Apparatus and method for measuring strain in bragg gratings

An apparatus for measuring strain of gratings written in an optical fiber comprising:

• a reference optical fiber network having a reference optical fiber;
• a coherent light source for transmitting optical radiation over a plurality of contiguous predetermined wavelength ranges into the reference optical fiber network and an optical fiber network under test, the optical fiber network under test including a reflector and a sensing optical fiber having a plurality of gratings written thereon, the transmission of optical radiation over each wavelength range into the reference optical fiber network and the optical fiber network under test producing a plurality of reference interference fringes and measurement interference fringes, respectively;
• a detector for detecting the reference and measurement interference fringes;
• a circuit for sampling the detected reference and measurement interference fringes such that each sampled value of the reference and measurement interference fringes is associated with a corresponding sample number;
• and
• a control system for (l) determining for each sample number the wavelength change of the reference optical fiber due to the wavelength of optical radiation, (ii) matching each wavelength change with a corresponding sampled value of each measurement interference fringe, (iii) transforming each sampled measurement interference fringe into a spatial domain waveform such that each spatial domain waveform corresponds to one of the contiguous predetermined wavelength ranges, (iv) summing the spatial domain waveforms to form a summation spatial domain waveform, (v) determining from the summation spatial domain waveform the location of each grating with respect to the reflector of the optical fiber network under test, (vi) determining from each spatial domain waveform the portion of the spatial domain waveform corresponding to a particular grating, (vii) transforming each portion into a frequency spectrum representation, and (viii) determining the strain on the grating at each wavelength of optical radiation by determining the difference between the current wavelength and an earlier, zero-strain wavelength measurement.

An apparatus for measuring strain of gratings written in an optical fiber comprising:

• a reference optical fiber network having a reference optical fiber;
• means for transmitting optical radiation over a plurality of contiguous predetermined wavelength ranges into the reference optical fiber network and an optical fiber network under test, the optical fiber network under test including a reflector and a sensing optical fiber having a plurality of gratings written thereon, the transmission of optical radiation over each wavelength range into the reference optical fiber network and the optical fiber network under test producing a plurality of reference interference fringes and measurement reference interference fringes, respectively;
• means for detecting the reference and measurement interference fringes;
• means for sampling the detected reference and measurement interference fringes such that each sampled value of the reference and measurement interference fringes is associated with a corresponding sample number;
• means for determining for each sample number the wavelength change of the reference optical fiber due to the wavelength of the optical radiation;
• means for matching each determined wavelength change with a corresponding sampled value of each measurement interference fringe;
• means for transforming each sampled measurement interference fringe into a spatial domain waveform such that each spatial domain waveform corresponds to one of the contiguous predetermined wavelength ranges;
• means for summing the spatial domain waveforms to form a summation spatial domain waveform;
• means for determining from the summation spatial domain waveform the location of each grating with respect to the reflector of the optical fiber network under test;
• means for determining from each spatial domain waveform a portion of the spatial domain waveform corresponding to a particular grating;
• means for transforming each determined portion into a corresponding frequency spectrum representation;
• and
• means for determining the grating at each wavelength of optical radiation by determining the difference between the current wavelength and an earlier, zero-strain wavelength measurement.