects of the thermal expansion coefficient and the Poisson's ratio on the cladding layer can be considered as
linear superposition of those on the interlayer and the upper cladding layer. However, this conclusion is unsuitable for
Young's modulus because of big coupling effect when the thermal expansion coefficient of the interlayer is large.
Thermal stress effects on the mode field characteristics of arrayed waveguides are studied precisely by finite element method (FEM) with plain strain model. The amplitudes of Ex and Ey are comparable for the Ex11 mode and the Ey11 mode in a homogeneous and isotropic optical channel waveguide. Considering the elasto-optic effect, the refractive indices are inhomogeneous and anisotropic. It is shown that the amplitude of Ey for the Ex11 mode and that of Ex for the Ey11 mode are much smaller than the other electric field components. The simplest method to apply thermal stress is to place a stress plate on the arrayed waveguides. It is shown that the effective indices of the two modes, especially the Ey11 mode, can be tuned by stress plates with different thickness. The order of magnitude of tune range is 10-4 whereas the material of plate is aluminum. The effective indices can be increased by attaching an aluminum plate with appropriate thickness under arrayed waveguides, and will be decreased whatever the thickness of the plate is set if attached over arrayed waveguides. However, the stress plate has small effect on the optical mode fields.
Analytical solutions are very useful to understand influence factors in the central wavelength temperature sensitivity of
AWGs (arrayed waveguide gratings) and convenient to be used in preliminary design. In this paper, the elastic multilayer
theory and stress concentration effect are combined to estimate thermal stresses in arrayed waveguides. Effective indices
and their temperature coefficients are estimated by use of effective index method. The effects of material properties on
temperature sensitivity in AWGs are completely studied. It is theoretically demonstrated that athermal AWGs can be
obtained by choosing proper material with negative TEC (thermal expansion coefficient) for the substrate. Subsequently,
the theory is extended to study the modified AWGs with a stress plate attached on the bottom, top, or both sides of
arrayed waveguides. It is shown that the temperature sensitivity can be controlled effectively by stress plates with
different TECs. After attaching a plastic plate on the bottom, the temperature sensitivity of central wavelength can be
reduced to -0.003 pm/K for TE mode and 3 pm/K for TM mode, respectively, whereas the thickness is 0.83mm.
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