Design Of Temperature Sensor And Pressure Sensor Based On Long-period Polymer Waveguide Gratings

Long-period gratings (LPGs) have become increasingly popular for applications as gain flatteners of erbium-doped fiber amplifiers (EDFAs), band-rejection filters, and various kinds of add-drop couplers and sensors. Presently, researches on the long-period fiber gratings (LPFGs) are extensive and mature, and more and more attention has been devoted to the study of long-period waveguide gratings (LPWGs), which have huge potential in a wide field, because of the advantages in the choice of materials, the variety of fabrication technology, the flexibility of waveguide and grating structures, and so on.Based on the composite slab waveguide mode theory and the effective index method (EIM), the sensitivity characteristics of the LPWGs to surrounding temperature and external pressure are analyzed in present dissertation. And then the elementary parameters of several kinds of temperature sensor and pressure sensor based on polymer LPWG are proposed.In chapter 2, the EIM in combination with composite slab waveguide mode theory is used to study the mode characteristics of different types of channel waveguide, such as strip-raised, ridge, strip-loaded and strip-embedded channel waveguide. The calculation results show that among the four types of waveguide the strip-loaded structure is the best for high temperature sensitivity and pressure sensitivity of LPWGs.In chapter 3, the general functions of the LPWGs'sensitivity to the temperature and the pressure are deduced from the phase-matching condition. Some conclusions were made from the sensitivity functions. Those include:1) The thermal-optical effects of guide core and cladding are the most important factors in determining the temperature sensitivity, while pressure sensitivity is closely related to the elasto-optical effects of both the core and the cladding, and the elastic deformation of the core principally.2) The closer to each other the refractive indexes of the core and the cladding are, the wider the adjustable extents of single mode condition are, as well as the higher the temperature and pressure sensitivities are. 3) The appropriate thicknesses of both the core and the cladding are important to temperature and pressure sensitivities, while the core width has less effect on the sensitivities.Along with the conclusions made above, several designs of temperature sensor and pressure sensor based on polymer LPWG are proposed. Epoxy (Optocast 3505) and COP (Cyclo-olefin Polymer), whose thermo-optical coefficients have a large difference, can be used to fabricate a temperature sensor with a temperature sensitivity of -82.82 nm/℃, while such a LPWG structure has a low pressure sensitivity of 1.38×10-5nm/Pa. Low temperature sensitivity could be obtained with materials of core and cladding whose thermo-optical coefficients are close to each other, such as BCB (Benzocyclobutene) and PMMA (Polymethyl-methacrylate). This effect can be applied to the pressure sensor. With the BCB strip-loaded waveguide on SiO2 substrate (PMMA cladding), pressure sensitivity of 4.44×10-6nm/Pa and zero temperature sensitivity could be realized. The pressure sensitivity could be increased for the materials with low Young's modulus, such as ENR (Epoxy Novolak Resin). The pressure sensitivity of ENR LPWG sensor is higher than 3.84×10-4 nm/Pa. But such a device can only be used in the case of constant temperature, because the device has high temperature sensitivity simultaneously.