| 摘要 |
PROBLEM TO BE SOLVED: To produce the preform which has a uniform refractive index and no refractive index distribution and in which reduction in transmissivity due to the irradiation with a laser beam, etc., is inhibited from occurring by heating a specific optical synthetic quartz glass base material in a gaseous hydrogen atmosphere. SOLUTION: In this production, an optical synthetic quartz glass base material in which a region having a maximum OH group concn. exists in the central part and the OH group concn. values are gradually decreased in the radial directions with this region as the center from the region toward the peripheral part and also, a refractive index distribution due to a virtual temp. distribution is formed so as to compensate the refractive index distribution due to this OH group concn. distribution, is heated to 610 to 790 deg.C in a gaseous hydrogen atmosphere whose pressure is 1×10<3> to 1×10<6> Pa. Thus, the objective preform in which a region having a maximum OH group concn. exists in the central part and the refractive index values are decreased in the radial direction with this region as the center from the region toward the peripheral part and also, a refractive index distribution due to a virtual temp. distribution is formed so as to compensate the refractive index distribution due to this OH group concn. distribution and which has a 5×10<5> to 1×10<15> molecule/cm<3> hydrogen concn., can be produced.
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