These are tunable Faraday rotators in which the plane of polarization of a laser beam is rotated through 45° as it passes through a high Verdet constant material, positioned in an axial magnetic field produced by permanent magnets.
Some optical glasses (e.g. FS57 and special grades of glass selected for their Verdet constant) may be used, but Leysop only uses the highest quality single crystal TGG (terbium gallium garnet). This offers the highest Verdet constant for the visible to near infrared region and provides high power handling with low optical distortion.
Rotation is additive for reflectors passing back through the isolator giving 90° special difference between the input and reflected polarization states at the input to the isolator. An input polarizer will therefore reject back reflections. An output polarizer is necessary to select the correct polarization state for transmission back through the isolator.