Optimization of coupling between He11-waveguide mode and Gaussian beam

Megawatt microwave power transmission systems often use a combination of quasi optical mirror lines andovermoded corrugated circular waveguides. The optimum coupling between a free space Gaussian beam and acorrugated waveguide can be achieved by particular mode mixtures of HEll and HE12 hybrid modes [1]. Thewanted amount of the HE12 mode can be generated from the HEll in a corrugated mode converter with periodicdiameter variation or in a corrugated taper. In a previous publication [2] the design and first measurements onperiodic mode converters have been presented. The promising detailed measurements (section A) of these ratherlong devices stimulated the development of very compact converters - i.e. nonlinear tapers. Their design ispresented in section B.