Abstract:
A multiplexed fiber optic sensor system. The system has an array of sensor elements, each responsive to a respective measurand, a light source, a fiber optic waveguide for directing light from the source to the array, a scanner for providing relative motion between the array and the light, a beamsplitter for receiving return light from the array so that the return light can be detected for analysis, and a photodetector for receiving the return light and providing an output signal in response thereto. The scanner is operable to scan the light over the sensor elements so that return light be collected from each respective element, whereby data can be determined concerning each respective measurand.

Description:
RELATED APPLICATION  
       [0001]     This application is based on and claims the benefit of the filing date of Australian patent application no. 2003904412 filed 15 Aug. 2003, the contents of which are incorporated herein by reference in its entirety.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to a multiplexed fiber optic sensor system of particular but by no means exclusive application in monitoring gases, micro-organisms and other substances for concentration, temperature and humidity.  
       BACKGROUND OF THE INVENTION  
       [0003]     Fiber optic sensor systems and optrodes have been developed intensively for over a quarter of a century. By the mid 1980s, the designs of most of the existing fiber optic sensors had been proposed and tested (see, for example, Dakin and Culshaw,  Optic Fibre Sensors,  IV: Analysis and Future Trends).  
         [0004]     In the following description, reference is made to “measurands” and to “optrodes”. A measurand is a parameter that is desired to be measured, such as temperature, humidity, oxygen, partial pressure or carbon monoxide concentration. An optrode is an optical arrangement connected to the tip of an optic fiber to allow optical determination of a measurand (cf. electrode).  
         [0005]     Although it is possible to devise an optic fiber/optrode system for any single desired measurand, the cost is generally much greater than for existing non-fiber based systems. Indeed, the cost may be 10 to 1,000 times greater owing to: 
        1. Development cost amortization;     2. Cost of fabrication of fiber link, connectorisation, etc;     3. Cost of light source, detector, optics and reference electronics; and     4. The complexity of providing an optical reference.        
 
         [0010]     This price disadvantage has inhibited the wide use of fiber optic sensor systems. However, fiber optic measurement systems have been successfully commercialized where: 
        1. The unique advantages (e.g. electrical isolation) justify the expense of a single point optical detector, such as in the form of a temperature sensor based on fluorescence delay time (used in industrial microwave ovens);     2. Where one set of optics and electronics is able to interrogate and quantify the measurand at a very large number of points along a fiber, by means of optical time domain reflectometry (for example in a distributed temperature sensor in which the fiber temperature can be measured at intervals of, say, 50 cm over a length of several hundred metres); and     3. Systems in which the whole length of the fiber acts as the sensor, such as for embedded strain measurement in concrete, for perimeter vibration detectors and for liquid petroleum gas spill alarm systems.        
 
       SUMMARY OF THE INVENTION  
       [0014]     The present invention provides, therefore, a multiplexed fiber optic sensor system, comprising: 
        an array of sensor elements, each responsive to a respective measurand;     a light source;     a fiber optic waveguide for directing light from sail source to said array:     a scanner for providing relative motion between said array and said light;     a beamsplitter for receiving return light from said array so that said return light can be detected for analysis; and     a photodetector for receiving said return light and providing an output signal in response thereto;     wherein said scanner is operable to scan said light over said sensor elements so that return light be collected from each respective element, whereby data can be determined concerning each respective measurand.        
 
         [0022]     The waveguide preferably comprises a single fiber or a fiber bundle.  
         [0023]     The return light is preferably either reflected light, fluorescent light or both reflected and fluorescent light.  
         [0024]     Thus, the invention provides a system in which, for example, a fiber or a bundle of fibers can be used to measure the signal from a number of closely spaced measuring points. The scanner can comprise, for example, any of the miniaturized scanning devices developed by Optiscan Pty Ltd, can be installed remotely from the photodetector and can readily be adapted to perform the scan over an array of elements at closely spaced measuring points.  
         [0025]     One application of such a system is as an optic time domain reflectometer system used to monitor the temperature of a multiplicity of points in a single fiber, woven between bricks in the outer layer of large furnaces.  
