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A burner for a vapour deposition process has a central nozzle for ejecting a glass precursor material, at least a crown of nozzles surrounding said central nozzle for ejecting a flame reactant, and a circular nozzle between said central nozzle and said crown of nozzles for ejecting an innershield ga

A burner for a vapour deposition process has a central nozzle for ejecting a glass precursor material, at least a crown of nozzles surrounding said central nozzle for ejecting a flame reactant, and a circular nozzle between said central nozzle and said crown of nozzles for ejecting an innershield gas, wherein the central nozzle has a concave shape in a cross section in the axial plane and, preferably, a symmetry about an axial plane.

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1. A burner for a vapour deposition process, comprising: a front face;a precursor exit port defined in said front face for ejecting a glass precursor material, the precursor exit port having a center centrally-located on said front face, said precursor exit port having a concave shape at said front

1. A burner for a vapour deposition process, comprising: a front face;a precursor exit port defined in said front face for ejecting a glass precursor material, the precursor exit port having a center centrally-located on said front face, said precursor exit port having a concave shape at said front face comprising a circular central region and a plurality of radial regions extending from the central region and each radial region is defined by a pair of walls converging toward the axis and an end wall, respectively; andan annular orifice defined in said front face for ejecting a gas, said annular orifice surrounding said central orifice. 2. The burner according to claim 1, wherein said precursor exit port has a symmetry about an axial plane. 3. The burner according to claim 1, further comprising a ring of nozzles, each nozzle in the ring of nozzles being defined by a respective surface, the ring of nozzles surrounding said precursor exit port for ejecting a flame reactant. 4. The burner according to claim 3, wherein the annular orifice is located between said precursor exit port and said ring of nozzles. 5. The burner according to claim 4, wherein said precursor exit port has first angular sectors of minimum radial dimensions and second angular sectors of maximum radial dimensions. 6. The burner according to claim 5, wherein said ring of nozzles comprises a first set of nozzles in the same angular positions of said second angular sectors and a second set of nozzles in the same angular positions of said first angular sectors. 7. The burner according to claim 6, further comprising a first set of ducts terminating in said first set of nozzles and a second set of ducts terminating in said second set of nozzles, the ducts of the first set being inclined at a first angle with respect to a central axis of said burner and the ducts of the second set being inclined at a second angle with respect to said central axis, said second angle being greater than said first angle. 8. The burner according to claim 1, having a central duct terminating in said percursor exit port for the passage of said glass precursor material, and comprising a central member positioned inside the central duct for forcing the glass precursor material toward an external boundary of said central orifice. 9. The burner according to claim 8, wherein the central member has at least an enlarged portion that substantially fits with external walls of said central duct. 10. The burner according to claim 1, wherein the plurality of radial regions extending from the central region of the precursor exit port are evenly-spaced with respect to one another. 11. The burner according to claim 1, wherein the plurality of radial regions extending from the central region of the precursor exit port are triangular in shape. 12. A burner for a vapour deposition process, comprising: a face block having a front face;a central duct for the passage of a glass precursor material, the central duct exiting in a precursor exit port on said front surface;a circular duct for the passage of a gas reactant, said circular duct extends around said central duct and terminates in a circular reactant exit port on said front surface;wherein said precursor exit port has a concave shape at said front face increasing contact surface between the precursor and the reactant, said concave shape comprising a circular central region and a plurality of radial regions extending from the central region and each radial region is defined by a pair of walls converging toward the axis and an end wall, respectively. 13. The burner according to claim 12, wherein the radial regions are evenly-spaced around the circular central region so as to define non-contiguous radial regions. 14. The burner according to claim 12, wherein the end wall of each radial region lays on a same circumference as the end walls of the other radial regions. 15. The burner according to claim 12, wherein the end walls of the radial regions lays close to the internal boundary of the circular reactant exit port. 16. The burner according to claim 12, wherein the radial regions have a triangular shape. 17. The burner according to claim 12, further comprising a first passage and a second passage terminating each in two rings of orifices. 18. The burner according to claim 17, wherein the circular reactant exit port is located between said precursor exit port and said rings of orifices. 19. The burner according to claim 17, wherein the orifices of the inner ring of orifices are in a number that is twice the number of the radial regions of the circular reactant exit port.