The Research Of Terahertz Band Stop Filter Based On The Frequency Selective Surface

The Frequency Selective Surface(FSS) is an infinite plane periodic metal structure composed of patch or aperture unit structure, it can be arranged in a specific style with a single, double or multi-layered periodic structure. When the plane electromagnetic wave irradiates on the plane with periodic metal structure, different surface currents can be induced on different metal unit structures. The plane electromagnetic wave will be totally reflected when it irradiates on the surface of patch unit structure, and totally transmitted when it irradiates on the surface of aperture unit structure. The unique spatial filtering characteristic of FSS to incident electromagnetic waves with different polarizations and angles of incidence activates its extensive use in electromagnetic domain.In this paper, the basic concepts and principles of the FSS are briefly introduced. The FSS is designed to apply in quasi-optical mode converter(QOMC) used on the gyrotron oscillator as a spatial filter.(1) A terahertz spatial filter in which the linear polarized beam at 0.11 THz is reflected but the beam at 0.22 THz is transmitted based on FSS is designed to separate radiations from a dual-frequency operation coaxial gyrotron with two electron beams. A quasi-optical technique is used to converted the operation modes TE02 and TE04 in the dual-frequency operation gyrotron into the linear polarized Gaussian beams. Then the linearly polarized beams with different frequencies are oblique incident toward a spatial filter based on frequency selective surface, the beam at 0.11 THz is totally reflected, the beam at 0.22 THz is transmitted and the transmittance is about 95%. The transmission characteristics of the spatial filter change little when the incident angle ranges from 0 to 45 degree.(2) A terahertz spatial filter in which the linear polarized beam at 0.22 THz is reflected but the beam at 0.42 THz is transmitted based on FSS is also designed for a second harmonic terahertz gyrotron. The transmission coefficient is about-20 dB at 0.22 THz but only-0.2dB at 0.42 THz. it means that the beam at 0.22 THz is totally reflected but the beam at 0.42 THz is transmitted. The transmission characteristics of the spatial filter change little when the incident angle ranges from 0 to 40 degree.The preliminary experiment for the transmission coefficient of the spatial filter operated at 0.11 THz and 0.22 THz is carried out with THz-TDS system TERAVIEW TPS 3000 when the incident angle is 0 degree, the trend of the transmission coefficient from the theoretical predictions agrees well with the experimental results, but there is a little difference between the theoretical predictions and the experimental results at 0.11 THz and 0.22 THz due to the fabrication error.