Apodised complex filter

1. An optical filter composed of a concatenation of elementary filters consisting of chirped slanted Bragg gratings written on a continuous portion of an optical waveguide, characterised in that the various gratings in the concatenation have the same angle, identical lengths and a chirp, and further characterized in that each elementary filter in the concatenation is apodised with a uniform mean refractive index (Δn mean ), the mean refractive index being constant over all the gratings in the concatenation.

2. An optical filter according to claim 1 , characterised in that the chirp of the various gratings is linear over the entire concatenation.

3. An optical filter according to claim 1 , characterised in that the optical waveguide is an optical fibre.

4. An optical filter according to claim 1 , characterised in that the optical waveguide is a planar waveguide.

5. A method of writing an optical filter on a portion of optical waveguide, characterised in that a plurality of elementary filters consisting of chirped slanted Bragg gratings are written through a single chirped phase mask so as to form the optical filter by concatenation of the said elementary filters, each grating being written through a slot moved along the phase mask, and further characterized in that the writing of each grating constituting an elementary filter is apodised to produce a uniform mean refractive index, the mean refractive index being constant over all the gratings in the concatenation.

6. A method of writing an optical filter according to claim 5 , characterised in that the movement of the slot is between 1 and 5 mm between two consecutive writings.

7. An optical gain flattening system comprising an optical filter composed of a concatenation of elementary filters consisting of chirped slanted Bragg gratings written on a continuous portion of an optical waveguide, characterised in that the various gratings in the concatenation have the same angle, identical lengths and a linear chirp, and further characterized in that each elementary filter in the concatenation is apodised with a uniform mean refractive index (Δn mean ), the mean refractive index being constant over all the gratings in the concatenation.

8. An optical filter according to claim 1 , wherein each elementary filter in the concatenation has a different apodisation.

9. A method of writing an optical filter according to claim 5 , wherein the apodisation of each elementary filter is achieved using the same irradiation time.

10. An optical gain flattening system according to claim 1 , wherein each elementary filter in the concatenation has a different apodisation.