Design And Experimental Study On The Optical Properties Of In-Fiber Bandpass Whispering Gallery Mode Resonator

Bandpass device is of crucial importance for signal extraction in optical communication system,wavelength selection in laser system and information perception in measurement system.The precision of channel selection,fineness of laser spectrum and accuracy of measurement can be further improved through narrowing the bandwidth.Whispering galley mode microcavity with high quality factor is an ideal candidate for narrow band filter,but the whispering gallery mode microcavity coupled by a single waveguide usually show band-rejection characteristics.Bandpass output can be achieved by employing a pair of coupling waveguides,while the integration and long-term stability are limited by the separated coupling structure between the waveguide and the microcavity.In order to achieve highly integrated,stable and fiber compatible bandpass whispering gallery mode microcavity,two coupling structures are proposed and fabricated in the standard single mode fiber.The main contents are as follows:(1)Based on the evanescent field coupling theory of the spherical whispering gallery mode microcavity,the effects of the sphere size,material and waveguide structure on the coupling results in the optical fiber integrated structure are analyzed and a twodimensional tapered fiber core coupling structure is proposed.The simulation and experimental results show that the proposed structure can enhance the intensity of evanescent field and reduce the phase transmission constant mismatch between the coupling waveguide and the whispering gallery mode in the spherical microcavity,which provides an effective solution for the fabrication of efficient coupled bandpass whispering gallery mode microcavity resonant structure in the fiber.(2)A transmission bandpass whispering gallery mode microcavity coupling structure based on two-dimension tapered fiber core and curvature waveguide is proposed and fabricated.The two-dimensional tapered fiber core bridges the standard single mode optical fiber core and the curvature waveguide,by which the loss resulted from mode convention can be reduced.Packaging the coupling structure and the spherical whispering gallery mode microcavity together can improve the long-term coupling stability and maintain the surface smoothness and purity of the spherical microcavity.The experimental results show that the bandwidth of the in-fiber transmitted bandpass whispering gallery mode microcavity is 0.15 nm and its quality factor remains unchanged during the thermo-optical tuning process,which can be used for temperature change measurement of sensor system,wavelength selection and linewidth compression of laser system.(3)A reflection bandpass whispering gallery mode microcavity coupling structure based on the two-dimensional tapered fiber core and a Y-shaped waveguide is proposed and fabricated.The two-dimensional tapered fiber core bridges the standard single-mode optical fiber core and the common end of the Y-shaped waveguide.The branches of the Y-shaped waveguide can simultaneously excite and collect the whispering gallery mode.A passband with a bandwidth of 0.0326 nm can be obtained by using the polymer microsphere as the whispering gallery mode microcavity.Such a device can be used for channel extraction of optical fiber communication systems,laser feedback frequency stabilization and linewidth compression.The integration and the quality factor of the device can be further improved with more advanced micro/nano machining tools,by which both the coupling waveguide and the whispering gallery mode cavity can be fabricated in optical fiber.