PURPOSE:To stabilize the reproducibility of the heating conditions by controlling the optical energy incident on an optical fiber. CONSTITUTION:The photo-intensity in the center of the light receiving end of an optical fiber 5 is sensed by a photo-sensor 8, which is located in the same central part, and the photo-intensity signal transduced by a photo-electric transducer 9 is fed to a calculator circuit 10, which calculates the optical energy incident to the optical fiber 5. The difference of the obtained value from the set value on an output command circuit 15 is amplified by an error amplifying circuit 13, and in compliance with the result therefrom the output of a power supply circuit 14 is varied to regulate the photo-quantity of a light emission lamp 1. Thus the optical energy incident on the optical fiber 5 is controlled into the set value for the output command circuit 15, and stable heating characteristics are obtained.
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