METHOD AND DEVICE FOR DETECTING FOREIGN OBJECT IN OPTICAL FIBER

PROBLEM TO BE SOLVED: To positively detect bubbles in an optical fiber by applying linear polarization light from the side of the optical fiber immediately after melting and spinning in a wire-drawing furnace, receiving transmitted light via an analyzer that is installed on the rear surface of the optical fiber, and performing synchronization detection with a lock-in amplifier. SOLUTION: An electrical signal from a photoreceptor 9 is inputted to a lock amplifier 10 for measuring the AC voltage of only a frequency component that is equal to a modulation frequency and an output voltage from the amplifier 10 is recorded by a recorder 11. The operation of the amplifier 10 is called synchronous detection. With parallel light through an optical fiber 5 to be measured, a voltage being measured by the amplifier 10 is nearly constant when there are no foreign objects such as bubbles in the fiber 5. When there are foreign objects such as bubbles in the fiber 5. When there are foreign objects such as bubbles in the fiber 5, parallel light is scattered in various directions according to the size and position of the bubbles or the like and the quantity of light through an analyzer 6 increases and a measurement voltage by the amplifier 10 changes. A polarizer 4 and the analyzer 6 are used for quenching, a noise level is lowered, and a signal is detected by the amplifier 10 for detecting a weak AC signal.