Author: Wu, Jushuai
Title: Optically μ-printed polymer microphotonic sensors
Advisors: Zhang, A. Ping (EE)
Tam, Hwa-yaw (EE)
Degree: Ph.D.
Year: 2018
Subject: Hong Kong Polytechnic University -- Dissertations
Photonics -- Materials
Polymers -- Optical properties
Department: Department of Electrical Engineering
Pages: xix, 134 pages : color illustrations
Language: English
Abstract: With the advance of micro/nano fabrication technologies, microphotonic devices, such as optical microresonators and microinterferometers, have drawn intensive attention in various areas ranging from optical sensors to nonlinear optics. Although inorganic materials are widely used in microphotonic sensors, polymer materials attract more and more attention owing to their advantages including low cost, easy processability, mechanical and optical diversity, and biocompatibility. In this thesis, three kinds of polymer microphotonic devices were fabricated and systematically investigated for sensing applications. Firstly, polymer optical whispering-gallery mode (WGM) resonators were fabricated by using an own-developed 3D μ-printing technology based on optical maskless exposure approach. Suspended-disk polymer WGM resonators with the radiuses of 230 and 160 μm were successfully fabricated. Optical fiber tapers with the minimum diameter of 2 μm were used to couple light into and out the polymer WGM resonators. The quality factor of the fabricated resonators was measured to be around 6×103. Numerical simulations using software COMSOL have been carried out to analyze the optical WGM resonators and compare with the measured results. The fabricated polymer WGM resonators are appealing for refractive index sensing and biosensors.
Secondly, optical fiber-tip pressure sensors were fabricated by using an own-developed in situ μ-printing technology. SU-8 Fabry-Perot (FP) interferometers with sealed air cavities were fabricated on the end face of a standard optical fiber for development of pressure microsensors. SU-8 suspended diaphragms were directly printed by using a dynamic optical exposure technology and then were further constructed by a followed printing process to form sealed air cavities. Multi-beam FP interferometric fringes of reflection spectra were measured for pressure measurement. The sensing performance of the optical fiber-tip pressure microsensor was tested in the experiments. For a fiber-tip pressure sensor with the air cavity length of 93 μm and SU-8 diaphragm thickness of 11 μm, the measured pressure sensitivity was 2.93 nm/MPa. Numerical simulations of a 3D structural model have been performed by using commercial software COMSOL, and the simulated results agree well with the measured data. Lastly, novel optical fiber-tip CO₂ sensors were developed by in-situ μ-printing of a functional polymer material, i.e. poly (1-allyl-3-vinylimidazolium bromide) (PAVB), on the end faces of single-core/multicore fibers to form micrometer scale FP interferometers. The PAVB FP interferometer can absorb CO₂ molecules, both physically and chemically, which results in an increase of the effective RI of PAVB polymer and a red shift of the resonant wavelength in the reflection spectrum. As such FP interferometers are also sensitive to the temperature of the surrounding environment, another FP interferometer made of SU-8 was fabricated on the end face of the same fiber as a reference temperature sensor. The measured spectra were analyzed by using fast Fourier transformation to calculate the length of the polymer FP cavities. The fiber-tip CO₂ sensors show a linear response to the change of CO₂ concentration with the sensitivity of 34.92 pm/vol% in the range from 0 vol% to 75 vol%. The temperature sensitivity of the PAVB FP interferometer is 0.704 nm/°C, whose cross sensitivity effect can be compensated by the SU-8 FP interferometer with the temperature sensitivity of 0.059 nm/°C. The rise and fall times of the dynamic response of the fiber-tip CO₂ sensors were measured to be 6.1 and 8.0 min, respectively.
Rights: All rights reserved
Access: open access

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