Single loop and double loop filters based on silicon on insulator（SOI）are fabricated

and their filtering characteristics are studied.At the same time

the influence of optical coupling coefficient on the output characteristics of filters is analyzed.Aiming at the weak tuning ability of evanescent wave coupling structure

an improved structure of Mach Zehnder interferometer（MZI）coupler is proposed.The length of micro ring cavity is 1 218 μm.For single and double loop filters

when the electrode power is in the range of 18～37 mW and 20～29 mW respectively

the filter can be tuned from under coupling to critical coupling

and the center wavelength of the double loop filter shifts by 1 nm in the range of 0～26 mW

which is equivalent to a free spectrum width.