STRAIN CHANGE MEASURING DEVICE AND STRAIN CHANGE MEASURING METHOD
An object of the present disclosure is to measure, at a high sampling rate, a distribution of a vibration pattern applied to each core after being branched. A strain change measurement apparatus according to the present disclosure inputs, at a reference time, first chirped pulsed light having a chirp having a frequency changing linearly with time to a measurement target optical fiber branched by a coupler, acquires a signal of first scattered light regarding the first chirped pulsed light, inputs, at each monitoring time, second chirped pulsed light having a chirp obtained by time axial inversion with respect to the first chirped pulsed light to the measurement target optical fiber to acquire a signal of second scattered light regarding the second chirped pulsed light, determines a shift amount allowing for maximization of a correlation between a waveform obtained by inverting the signal of the first scattered light around the time axis and further shifting and a waveform of the signal of the second scattered light, and uses the shift amount to calculate a change in strain amount in the measurement target optical fiber from the reference time to each monitoring time.
1 . A strain change measurement method comprising:
by a strain change measurement apparatus, inputting first chirped pulsed light having a chirp having a frequency changing linearly with time to a measurement target optical fiber branched by a coupler to acquire a signal of first scattered light regarding the first chirped pulsed light;
by the strain change measurement apparatus, inputting second chirped pulsed light having a chirp obtained by time axial inversion with respect to the first chirped pulsed light to the measurement target optical fiber to acquire a signal of second scattered light regarding the second chirped pulsed light; and
by the strain change measurement apparatus, determining a shift amount allowing for maximization of a correlation between a waveform obtained by inverting the signal of the first scattered light around the time axis and further shifting and a waveform of the signal of the second scattered light, and using the shift amount to calculate a change in a strain amount in the measurement target optical fiber.
2 . The strain change measurement method according to claim 1 , wherein the measurement target optical fiber includes a branched optical fiber being a portion branched by the coupler and a reflection element provided at an end of the branched optical fiber,
the first scattered light is light scattered in the measurement target optical fiber before reaching the reflection element (hereinafter, referred to as a normal signal),
the second scattered light includes a signal obtained by detecting backscattered light that is generated in accordance with a propagation of the second chirped pulsed light returning in a direction of an incident end after reflecting the second chirped pulsed light using the reflection element, that is propagated in a direction of the reflection element, that is further reflected by the reflection element and that is propagated in the direction of the incident end (hereinafter, referred to as a ghost signal), and
the shift amount is a shift amount that maximizes a correlation value between a time waveform of the ghost signal and a time waveform obtained by temporally inverting and shifting the normal signal, calculated in a range of a predetermined section determined by the chirp pulse.
3 . The strain change measurement method according to claim 1 , wherein the measurement target optical fiber includes a branched optical fiber being a portion branched by the coupler and a reflection element provided at an end of the branched optical fiber,
the first scattered light includes a signal obtained by detecting backscattered light that is generated in accordance with a propagation of the second chirped pulsed light returning in a direction of an incident end after reflecting the second chirped pulsed light using the reflection element, that is propagated in a direction of the reflection element, that is further reflected by the reflection element and that is propagated in the direction of the incident end (hereinafter, referred to as a ghost signal),
the second scattered light is light scattered in the measurement target optical fiber before reaching the reflection element (hereinafter, referred to as a normal signal), and
the shift amount is a shift amount that maximizes a correlation value between a time waveform of the ghost signal and a time waveform obtained by temporally inverting and shifting the normal signal, calculated in a range of a predetermined section determined by the chirp pulse.
4 . The strain change measurement method according to 1 , further comprising:
acquiring the signal of the first scattered light in advance;
acquiring the signal of the second scattered light at individual monitoring times for tracking a vibration pattern; and
calculating a strain change of the signal of the second scattered light at the individual monitoring times with respect to the signal of the first scattered light acquired in advance and acquiring a temporal change in strain based on the calculated strain change to measure a distribution of a vibration pattern of the measurement target optical fiber.
5 . The strain change measurement method according to claim 1 , further comprising:
alternately inputting the first chirped pulsed light and the second chirped pulsed light to the measurement target optical fiber at individual monitoring times for tracking a vibration pattern to acquire the signal of the first scattered light and the signal of the second scattered light; and
calculating a strain change of the signal of the second scattered light at a monitoring time with respect to the signal of the first scattered light acquired at a time different from the monitoring time and acquiring a temporal change in strain based on the calculated strain change to measure a distribution of a vibration pattern of the measurement target optical fiber.
6 . A strain change measurement apparatus configured:
to input, at a reference time, first chirped pulsed light having a chirp having a frequency changing linearly with time to a measurement target optical fiber branched by a coupler to acquire a signal of first scattered light regarding the first chirped pulsed light;
to input, at individual monitoring times, second chirped pulsed light having a chirp obtained by time axial inversion with respect to the first chirped pulsed light to the measurement target optical fiber to acquire a signal of second scattered light regarding the second chirped pulsed light; and
to determine a shift amount allowing for maximization of a correlation between a waveform obtained by inverting the signal of the first scattered light around the time axis and further shifting and a waveform of the signal of the second scattered light, and use the shift amount to calculate a change in strain amount in the measurement target optical fiber from the reference time to each of the monitoring times.