Abstract:
A sensor network having a series arrangement of fiber-coupled, reflective sensors is disclosed. In operation, a first light signal having multiple wavelength bands is launched in an upstream direction on a fiber bus. Each sensor includes a wavelength filter and an FP sensor that is sensitive to a parameter. Each wavelength filter (1) selectively passes a different one of the wavelength bands to its FP sensor and (2) reflects the remaining wavelength bands back into the fiber bus to continue upstream. The FP sensor imprints a signal based on the parameter onto its received light and reflects it as a second light signal. The collimator, wavelength filter, and FP sensor of each sensor are arranged such that each second light signal is returned to the fiber bus, which conveys them in a downstream direction to a processor that measures them and estimates the parameter at each sensor.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/930,068 filed Jan. 22, 2014. The entire disclosure of U.S. Provisional Application No. 61/930,068 is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to sensors in general, and, more particularly, to optical sensing systems. 
     BACKGROUND OF THE INVENTION 
     A sensor is a device that is designed to detect changes in a quantity (i.e., a measurand) and provide a corresponding output. All-optical, fiber-coupled sensors have many advantages over many other conventional sensors (e.g., acoustic sensors, etc.), such as small size and weight, ability to operate in chemically and/or electrically harsh environments, ease of multiplexing large numbers of sensors, and compatibility with fiber-optic networks for data transport and processing. They are therefore attractive for use in many applications. 
     Such sensor networks often rely upon fiber-Bragg grating (“FBG”) sensors to measure a measurand (e.g., temperature and/or strain). It is relatively easy to integrate multiple FBG sensors in a single-fiber network because each sensor has only a limited number of operating wavelengths. Prior-art FBG sensor systems usually employ swept-wavelength sources so that all of the sensors in the network can be interrogated by a single source. Although easy to implement, the dynamic range of a typical FBG sensor is relatively poor and this has limited their deployment in many applications. 
     Fabry-Perot (“FP”) cavity-based sensors are often used in applications in which the limitations of FBG sensors are not easily overcome. FP sensors are widely used, for example, in many optical-sensor-based accelerometer and pressure sensing applications. But FP sensors are not without their own limitations. For instance, it is challenging to multiplex FP sensors in a single-fiber network because of inter-sensor interference. 
     An optical-sensor based network that has high dynamic range and is easily implemented would provide an attractive alternative to sensor networks known in the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention enables a sensor network without some of the costs and disadvantages of the prior art. An embodiment of the present invention includes one or more FP sensors, where each FP sensor is operatively coupled with a wavelength-selective filter that limits the wavelengths upon which the FP sensor operates. As a result, a single-fiber network can include multiple sensors without giving rise to inter-sensor interference that can degrade the signal-to-noise ratio of the system. 
     An illustrative embodiment of the present invention includes a light source that launches a range of wavelengths, comprising a plurality of wavelength bands, onto an optical fiber bus (hereinafter referred to as a “fiber bus”) to which a plurality of sensors is optically coupled. Each sensor operates in reflection mode to reflect a signal back into the fiber bus, where the reflected signal from each sensor is within a different wavelength band that is uniquely identified with that sensor. A wavelength-response function for each sensor is then determined from its respective reflected signal and analyzed to determine a value for the measurand for which that sensor is sensitive. In some embodiments, each of the sensors is sensitive for the same measurand. In some embodiments, at least one of the sensors is sensitive for a first measurand and at least one of sensors is sensitive for a second measurand. 
     Each sensor includes a Fabry-Perot cavity that is optically coupled with a reflective wavelength filter, where each of the wavelength filters is selectively transmissive for a different one of the plurality of wavelength bands. As a result, each of the Fabry-Perot cavities receives a different one of the plurality of wavelength bands. At each sensor, the wavelength filter is optically coupled with the fiber bus via a serially coupled dual-fiber collimator. The collimator receives light from the source on a first fiber portion and couples light reflected from the wavelength filter into a second fiber portion. Light reflected by the wavelength filter returns through the second fiber portion to the fiber bus, which conveys the reflected light to the next sensor in line. 
     Light transmitted by the wavelength filter is received by the Fabry-Perot cavity, which reflects a portion of this light. The (light) signal reflected from the cavity is based on its cavity length, which is a function of the magnitude of the measurand for which that sensor is sensitive. The Fabry-Perot cavity and the wavelength filter are arranged such that this reflected signal returns through the first fiber portion to the fiber bus, which then conveys the reflected signal to a receiver and processor. 
