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Description  |
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FIELD OF THE INVENTION
The present invention relates generally to fiber optic sensors. More
specifically, the present invention relates to inline time division
multiplexed fiber optic interferometric sensor arrays having reduced
signal content due to environmental stresses.
BACKGROUND OF THE INVENTION
FIG. 1A shows a typical fiber optic sensor 1 for use in an inline time
division multiplexed (pulsed) array. Sensor comprises a fiber optic sensor
coil 10 and a reference line 12, each formed from an optical fiber cable
generally denoted 22, disposed between a pair of couplers generally
denoted 14. Typically the reference line 12 is physically isolated from
perturbation by external factors. Each coupler 14 typically is a fiber
optic coupler and has two input ports and two output ports. A pulsed
coherent light source 18 supplies a series of light pulses to sensor 1. To
interrogate sensor 1, a compensating interferometer or compensating sensor
26 is required between the output of sensor 1 and a photodetector 20.
Alternatively, as shown in FIG. 1B, a sensor 1' comprises a coil 10 coupled
between a first output port of a coupler 14 and a discrete reflector or
mirror 24a. A second reflector or mirror 24b is coupled to the second
output port of coupler 14. Reflectors 24a and 24b are full reflectors
reflecting all of the impinging coherent light. The configuration of FIG.
1B is functionally identical to that of FIG. 1A.
An illustrative example of sensor 1 operation will now be discussed with
reference to FIG. 1A. During operation, coherent light source 18 is
coupled to both coil 10 and reference line 12 via coupler 14a and a fiber
cable 22a. Sensor 1 responds to a measurand, such as acoustic waves
impinging on sensor coil 10, by changing the length of the coil 10 as a
function of the magnitude of such measurand. The compensating
interferometer 26 is responsive to the optical output of coherent light
produced by coil 10 for developing an optical interference pattern, which
is detected by a photodetector 20. Photodetector 20 generates an
electrical signal in response to the optical interference pattern, thereby
providing an electrical signal representative of the impinging acoustic
wave.
The use of a plurality of fiber optic acoustic sensors as described
hereinabove in an inline array of such sensors is also known. As
illustrated in FIGS. 2A-2D, a plurality of sensors 1 and 1' (FIGS. 1A and
1B) are combinable in various series arrangements to form inline fiber
optic acoustic sensor arrays 2 through 5, respectively. FIG. 2A shows a
Fabry-Perot array 2 formed from a plurality of sensors 1'. Coupler 14 and
reflector 24b of sensor 1' (FIG. 1B) are replaced by a partial reflector
24', as shown, thereby reducing the number of elements needed to form an
inline array of sensors 1'. FIG. 2B shows a tapped serial array 3 formed
by a plurality of sensors 1 as disclosed in U.S. Pat. No. 4,889,986. FIG.
2C shows a Stanford ladder array 4 produced by an alternative
configuration of sensor 1, while FIG. 2D shows an inline Michelson array 5
produced by an alternative configuration of sensor 1'. It will be
appreciated that a complete sensor is defined between each adjacent pair
(set in the case of a Stanford ladder array) of couplers or reflectors.
Fiber cables 22 employed in forming sensing coils 10 are sensitive to a
large number of environmental effects, such as temperature fluctuations
and pressure variations. It is known to provide a compensating
interferometer 26, having the configuration of sensor 1 in FIG. 1A, in
each of the arrays 2 through 5 to compensate for the path differences of
individual sensors 1 and 1' in the arrays 2 through 5.
Referring to FIGS. 3A-3D, in accordance with conventional techniques for
packaging arrays 2 through 5 into deployable assemblies, repeating
segments of each array are packaged as identical sensor units 30 located
along each array 2 through 5, respectively. For example, as shown in FIG.
4 with respect to array 3 of FIG. 3B, each sensor unit 30 in an inline
array is conventionally formed by mounting together a sensing coil 10
wrapped on or embedded in a compliant medium 11, a reference line 12 and
one of the couplers 14 (reflectors 24' in the case of the FIG. 3A array)
associated with a given sensor 30. A housing 40 can be provided which
encloses the sensing coil 10, reference line 12 and the associated coupler
14. A perforated aluminum tube from 5-10 inches long and approximately
11/2 inches in diameter is commonly used as housing 40. A potting medium
(not shown) is used to secure reference line 12 and the coupler 14 within
housing 40. Alternatively, instead of providing a separate housing for
each sensing unit, the entire array is typically disposed within a
protective hose or other tubular member 23 for deployment. Thus, each
sensor unit 30 so packaged contains only a portion of a complete
functional sensor (sensor 1 shown in FIG. 1A for the FIG. 3B array, and
sensor 1' shown in FIG. 1B for the FIG. 3A array).
