| 摘要 |
PURPOSE:To measure accurately true elongation even if an optical fiber is twisted in a cable or is laid in a complicated shape, by establishing a certain relation between the delay of propagation time of light due to elongation of the optical fiber and an elongation DELTAL of the optical fiber. CONSTITUTION:The light modulated by a sine wave signal modulator 24 is made incident to an optical fiber 10. At this time, a sine wave signal A2 obtained by not allowing the light to pass the optical fiber but allowing it to pass an attenuator 26 is superposed onto an output A1 of a photodetector 16 and is inputted to a spectrum analyzer 28. Sweep is performed with a modulating frequency (f), and the level of the synthesized signal in the frequency is measured by a frequency synchronized with the frequency. The synthesized signal A1+A2 is described as the output of the spectrum analyzer 28 on a cathode-ray tube. A delay time variation DELTAgamma is obtained as DELTAgamma=(f'0-f0)/2f0 in accordance with the first zero point f0 and a frequency f'0, which generates a strain in the optical fiber 10 and gives a new zero point when the propagation time of the optical signal becomes gamma1+DELTAgamma, on the frequency base. Consequently, DELTAgamma is measured to obtain the elongation DELTAL. |
