Apparatus and method for multiplexing fiber optic sensors

The pulsed input light source of the fiber-optic sensor array is divided into a plurality of input light sources for the respective sensors of the array by a network of low-loss single mode fixed ratio fiber-optic couplers. The input light is applied to the input fiber of a first fixed ratio coupler, the output fibers thereof providing the input fibers of further fixed ratio couplers and so forth until the light is divided into the appropriate number of sources. The divided light sources are applied to the input fibers of the variable ratio fiber-optic coupler sensors of the array through differing lengths of optical fiber so that the input light pulses impinge upon the sensors of the array at different times. The output fibers of the sensors are coupled to multimode busses through low-loss taps. In an alternative embodiment for highly imbalanced ratio fiber-optic coupler sensors, the pulsed input light is applied to the input fiber of a first sensor. The high percentage output leg of the first sensor provides the input to a second sensor of the array, the high percentage output fiber thereof providing the input to the next array sensor. The low percentage legs of the sensors, which carry the information, are tapped into a multimode fiber-optic bus.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to arrays of fiber optic sensors, particularly with 
respect to multiplexing arrangements therefor. The invention is 
specifically applicable to multiplexing arrays of variable coupler fiber 
optic sensors. 
2. Description of the Prior Art 
The variable coupler fiber optic sensor is adaptable for a variety of 
parameters such as temperature, pressure, sound and the like. The sensor 
is described in U.S. Pat. No. 4,634,858, issued Jan. 6, 1987, entitled 
"Variable Coupler Fiber Optic Sensor" and assigned to the assignee of the 
present invention. Said U.S. Pat. No. 4,634,858 is incorporated herein by 
reference. Applications of the sensor are disclosed in U.S. patent 
application Ser. No. 376,342, filed July 6, 1989, entitled "Variable 
Coupler Fiber Optic Sensor Hydrophone", by David W. Gerdt; and Ser. No. 
444,920, filed Dec. 4, 1989, entitled "Method of Monitoring Cardiovascular 
Signals and Fiber Optic Coupler Phonocardio Sensor Therefor", by David W. 
Gerdt. Said Ser. Nos. 376,342 and 444,920 are assigned to the assignee of 
the present invention and are incorporated herein by reference. 
Briefly, as described in said U.S. Pat. No. 4,634,858, the sensor comprises 
a plurality of input optical fibers, each having a core, the cores of the 
optical fibers being merged and fused in a waist region to form a common 
optical core wherefrom a plurality of output optical fibers emerge. Light 
energy from, for example, a laser or light emitting diode incident to one 
of the input fibers is distributed to the plurality of output fibers. The 
waist region is encapsulated in material having a refractive index 
variable with stress applied thereto and the applied stress varies the 
distribution of output light energy. A differential detector such as a 
plurality of photodiodes is coupled to the output fibers for providing 
signals representative of the optical energy distribution in the output 
fibers. The sensor may be constructed in accordance with teachings in U.S. 
patent application Ser. No. 240,986, filed Sept. 6, 1988, entitled "Fiber 
Optic Fabrication Furnace", by David W. Gerdt. Said U.S. patent 
application Ser. No. 240,986 is assigned to the assignee of the present 
invention and is incorporated herein by reference. 
Thus, a sensor includes at least one input fiber requiring a light source 
and generally two output fibers coupled to a detector for providing a 
signal related to the physical parameter being sensed. 
It is often desirable to configure such sensors into arrays such as linear 
arrays or nets of plural sensors, each requiring a light source and output 
detectors. For example, an array of hyrophone sensors may be mounted on 
the outer hull of a marine vessel such as a submarine or an array of 
pressure sensors may be utilized to detect levels of liquids such as in 
oil tankers. In such arrays, it is desirable to minimize the number of 
input and output leads so as to minimize hull penetration fittings. 
Independent sensors for constructing such arrays, each sensor requiring a 
separate light source and detector, results in excessive complexity and 
cost. 
It is a desideratum to reduce the large number of input and output fibers 
from individual sensors utilized in such arrays. A variety of techniques 
are known in the art for multiplexing a plurality of fiber optic sensors 
of types other than the variable coupler fiber optic sensor, such as the 
interferometric type. The prior art does not provide a useful multiplexing 
configuration for the variable coupler fiber optic sensor. It is believed 
that the prior art does not provide a useful multiplexing configuration 
for fiber optic sensors of the intensity type such as the micro-bending 
sensor. 
Configurations for multiplexing fiber optic sensors of the interferometric 
type are discussed in "Fiber-Optic Multisensor Networks", SPIE 
Proceedings, Volume 985, (1988) by Kersey and Dandridge. Such multiplexing 
configurations are undesirably complex since interferometric sensors 
require preservation of precise phase information. A reference leg and a 
sensor leg are always required for Michelson and Mach-Zehnder sensors. 
