The asymmetric transmission realized by cascading an acousto-optic device and a whispering gallery mode microcavity

Abstract

In this paper, An asymmetric transmission scheme is proposed by cascading an acousto-optic device and a coupled whispering gallery mode (WGM) microsphere cavity, and it is demonstrated by theory and experiment. With the acousto-optic interaction in a fiber, the vector modes of the fundamental core mode could be converted to the different vector modes of a cladding (linear polarization -LP) mode, and because of the optical path difference between the cladding vector modes, the polarization of the cladding mode will be changed. The cladding mode could be converted back to the core fundamental mode by coupling with a WGM microcavity. By calculating the overlapping of the mode fields in the tapered fiber and the microcavity at the resonance wavelength, the coupling coefficients between different LP modes and WGM will be solved. And, the transmittivity and conversion coefficient of the two fiber modes, under the condition that when the polarization of the incident light does not coincide with the polarization orientation of the WGM, can be got. The transmission spectrum of the coupled WGM microcavity is calculated by using MATLAB program under eight states, including the states at different incident directions, different incident polarization of input and whether the acoustic wave is on or off. The results show that the conversion coefficient from the cladding modes to the core mode is completely different from that of the contrary process when the acoustic wave is working. And the forward and backward incident lights have completely different transmission characteristics, thus achieved the asymmetric transmission. The transmittances of forward incidence and reverse incidence at different polarizations are also studied, both of them change periodically over the polarization angle, and their phase difference is equal to the polarization change caused by acousto-optic interaction in the fiber. In the experiment, a two-stage tapered fiber is used to realize the working of acousto-optic interaction and the coupling of whispering gallery mode at the same time. By controlling the working states of the system, the same 8 states as in the calculation had been studied experimentally. In the results, due to the polarization-selection effect of the WGM, the light energy incident from the opposite directions will show different transmission characteristics. While the forward transmittance reaches the maximum value (about 0.505), the reverse transmittance reaches about the minimum value (0.010), and the transmission isolation reaches about 17 dB. The transmittance in two directions was measured at different incident polarization angles, the transmission isolation was analyzed, and the polarization change of cladding mode in the fiber was verified to be about 80°. The measured results coincide with the calculations of the developed theory well. At last, the shortcomings and optimization methods of the scheme are discussed. The asymmetric transmission scheme in this paper inherits the advantages of rapid response and good tuning of acousto-optic device, and has an all-fiber structure, which has important application potential in optical switch and isolator.