         [0026]     The system would have some important advantages, for example by providing an auto-referencing signal with compensation for factors such as photo bleaching or environmental degradation of the optical material. It would permits novel areas of fiber optic sensing in which small changes in the position of objects in the sensor visual field are used to give information on the measurand.  
         [0027]     The invention could operate with multimode fibers or single mode fibers, and can could use blue LEDs and potentially any scan mechanism including tuning fork or mirror scanning mechanisms. It could also be configured to scan without a lens set separate from the optical fiber depending on the actual dimensions of the sensor array. In other words, a single tightly focused diffraction limited spot scan may not be required.  
         [0028]     It is envisaged that in some embodiments, it would be possible for one fiber to interrogate and give numerical readouts on the fluorescence or reflection of up to a million separate sensor points in just over one second.  
         [0029]     Much of the work on the detection of chemicals using optic fiber sensors has been directed to the development and testing of optrode materials for military objectives (e.g. at the US Naval Research Laboratories). The detection of airborne and waterborne toxic agents and bacteria are areas in which it is anticipated that this claimed invention could be applied. These are of particular relevance at the present time and as potential applications of the present invention.  
         [0030]     The light source is preferably a laser source or an LED, and may be monochromatic, have a spectrum comprising two or more wavelengths, or comprise a broad band spectrum.  
         [0031]     In one embodiment, said light source comprises a plurality of separate light sources, and the system includes means for combining light from each of said sources into a single beam of light. Preferably each of said plurality of light sources has an output of different wavelength (or different color), and more preferably each comprises an LED.  
         [0032]     In one embodiment, the light comprises at least two wavelengths so that data can be calibrated on the basis of a comparison of the response of each of said sensor elements to the respective wavelength components of said light.  
         [0033]     The system may include a fiber optic waveguide for collecting return light from said array. This may be a further waveguide, or a single fiber optic waveguide can be used to transmit both the incident light and the return light. In either case, the fiber optic waveguide or waveguides may each comprise a single fiber or a fiber bundle.  
         [0034]     The array may additionally include at least one reference sensor element of known characteristics so that data from at least one other of said sensor elements can be normalized or calibrated against said reference sensor element.  
         [0035]     Thus, compensation can be applied to measurements for the undesired effects of, for example, photo bleaching of optrode material, spontaneous degradation, and optical losses in the system (including due to bending of fibers, etc). The reference element is preferably insensitive to changes in the measurand of the sensor element for which it acts as a reference.  
         [0036]     In one embodiment, the array additionally includes a plurality of reference sensor elements, one for each respective sensor element sensitive to a respective measurand.  
         [0037]     Thus, for each measurand there would be, in this embodiment, a corresponding reference sensor element.  
         [0038]     The present invention also provides a method of performing multiplexed fiber optic sensing, comprising:  
         [0039]     transmitting light by means of a fiber optic waveguide to an array of sensor elements, each responsive to a respective measurand;  
         [0040]     scanning said light over said array;  
         [0041]     collecting and detecting return light from said array and generating a signal indicative thereof;  
         [0042]     whereby data can be determined concerning each respective measurand.  
         [0043]     It will be appreciated that the method may include steps corresponding to the various functions provided by the optional features of the above-described system. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0044]     In order that the invention may be more clearly ascertained, preferred embodiments will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0045]      FIG. 1  is a schematic view of a multiplexed fiber optic sensor system according to one embodiment of the present invention;  
         [0046]      FIG. 2  is a schematic view of a multiplexed fiber optic sensor system according to another embodiment of the present invention;  
         [0047]      FIG. 3A  is a detail of the systems shown in  FIGS. 1 and 2 ;  
         [0048]      FIG. 3B  is a detail of a variation of the systems shown in  FIGS. 1 and 2  in which scanning is effected by means of a movable lens;  
         [0049]      FIG. 4  is a schematic view of a multiplexed fiber optic sensor system according to further embodiment of the present invention;  
         [0050]      FIG. 5  is a schematic view of a multiplexed fiber optic sensor system according to a still further embodiment of the present invention; and  
         [0051]      FIG. 6  is an image obtained by means of the system of  FIG. 5 . 
