Obviously the most important optical element in a Faraday Rotator and isolator are the Magneto Optical Glass / Faraday rotator Rod.
The characteristics that one looks for in a Faraday rotator optic, include a high Verdet constant,low absorption coefficient,low non-linear refractive index and high damage threshold. Also,to prevent self-focusing and other thermal related effects,the optic should be as short as possible.The two most commonly used materials for the 400-1600nm range are terbium doped borosillicate glass (MR3-2) and terbium gallium garnet crystal (TGG).But TGG crystal can't grow large size and too expensive