Polarization beam splitter or combiner

This invention relates to elements such as birefringent crystals, which are used to separate a non-collimated input beam into two orthogonally polarized sub-beams or to combine two orthogonally polarized beams into a single beam. The optical device lessens or obviates the optical path length difference encountered in prior art devices and provides a polarization beam splitter / combiner that has substantially same optical path lengths for two split or combined beams propagating therethrough. Alternatively, the device is designed to provide a selected path length difference to compensate for polarization mode dispersion in other optical devices. The polarization beam splitter/combiner in accordance with the invention has a first uniaxial crystal having an o-ray path and an e-ray path and having the first port disposed at an end face thereof; a second uniaxial crystal having an o-ray path and an e-ray path, the e-ray path of the second uniaxial crystal being optically coupled with the o-ray path of the first uniaxial crystal and the o-ray path of the second uniaxial crystal being optically coupled with the e-ray path of the first uniaxial crystal. Alternatively, the axis of the second crystal is aligned in such a manner that the o- ray path is retarded by an extraordinary index of refraction of the crystal and the e-ray path is retarded by an ordinary index of refraction to equalize the optical path lengths.