Optimal Design And Fabrication Of Polymeric Planar Optical Waveguide Bio-chemical Sensor

To withstand the growing threat of terror attacks and great demands of medicaldiagnosis and environmental monitoring, optical bio-chemical sensor is attractingmore and more attention. Same as fiber sensor, planar optical waveguide sensorbenefits from high sensitivity, fast response and resistance to electromagneticinterference etc. Moreover, flexible design in structure and function makeswaveguide sensor a promising solution for future optical bio-chemical sensor.However, the waveguide devices based on inorganic materials fail to meet therequirements of mass production due to the complicated fabrication process andexpensive equipments.Organic polymers for optical communication are not only cost-effective insynthesis and processing, but also highly compatible with samples as metal ions,proteins and virus. Polymers include a wide range of structures, which offer lots ofchoices. Therefore, it is more profitable to use polymers to fabricate opticalbio-chemical sensors than inorganic materials. This dissertation combines withexisting technical problems and the trend of developments to optimize severaldifferent structures of polymeric waveguide bio-chemical sensor, includingtheoretical design and fabrication technology. The main work is as follows:1. On the basis of the theory of optical waveguide mode, taking certainboundary conditions into account, we solved the characteristic equations ofasymmetric three-layer slab waveguide. Then using effective index method, wededuced the equations of guiding mode of rectangular waveguide. This dissertationexplained the working principle of optical waveguide sensors, which was the theoryof evanescent field and two kinds of sensing model: uniform sensing and surfacesensing. We also introduced the parameters of waveguide sensor, includingsensitivity, detection limit, resolution, range and linearity in detail. And we enumerated the experimental drugs and instruments for the fabrication of polymericwaveguide sensors in this dissertation.2. Aiming at the bio-chemical sensor based on Mach-Zehnder interferometer(MZI) structure, whose output signal is periodic, we presumed to set the workingpoint of the device and achieve high linearity in a large measuring rang bycontrolling the length of sensor window. First, according to the refractive index ofwaveguide materials and dimension and insertion loss of the device, using thecalculation of Matlab software and simulation of Rsoft software, we obtained thatthe optimum length of sensing window was11515μm. At the same time, weimproved the fabrication technology of Inductively Coupled Plasma (ICP) etchingprocess for sensing window by deeply investigating etching parameters, includingantenna power, bias power, gas ratio, chamber pressure and etching time. Due to theprecise control of etching, the water contact angle on the waveguide surface wasdecreased from130°to30°, and no additional insertion loss was brought in afteretching. The results of sensing test demonstrated that the refractive index measuringrange for liquid, sensitivity and detection limit of the optimized waveguide sensorwere from1.31to1.42,225.4dB/RIU and10-6RIU, respectively. Waveguidesensors based on MZI structure with sensing window lengths of10000μm and15000μm were fabricated to compare with the optimized sensor. The sensitivitieswere93.3dB/RIU and94.2dB/RIU, which were much lower than225.4dB/RIU.Finally we evaluated the long-term stability of the waveguide sensor chip.3. The MZI-structure waveguide bio-chemical sensor was further improved indesign and fabrication. Through calculation and simulation, the Y-shape splitter andcombiner were designed as different lengths in horizontal direction to form anasymmetric MZI structure, which kept the sensor at the working point. Then weprepared the sensing layer based on silane coupling agent KH590with sol-gelmethod. This layer was deposited on the waveguide core, which greatly enhance theevanescent field. The simulation results showed that12%of the optical field in thesensing layer distributed in the sensing layer. After the modification on thewaveguide core, detection for copper ions was conducted and the minimum detectable concentration was20μg/ml.4. The advantages of micro-ring waveguide in sensing applications wereanalyzed. The working principles of waveguide coupling and polymeric microringresonators were detailedly introduced. Then we calculated the radius of micro-ring,the distance of coupling gap and structure parameters. The output power andspectrum of the micro-ring sensor were studied in consideration of linearity andmeasuring range.This dissertation has carried out the research on the optimization of theoreticaldesign and fabrication technology for polymeric planar optical waveguidebio-chemical sensor. Due to the advantages of function and quantity integration, it islikely to fabricate high-performance, small-size and low-cost sensor chip withpolymeric waveguide devices of and realize large-scale production, which serves inhealth care, food security and other important areas.