Characterisation of UV-induced Birefringence in Optical Fibre Rocking Filters

Abstract

In single-mode birefringent fibre, a mode launched along a principal polarisation axis maintains that polarisation state. Optical fibre rocking filters (or polarisation mode converters) are structures that rotate the polarisation state of the optical field. To achieve this, the fibre’s principal axes are periodically rocked through a small angle θ by either oscillating the preform during the draw process [1] or by using UV light to externally irradiate the fibre periodically [2]. Since the birefringence beat length is wavelength dependent, there is a resonant wavelength at which the beat length is equal to the rocking filter twisting period. At this wavelength there is complete coupling from one polarisation mode to the other, while at other wavelengths, there is only partial coupling. At resonance, the state of polarisation is rotated by an angle 2θ per period and thus for complete power transfer, π/(4θ) coupling steps are required.