     The receiver receives the reflected signal from each sensor in a different wavelength band and the processor processes each received reflected signal to determine the wavelength at which a minima occurs in its respective wavelength band. The spectral position of each minima is then used to determine the cavity length of each Fabry-Perot cavity and, thus, a value for the measurand of interest at each sensor. 
     In some embodiments, the light source is a swept-wavelength source. In some other embodiments, the light source is a broadband source. 
     A method in accordance with the present invention is suitable for measuring the wavelength-response function for each sensor and dynamically determining the magnitude of the measurand being measured by each sensor. In some embodiments, the method comprises processing a light signal reflected by a sensor and determining the wavelength at which the light signal exhibits a minima. The method further determines the cavity length of the Fabry-Perot cavity included in the sensor and estimates a magnitude for its respective measurand based on this cavity length. Changes in the wavelength are then monitored and used to determine changes in the measurand over time. 
     An embodiment of the present invention is a sensor network comprising: (1) a source, the source being operative for providing a first light signal that includes a first plurality of wavelength bands; (2) a fiber bus; and (3) a plurality of sensors, each sensor being a reflective sensor that is optically coupled with the fiber bus, and each sensor comprising; (a) a wavelength filter; and (b) a Fabry-Perot (FP) sensor that is optically coupled with the wavelength filter, the FP sensor being sensitive for one of a plurality of measurands; wherein each wavelength filter of the plurality thereof is operative for selectively providing a different one of a second plurality of wavelength bands to its respective FP sensor, and wherein the first plurality of wavelength bands includes the second plurality of wavelength bands. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a schematic drawing of a sensor network in accordance with an illustrative embodiment of the present invention. 
         FIG. 2  depicts operations of a method for measuring a measurand in accordance with the illustrative embodiment of the present invention. 
         FIG. 3  depicts a sensor in accordance with the illustrative embodiment of the present invention. 
         FIG. 4  depicts a schematic drawing of a side view of an FP sensor in accordance with the illustrative embodiment of the present invention. 
         FIG. 5  depicts a typical theoretical response function of an FP sensor in accordance with the illustrative embodiment of the present invention. 
         FIG. 6  depicts a typical theoretical response function of a sensor in accordance with the present invention. 
         FIGS. 7A-C  depict plots of minima in the wavelength-response function of a sensor detected at three different times in accordance with the illustrative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts a schematic drawing of a sensor network in accordance with an illustrative embodiment of the present invention. Network  100  is a distributed pressure-sensor network operative for providing a spatial map of pressure based on measurements made at N locations. In some embodiments, network  100  is operative for monitoring a plurality of measurands at one or more locations. In some embodiments, network  100  is operative for monitoring one or more measurands at one or more locations. For the purposes of this Specification, including the appended claims, a “measurand” is defined as the quantity measured by a sensor. 
     Network  100  includes source  102 , fiber bus  104 , sensors  106 - 1  through  106 -N, receiver  108 , and processor  110 , interrelated as shown. It will be clear to one skilled in the art, after reading this Specification, how to specify, make, and use alternative embodiments wherein a network includes any practical number of sensors. 
       FIG. 2  depicts operations of a method for measuring a measurand in accordance with the illustrative embodiment of the present invention. Method  200  begins with operation  201 , wherein light signal  116 - 1  is provided on fiber bus  104 . Method  200  is described herein with continuing reference to  FIG. 1 , as well as to  FIGS. 3-7 . 
     Source  100  is a broadband source that provides light signal  116 - 1  to fiber bus  104  via isolator  112 . Light signal  116 - 1  has a spectral width greater than or equal to the combined wavelength range of sensors  106 - 1  through  106 -N (referred to, collectively, as sensors  106 ). As a result, light signal  116 - 1  includes light within N wavelength bands (i.e., wavelength bands λ 1  through λN), where each wavelength band spans at least one free-spectral range of the FP sensor included in a corresponding sensor  106 , as discussed below and with respect to operation  206 . In some embodiments, source  100  is a swept-wavelength source that sweeps the wavelength of light signal  116 - 1  through a range of wavelengths equal or greater than the combined wavelength range of sensors  106 . 
     At operation  202 , for each of i=1 through N, sensor  106 - i  receives light signal  116 - i.    
