Volume Waveguide Grating Couplers for Optical Interconnects: Analysis, Fabrication and Application
|Keywords||volume waveguide grating equivalent transmission line method grating coupler two-photon initiated photopolymerization|
Waveguide grating structures have been playing an important role in the development of integrated optics. They provide a means of coupling light into or out of waveguide. This attractive coupling method reduces the size and weight of optical systems, integrated waveguides and electronic components to form photoelectric systems. In contrast to surface relief gratings, volume index grating is usually positioned inside the guided wave layer. In this position, the amplitude of the guided mode, especially fundamental mode, reaches a maximum and the interaction between guided mode and volume index grating is expected to be very strong. Therefore, a higher coupling efficiency can be achieved. Volume gratings have many advantages over their competitors, such as simpler fabrication method and smoother surface. Unfortunately, it seems to be very difficult to produce stable high-resolution volume index gratings in polymeric planar waveguide for practical application. At present, most methods utilize basically physical change of materials, and do not induce natural chemical change within the material. So it is very difficult to guarantee the long-term stability of the gratings. The two-photon initiated polymerization as a technique for the direct fabrication induces essential chemical processes and results in a permanent change of the refractive index of materials. This technique will achieve the flexibility of one-step fabricating gratings and be helpful for further procedures in integrated optics. The purposes of this dissertation are to study the coupling characteristics of volume waveguide gratings and to propose a novel method for fabricating volume waveguide gratings using two-photon initiated photo-polymerization. The main research content and achievements are shown as following.Basing upon equivalent transmission line method, a perturbation analysis on the guiding properties of volume waveguide gratings is presented. The leakage parameter is important for volume gratings used as input or output couplers. The leakage parameter, which influences the leakage energy into the diffracted orders scattered by the grating, is achieved as functions of the structure parameters of the volume grating, for example, grating period and thickness of the waveguide and index modulation. We provide an analysis in quality for the variation of the grating parameters. The result enables one to predict the changes of a with the parameter variation in coupling performance. It is therefore useful for design of volume waveguide gratings.By using the prism-in coupling method, the transmission modes have been obtained, and the optical transmission losses of guided modes have been measured at 632.8nm for the polymer film waveguides. By controlling the concentration of resin solution, waveguides of different guided modes can be achieved. The optical loss of the fundamental mode is 0.38dB cm-1 for TE polarization and 0.41dB cm-1 for TM polarization, respectively.This thesis first reports the existence of two-photon initiated photopolymerization in solid polymer waveguide film. Criterion of the laser beam focused by a high numerical aperture objective just lying in the waveguide film is presented. Influence of the system performance including the shake of the translator and the scanning speed on the quality of volume gratings is studied.The fabricated volume waveguide gratings were illustrated with a phase-contrast optical microscope. The surface topography of the grating samples was investigated by an atom force microscope. Characteristic microstructure simultaneously appears on the surface of the polymer film with increased incident power during the two-photon initiated photopolymerization. It is confirmed that the stable morphological change is attributed to material swelling.The corresponding index modulation depth of volume grating reaches 5.7×10-3. The input coupling characteristic of volume waveguide grating without surface morphology is investigated. We successfully demonstrated the implementation of input coupling by the volume waveguide grating at the wavelength of 632.8nm. The measured coupling efficiency was about 11%. Also a simple approach to measure the refractive index and thickness of the polymeric waveguide film is presented.