Wideband Windows for Millimeter- and Submillimeter-Wave Vacuum Devices
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Radiophysics and Quantum Electronics, Vol. 63, No. 2, July, 2020 (Russian Original Vol. 63, No. 2, February, 2020)
WIDEBAND WINDOWS FOR MILLIMETER- AND SUBMILLIMETER-WAVE VACUUM DEVICES S. N. Vlasov,∗ E. V. Koposova, S. Yu. Kornishin, V. V. Parshin, D. A. Perminov, and E. A. Serov
UDC 537.86
We present the results of a theoretical calculation and an experimental study of double-sided antireflective processing of a dielectric plate by creating a specially profiled corrugated structure on its surface. Such a structure allows one to minimize the reflection coefficient of an electromagnetic wave to a level of about −30 dB within a range of at least one octave. This antireflective effect can be used in energy output windows of frequency-tunable generators operated in the millimeter- and submillimeter-wave bands. It can also be applied in windows of wideband spectroscopy devices used to diagnose a broad range of materials including both gases and condensed media, as well as in medical devices, in which the studied sample/object is placed in a vacuum-tight box. Theoretical calculations are based on solving numerically the complete equations of the electromagnetic field. Reflections from double-sided corrugated plates were studied experimentally using a Michelson interferometer. It is shown that the reflection coefficient is equal to no more than −23 dB in the 50–220 GHz frequency range. The antireflective profile was manufactured using a plate of radiation-processed fluorine F-4 (Raflon). This new material allowed us to manufacture an antireflective structure with a required accuracy.
1.
INTRODUCTION
Currently, frequency-tunable generators (gyrotrons) producing moderate powers (10–1000 W) in the millimeter- and submillimeter-wave bands are being actively designed [1–4]. The quasioptical energy output windows in the form of a plane-parallel dielectric plate with the resonance thickness, which are used now, are efficient in a narrow frequency range (less than 1%). However, both the continuously and discretely tunable multifrequency gyrotrons [2–4] require quasioptical windows that reflect less than one percent in the range of about one octave [3]. The same windows are required for wideband spectroscopic instruments used for diagnostics of both gases and condensed media [5, 6], especially for those medial applications, where the studied object is placed in a closed box. In this equipment, reflections from windows, even if the windows are inclined to an angle of about 10◦ , create a standing wave in the transmit-receive line, which makes information processing significantly more complicated, and the temperature instability of the line introduces additional uncertainty to the measurement results by changing phase relationships in the standing wave. The use of a plane-parallel plate inclined to the Brewster angle requires that its size should be much bigger. This leads to a greater thickness of the plate and greater values of the ohmic loss in it, which is not always acceptable [7]. This paper is based on the results of works [8–10]. It descri
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