The millimeter wave bandpass filter using the Whispering-Gallery mode (WG mode) dielectric disk resonators is presented in this paper. A 4 stage maximally flat bandpass filter is constructed with the PTFE disk resonators. For the filter design, the coupling coefficients of this mode in the coupled disk resonators are calculated by an approximated separation of variables method. Furthermore, the external Q values of the disk resonator excited by a dielectric waveguide are investigated experimentally. Designed center frequency is 60 GHz and 3 dB band width is 150 MHz. Furthermore, as an attempt to improve the spurious characteristics, another filter structure which consists of some kinds of dielectric disk is tested. As a result, some spurious responses are reduced considerably.

Rugate thin film optical filters are useful for designing arbitrary-shaped spectra, such as multistep or triangular spectra. A technique for synthesizing the refractive index distribution of rugate filters was used to suppress unwanted ripples on the spectrum. The refractive index of an amorphous hydrogenated silicon oxide (a-SiOx:H) rugate thin film was minutely controlled with a resolution of 0.001 using radio-frequency (RF) magnetron sputtering. The fabricated rugate filters had multistep bands over a wavelength range of 1260-1670 nm or good linearity over 1290-1650 nm.