The arrays 2-5 shown in FIGS. 3A-3D, respectively, are all used with
conventional signal processing circuitry such as that disclosed in U.S.
Pat. No. 4,889,986, wherein sensor responses are determined based on the
travel time of coherent light pulses through each sensor of a sensor
array. However, as noted above, inline sensor arrays 2-5 with sensor unit
packaging as shown in FIGS. 3A-3D do not include clearly defined sensors.
In addition, the required spacing between sensor units 30 in the arrays 2-5
necessitates long leads on coils 10 and reference lines 12. This further
complicates measurement of the signals of interest from sensors 1, 1',
because environmental stresses produced in the portions of fiber cables 22
connecting the sensor units to each other are indistinguishable from the
stresses produced within the sensor units in response to the acoustic
pressure waves of interest.
Heretofore, an improved inline sensor array which decouples environmental
stress in the portions of fiber cables 22 connecting the sensor units to
each other in the array has not been achieved.
SUMMARY OF THE INVENTION
An object of the invention is to provide an improved inline fiber optic
sensor array which decouples environmental stress and thereby provides
improved sensor signal-to-noise ratio.
These and other objects and advantages are achieved in accordance with the
present invention by packaging an inline fiber optic acoustic sensor
array, the array comprising first and second fiber optic sensor units for
sensing a desired measurand connected in a linear array, such that each
sensor unit incorporates a complete functional sensor, and the two sensor
units are separated from each other by an intermediate delay element
responsive both to the desired measurand and to environmental stress
connected between the first and second sensors for providing time
separation between desired measurand signals produced by the first and
second sensor units and signals produced by the array in response to
environmental stress impinging on the delay element.
According to one aspect of the present invention, each fiber optic sensor
produces a modulated coherent light beam in response to an impinging
desired measurand. The delay element produces a modulated coherent light
beam in response to both an impinging desired measurand and environmental
stresses. Modulated coherent light beams produced by each fiber optic
sensor are time separated from modulated coherent light beams produced by
the adjacent delay element. Conventional time discriminating signal
processing techniques are used to interrogate only the sensor units, or to
otherwise eliminate electrical signals corresponding to the modulated
coherent light beam produced by the delay element, thus decoupling
environmental stresses from the electrical signals being processed.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects, features and advantages of the invention are
disclosed in or apparent from the following detailed description of
preferred embodiments. The preferred embodiments are described with
reference to the drawing, in which like elements are denoted by like
reference numbers, and in which:
FIGS. 1A and 1B diagrammatically show illustrative examples of alternative
conventional arrangements of fiber optic acoustic sensors;
FIGS. 2A-2D diagrammatically show illustrative examples of alternative
arrangements of conventional inline sensor arrays formed with the sensors
shown in FIGS. 1A and 1B;
FIGS. 3A-3D diagrammatically show conventional packaging of the inline
acoustic sensor arrays shown in FIGS. 2A-2D;
FIG. 4 is a partly diagrammatic more detailed sectional side view of a
conventional packaging arrangement for a sensing unit in the inline
acoustic sensor array shown in FIG. 3B;
FIG. 5 is a partly diagrammatic sectional side view of a fiber optic sensor
packaged according to the present invention; and
FIGS. 6A-6D are illustrative examples of preferred embodiments of inline
sensor arrays packaged in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Although the invention will be described in terms of a fiber optic
hydrophone application, any inline, time division multiplexed fiber optic
array sensing a measurand, such as a magnetic field, temperature, pressure
or an electric field, is also within the purview of the invention.
Referring to FIG. 5, a fiber optic sensor 1 packaged as a sensor unit 30'
in accordance with a preferred embodiment of the present invention
comprises a coil 10 wrapped on or embedded in a compliant medium 11,
reference line 12, and a pair of couplers 14a and 14b all mounted
together. Each sensor unit 30' is thus a complete functional sensor 1.
