Research On Coupling Property And Package Technology Of Optical WGM Microcavity

Over the last two decades, optical dielectric microresonators supporting optical whispering gallery modes (WGMs) have attracted considerable research interests because of their high quality factors (Q) and low mode volumes. They are very attractive in many applications ranging from fundamental physics to key components in modern photonics such as optical switches, modulators, narrow-band filters and high-sensitivity sensors. Although the WGM resonators provide potentials in many fields, so far, they are only investigated in the lab environment and there are some challenges by using them into practical applications. Widely known, the resonant property of WGM is sensitive to the environment outside the resonator. First, the adsorption of molecular water and deposition of micro-dust particles on the microresonator surface significantly reduce the Q factor. Second, the resonance wavelength of the WGM is susceptible to the thermal fluctuations resulting from the ambient temperature variation. These can both degrade the sensor performance and limit the development of the microresonator-based practical devices. Moreover, to achieve a high coupling efficiency, the fiber taper is commonly used to excite the WGMs of the microresonator, which makes the coupling system fragile and lack robustness. An external tiny vibration may change the coupling condition between the resonator and fiber taper. In past research, a hermetic box is usually used to guarantee the super clean environment and the gas stability around the coupling system. Although the contaminations can be excluded effectively and the temperature fluctuations can be reduced significantly, the box and the discrete coupling structures including high-resolution 3D translation stages are heavy and occupy a large space, making the WGM resonators appropriate in fundamental research in a laboratory but not applicable in practical use. To promote the microresonators into practical applications, designing a suitable package is a feasible approach.To solve the aforementioned problems, my work listed below focuses on two main parts:simulations and experiments. First, we investigated the three-layer-coated microsphere resonator by using finite difference time domain (FDTD) method and PDE solver of the COMSOL software. The simulation results demonstrate that this new-style structure can be used as a sensor which not only reduces the thermal effect in RI sensing but also detects temperature change simultaneously. Second, we experimentally studied the coupling properties of microsphere, cylinder and bottle resonators. And we package them into a whole device that can be moved freely. Moreover, thermal sensing experiments and vibration tests of the packaged device are demonstrated. Detailed research contents are shown as follows:(1) With FDTD method, whispering gallery modes (WGM) in a microsphere coated with three layers of high, low, and high refractive index (RI) are simulated. We investigate the effects of the waveguide RI and the thickness of the low-RI layer on resonance characteristics. It is found that each of the two high-RI layers can sustain its own WGM if the values of RI and thickness of the three layers are appropriate. Furthermore, the effect of the RI of the surrounding medium on resonance characteristics is also studied. The simulation results show that a RI change of the surroundings will only change the resonance wavelength of the outer layer, and will not affect the WGM of the inner layer. Such property makes it feasible for a potential application in high-precision RI and temperature sensing.(2) With the FEM and perturbation theory, a numerical model is developed to calculate the sensitivities in RI and temperature of the three-layer-coated resonator. The results show that the inner and outer modes, centered on the inner and outer high-RI layers respectively, have nearly identical changes with a temperature variation yet significant differences in sensitivity to the Rl change in the surrounding medium. The coatings thickness effect on the WGMs of the inner and outer modes is investigated, and the results show that the thermal noise can be eliminated by optimizing the outer layer thickness.(3) We demonstrated a novel method to package the micrbsphere-taper coupling system into a whole device. With a glass tube, two glass plates, and the UV glue, we achieve a packaged coupling structure without changing the traditional coupling condition. A Q factor as high as 1.08×108 is obtained under the condition of contact between the fiber taper and the microsphere, which makes the transmission spectrum more stable and insensitive to the external vibration and air flow. The effects of the position and orientation of the deformed microsphere relative to the fiber taper on the resonant spectrum are studied in detail, and it is found that different spectra and Q factors can be required by adjusting the sphere position and orientation carefully. To seal the packaged structure in a hermetic and transparent organic glass box, which can eliminate the environmental factors that worsen Q, an excellent performance to maintain the high Q is realized. In addition, we also fabricate a novel packaged dual-tapered-fiber coupled microsphere resonator, which can be used as an Add-Drop filter. The device possesses a Q factor as high as 2.7× 106, a FSR of about 0.016 nm, and a maximum drop efficiency of 42%.(4) By using a standard fiber fusion splicer, we fabricate the cylinder and bottle resonator with high Q factor. Selective excitation of resonant modes were obtained by using a tapered fiber coupled at different positions along the axis of the resonator. With the help of our numerical and theoretical models, the spatial and spectral mode properties were identified, showing excellent agreement with the experimental results, such as the optical field distribution and the frequency spacing. In addition, controlled and stable coupling was demonstrated experimentally by vertically moving the resonator. Finally, the packaged device for the cylinder and bottle resonator are obtained. Due to the diameter uniformity of the cylindrical resonator, a vibration performance test shows that the packaged microcylinder device is more robust than the packaged microsphere and bottle device. We especially believe that this portable and robust device can be used in strong vibration environments such as rockets, missiles, and airborne crafts.