Abstract

Correction of modal dispersion of light transmitted through multimode optical fibres has been the subject of extensive research to date. At low optical powers photorefractive crystals are typically used to produce phase conjugate correction at the output end of the fibre in double-pass schemes [1]. Double-pass correction schemes vary in their approach to overcome the apparent problem that only one (the e-polarised) polarisation state of the scrambled output from the fibre, can undergo phase conjugate correction. We report the results of our study on the implementation of phase conjugate correction of modal distortion in highly multimode, (300 micron diameter), short (~1m) passive optical fibres using a non-polarisation preserving phase conjugation (NPPPC) scheme [2] and its application to a non-reciprocal transmission element. Figure 1 shows a schematic of the set-up used to demonstrate phase-conjugate non-reciprocal transmission characteristics in multimode fibres. An initial vertically polarised Gaussian input beam from an Ar-ion laser is launched into fibre A where it becomes polarization and modally scrambled as it travels along fibre A’s length. The optical power coupled into fibre B, and hence the forward transmission factor is dependant on the distance, z, between fibres A and B. The field from fibre B is then sent to a NPPPC consisting of a self-pumped BaTiO₃ crystal. Previous work has shown that the field reflected from such a NPPPC consists of a phase conjugate part which will all couple back from fibre B to A in the reverse direction and a non-phase conjugate part which will have the same transmission characteristics as the forward travelling wave leading to different transmissions for the forward and backward direction.

© 2001 EPS