Additionally, interferometric sensors are difficult to deploy and are 
extremely expensive. 
SUMMARY OF THE INVENTION 
A single light source provides the input to each sensor of an array of 
fiber optic sensors. Light from the source is launched into a tree of 
fixed ratio fiber optic couplers that divides the light into plural light 
sources for the sensors of the array. The source light is launched into 
one input fiber of a first fixed ratio fiber optic coupler which divides 
the light energy between the output fibers thereof. Further, fixed ratio 
fiber optic couplers receiving input light from the outputs of the first 
coupler further subdivide the light from the first coupler until the 
desired number of sensor input sources are generated. The sensors of the 
array receive the input light with relative delays therebetween. The 
output fibers of the array sensors are tapped into multimode busses to 
maintain time separation between the sensor outputs. The outputs of the 
multimode busses are coupled to detectors for providing the array outputs. 
Preferably, the array sensors comprise variable ratio fiber optic coupler 
sensors. 
An alternative embodiment of the invention utilizes a linear array of 
highly imbalanced variable ratio fiber optic coupler sensors where the 
high light output fiber of each sensor is coupled to the input fiber of 
the next sensor in the array. The low light output fiber of each sensor 
provides the sensed data and is tapped into a multimode bus. Light is 
launched into the input fiber of the first sensor in the array.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a multiplexing configuration for an array 10 of 
variable ratio fiber optic coupler sensors 11-14 is illustrated. Each of 
the sensors 11-14 is of the type described in the above-referenced U.S. 
patent and U.S. patent applications and forms an extended array of coupler 
sensors. The input optical energy for each of the sensors 11-14 of the 
array 10 is provided by an optical source 15. The optical source 15, for 
example, comprises a pulsed light emitting diode or pulsed laser diode 
appropriately pigtailed to input fiber 16 of a low-loss, fixed ratio, 
environmentally insensitive fiber optic coupler 17. The coupler 17 
functions as a stable fiber optic beamsplitter dividing the pulsed light 
that is coupled or launched into the input leg 16 into light pulses on 
output fibers 18 and 19. The coupler 17 preferably provides a division 
ratio into the output fibers 18 and 19 of approximately 50:50 and is 
generally of the type described in said U.S. Pat. No. 4,634,858, except 
that the division ratio into the output fibers is fixed. A stable fixed 
ratio coupler may be implemented if the sensor of said U.S. Pat. No. 
4,634,858 is not encapsulated in stress sensitive encapsulant. Preferably, 
an ultra-low loss etched coupler may also be utilized. The output fibers 
18 and 19 of the coupler 17 form input fibers to further fixed ratio 
couplers 20, the output fibers of which form the input fibers of still 
further fixed ratio couplers 21. Output fibers 22 of the couplers 21 form 
the input fibers of the sensors of the array 10. Each of the couplers 20 
and 21 is identical to the coupler 17 and comprises a single mode fiber 
optic coupler. 
Thus, the couplers 17, 20 and 21 form a beamsplitter network or tree 23 
functioning to divide the pulse from the light source 15 into as many 
light sources on the fibers 22 as required for input into the array 10. 
The light pulses propagating through the couplers 21 simultaneously exit 
the network 23 on the fibers 22. Each of the fibers 22 forms the input 
fiber to one of the variable ratio coupler sensors 11-14. 
Sensors of the array 10 are spaced at varying distances from the network 23 
so that light pulses on the fibers 22 arrive at respective sensors of the 
array 10 at different times. The relative delays of the input pulses to 
the sensors 11-14 are controllable by varying the relative distances of 
the sensors 11-14 from the network 23 or by inserting conventional fiber 
optic delay lines between the sensors 11-14 and the network 23. A fiber 
optic delay line may be implemented by a length or coil of fiber which 
increases the propagation time of a light pulse to the input of a sensor. 
For example, the relative delay between the sensors 11 and 12 results from 
a fiber length 24. 
As described in the previously referenced U.S. patent and patent 
applications, a ratio change of the light output occurs at each of the 
sensors of the array 10 which change is related to the physical parameter 
being sensed, for example, acoustic energy, temperature, pressure and the 
like. At each of the sensors 11-14, a single pulse enters the sensor and 
two pulses, split by the coupler sensor exits on output fibers 25 
therefrom. The ratio of the exit pulses from a sensor contains the 
information with respect to the physical parameter being measured. The 
sensor exit pulses travelling along the fibers 25 are conveyed to 
multimode bus or trunk fibers 26 and 27. The sensor exit pulses enter the 
trunks 26 and 27 at low-loss taps. Thus, each of the taps launching exit 
pulses into the trunks 26 and 27 couples the single mode output fibers 
from the sensors 11-14 into the multimode trunks 26 and 27. For example, 
an output fiber 28 of the sensor 13 launches the exit pulses thereon into 
the trunk 26 at a tap 29 and an output fiber 30 from the sensor 13 
launches the pulses travelling thereon into the trunk 27 at a tap 31. In a 
similar manner, output fibers 32 and 34 of the sensor 14 launch the exit 
pulses thereof into the trunks 26 and 27 at taps 33 and 35, respectively. 
