| As one of the priorities in scientific developments in 21st century, sensors have an irreplaceable status in obtaining and delivering information. They become the instrument of many professions such as automation, measurement, space technolog, military project and medical diagnostics etc. Developing novel, efficient, low-cost sensors is bound to promote scientific development. Integrated optical waveguide sensors not only possess the advantages of classic sensors, for instant, high sensitivity, rapid response, but also overcome the defects in miniaturization, intelligentization and integration. Combined with the outstanding superiority of organic polymers, utilizing the technology of integrated optical waveguide to the production of novel sensors, will undoubtedly be a highly potential research area.This thesis is aimed at the basic research work of optical waveguide biochemical sensors with a Mach-Zehnder structure based on organic polymers. Main contents include the following aspects:Part One: The significances of optical wavguide sensors are introduced, a unique advantage by applying organic polymer to the fabrication of optical waveguide bio-chemical sensors is brought forward and the development of optical waveguide bio-chemical sensors in China and other countries is briefly introduced;Part Two: The concept of waveguide mode is expatiated based on the theory of optical waveguide. We also deduce penetration depth, eigenvalue equations of waveguide mode and the conditions for cut-off waveguide. On the basis of the slab waveguide theory, the condition for single mode transmission and cut-off conditions of channel waveguides are calculated by ways of Marcatili and effective refractive index respectively, which can be considered as the theoretical foundation for the analysis and design of the structure of optical waveguide sensors.Part Three: Every main character of the waveguide sensor with an M-Z structure is analyzed in details in this part, which contributes a lot to the fabrication and test of sensor devices. The specific work can be separated into three sections below:First of all, the principle of evanescent field, as the sensing principle of waveguide sensors is detailedly expatiated. The reason why most waveguide sensors work on the principle of evanescent field is that, the evanescent field is quite sensitive to the change of phase. Therefore, it is the most effective method to measure the refractive index of liquids by evanescent field. Besides, the detection based on evanescent field is real-time and non-invasion.Compared with other common structures of waveguide sensors based on evanescent field, we choose the M-Z structure as our structure of devices. The M-Z structure not only performances well in sensitivity, but also is conveniently achieved by mature semiconductor process. Next, the working principle of waveguide sensor is introduced: one of the two waveguides is selected as a reference waveguide, and the other one is sensing waveguide. As the upper cladding layer of the sensing waveguide is removed, the core layer of waveguide is exposed. So a sensing window on the sensing waveguide is formed. By adding liquid sample with certain concentration, the evanescent field of the light in waveguide is altered according to the sample; at last, the changes in phase transform to the modification of the output power.In the third section, the features of the waveguide sensor with an M-Z structure are specifically illustrated. The function of power transmission is deduced by transmission matrix and main factors which affect sensitivity are analyzed. Upon these, requirements for the design of the waveguide sensor with an M-Z structure are advanced:(1)The conditiong for single mode transmission is calculated through the way of effective refractive index.(2)The changes of phase must be confined at a monotone interval with a length ofπ, because the functions of power transmission and sensitivity are both periodic. Moreover, the sensor should work at aroundπ/2 + kπ, which is the linar area. By controlling the size of sensing window accurately, it can be achieved to promote the performance of sensors.(3)By utilizing a reverse symmetry structure to enhance the evanescent field on sensing waveguide, we can obtain a previous improvement in the sensitivity.Part Four: This is the key work in this thesis, several parts are included:(1)According to the requirements of material used in planar waveguide devices, PMMA is selected as the waveguide material to study the basic fabrication procees such as spin-coating, evaporation, RIE and lithography. The structure of waveguide is observed by microscope and scanning electron microscopy. The familiarity with fabrication process lays an important foundation for the design and fabrication of waveguide sensor in the next step.(2)The SU-8—negative,epoxy resin based UV photoresist is studied. As the refractive index of SU-8 can be modified easily, SU-8 resist is directly employed as the core/cladding materials herein. The capability of the straight waveguide with the structure of inverted ridge fabricated as the process of the SU-8 waveguide fabrication is tested. And we get good output near-field spots which verify the possibility that SU-8 fits for the main material of waveguides.(3)In order to adjust the refractive indexes of core/cladding layer, DR1 is doped into SU-8 to form DR1/SU-8 which can be used as core layer with high refractive index according to the conditiong for single mode transmission of rectangle wavguides. After removing the flat layer of the core layer by RIE, better output near-field spots are obtained. After lithography, the sensing window is formed. The whole fabricationg of waveguide sensor is complete. We drip Nacl solution to the sensing window when the output is steady; we can observe a change in the output power with nearly 3.5dB, by using optical power meter. |
PDF Full Text Request