| 摘要 |
PURPOSE:To detect fault points such as breakage with high sensitivity by making the light beam modulated with repetitive signals incident to an optical fiber, and detecting the difference between the modulation frequencies of the modulated incident light beam and the backward scattered and reflected light beam. CONSTITUTION:An oscillator 21 changes an oscillation frequency at an F0[Hz/sec] ratio with respect to a time, and this signal is amplified with an amplifier 22, whereby a laser 23 is modulated. The modulated light 14 from the laser 23 is made incident to an optical fiber 11 past a photodirectional coupler 24. The backward scattered and reflected light beam 15 from the fiber 11 again passes through the coupler 24, is detected with a photo-electric transducer 25 and is amplified with an amplifier 26, after which it is frequency-mixed with the output of the oscillator 21 with a mixer 27, whereby the signal of the differential frequency is created. Only the differential frequency signal is extracted by a band-pass filter 28, and the frequency components are observed with a spectrum analyzer 29. From the difference between the modulating frequency F1[Hz] of the incident light beam and the frequency F2[Hz] extracted from the reflected light, the distance S1[m] between the incident point and the reflection point is determined by S1=VX¦F1-F2¦/2XF0. |
