Tunable filter based on whispering gallery modes

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Abstract
It is interesting to note that the dependence Dn(EZ) has a hysteresis feature when a large DC electric field (EZ>2 MV=m) is applied to the cavity. The rapid change of the applied voltage results in an incomplete compensation of the mode shift, i.e. Dn(EZ ¼0) 6¼0, and the resonance frequency returns to its initial position several seconds after the electric field is switched off. The maximum frequency tuning of the filter in this nonlinear regime was approximately 40 GHz. The physical origin of the hysteresis will be discussed elsewhere. The insertion losses in our scheme are primarily due to the inefficient coupling technique with the diamond prism configuration. We believe that antireflection coating of the coupling prisms or use of a special grating placed on a high-index fibre may reduce the losses significantly. To demonstrate the filter performance we used it in an optical fibre line to transmit a video signal. Such transmission lines might be important for the development of portable optical domain microwave navigation and communication devices that can provide significantly higher capability in applications such as NASA planetary explorations. The scheme of the transmission line used in the experiment is shown in Fig. 3. A video signal with an approximately 20 MHz FWHM band- width and zero carrier frequency is sent from a CCD camera to a mixer, where it is mixed with a 10 GHz microwave carrier. The resulting modulated microwave signal is filtered to suppress the higher harmo- nics, and is amplified and upconverted into light using a Mach-Zehnder electro-optic modulator. The modulated signal is then sent through our filter, heterodyned and detected with a fast photodiode. The photodiode output is mixed with a microwave carrier to restore the initial signal. An example of pictures transmitted with and without the filter is shown in Fig. 4.

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