Abstract: A method of treating a glass fiber-based prepreg for use in production of laminates, copper-clad laminates, multilayer printed wiring boards, or the like by heat treating a B-staged, glass fiber-based prepreg cut into a predetermined size with a far infrared ceramic heater to a temperature not higher than the softening point of a matrix resin of the prepreg for a short period of time.

Abstract: This invention provides a magneto-optic garnet grown by liquid phase epitaxy on a nonmagnetic garnet substrate and having a composition of the following formula (1)Ho.sub.x Tb.sub.y Bi.sub.3-x-y Fe.sub.5 O.sub.12 (1)wherein 0.3.ltoreq.y/x.ltoreq.1.0 and x+y<3.0.According to this invention there is provided a magneto-optic garnet as a Faraday rotator for use in an optical isolator, optical circulator, etc., utilizing Faraday effect, which has a very large Faraday rotation coefficient, a small difference in lattice constant from a nonmagnetic garnet substrate, exhibits a mirror face without causing a film defect (or so-called pit), and has a small temperature dependency.