Optical waveguides – Optical waveguide sensor
Reexamination Certificate
1998-05-31
2001-09-04
Arroyo, Teresa M. (Department: 2881)
Optical waveguides
Optical waveguide sensor
C385S013000
Reexamination Certificate
active
06285806
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to an interferometric sensor array which provides a large number of individually addressable sensor locations with high spatial accuracy and in particular to such an array as applied for detection of acoustic or other vibrations, disturbance or the like.
DESCRIPTION OF RELATED ART
It is known in the art to form a sensor array by providing an optical fiber with multiple sensing segments separated by weakly reflecting portions such as fiber Bragg grating reflectors. The sensing segments undergo a change in refractive index in response to a physical condition to be detected, such as stress, strain or sound. Typically, one short light pulse is sent into the fiber, and the time delay of the return pulse identifies the weakly reflecting portion which reflected the return pulse. The weakly reflecting portions are spaced far enough apart that the propagation time between them is at least equal to the width of the short light pulse. Propagation time is in turn determined by the speed of light in a fiber, which is given by c
, where c is the speed of light in a vacuum, and n is the index of refraction of the fiber. For many commercially available optical fibers, n≈1.5.
Concepts relating to such sensor arrays are set forth in detail in the following references:
U.S. Pat. No. 4,775,216 to Layton, Oct. 4, 1988;
U.S. Pat. No. 4,778,248 to Arzur et al, Oct. 18, 1988;
U.S. Pat. No. 4,889,986 to Kersey et al, Dec. 26, 1989;
U.S. Pat. No. 5,144,690 to Domash, Sep. 1, 1992;
U.S. Pat. No. 5,208,877 to Murphy et al, May 4, 1993;
U.S. Pat. No. 5,323,404 to Grubb, Jun. 21, 1994;
U.S. Pat. No. 5,436,988 to Narendran, Jul. 25, 1995;
U.K. Published Patent Application 2,189,880 A to Lamb, published Nov. 4, 1987;
U.K. Published Patent Application 2,214,636 A to Lamb, published Sep. 6, 1989; and
H. S. Al-Raweshidy et al, Spread spectrum technique for passive multiplexing of interferometric optical fiber sensors,
SPIE Vol.
1314
Fibre Optics
90, pp. 342-7.
Pseudo-random bit sequences (PRBS's) are known in such arts as radar and code-division multiple-access (CDMA) communication systems. An important characteristic of a PRBS is that it comprises a plurality of segments, each of which can be easily distinguished from the others. This characteristic allows demultiplexing by correlation. The characteristics of PRBS's have been explored in detail in Sarwate et al, Crosscorrelation Properties of Pseudorandom and Related Sequences,
Proceedings of the IEEE,
Vol. 68, No. 5, May, 1980, pp. 593-620.
FIG. 1
shows a schematic diagram of a known interferometric sensor array using code-division multiplexing. In sensor array
100
, laser
102
emits coherent light. Pseudo-random bit sequence (PRBS) generator
104
generates a pseudo-random bit sequence, which is input to modulator
106
. Modulator
106
modulates the coherent light from laser
102
to produce PRBS optical input
108
. PRBS optical input
108
is input to fiber
110
. Fiber
110
includes N sensors
112
-
1
,
112
-
2
,
112
-
3
, . . . ,
112
-N separated by lengths of fiber
114
-
1
,
114
-
2
, . . . ,
114
-(N−1).