Sensor units 30' are connected in an inline array with an intermediate
delay element 10a coupled between adjacent sensor units 30' by optical
fiber cables 22. A fiber optic coil advantageously comprises delay element
10a, as shown. It will be appreciated that coherent light travelling from
a first sensor unit 30' to a second sensor unit 30' through coil 10a is
separated in time by the time required for a coherent light beam to travel
through coil 10a. For protection, the entire array, including delay
elements 10a, is enclosed within, for example, a hose 23.
Referring to FIGS. 6A-6D, inline arrays 2'-5' corresponding to the arrays
2-5 shown in FIG. 3A-3D, but packaged according to the present invention,
will now be described. Referring specifically to FIG. 6A, sensor array 2'
comprises a series of sensor units 30' incorporating complete sensors 1'
each separated by a coil delay element 10a. Thus, array 2' is formed from
a plurality of fiber optic sensors 1' having adjacent pairs of sensors 1'
serially coupled by a delay element 10a.
Referring to FIGS. 6B and 6C, sensor arrays 3' and 4', respectively,
comprise a plurality of linearly connected sensor units 30' incorporating
complete sensors 1 each separated by a delay element 10a, and connected in
parallel with a fiber cable 22 terminating in compensating interferometer
26, as shown. Thus, arrays 3' and 4' are both formed from a plurality of
fiber optic sensors 1 with each adjacent pair of sensors 1 separated by
one delay element 10a.
Referring to FIG. 6D, sensor array 5' similarly comprises a series of
sensor units 30' incorporating complete sensors 1' serially connected by
intermediate delay elements 10a.
The arrangement of complete sensors 1 or 1' separated by delay elements 10a
as shown in FIGS. 6A-6D increases the signal-to-noise ratio of the array
3' by decoupling each sensor 1, 1' from induced environmental stresses.
Sensors 1, 1' in each array produce modulated coherent light beams in
response to impinging acoustic pressure waves. Delay element 10a produces
modulated coherent light beams in response to impinging acoustic pressure
waves and environmental stress on the array. The modulated coherent light
beams produced by sensors 1, 1' are time separated from modulated coherent
light beams produced by delay element 10a. Conventional time
discriminating signal processing techniques can thus be readily used to
separate signals corresponding to modulated coherent light beams produced
in sensors 1, 1' of arrays 2'-5' in response to impinging acoustic
pressure waves from signals corresponding to acoustic pressure waves and
environmental stress produced in other parts of the arrays. Thus the
output of an array 3' in accordance with the present invention can be
processed so as to effectively only interrogate the sensor portions of the
array.
Referring to array 3' shown in FIG. 6B, an illustrative example will now be
described wherein coil 10 in each complete sensor 1 and delay element 10a
are each formed from a fiber cable 22 one hundred meters long. In the
illustrative case, it is assumed that a coherent light beam has a velocity
of 1 kilometer per 5 microseconds (.mu.sec) in fiber cable 22 and, thus,
the coherent light beam travels though each coil 10, and delay element
10a, in 0.5 .mu.sec. Thus, a coherent light beam traveling from a first
sensor 1 to a second sensor 1 via delay element 60 is time separated by
0.5 .mu.sec. Conventional signal processing, based on the known time
separation between sensors 1, processes signals corresponding to modulated
coherent light beams produced by sensors 1 but does not process signals
corresponding to modulated coherent light beams produced by delay element
10a. Thus, environmental stresses which are coupled to array 3' in delay
element 10a are decoupled from sensors 1.
The array gain (AG) for a generalized array is given as AG=10.times.log
N.times.SNR, where N is the number of sensors employed in the array and
SNR is the conventional signal-to-noise ratio for the best sensor in the
array. For sensors 1 and 1' packaged according to the present invention,
decoupling environmental stresses provides an overall improvement in SNR
for each sensor 1, 1' and, thus, provides an overall increase in AG for
arrays 2'-5' over inline sensor array 2-5 having equivalent numbers of
sensors 1, 1'.
Other modifications and variations to the invention will be apparent to
those skilled in the art from the foregoing disclosure and teachings.
Thus, while only certain embodiments of the invention have been
specifically described herein, is will be apparent that numerous
modifications may be made thereto without departing from the spirit and
scope of the invention.
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