Brillouin scattering measurement method and Brillouin scattering measurement device

A Brillouin scattering measurement method that uses a composite pulse train composed of composite pulses with an interval of the composite pulse train longer than a phonon lifetime;

• each composite pulse being formed of two kinds of optical pulses having different durations generated from a laser light from a laser source, by combining both optical pulses as a pair to be located at respective predetermined time positions;
• and injects the composite pulse train into one end of an optical fiber provided to a measurement object, to measure physical quantities of the measurement object from frequency shift change of Brillouin backscattered light generated by the composite pulse train in the optical fiber, the Brillouin scattering measurement method comprising the steps of:
• phase-modulating one of the optical pulse on the basis of values of each row of a Hadamard matrix;
• optically heterodyne-receiving the Brillouin backscattered light from each composite pulse with a reference light from the laser light source;
• heterodyne-receiving the first signal with a signal having a predetermined frequency and then passing the heterodyne-received signal through two kinds of low-pass filters corresponding to the optical pulses, to output the filtered signals as second signals;
• calculating cross-spectrum of one of the second signals and a complex conjugate signal of the other second signal;
• calculating a synthetic spectrum from the calculated cross-spectrum;
• and
• accumulating the synthetic spectrum using an inverted Hadamard matrix and averaging the accumulated synthetic spectrums.

A Brillouin scattering measurement system configured to measure a frequency shift of Brillouin backscattered light, comprising:

• a laser light source;
• a probe light generator configured to generate a probe light for measuring physical quantities of a measurement object, the probe light generator including:
• a pulse generator generating, from a laser light from the laser light source, two kinds of optical pulses having durations different from each other;
• a phase selector having a plurality of pieces of phase information based on two Golay code sequences, and selecting a piece of the phase information to be sent;
• a phase shifter phase-modulating one of the optical pulses to impart to the one optical pulse the pieces of phase information received from the phase selector;
• and
• a pulse combiner combining as a pair the phase-modulated pulse and the other pulse and locates both pulses at respective predetermined time positions,
• an optical heterodyne receiver optically heterodyne-receiving with a reference light from the laser light source a Brillouin backscattered light generated by the probe light injected into an optical fiber provided to the measurement object;
• a heterodyne receiver heterodyne-receiving the signal output from the optical heterodyne receiver with a signal having a predetermined frequency from an oscillator;
• and
• a signal processor passing the signal output from the heterodyne receiver through low-pas filters corresponding to the short pulse and the long pulse, and calculating respective spectra of the filtered signals, to calculate a cross-spectrum of the spectra.