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We present novel concepts for tunable optical filters. Long resonant cavities of about 30-μm length have been realized with two-chip designs. GaAs technology has been applied to filters that are designed for the use in dense wavelength-division multiplexing (WDM) at wavelengths around 1550 nm. A finesse of 46.7, a full-width at half-maximum (FWHM) of 1.2 nm and electrostatic tuning over a range of 103 nm with an applied voltage of 35 V has been achieved. An alternative tuning concept that allows to tune the resonator length 4 μm by heating Ni-Cr resistors placed on the suspending beams of a membrane with an applied voltage of 2.7 V has been realized.