An overlapped chirped fiber bragg grating sensing fiber and methods and apparatus for parameter measurement using same

Abstract

An optical sensor includes an optical fiber inscribed with densely-overlapping, chirped-frequency fiber Bragg gratings at multiple locations along the optical fiber such that light reflected from a location on the optical fiber is reflected at multiple frequencies in a range of frequencies. An entire length of the optical sensor includes densely-overlapping, chirped-frequency fiber Bragg gratings. At least two of the densely-overlapping, chirped-frequency fiber Bragg gratings overlap at every measurement point along the entire length of the optical sensor. An optical sensing system uses the optical sensor. A method of making the optical sensor is described.

1 . An optical sensor comprising:
 an optical fiber inscribed with densely-overlapping, chirped-frequency fiber Bragg gratings at multiple locations along the optical fiber such that light reflected from a location on the optical fiber is reflected at multiple frequencies in a range of frequencies,   wherein an entire length of the optical sensor includes densely-overlapping, chirped-frequency fiber Bragg gratings, and   wherein at least two of the densely-overlapping, chirped-frequency fiber Bragg gratings overlap at every measurement point along the entire length of the optical sensor.   
     
     
         2 . The optical sensor in  claim 1 , wherein the optical fiber comprises multiple optical light guiding cores within the optical fiber, each of the multiple optical light guiding cores being inscribed with the densely-overlapping, chirped-frequency fiber Bragg gratings. 
     
     
         3 . The optical sensor in  claim 1 , wherein each of the densely-overlapping, chirped-frequency fiber Bragg gratings is arranged to reflect light from a light source at multiple discrete frequencies at a single location on the optical fiber. 
     
     
         4 . The optical sensor in  claim 1 , wherein a first one of the densely-overlapping, chirped-frequency fiber Bragg gratings is arranged to reflect light from a light source at a first set of multiple discrete frequencies at a first location on the optical fiber, and a second one of the densely-overlapping, chirped-frequency fiber Bragg gratings is arranged to reflect light from the light source at a second set of multiple discrete frequencies and at a second different location on the optical fiber, the second set of multiple discrete frequencies different from the first set of multiple discrete frequencies. 
     
     
         5 . The optical sensor in  claim 1 , wherein each of the densely-overlapping, chirped-frequency fiber Bragg gratings is arranged to reflect light from a light source at multiple discrete frequencies that depends on a slope associated with each of the densely-overlapping, chirped-frequency fiber Bragg gratings. 
     
     
         6 . The optical sensor in  claim 1 , wherein each of the densely-overlapping, chirped-frequency fiber Bragg gratings reflects a corresponding first set of wavelengths of light and transmits a corresponding second set of wavelengths of light different from the corresponding first set of wavelengths of light. 
     
     
         7 . The optical sensor in  claim 1 , wherein each of the densely-overlapping, chirped-frequency fiber Bragg gratings is inscribed in a light guiding core and creates a periodic variation of refractive index of the light guiding core. 
     
     
         8 . The optical sensor in  claim 1 , wherein each of the densely-overlapping, chirped-frequency fiber Bragg gratings is inscribed in a light guiding core and creates an aperiodic variation of refractive index of the light guiding core. 
     
     
         9 . The optical sensor in  claim 1 , wherein a reflected frequency of light at a particular point along a length of the optical sensor depends on a location of the particular point along the length of the optical sensor. 
     
     
         10 . An optical sensing system comprising:
 an optical sensor including an optical fiber inscribed with densely-overlapping, chirped-frequency fiber Bragg gratings at multiple locations along the optical fiber such that light reflected from a location on the optical fiber is reflected at multiple frequencies in a range of frequencies, wherein an entire length of the optical sensor includes densely-overlapping, chirped-frequency fiber Bragg gratings, and wherein at least two of the densely-overlapping, chirped-frequency fiber Bragg gratings overlap at every measurement point along the entire length of the optical sensor;   optical interrogation circuitry coupled to the optical sensor; and   control circuitry, coupled to the optical sensor, configured to:
 detect measurement reflection data from the optical sensor over the range of frequencies, 
 determine a change in the detected measurement reflection data over the range of frequencies, and 
 determine a parameter describing a state of the optical sensor based on the determined change in the detected measurement reflection data. 
   
