Traditional vs. advanced Bragg reflectors for oversized circular waveguide

This paper compares two types of distributed Bragg reflector, based on the periodic wall perturbation of an oversized circular waveguide. The first type is a traditional mirror, where wall ripples with a period of half a guided wavelength for the working mode couple forward and backward waves. The other type is an advanced reflector with a ripple period of about a guided wavelength, exploiting an intermediate conversion into a quasi-cutoff mode. The design of both reflectors has been optimized with a mode matching code to deliver a reflectivity >96% for the TE5,3 mode at 250 GHz and a power to gun <0.5% in copper waveguides with a diameter of 15 mm. Such specifications are relevant to the upstream mirror of a cyclotron auto-resonance maser under development at ENEA Frascati. The two types of reflector are compared in terms of mechanical dimensions, reflectivity, bandwidth and losses.