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9 May 2018 Fabrication techniques for micro-optical hollow resonator used in high-bandwidth sensing applications
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The purpose of this paper is to identify fabrication methods for creating hollow optical whispering gallery modes (WGM) micro-resonators. These resonators are typically made from high optical polymeric materials like polydimethylsiloxane (PDMS) in solid shape which make it work as optical sensors once the round trip of the light is equal multiple integer of the wavelength that we used. The solid WGM resonators proved that it could reach a very high level of sensitivity for a certain input like (electric field, magnetic field and angular velocity,…etc.) but with a low bandwidth; because it behaved as an over damped system. On the other side, theoretically the hollow resonators can get the same level of sensitivity for the same input with an ultra-high bandwidth in kilo Hertz. A lot of sensing applications could be enhanced just using such a proposed sensor. Experimentally, we may consider the hollow resonators as simple harmonic oscillators. Accordingly, two parameters could be adjusted; the stiffness and the mass of the resonator. Since altering the stiffness coefficient requires us to change the material properties of the polymer we use and since there is a lot of constrains on the polymer material that avoid us to deal with it, based on that; changing the mass of the resonator is the only way for increasing its bandwidth. In conclusion, decreasing the mass of the sensor, making it hollow, can significantly affect its mechanical natural frequency leading to increase its bandwidth. Our challenge now is to fabricate these hollow resonators, since its typical size should range between couple of hundreds of micro-meters with fixed wall-thickness to let the optical resonance to occur. Different methods are being tested in this paper and currently we are finding more reliable ways to do it successfully. The first method will be by coating the hollow polymethylmethacrylate (PMMA) sphere with mechanically controlled syringe then a layer of PDMS base would be used as an outer layer which support the optical light to propagate through the resonator. The second method is to use the same hollow sphere but this time with the UV curable PDMS outer layer. The last method when using the microfluidic channels to create the hollow channels to create resonators using the gravitational force and its surface tension. All of these methods to produce these promising micro-optical hollow resonators will be shown with experimental analysis.
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Amir R. Ali and Ahmad M. Monier "Fabrication techniques for micro-optical hollow resonator used in high-bandwidth sensing applications", Proc. SPIE 10680, Optical Sensing and Detection V, 106801Z (9 May 2018);

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