Method of detecting and/or measuring physical magnitudes using a distributed sensor

A method of detecting and/or measuring changes in a physical magnitude using a distributed sensor, changes being detected and/or measured at a plurality of measurement points along said sensor, said sensor comprising an optical fiber having an optical core for guiding a majority of input light waves, said optical core including a plurality of diffraction gratings distributed along said optical fiber for reflecting a portion of said input light waves to create reflected signals, each diffraction grating situated at one of said measurement points, said diffraction gratings each having a central reflection wavelength in the absence of strain which is substantially the same for all diffraction gratings, and said diffraction gratings all having a reflection spectrum;

• the method comprising the following steps:
• injecting a detection light signal into an inlet end of said optical fiber, the light signal having a wavelength close to said central reflection wavelength;
• determining the power of one of said reflected signals, referred to as detected reflected power as a function of time;
• said detected reflected power having an amplitude;
• comparing said detected reflected power as a function of time with an idle reflected power which is power reflected as a function of time by said sensor in the absence of any change in said physical magnitude;
• and
• detecting a change of a physical magnitude at each measurement point by determining a ratio of the detected reflected power to the idle reflected power, and by comparing said ratio with a predetermined threshold.

A method of detecting and/or measuring changes in a physical magnitude using a distributed sensor, changes being detected and/or measured at a plurality of measurement points along said sensor, said sensor comprising an optical fiber having an optical core for guiding a majority of input light waves, said optical core including a plurality of diffraction gratings distributed along said optical fiber for reflecting a portion of said input light waves to create reflected signals, each diffraction grating situated at one of said measurement points, said diffraction gratings each having a central reflection wavelength in the absence of strain which is substantially the same for all diffraction gratings, and said diffraction gratings all having a reflection spectrum;

• the method comprising the following steps:
• injecting a detection light signal into an inlet end of said optical fiber, the detection light signal comprising two consecutive light pulses, a pulse whose wavelength belongs to the reflection spectrum of the diffraction gratings and is less that said central reflection wavelength, and a pulse whose wavelength belongs to the reflection spectrum of the diffraction gratings and is greater than said central reflection wavelength;
• determining the power of said reflected signals for each of the two consecutive light pulses, referred to as a first detected reflected power and a second detected reflected power, respectively, as a function of time;
• each detected reflected power having an amplitude;
• comparing said first detected reflected powers with the second detected reflected power to produce a ratio of the detected reflected power having a value;
• and
• detecting a change in physical magnitude by comparing the ratio of the detected reflected powers with a predetermined value.