  We propose a polarization-independent optical isolator that does not use
walk-off between the two orthogonal polarizations. The design is based on two
Faraday rotators in combination with two half-wave plates in a closed,
Sagnac-interferometer-like, configuration. An experimental prototype is tested
successfully under variation of the input light polarization state with
isolation level between 43 dB and 50 dB for all input polarizations (linear,
circular, or elliptical).
