PURPOSE: To prepare a parent material for a light transmitting body having low transmitting loss and suitable for wide range of the wavelength of light by heat- treating a quartz-based parent material of porous glass in a gaseous mixture consisting of SOCl2 and He at a temp. below a vitrification temp. of the glass, and heat-treating further in an atmosphere contg. O2.
CONSTITUTION: A quartz-based parent material for the porous glass 1 prepd. by the VAD process, etc. is placed in a heating furnace 11 provided with a carbon heater 17 and an intermediate cylinder 16 included therein, which is interposed between a bottom cylinder 13 provided with a gas inlet 12 and a top cylinder 15 provided with a gas outlet 14, and the parent material is heat-treated by rotating and moving downward. The heat-treatment is performed in the first stage in SOCl2, then in a gaseous mixture consisting of about 0.25W2.5vol% SOCl2 and He, at a temp. below the vitrification temp. of the glass, i.e. 700W1,400°C. Then, the heat-treatment is carried on further at ≥700°C preferably by changing the atmosphere in the furnace to the atmosphere contg. O2 to remove chlorine in the parent material. Finally, the heat-treatment is executed at ≥1,300°C to cause vitrification.
FURUGUCHI MAKOTO
OGURA KUNIO
IINO AKIRA
ORIMO KATSUMI
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