Birefringent photosensitivity in monomode optical fibre: application to external writing of rocking filters

Efficient, narrowband rocking filters have been made in polarisation-maintaining monomode optical fibre by photoinducing periodic birefringent gratings along the length of the fibre. These gratings, fabricated with a period chosen to provide synchronous coupling between the two principal states of polarisation of the LP₀₁ mode at a preselected wavelength, gently rock to and fro along the principal polarisation axes of the fibre. To photoinduce the grating, the fibre is exposed periodically along its length to a UV light beam which is incident at right angles to the geometrical axis of the fibre and is directed pi /4 rad with respect to the principal polarisation axes. Efficient polarisation mode conversion at 1.3 mu m with the birefringent fibre grating is demonstrated.