Study Of Polymer-based Microring Resonators Operating At Near Infrared Wavelengths

In recent years,integrated optics has been playing important roles in the optics communication and sensing areas in recent years.Polymer materials for fabricating planar integrated optical circuits have attracted great interest,since they may open a new era toward the fabrication of photonics components and integrated optical devices.This is due to their extraordinary advantages such as high optical transmittance,mechanical and chemical stability and their potential for low-cost production.One of these integrated photonic devices,the planar waveguide microring resonator,has received considerable attention due to its small footprint and ease of integration with other optical and fluidic components.In order to confine the light inside the microring resonators,several materials have been used,such as silica,silicon on insulator(SOI),silicon nitride,glass and polymers.Polymer materials are frequently chosed to construct microring resonators because they have a much greater chemical flexibility than semiconductor materials,it is possible to tune more easily their refractive index.In general polymer waveguide fabrication process is more simple compared to the chemical or physical depsoition and dry etching procedure required for silicon based materials.Moreover,they are transparent in the visible range,therefore we can extend the usual working window of integrated sensors.A lot of work on polymer-based microring resonators at infrared wavelengths has been studied.However,there is an urgent demand for the development of polymer-based device closer to/or at visible wavelengths where the absorption of water is about 2000 times lower than in near infrared regions and lower cost light sources are available in this range.The main works describe the investigation of micro-resonator with multimode interference coupler and slot-waveguide microring,which are operating at very-near-infrared wavelengths(around 890 nm)based on polymer materials.(1)Polymer-based microring resonators(MRR)have attracted extensive attentions due to their various advantages such as high Q factor,cost-effective and easy-fabrication.A microring resonator incorporating an MMI coupler with a 50:50 splitting ratio has been demonstrated on the cost-effective polymer platform.The radius and the structure of the MMI coupler are simulated and optimized using BPM in order to reduce the total loss of the microring resonator.For conventional NIL,silicon and metal are normally utilized to fabricate the hard mold by etching.However,the etching process will result in surface roughness problems which increase the scattering loss of the waveguide.In this work,UV-based soft nanoimprint lithography(UV Soft NIL)is proposed.The negative photoresist(SU8-2)prepared by photolithography on fused silica substrate is used as the master mold in order to avoid the etching process.In the replica process,we make full use of PFPE composite molds,which demonstrates high resolution and fidelity without applying pressure.Ormocore is used as low-cost core materials which is transparent at very-near-infrared wavelengths.By optimizing the proportion between Ormocore and the thinner maT,the devices can be fabricated almost without residual layer.The fabricated device with Q-factors up to 23000 was demonstrated,which shows high potential for biosensing applications.(2)Slot waveguide is a specific structure which could confine light in a low refractive index material.This property makes slot waveguide very attractive for sensing application.Typically,slot waveguides have been fabricated from semiconductors operating in the near infrared(NIR)wavelength region.The objective of this work was to demonstrate the fabrication of slot waveguide on low-cost polymer platform.A polymer-based microring operating at very-near-infrared region(around 890 nm)is proposed and designed.A detailed analysis of modal characteristics of the slot waveguides were carried out.The relationships between the sensitivity and the waveguide height,width and slot width are analyzed for refractive index sensing.Conventional bend slot waveguide has a large propagation loss,which has a bad influence on the quality factor and the extinction ration of the microring.An asymmetric waveguide was designed which could maintain the mode propagate in the center of the slot in order to decrease the bending loss.A strip-slot mode converter using the multimode interference(MMI)structure was introduced for better coupling between strip and slot waveguides.Simulations show that the sensitivity of the designed microring resonator could reach 109 nm/RIU.A silicon master mold was fabricated using electron-beam lithography.A unique class of fluorinated polymer,perfluoropolyether(PFPE),was used to fabricate the flexible soft mold,which was replicated from the silicon mold successfully.The slot waveguide was developed based on UV-based soft nanoimprint technique.The fabricated slot walls with a high aspect ratio are shown compatible with low cost mass production processes.