     
    
     DETAILED DESCRIPTION  
       [0052]     A multiplexed fiber optic sensor system according to one embodiment of the present invention is shown schematically in  FIG. 1 . A light beam  10  is emitted from a light source  12 , passes through a beamsplitter  13  and is then focussed by a lens into the core at the proximal or entry tip  14  of an optic fiber  15 . The light energy passes along the fiber and emerges from the distal or exit tip  16  of the fiber  15  where it is directed to impinge upon a single sensor element  17 , which is in close proximity to the fiber exit tip  16 .  
         [0053]     Sensor element  17  is one of a plurality of sensor elements constituting a sensor array  18 . Notably, no objective is employed: the fiber tip  16  is simply located sufficiently close to array  18 . It is envisaged that the separation would be from a few microns to 10 or 20 microns, but this should be adjusted—as will be appreciated by those skilled in the art—according to light source, etc. The array  18  additionally includes reference elements, one for sensor element and in each located adjacent to its corresponding sensor element. These reference elements are selected to be insensitive to the measurand of the corresponding sensor element, so that the data from the sensor element can be corrected or normalized for the undesired effects of, for example, photo bleaching of optrode material, spontaneous degradation, and optical losses in the system (including due to bending of fibers, etc). The system can therefore be described as “self-referencing” or “self-normalizing”.  
         [0054]     The array  18  can be any desired shape, and this choice will generally depend on the scanning technique employed. If scanning is effected by means of a tuning fork in one (fast) direction and a slower scan in the other direction, a rectangular array  18  may be appropriate. However, if scanning is effected in a manner that produced circular fiber or array motion, a circular array  18  may be preferred.  
         [0055]     Some light from the sensor element  17  (which may be reflected light, fluorescence, etc., emitted in response to the incident light energy) is coupled back through the fiber tip  16  as return light, and hence along fiber  15  to re-emerge from proximal fiber tip  14 . This return light is directed by beamsplitter  13  to a lens  19 , which focusses the return light to pass through a spatial filter  20 . The return light then impinges upon photo transducer  21 . The electrical signal output by transducer  21  is passed to the central, control electronics  22 , which is also connected to an actuator  23  that moves the fiber exit tip  16  to optically couple that tip to each of the sensor elements sequentially.  
         [0056]     A synchronization signal allows the output from the phototransducer  21  to be correlated with the instant sensor element of array  18  being observed. This allows a quantification of each parameter to be obtained and displayed on display  24 . It is preferred that the scanning motion of exit tip  16  should be resonant to reduce energy requirements. Additional scanning in an orthogonal direction or directions (whether x-y, x-z or x-y-z scanning) is also possible. This would increase the number of sensor spots that can be scanned and thereby interrogated. It is anticipated that each sensor array  18  could be produced using gene chip production techniques.  
         [0057]     A multiplexed fiber optic sensor system according to another embodiment of the present invention is shown schematically in  FIG. 2 . The system of  FIG. 2  employs a fused biconical taper coupler or other in-fiber beamsplitter device as the beamsplitter to direct the return signal light to the photodetector.  
         [0058]     Referring to  FIG. 2 , light from light source  31  is focussed by lens  32  into a fiber  33 . The light travels along the core of fiber  33  until it reaches a fiber coupler  34  and from the coupler to the exit tip  35  of coupler leg  36 . Return light returning from a particular sensor element  37  in close proximity to the fiber exit tip  35  travels to the coupler  34 , and a portion of the light is conveyed along coupler leg  38  to a photodetector  39 . The control electronics  40 , fiber scanning mechanism  41  and display  42  are the same as the corresponding elements of the embodiment shown in  FIG. 1 .  
         [0059]     The scanning interrogation of the sensor array  43  can be achieved by moving the fiber tip  35  as described above, or alternatively by moving (preferably resonantly) the sensor array  43  itself. It may be desirable that the motion of the array  43  be arranged so that it produces a flow of a gas being monitored over the sensor elements  37 . This would speed up the reaction time by creating turbulence.  
         [0060]     Alternatively, a fan can be used to force the monitored gas over the array  43  and thereby speed up the reaction time.  
         [0061]     It is possible, and indeed desirable in some embodiments, that a lens be interposed between the fiber exit tip  35  and the array  43  of sensor elements  37 , in order to converge the optical energy emanating from the fiber tip  35  to focus into or onto each individual sensor element  37  in turn. The optical energy returned by each sensor element  37  is re-converged into the fiber tip  35 , where it is coupled as bound energy modes and travels back to the photodetector  39 .  
