| 摘要 |
PROBLEM TO BE SOLVED: To narrow the variation distribution of refractive index values of the preform by heating an optical synthetic quartz glass base material having an OH group concn. distribution and thereafter, slowly cooling the material under specified conditions. SOLUTION: In this production, an optical synthetic quartz glass base material which is produced by a vapor-phase axial deposition(VAD) method or direct synthesis and has a 600 to 1,000ppm OH group concn. and a 20 to 100ppm chlorine concn. is heated to a temp. in the range of 1,500 to 2,000 deg.C and thereafter, slowly cooled at a <=5 deg.C/min cooling rate from the temp. to room temp. to form a virtual temp. distribution so as to compensate the refractive index distribution due to the OH group concn. distribution. Thus, the objective preform in which a region having a maximum OH concn. exists in the central part and the OH concn. values are gradually decreased in the radial directions with this region as the center from the region toward the peripheral part to form a refractive index distribution due to this OH group concn. distribution and which has a <=45ppm difference between the maximum and minimum OH group concn., a 200 to 300mm diameter, a 60 to 150mm length, a refractive index variation width (Δn) of <=1×10<-6> and a <=1ppm chlorine concn., can be produced.
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