Strongly Confined SU-8Optical Waveguides And Devices

As optical communication and sensing technologies progress rapidly, optical integrated devices, especially optical filters, attract more and more interests because of their low cost and compactness. Among various materials for optical integration, polymer shows superior performances in terms of cost reduction and low loss. On the other hand, to achieve lower cost and better performance, higher integration density is desirable for optical integrated devices, where the strongly confined waveguide with a high refractive index contrast is a prerequisite. In this work, two kinds of strongly confined SU-8polymer waveguides, namely, ridge and suspended waveguides are studied systematically. Based on these waveguides, two typical optical filters, i.e., arrayed waveguide gratings (AWG) and microring resonators (MRR), are developed for practical use.The fundamentals of optical waveguides are briefly reviewed. While the slab waveguide mode can be solved analytically, numerical simulation is necessary to solve waveguide problems with more complex situations. Several numerical simulation methods, such as finite difference method (FDM), beam propagation method (BPM), and the finite-difference time-domain (FDTD) method, are introduced. These methods are intensively used in the following.The SU-8polymer ridge waveguides with strong light confinement are studied. The fabrication process is optimized and the fabricated waveguides perform well with a low loss of0.24～0.15dB/mm. According to the measurement results, this kind of waveguide can afford a bending radius as small as75μm, indicating its large potential to increase the integration density.Two typical optical filters, i.e., AWG and MRR, are realized based on the SU-8polymer ridge waveguides. By using this strongly confined waveguide, the fabricated AWG is only1/40in size of that based on the traditional buried SU-8polymer waveguide, showing a considerable increase of the integration density. By adjusting the size of the waveguide, the polarization dependency is eliminated. Besides, the present AWG shows fairly good thermal-optic tuning performances in terms of a high tuning efficiency and a large tuning range. MRR based on SU-8polymer ridge waveguides is also investigated. The waveguides in the coupling region are locally narrowed to improve the light coupling while maintaining a reasonable gap size. The spectral response of the fabricated MRR is characterized. To further enhance the light confinement and reduce the device size, we developed the SU-8suspended waveguide by etching away the buffer layer of the SU-8ridge waveguide. Since this waveguide is strongly confined in both lateral and vertical directions, the bending radius can be reduced to7μm, which is an order smaller than that of the ridge waveguide. While crossing structures (as the supporting elements) are inevitable to realize the suspended waveguide, low-loss (<0.1dB) supporting structures are developed. The microdisk resonators based on the suspended waveguide are fabricated and show a large free spectral range and a high extinction ratio.