         [0062]     In embodiments where a lens is used in this manner, the scanning of the spot of light across the array  43  of sensor elements  37  can also be achieved by motion of that lens itself. Where such a lens is not used (see  FIG. 3A ), light  51  emerges from the tip  52  of the optic fiber  53  and impinges on sensor element  54 . Reflected light or fluorescence from the sensor element  54  returns into the cora  55  of fiber  53  and is carried back to the beamsplitter (such as beamsplitter  13  of  FIG. 1  or fiber coupler  34  of  FIG. 2 ). Scanning is achieved by moving the fiber tip  55  or by moving the sensor array  56  (or both) to achieve relative motion in the directions indicated by arrows  57  and  58 .  
         [0063]     In embodiments where a lens is in fact used in this manner (see  FIG. 3B ), a lens  61  may be located between the fiber exit tip  62  and the instant sensor element  63 . This converts the system from a near field-scanning mode to a confocal mode of operation. In this case the scanning of the array  64  of sensor elements  63  may be carried out by movement of the lens  61  in the direction indicated by arrow  65 .  
         [0064]     Scanning can also be carried out by means of a movable mirror. A multiplexed fiber optic sensor system in which a scanning mirror is used, according to a further embodiment of the present invention, is shown schematically in  FIG. 4 . This embodiment also uses a bulk optic beamsplitter at the distal (sensor head) end. This totally separates the outgoing and returning optical energy paths in the fibers(s).  
         [0065]     Thus, referring to  FIG. 4 , a light beam  71  from light source  72  is focussed by a lens  73  into the proximal tip  74  of an optic fiber  75 . The light travels along the fiber  75  to the distal or exit tip  76 . The optical energy emerges from the fiber tip  76 , is collimated by a lens  77  and passes through a beamsplitter  78 . It then is reflected by a scanning mirror  79 . The mirror  79  is connected to an actuator  80  that is controlled by control electronics  81  to move the mirror  79 .  
         [0066]     After reflection from the scanning mirror  79 , the beam is focussed by a further lens  82  onto an individual sensor element  83  in an array  84  of sensor elements. Return light re-emanated from the sensor element  83  returns via lens  82  and scanning mirror  79  to beamsplitter  78 . A portion of the return light is diverted from its path by the beamsplitter  78  to mirror  85 , and reflected to lens  86  which focuses the return light into the core at the tip  87  of an optic fiber  88 . The light is transmitted by fiber  88  to its other end  89  from which it emerges to impinge on the phototransducer  90 . The output signal of the phototransducer  90  is transmitted to the control electronics  81 , where it is processed using the actuator feedback signal from actuator  80  to correlate the position of the scanning mirror  79  and hence identity of sensor element  83  with the data being received from the phototransducer  90 , to provide a read-out of the parameters that are being measured. This read out is displayed on display  91 .  
         [0067]     In some existing sensor systems, the measurement is made using a change in color of the optrode material using reflected light. The embodiments described herein of the present invention preferably use monochromatic light to provide a reference reflection from an adjacent spot on the sensor array (constituting a reference sensor element) to compensate for fiber transmission variations. It is also possible to use the ratio of two wavelength or color spectral regions (as shown in  FIG. 5 ). A white light source or two separate colored sources (such as LEDs) could be used for this purpose.  
         [0068]     Thus, a multiplexed fiber optic sensor system according to a still further embodiment of the present invention is shown schematically in  FIG. 5 . Optical energy from a first light source in the form of first LED  101  is collimated by means of a first lens  102  and combined with light from second light source in the form of second LED  103  that has been collimated by a second lens  104 . LEDs  101  and  103  have light outputs of different wavelengths.  
         [0069]     The light is combined by means of a dichroic beamsplitter  105  which directs the combined light to focussing lens  106 . Lens  106  focusses the light into optic fiber  107 , and transmits the light to a wavelength independent beamsplitter  108 . The light is scanned by scanner  109  (which can be of any suitable form) and focussed by lens  110  onto the sensor element  111  of array sensor  112  in turn.  
