Dynamic gain equalizing filter

1. A tuneable optical filter comprising a long-period Bragg grating ( 5 ) written in a portion of an optical waveguide ( 10 ) comprising a core and an optical cladding ( 11 ), characterised in that the potion of the waveguide comprising the grating ( 5 ) is at least partially immersed in an external environment consisting of two distinct sections, a first section ( 1 ) whose refractive index is higher than that of the optical cladding ( 11 ) and a second section ( 2 ) whose refractive index is lower than that of the optical cladding, the long-period grating being at least partially immersed in one of the said sections so as to almost independently tune the wavelength and/or the contrast of the spectral response of the filter.

2. An optical filter according to claim 1 , characterised in that the grating ( 5 ) is at least partially immersed in each of the sections ( 1 , 2 ) of the external environment.

3. An optical filter according to claim 1 , characterised in that the level of immersion of the grating ( 5 ) in the sections ( 1 , 2 ) of the external environment is dynamically variable.

4. An optical filter according to claim 1 , characterised in that the sections ( 1 , 2 ) of the external environment are composed of fluids (L 1 , L 2 ).

5. An optical filter according to claim 1 , characterised in that the sections ( 1 , 2 ) of the external environment are composed of gels.

6. An optical filter according to claim 1 , characterised in that the sections ( 1 , 2 ) of the external environment are composed of polymers.

7. An optical filter according to claim 1 , characterised in that the optical waveguide is an optical fibre or a planar waveguide.

8. A device for packaging the filter according to claim 1 , characterised in that the sections ( 1 , 2 ) of the external environment are disposed in movable reservoirs ( 41 , 42 ) associated with electromagnetic control means ( 415 ).

9. A packaging device according to claim 8 , characterised in that each reservoir ( 41 ) is composed of a receptacle ( 410 ) containing a material ( 411 ) with a refractive index distinct from that of the optical cladding, the receptacle ( 410 ) being surrounded by electromagnetic control means ( 412 ).

10. A packaging device according to claim 8 , characterised in that the electromagnetic control means ( 415 ) comprise a plurality of coils supplied separately with voltage and current.

11. A packaging device according to claim 8 , characterised in that the electromagnetic control means comprise a magnetic ring ( 416 ).

12. An optical gain flattening system comprising a plurality of tuneable optical filters (S 1 , S n ) each comprising a long-period Bragg grating ( 5 ) written in a portion of an optical waveguide ( 10 ) comprising a core and an optical cladding ( 11 ), characterised in that the portion of the waveguide comprising the grating ( 5 ) is at least partially immersed in an external environment consisting of two distinct sections, a first section ( 1 ) whose refractive index is higher than that of the optical cladding ( 11 ) and a second section ( 2 ) whose refractive index is lower than that of the optical cladding, the long-period grating being at least partially immersed in one of the said sections so as to almost independently tune the wavelength and/or the contrast of the special response of the filter, characterised in that each filter is tuned by means of a filter packaging device according to claim 8 .

13. An optical gain flattening system comprising a plurality of optical filters (S 1 , S o ) according to claim 1 .