Each sensor
112
-
1
,
112
-
2
,
112
-
3
, . . . , except last sensor
112
-N, includes a corresponding first coupler
116
-
1
,
116
-
2
, . . . ,
116
-(N−1), which splits off a portion of the light flux of PRBS optical input
108
in fiber
110
. In each detector
112
-n, n=1, 2, . . . , N, the split-off portion of the light enters second coupler
118
-n, which divides the flux between first fiber
120
-n and second fiber
122
-n, the first and second fibers having equal optical lengths. First fiber
120
undergoes a change in its refractive index when exposed to the condition to be sensed (e.g., such measurands as an acoustic wave, temperature change, distension because of stress or strain, etc.), while second fiber
122
undergoes no such change. The fluxes are recombined in third coupler
124
, where they interfere to produce PRBS output signal
130
-
1
, . . . ,
130
-N. Each PRBS output signal is time-delayed by the total length of fiber between laser
102
and the corresponding third coupler
124
-n. Fourth coupler
126
-n couples the PRBS output signal to return fiber
128
. Last sensor
112
-N has the same construction as the other sensors, except that first coupler
116
and last coupler
126
are unnecessary. PRBS output signals
130
-
1
, . . . ,
130
-N add in return fiber
128
to produce total output
132
. Total output
132
is detected by detector
134
.
Total output signal
132
must be demultiplexed to rederive each of the PRBS output signals. To effect this demultiplexing, time delay circuit
136
receives the PRBS from PRBS generator
104
and applies a time delay to the PRBS corresponding to the time delay of each PRBS output signal. The time-delayed PRBS is correlated with the output of detector
134
in correlation circuit
138
. The result of the correlation is applied through low-pass filter (LPF)
140
to reduce high frequency noise, and is output at sensor array
100
. Thus, each sensor is addressable.
However, sensor array
100
has the following drawbacks. First, because sensor array
100
requires four couplers for each sensor except the last and also requires return fiber
128
, sensor array
100
is complicated and expensive to build. Second, because of the length of the fibers required and imperfect transmission in any real-world optical fiber, sensor array
100
suffers from a significant loss of light flux. A particular disadvantage arising from such a loss is a limitation on the number of sensors.
SUMMARY OF THE INVENTION
An object of the invention is to reduce number of sensors necessary to do remote sensing, e.g. of the kind done by the apparatus of FIG.
1
.
Another object is to reduce amount of optical fiber necessary to do remote sensing such as is done by the apparatus of FIG.
1
.
Another object is to provide an optical fiber sensor array which has a simple design and is inexpensive to build.
To achieve these and other objects, the present invention concerns an optical system and method employing an optical fiber with a plurality of partially reflective elements, an optical source to launch an optical signal into the fiber, and a phase detector disposed effective to determine the phase between the optical signal and light reflected from at least one preselected element. By using reflected light, the invention requires less optical fiber for the same number of sensors because the invention need not employ an additional return line, such as line
128
of FIG.
1
. Moreover, because the invention uses reflected light, rather than plural sensor taps (e.g. sensors
120
-n in FIG.
1
), it can dispense with the numerous couplers needed in each of these taps, saving on hardware, and the inherent lossyness of such couplers. Consequently, the invention provides an improved optical budget for the user, permitting a larger number of sensors for the same optical power, and permits one to do so with a simpler apparatus using less hardware.
These and other objects are further understood from the following detailed description of particular embodiments of the invention. It is understood, however, that the invention is capable of extended application beyond the precise details of these embodiments. Changes and modifications can be made to the embodiments that do not affect the spirit of the invention, nor exceed its scope, as expressed in the appended claims. The embodiments are described with particular reference to the accompanying drawings, wherein:
REFERENCES:
patent: H1626 (1997-01-01), Kersey
patent: 3633183 (1972-01-01), Cobb
patent: 4775216 (1988-10-01), Layton
patent: 4778248 (1988-10-01), Arzur et al.
patent: 4889986 (1989-12-01), Kersey et al.
patent: 5144690 (1992-09-01), Domash
patent: 5208877 (1993-05-01), Murphy et al.
patent: 5323404 (1994-06-01), Grubb
patent: 5436988 (1995-07-01), Narendran
patent: 5973317 (1999-10-01), Hay
patent: 5987197 (1999-11-01), Kersey
patent: 2189880 (1987-11-01), None
p
Dandridge Anthony
Kersey Alan D.
Vohra Sandeep T.
Arroyo Teresa M.
Ferrett Sally A.
Inzirillo Gioacchio
Karasek John J.
The United States of America as represented by the Secretary of
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