Two methods of apodisation of fibre-Bragg-gratings

Two new methods of apodising fibre-Bragg-gratings are theoretically examined in this paper. Both work on the principle of providing a constant average effective refractive index along the grating. The first method takes advantage of the finite coherence length of the grating inscribing laser. The second uses a double grating writing technique by introducing a path-length difference across the wavefront of one of the two beams of the writing interferometer. The apodised grating is formed by first writing a uniform grating and then a second, centre-phase-matched grating using a phase-plate, the sum of which produces an apodised moire grating profile. Experimental results are presented to support the principle of apodisation using the second scheme.