Abstraef -The design, analysis, characterisation, manufacture and application of new, large area, permanently magnetised quasi-optical Faraday rotators is discussed. As isolators, these have given state of the art performance at W-band with isolations -6odB and insertion loss -0.35dB. at spot frequencies. It is believed that the wideband performance of these isolators is primarily limited by the matching of the femte to free space. lhtroduction Free-space isolators using femtes with extemal magnets have been described before at 40GHz [ 11 and 285GHz [2] with respective isolations of 40dB and 18dB, and respective insertion losses of 0.ldB and 2dB. However they required large extemal magnets and had relatively small apertures. Free-space isolators using permanently magnetised ferrites were first examined by D.Martin and R.Wylde who surveyed a number of femtes and produced an isolator with 17dB isolation and 1.W insertion loss at 115GHz [3]. This work was extended and improved upon by M.R.Webb [4] who produced isolators with isolations -3OdB at W-band, with losses 4.5dB. However, these isolators also had very small apertures which made it difficult to incorporate them easily into existing quasi-optical systems.Faraday rotators of size 100"x 100" which consist of a thin permanently magnetised femte sheet with appropriate quarter wavelength matching layers. Requiring no extemal field, and exhibiting low loss, these are extremely easy to use and incorporate into existing quasi-optical systems and have allowed a number of new quasi-optical techniques and systems to be utilised at W-band. When used as a 45 degree rotator and positioned between two angled wire-grid polarisers they can be used as a high performance 4 port circulator [4] as illustrated in Figure 1. This paper reports the manufacture of large area Figure 1. Schematic diagram of a quasi-optical 4 port circulator using a Faraday rotator between angled polarisers. Any reflection back from the detector at port 3 will be dumped into the absorbing load at port 2. TheoryAfter a permanently magnetised femte has been magnetised into saturation by a strong extemal magnetic field, and removed, it will have an intemal magnetic field given by:where MI is the magnetisation (remnance field), HA is the effective field due to the crystal anisotropy and Hd is the demagnetisation field. The anisotropy field HA is inversely proportional to the magnetism and normally represented as 2K/M, where K is the anisotropy constant. The demagnetisation field Hd is given by NdM, where Nd is the demagnetisation factor and is geometry dependent. In the case where the magnetisation is perpendicular to a large thin sheet Nd -1 (SI units) and Hd will have almost the same magnitude as the magnetisation.The physical basis of Faraday rotation is that the magnetisation field MI (but not the anisotropic or demagnetisation field) undergoes forced precession when it interacts with a propagating electromagnetic beam. The torque acting on MI is due to the anisotropic field HA and the demagnetisation fi...

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