Method of measuring acceleration using a fiber optic particle motion sensor

Abstract

A method of measuring acceleration using an optical sensor. In the optical sensor, acceleration, acoustic velocity, or displacement (vibration) causes a corresponding shift in the center wavelength of the sensor output. The sensor can be coupled to a high-speed interferometric interrogator through an unbalanced fiber interferometer. The unbalanced interferometer functions to translate optical wavelength shift into phase shift, which is easily demodulated by the interrogator.

1 . A method of measuring acceleration comprising:
 providing a sensor containing a section of an optical fiber having a grating inscribed therein;   illuminating a first end of the optical fiber with a light source;   coupling a second end of the optical fiber to an interferometer;   subjecting the sensor to an acceleration;   converting the acceleration to longitudinal strain within the grating;   converting the longitudinal strain to a peak in the grating reflection or transmission spectrum within the grating;   converting the change in the peak in the grating reflection or transmission into a phase shift within the interferometer; and   measuring an intensity output from the interferometer, in which the intensity is proportional to the phase shift in the interferometer and, therefore, proportional to the sensed acceleration.   
     
     
         2 . The method of  claim 1  wherein the grating comprises a fiber Bragg grating. 
     
     
         3 . The method of  claim 1  wherein the grating comprises a phase shifted grating. 
     
     
         4 . The method of  claim 1  wherein the grating comprises a pair of gratings comprising a Fabry-Perot interferometer. 
     
     
         5 . The method of  claim 1  wherein the grating comprises a pair of gratings that are phase shifted relative to one another. 
     
     
         6 . The method of  claim 5  wherein the phase shift between the gratings is π radians. 
     
     
         7 . The method of  claim 1  wherein the grating has a central transmission peak. 
     
     
         8 . The method of  claim 1  wherein the grating has a transmission peak with a spectral width of approximately 0.3 picometers. 
     
     
         9 . The method of  claim 1  wherein the grating has a stop band of low transmission extending above and below a wavelength of the peak transmission. 
     
     
         10 . The method of  claim 1  wherein the grating has a stop bandwidth of approximately  1  nanometer. 
     
     
         11 . The method of  claim 1  wherein the phase shift is measured using an interferometric interrogator coupled to the interferometer. 
     
     
         12 . The method of  claim 11  wherein the interferometric interrogator is a closed loop interrogator. 
     
     
         13 . The method of  claim 11  wherein the interferometric interrogator is a demodulator. 
     
     
         14 . A method of measuring acceleration comprising:
 providing a sensor containing a section of an optical fiber having a grating inscribed therein, the grating (a) having a central transmission peak and a stop band of low transmission which extends above and below a wavelength of the peak transmission, and (b) including at least one of a fiber Bragg grating, a phase shifted grating, a pair of gratings comprising a Fabry-Perot interferometer, or a pair of gratings that are phase shifted relative to one another by π radians;   illuminating a first end of the optical fiber with a light source;   coupling a second end of the optical fiber to an interferometer;   subjecting the sensor to an acceleration;   converting the acceleration to longitudinal strain within the grating;   converting the longitudinal strain to a peak in the grating reflection or transmission spectrum within the grating;   converting the change in the peak in the grating reflection or transmission into a phase shift within the interferometer; and   measuring an intensity output from the interferometer, in which the intensity is proportional to the phase shift in the interferometer and, therefore, proportional to the sensed acceleration.   
     
     
         15 . The method of  claim 14  wherein the transmission peak has a spectral width of approximately 0.3 picometers. 
     
     
         16 . The method of  claim 14  wherein the grating has a stop bandwidth of approximately 1 nanometer. 
     
     
         17 . The method of  claim 14  wherein the phase shift is measured using an interferometric interrogator coupled to the interferometer. 
     
     
         18 . The method of  claim 17  wherein the interferometric interrogator is a closed loop interrogator. 
     
     
         19 . The method of  claim 17  wherein the interferometric interrogator is a demodulator. 
     
     
         20 . A method of measuring acceleration comprising:
 providing a sensor containing a section of an optical fiber having a grating inscribed therein, the grating (a) having a central transmission peak with a spectral width of approximately 0.3 picometers and a stop band of low transmission and approximately 1 nanometer which extends above and below a wavelength of the peak transmission, and (b) including at least one of a fiber Bragg grating, a phase shifted grating, a pair of gratings comprising a Fabry-Perot interferometer, or a pair of gratings that are phase shifted relative to one another by π radians;   illuminating a first end of the optical fiber with a light source;   coupling a second end of the optical fiber to an interferometer;   subjecting the sensor to an acceleration;   converting the acceleration to longitudinal strain within the grating;   converting the longitudinal strain to a peak in the grating reflection or transmission spectrum within the grating;   converting the change in the peak in the grating reflection or transmission into a phase shift within the interferometer; and   using an interferometric interrogator coupled to the interferometer to measure an intensity output from the interferometer, in which the intensity is proportional to the phase shift in the interferometer and, therefore, proportional to the sensed acceleration.