| The Whispering Gallery Mode(WGM)optical microcavity limits the light to the edge of the resonator through the internal total reflection,which has a higher quality factor Q and a smaller mode volume V than the conventional optical microcavity,such as FP resonator.This pattern feature greatly enhances the interaction of light and materials.It is widely applicated in microcavity quantum kinetics,ultra low threshold lasers,nonlinear optics,microwave photonics,high sensitivity biosensors and so on.Here,we propose a new method of fabrating spherical resonators to get the bigger resonator and higher Q factor.Due to the rich resonances and high Q characteristics of the spherical resonator,the experimental platform of pumping,measuring and analyzing the quartz spherical cavity Brillouin laser is constructed.The low threshold Brillouin laser is realized and the output characteristics are measured.In addition,a coupling system of the taper and the resonator is designed to obtain a high Q mode resonance.The stable Brillouin laser output lays the foundation for the optical application.The main research contents include:1.The coupling theory of the taper and the WGM microcavity.Based on the Maxwell equations in the classical electromagnetic theory,we construct the theory of optical waveguide in the tapered fiber and analyze the influence of fiber size on the propagation constant,the field and energy distribution of different waveguide modes.Based on the Helmholtz equation under the spherical coordinates,the eigenequation of the quartz spherical cavity is deduced and the field distribution of the WGM microcavity is obtained.We construct the coupling system of the taper and the spherical cavity and analyze the the coupling characteristics.The transmittances of the taper and the Q factors of the microcavity are compared respectively under three different coupling conditions of under-coupling,critical-coupling and over-coupling.2.The preparation of the WGM quartz sphereical cavity.Here,we propose a novelty method to fabricate the quartz spherical resonators by using the CO2 laser to heat the surface of the quartz-rod,driven by the motor.The top of the millimeter bar is scanned in the radial direction.The resonator has a smooth surface and a diameter in the range of 200?m to 800?m.In addition,we respectively measure the Q factor of resonators at low speed and high speed.With low-speed sweeping,the Q factor is fitted by Lorentz.With high-speed sweeping,the corresponding Q factor is estimated by analyzing the ringing-phenomenon,the highest Q factor is up to 5×108.3.Realization of WGM microcavity 15-order cascaded Brillouin laser.The principle of stimulated Brillouin scattering is introduced and the experimental platform of microcavity Brillouin laser is constructed.With continuous light pumping,a low threshold Brillouin laser has been realized on the quartz resonator with the diameter of 760?m.The threshold of the Brillouin is estimated about 96?W.The 11 GHz and 22GHz microwave beating signals are measured between the pump and the Stokes,the Stokes themselves,which match the Brillouin frequency shift of the quartz materials.The 15-order Brillouin laser is realized by pulsed light pumping(The peak power up to 380 mW),which is the highest order Brillouin lasing in the quartz spherical cavities.4.The packaging and characteristics of the cavity-taper coupling structure.Here,we design a package structure of cavity-taper coupling fixed by UV curing adhesive and test its perfoamance.(1).The packaged device has a high Q factor,remaining at the magnitude of 108.(2).The time stability of the packaged structure is obviously improved.(3).The packaged coupling device achieves a more stable cascaded Brillouin laser,the fluctuation range is less than 0.1 dB in 30 minutes.The signal to noise ratio and the side-mode suppression ratio of the output spectrum is up to 68dB and 39dB.Due to the strong Rayleigh scattering of the pump,the Brillouin signal has been suppressed and it is the best result in quartz spherical resonators. |
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