Investigation of ultraviolet-written polymer optical waveguides and bottom-electric-heating thermo-optic devices
Student thesis: Doctoral Thesis
Related Research Unit(s)
In recent years, there has been significant progress in the development of polymer materials for the realization of integrated optical devices because of their many nice optical properties, compatibility with other materials, and potential for mass production. Polymer optical waveguides can be fabricated in many ways. The conventional micro-fabrication process based on lithography and etching can produce high-quality waveguides but the process is complicated and slow. In this thesis, ultraviolet (UV) writing and imprinting techniques for the fabrication of polymer waveguides are investigated. These techniques are simple, fast, and inexpensive to implement, and therefore can lower the manufacturing cost of planar lightwave circuits based on polymer waveguides. With the help of these techniques, various useful polymer waveguide devices, such as optical power splitters, optical modulators, and wavelength-tunable filters, are designed and fabricated. Many of these devices explore the large thermo-optic effect of the polymer materials. The thesis consists of two parts. The first part focuses on the development of a UV-writing technique for the fabrication of buried polymer waveguide devices, which is based on a UV lamp as the light source and commercial benzocyclobutene (BCB) and epoxy OPTOCAST 3505 as the waveguide materials. The light emitted from the UV lamp can induce a large refractive-index change (up to ~0.012) in BCB, while a negligible refractive-index change in the epoxy OPTOCAST 3505, which makes possible the writing of buried waveguide devices in BCB by exposing an epoxy-clad BCB film to the UV light through a mask. With this material system, several examples of UV-written buried waveguide devices, such as straight channel waveguides, Y-junction waveguide branches, and long-period waveguide gratings (LPWGs), are demonstrated to show the potential of this UV-writing technique. In the writing of LPWGs, the UV-writing technique allows real-time monitoring of the growth of the transmission spectrum of the grating, and thus greatly simplifies the design and fabrication of such devices. The second part of the thesis focuses on the demonstration of a new heating approach of realizing thermo-optic polymer waveguide devices, where the electrode heater is introduced at the bottom of the waveguide substrate. In contrast to the conventional top-heating approach, the bottom-heating approach can minimize the possibility of damaging the polymer material because waveguide formation is the last step in the fabrication process. Moreover, it allows electric wires to be accessed from the bottom of the device, which can facilitate device packaging. To demonstrate the advantages of this new approach, several thermo-optic Mach-Zehnder interferometer (MZI) switches and tunable phase-shifted LPWGs are fabricated with the imprinting technique. The best MZI can be switched with an electric power of 17 mW and a response time of 12 ms, while the best LPWG requires an electric power of only 1.74 mW to produce a -phase shift with a response time of ~60 ms. The experimental results agree well with theoretical simulations.
- Optical wave guides, Polymers