         [0070]     Fiber  107  is multimode, which allows the transmission of the two wavelengths and, as the fiber  107  therefore has a greater core diameter, also increases the intensity of light that can be transmitted. This makes this embodiment particular suitable for reflection systems.  
         [0071]     The return light is separated by the beamsplitter  108  and transmitted by means of a further optic fiber  113  (whose output is collimated by lens  114 ) to a dichroic beamsplitter  115 , to which splits the two wavelength components and diverts them to respective photomultipliers  116  and  117 . The output signals of photomultipliers  116  and  117  are inputted into control electronics  118 . The ratio of the signals from photomultipliers  116 ,  117  provides a value for the measurand at each of the sensor spots. The results are then displayed on display  119   
       EXAMPLE  
       [0072]      FIG. 6  is an image taken with the system shown in  FIG. 5 . The image is of an electron microscope grid  120 ; such a grid could be used as a holder of optrode sensor material and hence act as the substrate of a sensor array.  
         [0073]     The vertical and horizontal units in  FIG. 6  are arbitrary.  
         [0074]     In this instance the image was taken with synchronized acquisition electronics using blue 488 nm light from an Argon ion laser as illumination and using the longer wavelength fluorescence to acquire the image.  
         [0075]     When in use as a sensor array substrate, some of the interstices  121 ,  122  and  123  in the grid  120  would be filled with materials that change fluorescence intensity or color when exposed to the gases that are desired to be measured. Interstice  124  would be filled with a material that does not change fluorescence intensity or wavelength when exposed to these gases, while interstice  125  would be filled with a fluorescent substance that changes fluorescence with temperature. Interstice  126  would be filled with a substance that changes fluorescence with humidity.  
         [0076]     The output intensity of the fluorescence from interstice  124  would be used to normalize the outputs from the materials at interstices  121 ,  122  and  123  (i.e. the intensity of the return fluorescence from interstice  124  is used to compensate for optical losses in the system). The signals from the materials at interstices  125  and  126  would used to compensate for temperature and humidity dependence of the (fluorescent) sensor materials at interstices  121 ,  122  and  123 . Cross sensitivities between the gases could also be compensated for.  
         [0077]     The use of multimode optical fiber results in a concomitant trade-off in that a larger core will need correspondingly larger sensor spots and the dimensions of the sensor head would need to be increased, or alternatively only a smaller number of spots would be monitored.  
         [0078]     For example if a Pentax brand insert is used as the basis of this system using single mode fiber, core diameter 3 microns, it is expected that it would be feasible to monitor up to 10,000 sensor elements. If multimode fiber is used and the core size is 30 microns (capturing 100 times the light from an incoherent source) then the number of sensor elements that could be interrogated would be reduced by a factor of 100.  
         [0079]     There would be some advantage in having the scanning carried out by means of pneumatic mechanism. This could be a simple vibrating reed type design carrying the optic fiber. A vacuum tube line to the head could also simultaneously increase the airflow over the sensor elements, which would increase the response time. If such a system was implemented it would be to possible generate the synchronization signal from the waveform of the optical return signal as in phase locked loop systems.  
         [0080]     In conclusion, it is envisaged that the present invention can be used to provide the following advantages over existing systems: 
    1. One sensor head can measure a great number of parameters simultaneously;     2. Several measurements can be combined to eliminate interferences between different parameters;     3. Reference elements can be included in the sensor array to allow compensation for effects such as photo bleaching of optrode material, spontaneous degradation, optical losses in the system (including due to bending of fibers, etc);     4. One sensor design can be used for a great variety of applications by changing the sensor spot array plate;     5. The sensor spot array plate can be made by standard gene chip techniques;     6. Can be made with low cost light sources using MM fiber.     7. The technique cancels out most variation problems, and allows various unique designs to be implemented where motion of a sensor element or object is to be quantified, such as temperature via a bimetal element, humidity where swelling of a humectant can be monitored, refractive index, and motion of a structure.    
 
         [0088]     Modifications within the scope of the invention may be readily effected by those skilled in the art. It is to be understood, therefore, that this invention is not limited to the particular embodiments described by way of example hereinabove.  
         [0089]     In the claims that follow and in the preceding description of the invention, except where the context requires otherwise owing to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.