Because time delays have been introduced between the exit pulses from the 
respective sensors 11-14, no mixing of the pulses on the trunks 26 and 27 
occurs. The trunk lines 26 and 27 convey the temporally ordered pulses to 
detectors 36 which preferably comprise a plurality of photodiodes 37 and 
38. Thus, the multimode trunks 26 and 27 convey the information sensed by 
the coupler sensors 11-14 to the detectors 36. The receiver photodiodes 37 
and 38 convert the optical energy of the pulses to electrical energy to 
provide an electrical signal representative of the physical parameter 
sensed by the sensors of the array 10. Circuitry not shown performs 
electrical amplification and information processing as required. 
The temporal spacing between the pulses on the trunks 26 and 27 can be 
adjusted by utilizing delay lines. Additionally, the pulse repetition 
frequency of the sensor pulses into the detection circuitry 36 can be 
varied so as to match the data processing capabilities of the detection 
circuitry as a function of the distance between sensors. 
In an alternative arrangement, one of the multimode trunks 26 and 27 can be 
tapped into the other trunk thus eliminating one output lead with a 
concomitant elimination of detection circuitry. With this arrangement, 
storage is required within the detection circuitry 36 to store the first 
to exit pulse of a pulse pair from a sensor so that ratio processing can 
be performed. With this arrangement, the multiplexed array has only one 
input fiber and one output fiber. 
When the arrangement of FIG. 1 is utilized to implement an array for use 
with marine vessels, only two or three hull penetration fittings depicted 
at 39 are required. The above-described process is a form of time division 
multiplexing. 
Referring to FIG. 2, a multiplexing configuration for use with variable 
ratio fiber optic coupler sensors having highly unbalanced output ratios 
is illustrated. As generally described in the above-referenced U.S. patent 
and patent applications, the sensor output ratio for a null stress field 
can be set during the sensor fabrication process to a desired ratio. A 
highly unbalanced ratio such as 95:5 or 97:3 may be obtained in order to 
provide a simplified multiplexing arrangement. Generally for such 
unbalanced coupler sensors, it is still possible to obtain the necessary 
information signal which is adequately conveyed by the output fiber with 
the low ratio factor. The output fiber with the large ratio factor 
exhibits negligible relative variation in the presence of a varying stress 
field and is therefore relatively insensitive. Thus, the output fiber with 
the large ratio factor provides light output power that is relatively 
constant and clean and can be provided as a light source into another 
coupler sensor. The information pulses on the output fibers having the low 
ratio factor can be collected onto a multimode bus. 
FIG. 2 illustrates a linear array 50 of highly unbalanced variable coupler 
fiber optic sensors 51-53. Light pulses from a suitable light source 54, 
such as a light emitting diode or laser, is launched into an input fiber 
55 of the sensor 51. The sensor 51 may be unbalanced with a ratio of 98:2. 
Thus, the sensor 51 divides the light energy on the input fiber 55 into 
approximately 98% on an output fiber 56 and approximately 2% on an output 
fiber 57. Disregarding unrecoverable excess loss, the sensor 51 transduces 
variations in the parameter measured into optical variations. The optical 
variations occur as relatively large amplitude variations on the 2% leg 57 
compared to the total amount of light on the leg 57. Variations also occur 
on the 98% leg 56 but these variations are small compared to the total 
amount of light present on the leg 56. Thus, the light from the leg 56 is 
relatively constant and clean and is therefore suitable as the light 
source for the input fiber of the following sensor 52. The output fiber 57 
is coupled to a multimode bus 58 at a low loss tap 59. 
Thus, the output fiber 56 of the sensor 51 is applied as the input fiber to 
the sensor 52 which, for example, may be configured to have a ratio of 
96:4. Thus, an output fiber 60 from the sensor 52 conveys approximately 
96% of the light energy therefrom and an output fiber 61 conveys 
approximately 4% of the light energy therefrom. The output fiber 61 is 
coupled to the multimode bus 58 at a low loss tap 62. Since the sensor 51 
receives more input light than the sensor 52, but the information leg 61 
of the sensor 52 has a larger ratio factor than the information leg 57 of 
the sensor 51, approximately the same amount of optical energy is provided 
by the sensors 51 and 52 into the trunk 58 at the taps 59 and 62. 
