| 摘要 |
A multi-nozzle type burner is used for producing a porous glass preform, the burner having small variations in deposition efficiency with the burner tip being not burned even when axial shift occurs at the concentric multi-tube part of the burner. The present invention provides a burner for producing a porous glass preform with a concentric multi-tube structure, comprising a glass material gas jet port in a center, a plurality of gas jet ports concentrically disposed outside the glass material gas jet port, and small-diameter gas jet ports which are disposed in a line or a plurality of lines concentrically to the glass material gas jet port so as to be enclosed in one of the gas jet ports other than the gas jet ports in the center and at an outermost side, the small-diameter gas jet ports in the same line having an identical focal length. In the present invention, the gas jet ports disposed outside the gas jet port enclosing the small-diameter gas jet ports are each reduced in diameter in a direction toward a burner tip, and satisfy a relationship of L1>L2>L3> . . . >Ln, where L1, L2, L3, . . . Ln represent lengths from the burner tip to diameter-reduction start positions of the outside-disposed gas jet ports in this order from the inside, respectively, so that the more outward the gas jet port is located, the closer to the burner tip the diameter-reduction start position of the gas jet port is.
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