Experimental Study On The Tunable Optical Whispering Gallery Mode Of Micro-resonator

In recent years, whispering gallery mode(WGM) resonators have attracted many researchers’ attentions. WGM has been widely applied in fundamental scientific research and technological applications owing to its unique properties, including ultra-high qualities factor(Q), small mode volume and ultra-strong power density, et al. Tunable WGM has tremendous value in optical filter, optical switch and sensing measurements. In this paper, due to the significant thermo-optic nonlinear effect, free carries absorption and multiphoton absorption effect in silica-based materials, we choose microtube and photonic crystal fiber as optical resonators to study tunable properties of WGM’s resonant frequency and amplitude, respectively. The main contents are as followings:① The transmission characteristics of microtube resonator’s oscillation frequency have been theoretically analyzed by using the transmission matrix method, applying finite element methods and COMSOL software to theoretically stimulate the resonator’s WGM which have been excited by coupling of microfiber, and studying coupling distance and wall thickness of microtube on resonances. Moreover, coupling distance and optical wave’s polarizations exerting effects on WGM have been experimentally studied.② Based on flame-heated draw technique, the tunable WGM’s resonator has been fabricated by using silica microtube and heating wire. The widely tunable range of WGM can be achieved by electric heating from the theoretical derivation. The WGM’s free spectrum range is 0.8nm, Q value is on the order of 104, and signal noise ratio is 4.3d B in the experiment. When 200 m A current intensity flows through the heating wire, the WGM shifts 0.57 nm with consuming 0.1m W electric power. In addition, the response speed is hundreds of milliseconds when using as an optical switch, response time can be improved by decreasing the wall thickness and increasing the length of microtube.③ We propose an entirely new optical resonator based on photonic crystal fiber to obtain WGM with ultrahigh Q value and ultra-small modes volume. In the experiment, the Fano mode and WGM are achieved by photonic crystal fiber with six air-hole, both of the Q are on the order of 104, and the number of modes are less than silica microtube’s. Tuning of WGM’s and Fano’s amplitudes can be achieved by free carries absorption and multi-photon absorption effect. From the mode field distribution of photonic crystal fiber resonator, we preliminarily explain that the reason of Fano modes when taking the photonic crystal fiber as optical resonator. The high order modes is damaged due to the hole-shape structure in the center of PCF, the lower order WGM is near in the outer edge of resonator, both of which lead to the formation of Fano modes.In summary,experimental results indicate that large tuning range by thermo-optic nonlinear effect and decreasing the WGM’s amplitude employing free carries absorption and multiphoton absorption effect have been achieved, respectively. The photonic crystal fiber with special structure contributes to the formation of Fano mode. Both of tunable WGMs have a little of value in tunable laser, optical filter and optical switch etc.