In a manner similar to that described, the output fiber 60 from the sensor 
52 forms the input fiber to the sensor 53 which may be constructed to 
provide a ratio of 93:7. The sensor following the sensor 53 may be 
constructed with a ratio of 89:11 and so on until the large ratio factor 
leg of a sensor does not provide a sufficiently large, unvarying and clean 
signal to provide to the next coupler sensor. It is appreciated that in 
the example given, the percentage differences between successive sensors 
is 2, 3, 4, . . . until the light output from the large ratio factor leg 
deteriorates so as not to be useable as the input source to a further 
sensor. 
Thus, each of the coupler sensors 51-53 of the array 50 is powered by 
optical energy received by a previous sensor or by the input source 54. 
The low ratio factor output fibers from the sensors 51-53 comprise single 
channels from the respective sensors conveying information from the 
sensors to the multimode trunk 58 at the tap points therein. Thus, each of 
the taps 59, 62, . . . comprises a tap from a single mode sensor output 
fiber into the multimode trunk 58. 
The pulsed optical source 54 is utilized in a time division multiplexed 
mode since the coupler sensors 51-53 are spaced apart in distance and 
therefore in time. A detector 63, such as a photodiode detector, receives 
all of the time separated pulses from the sensors of the array 50 
travelling along the bus 58 and converts the optical energy thereof to 
electrical energy which provides the output signals of the array 50. 
If the array 50 is utilized external to the hull of a marine vessel, the 
electrical energy provided to the array 50 at the source 54 and received 
from the array 50 at the detector 63 may originate and be received at an 
internal ship location remote from the location of the array. The input 
fiber 55 and the multimode output trunk 58 enter the interior of the ship 
at hull penetration fittings 64 and 65. The electrical signal from the 
detector 63 may be amplified and processed as necessary at the remote 
location. 
In a comparison of the configuration of FIG. 2 with that of FIG. 1, it is 
appreciated that each of the sensors of FIG. 2 provides a single fiber 
output whereas the sensors of FIG. 1 provide dual outputs. Although in 
FIG. 2, the benefits of a differential dual output are lost, a simplicity 
results by eliminating the single mode, low-loss coupler beamsplitting 
network 23, eliminating one of the photodiode receivers, eliminating many 
connections and eliminating a significant amount of optical fiber. It is 
appreciated that in the above-described configurations, single mode 
coupler sensors tapping into multimode trunks result in a total loss of 
coupler sensor signal and throughput trunk signal of less than 0.1%. All 
phase information is, however, lost on the multimode return trunk. 
Consequently, the described multiplexing configurations cannot be used 
with interferometric sensors. Interferometric sensors, on the other hand, 
must utilize single mode couplers to multiplex onto a single mode trunk in 
order to preserve phase information. Sensor signal and trunk throughput 
losses resulting from optical coupler division ratios severely limit the 
number of interferometric sensors that can be multiplexed utilizing known 
multiplexing configurations for such sensors. 
The above-described invention applies to all-optical, fiber-optic sensors 
that are optically coupled together via fiber-optic light-guides into an 
array. The present invention permits a plurality of sensors to be 
multiplexed into a multisensor array where the array has a minimum of 
input and output fibers. The couplers of the network 23 of FIG. 1 
introduce negligible losses and partition the pulsed light from a single 
source to provide light to plural sensors. Fiber optic delay lines provide 
temporal separation of the pulses to any desired extent which decreases 
the capabilities required of the electronic signal processing circuitry. 
Since the output pulses from the sensors are not phase sensitive, 
multimode fiber trunks with low-loss taps can be utilized. 
The low loss taps of the sensor output fibers into the multimode busses may 
be of any suitable known design. Such suitable taps are described in 
documentation available from the Ensign-Bickford Optics Company, 16-18 
Ensign Drive, P.O. Box 1260, Avon, Conn., with respect to the EBOC Linear 
Tap System. 
It is appreciated that although the invention is described in terms of 
variable coupler fiber-optic sensors, the invention is also applicable to 
single sourced sensors such as the microbending sensor. The invention 
provides simple and inexpensive configurations to multiplex intensity-type 
fiber-optic sensors into arrays. In addition to the uses described above, 
such arrays may also be utilized in oil exploration, intrusion detection, 
and other appropriate applications. 
While the invention has been described in its preferred embodiment, it is 
to be understood that the words which have been used are words of 
description rather than limitation and that changes may be made within the 
purview of the appended claims without departing from the true scope and 
spirit of the invention in its broader aspects.