Study On The Key Technology Of In-fiber Whispering Gallery Mode Resonator

Whispering gallery mode(WGM)resonator confining light by continuous total reflection posses ultrahigh quality factor,which has become a study focus of optical resonators in recent years.For passive WGM resonators,the WGMs are usually excited and collected by evanescent field coupling between the coupler and the cavity,typically including tapered fiber with waist diameter less than 2?m and prism.The coupling efficiency of the tapered fiber based coupler can be as high as 99% while it is physically fragile,prone to be contaminated and difficult for transfer.The prism coupler demonstrates high physical strength while it is bulky.In addition,the WGM cavity and the coupler are spatially separated in order to reach critical coupling state,which means the coupling structure is prone to be disturbed by the external vibration or air disturbance.To overcome the aboved mentioned problems,integrated and optical fiber compatible WGM resonator is demonstrated and studied in this thesis,including the resonant characteristics and the corresponding optimization method.The main contents are as follows:(1)The effects of cavity material,size and the surrounding medium on the resonant wavelength,effective refractive index and the field distribution of the in-fiber WGM resonator are studied theoretically.And the results show that the WGM can be effectively excited once the refractive index of the cavity is higher than that of its surrounding medium(e.g.,air or fiber cladding)and the WGM in the cavity matches with the core mode in phase.Moreover,it is easier to excite higher order WGM in the resonator with bigger size,which means that the field distribution is much more complicated.(2)A single mode fiber based in-fiber WGM resonator is demonstrated,where the fiber core functions as coupler while the micro structure fabricated by femtosecond puled laser sereves as the WGM cavity.The unique characteristics of such an in-fiber WGM resonator includes(1)highly integratable,by which the coupler and the WGM cavity are integrated in a piece of single mode fiber,meaning that the size of the WGM resonator is less than 125?m;(2)ease of transfer,which means that the transfer can be easily done and the distance between the coupler and the WGM cavity needs not be adjusted again benifitting the integrated structure;(3)optical fiber compatible,unlike the on-chip WGM resonator where a perpendicular grating is indispensable for coupling the light into the coupler,the coupling between the light source and the in-fiber WGM resonator is much more easier.Such an in-fiber WGM resonator can be used as narrow band filter or lasing cavity once the quality factor of the WGM resonator is improved by further smoothing the cavity surface through post process or other micromachining method.(3)An in-fiber Mach-Zehnder interferometer and WGM resonator coupling structure is fabricated to generate asymmetrical spectrum.Additional phase shift around the resonant wavelength of the WGM resonator is introduced to spectrum of the Mach-Zehnder interferometer,which means the asymmetric spectrum is sensitive to the phase shift.The stability of the coupling structure is improved by integrating the Mach-Zehnder interferometer and the WGM resonator into a piece of fiber,and asymmetric spectrum with slope of 117dB/nm is demonstrated experimentally,which can be used in edge filtering,optical switching and biochemical sensing with ultrahigh resolution.(4)Thermo-optically tunable in-fiber WGM resonator based on two kinds of microstructured fiber are demonstrated,including chemically etched grapefruit shaped fiber and femtosecond pulsed laser micromachined microstructured single mode fiber.The tuning efficiency of the grapefruit shaped microstructured fiber coupled polymethyl methacrylate spherical cavity with thermal-optical coefficient of-1.2×10-4/K is 460pm/K.More important is that the quality factor of the WGM keeps constant(5.9×104)during the tuning process since it is integrated into the fiber and contacts with the core of the microstructured fiber all the time.Compared with the grapefruit shaped microstructured fiber,the microstructured single mode fiber fabricated by femtosecond pulsed laser micromachining breaks the limit on the WGM cavity size while it possesses the comparable tuning efficiency.The above mentioned tunable in-fiber WGM resonators have potentials in tunable filtering and laser linewidth compressing.