Coherent reflectometric fiber Bragg grating sensor array

A method of detecting a physical condition, the method comprising:

• (a) generating a pseudo-random bit sequence;
• (b) emitting a light beam which is modulated in accordance with the pseudo-random bit sequence;
• (c) causing the light beam to enter an optical fiber so that the light beam propagates in the optical fiber in a first direction, the optical fiber comprising a plurality of fiber segments disposed in series, each of the plurality of fiber segments having an optical characteristic which varies in accordance with the physical condition, the fiber segments being separated by means for weakly reflecting portions of the light beam to form return optical signals which propagate in the optical fiber in a second direction which is opposite to the first direction, each of the return optical signals representing an effect of the physical condition on the light beam;
• (d) receiving the return optical signals and producing a photodetector output which represents a sum of the return optical signals;
• (e) producing a time-delayed pseudo-random bit sequence by producing a plurality of time-delayed pseudo-random bit sequences, each having a different time delay;
• (f) performing a correlation between the time-delayed pseudo-random bit sequence and the photodetector output to produce a correlation result representing one of the return optical signals by performing a plurality of correlations to produce a plurality of correlation results, each representing a different one of the return optical signals;
• and
• (g) calculating a difference between two of the return optical signals to determine an effect on the light beam by one of the fiber segments.

An optical monitoring system, comprising:

• an optical coupler, said fiber comprising a plurality of partially reflective and partially transmissive elements;
• an optical source disposed to launch an input signal into said fiber;
• a coupler in said fiber disposed to receive signal reflected from said elements;
• and
• a detector effective, responsive to said coupler, to determine the phase of at least a portion of said signal reflected from at least one of said elements, said detector comprising a correlator disposed to correlate said input signal with said at least a portion of said signal reflected from said at least one element.

An optical system comprising:

• an optical fiber comprising a plurality of partially reflective elements;
• an optical source disposed to launch an optical signal into said fiber;
• a phase detector disposed effective to determine the phase between said optical signal and light reflected from a preselected one of said elements, said phase detector comprising a correlation means;
• and
• a subtractor means for subtracting known phase shifts from a cumulative phase detected by said correlation means, thereby detecting a phase shift induced by a preselected one of said elements.

A fiber optic sensor array for detecting a physical condition, the fiber optic sensor array comprising:

• bit sequence generation means for generating a pseudo-random bit sequence;
• light source means, receiving the pseudo-random bit sequence, for emitting a light beam which is modulated in accordance with pseudo-random bit sequence, the light source comprising:
• a laser for emitting light;
• a modulator;
• a first coupler, disposed in a path of the light emitted by the laser between the laser and the modulator, for picking off a portion of the light emitted by the laser;
• an optical fiber disposed to receive the light beam so that the light beam propagates in the optical fiber in a first direction, the optical fiber comprising a plurality of fiber segments disposed in series, each of the plurality of fiber segments having an optical characteristic which varies in accordance with the physical condition, the fiber segments being separated by means of weakly reflecting portions of the light beam to form return optical signals which propagate in the optical fiber in a second direction which is opposite to the first direction, each of the return optical signals representing an effect of the physical condition on the light beam, the optical fiber further comprises a second coupler for picking off portions of the return optical signals;
• photodetecting means for producing a photodetector output which represents a sum of the return optical signals, the photodetecting means further comprises:
• a third coupler for coherently mixing the portion of the light picked off by the first coupler with the portions of the return optical signals picked off by the second coupler;
• and
• means for detecting a result of coherent mixing by the third coupler;
• time delay means for producing a time-delayed pseudo-random bit sequence;
• and
• correlating means for performing a correlation between the time-delayed pseudo-random bit sequence and the photodetector output to produce a correlation result representing one of the return optical signals.

A method of detecting a physical condition, the method comprising:

• (a) generating a pseudo-random bit sequence;
• (b) emitting a light beam which is modulated in accordance with the pseudo-random bit sequence to produce the light beam, and wherein step(b) further comprises picking off a portion of the light emitted by the laser;
• (c) causing the light beam to enter an optical fiber so that the light beam propagates in the optical fiber in a first direction, the optical fiber comprising a plurality of fiber segments disposed in series, each of the plurality of fiber segments having an optical characteristic which varies in accordance with the physical condition, the fiber segments being separated by means for weakly reflecting portions of the light beam to firm return optical signals which propagate in the optical fiber in a second direction which is opposite to the first direction, each of the return optical signals representing an effect of the physical condition on the light beam;
• (d) receiving the return optical signals and producing a photodetector output which represents a sum of the return optical signals, and wherein step(d) further comprises
• (i) picking off portions of the return optical signals;
• (ii) coherently mixing the portion of the light picked off in step(b) with the portions of the return optical signals picked off in step(d)(i);
• and
• (iii) detecting a result of the step of coherently mixing;
• (e) producing a time-delayed pseudo-random bit sequence;
• and
• (f) performing a correlation between the time-delayed pseudo-random bit sequence and the photodetector output to produce a correlation result representing one of the return optical signals.

An optical monitoring system, comprising

• an optical fiber, said fiber comprising a plurality of partially reflective and partially transmissive elements;
• an optical source disposed to launch an input signal into said fiber, said source further comprises:
• a laser for emitting light;
• a modulator;
• first coupler, disposed in a path of the light emitted by the laser between the laser and the modulator, for picking off a portion of the light emitted by the laser
• a second coupler for picking off portions of the return optical signals reflected from said elements;
• and
• a detector effective, responsive to said second coupler, to determine the phase of at least a portion of said signal reflected from said at least one of said elements, wherein the detector further comprises a third coupler for coherently mixing the portion of the light picked off by the first coupler with the portions of the return optical signals picked off by the second coupler.