     
     
         11 . The optical sensing system in  claim 10 , wherein the parameter is a measure of strain, optical phase, or delay along a length of the optical sensor. 
     
     
         12 . The optical sensing system in  claim 11 , wherein the control circuitry is configured to:
 obtain baseline reflection data for the optical sensor;   Fourier transform the baseline reflection data from a temporal domain into a spectral domain;   generate a first half spectral response of the baseline reflection data and a second half spectral response of the baseline reflection data;   Fourier transform the detected measurement reflection data from the temporal domain into the spectral domain;   generate a first half spectral response of the detected measurement reflection data and a second half spectral response of the detected measurement reflection data;   process the first half spectral response of the baseline reflection data and the first half spectral response of the detected measurement reflection data to determine a first result;   process the second half spectral response of the baseline reflection data and the second half spectral response of the detected measurement reflection data to determine a second result; and   determine a measure of the delay based on the first and second results.   
     
     
         13 . The optical sensing system in  claim 12 , wherein the control circuitry is configured to:
 obtain baseline reflection data for the optical sensor, and   determine the delay based on a comparison that uses the baseline reflection data and the detected measurement reflection data.   
     
     
         14 . The optical sensing system in  claim 13 , wherein the control circuitry is configured to use the delay to compensate for a misalignment between the baseline reflection data and the detected measurement reflection data. 
     
     
         15 . The optical sensing system in  claim 10 , wherein the control circuitry is configured to determine a measure of a phase slope associated with a chirp rate of a densely-overlapping, chirped-frequency fiber Bragg grating, compare the measured phase slope to a phase slope generated from a baseline reflection measurement of the optical fiber, and determine a measure of strain or delay at a location on the optical fiber based on the comparing the measured phase slope to the phase slope generated from a baseline reflection measurement of the optical fiber. 
     
     
         16 . The optical sensing system in  claim 10 , wherein the control circuitry is configured to extract a measure of delay along a length of the optical sensor by analyzing amplitude fluctuations in the densely-overlapping, chirped-frequency fiber Bragg gratings inscribed along the length of the optical sensor due to interference of the densely-overlapping, chirped-frequency fiber Bragg gratings. 
     
     
         17 . The optical sensing system in  claim 10 , wherein:
 the control circuitry includes OFDR circuitry,   a reflection wavelength of the overlapping chirped frequency fiber Bragg gratings exceeds a wavelength range of the OFDR circuitry, and   the OFDR circuitry is configured to measure light reflected from a location on the optical fiber having a spectral shift greater than a scan range of the OFDR circuitry.   
     
     
         18 . A method for making an optical sensor that includes an optical fiber, the method comprising the steps of:
 inscribing a first chirped-frequency, light refracting pattern on the optical fiber at every measurement point along at least a portion of a length of the optical sensor;   inscribing a second chirped-frequency, light refracting pattern on the optical fiber that overlaps the first chirped-frequency, light refracting pattern, the second chirped-frequency, light refracting pattern having a same pattern as the first chirped-frequency, light refracting pattern; and   inscribing a third chirped-frequency, light refracting pattern on the optical fiber that overlaps the first and second chirped-frequency, light refracting patterns, the third chirped-frequency, light refracting pattern having the same pattern as the first and second chirped-frequency, light refracting patterns,   wherein the optical fiber inscribed with the overlapping first, second, and third chirped-frequency, light refracting patterns reflects light from a location on the optical fiber at multiple frequencies in a range of frequencies, and   wherein the first, second, and third chirped-frequency, light refracting patterns overlap at every measurement point along an entire length of the optical sensor.   
     
     
         19 . The method in  claim 18 , wherein the inscribing the first, second, and third chirped-frequency, light refracting patterns are performed using an ultraviolet writing beam and a grating phase mask as the optical fiber is drawn from a glass preform. 
     
     
         20 . The method in  claim 18 , wherein at least two of the first, second, and third chirped-frequency, light refracting patterns overlap by more than 50%.