Optical isolator independent of input polarization direction utilizing a quarter-wave plate

A conventional optical isolator is composed of a pair of polarizers whose transmission axes make an angle of 45° with each other and a Faraday 45° rotator sandwiched by the polarizers. Hence, the insertion loss of an optical isolator significantly varies depending on the polarization angle of the input light. On the other hand, the authors have found an interesting phenomenon that when an elliptically polarized light is launched into a quarter-wave plate followed by a polarizer, the output light power is kept constant regardless of the principal axis direction of the input light when the birefringence axis of the quarter-wave plate and the transmission axis of the polarizer make an angle of 45°. Utilizing this unique phenomenon, input polarization direction dependence of the insertion loss of an optical isolator is reduced from 17 dB to 0.4 dB by attaching a quarter-wave plate in front of a conventional optical isolator.