       FIG. 3  depicts a sensor in accordance with the illustrative embodiment of the present invention. Sensor  106 - i  is representative of each of sensors  106 . Sensor  106 - i  comprises input fiber portion  120 - i  and output fiber portion  122 - i , collimator  302 , wavelength filter  304 - i , and FP sensor  306 - i . In the illustrative embodiment, each of sensors  106  is operative for sensing pressure. In some embodiments, at least one of sensors  106  is operative for measuring a different measurand, such as strain, acceleration, gravity, the presence of a chemical, temperature, magnetic field, and the like. In some embodiments, each of sensors  106  is operative for measuring a different measurand. 
     Light signal  116 - i  is conveyed to collimator  302  via input fiber portion  120 - i . It should be noted that each light signal  116 - i  includes all of the wavelength bands in light signal  116 - 1  except for those removed by the wavelength filters of each upstream sensors  106 , as discussed below and with respect to operation  204 . 
     Collimator  302  is a conventional dual-fiber collimator suitable for use in the wavelength range of light signal  116 - 1 . Collimator is optically coupled with input fiber portion  120 - i  and output fiber portion  122 - i  such that the collimator operates in pupil-division mode. 
     Collimator  302  provides light signal  116 - i  to wavelength filter  304 - i.    
     Wavelength filter  304 - i  is a thin-film wavelength filter that selectively passes wavelength band λi to FP sensor  306 - i  as light signal  310 - i  and reflects the remaining light in light signal  116 - i  back into collimator  302  as light signal  308 - i . Light signal  308 - i  is then coupled back into fiber bus  104  as light signal  116 - i +1 via output fiber portion  122 - i.    
     Because wavelength filter  304 - i  passed wavelength band λi to FP sensor  306 - i , light signal  116 - i +1 no longer contains this wavelength band. In other words, as light signal  116  travels upstream through the series of sensors  106 , it contains fewer of the wavelength bands originally contained in light signal  116 - 1 . For example, light signal  116 - 1  includes all of wavelength bands λ 1  through λN when it arrives at sensor  106 - 1 . After passing through sensor  106 - 1 , however, light signal  116 - 2  contains only wavelength bands λ 2  through λN. This process continues at each successive sensor along fiber bus  104  until light signal  116  reaches sensor  106 -N, at which it contains only wavelength band λN. 
     FP sensor  306 - i , wavelength filter  304 - i  and collimator  302  are arranged such that FP sensor  306 - i  reflects a portion of light signal  310 - i  as light signal  312 - i , which is coupled back into collimator  302  through the wavelength filter. Collimator  302  then couples light signal  312 - i  into input fiber portion  120 - i.    
     At operation  203 , FP sensor  306 - i  imparts a signal onto light signal  312 - i , where the signal depends on the pressure sensed by sensor  106 - i.    
       FIG. 4  depicts a schematic drawing of a side view of an FP sensor in accordance with the illustrative embodiment of the present invention. FP sensor  306 - i  includes stationary layer  402  and movable layer  404 . 
     Stationary layer  402  is a partially reflective mirror layer having reflectance of 50%. Stationary layer  402  includes surface  406 . 
     Movable layer  404  is a partially reflective mirror layer having reflectance of 50% for the wavelengths in light signal  116 - 1 . Movable layer  404  includes surface  408 . 
     Surfaces  406  and  408  collectively define optically resonant cavity  410 , which has a cavity length, d, equal to the separation between these surfaces. One skilled in the art will recognize that the response of optically resonant cavity  410  depends on the reflectance of the two mirrors that form the optical cavity, and cavity length, d, as follows: 
             T   =     1     1   -     F   ⁢           ⁢     sin   2     ⁢     ϕ   2                       R   =     1   -   T                 F   =       4   ⁢     R   0           (     1   -     R   0       )     2                   ϕ   =       4   ⁢   π   ⁢           ⁢   d     λ           
where T is the transmitted fraction of light, R is the reflected fraction, R 0  is the reflectance of each mirror in the cavity, d is the distance between the mirrors (i.e., cavity length), and λ is the wavelength of the light.
 
     Movable layer  404  is held above stationary layer  402  such that its position along the z-direction is based on magnitude of the measurand for which sensor  106 - i  is sensitive (i.e., in this example, pressure). As a result, cavity length, d, is a function of pressure and the wavelength-response function (WRF) of reflected light signal  312 - i  is, therefore, indicative of the pressure at sensor  106 - i.    
     One skilled in the art will recognize that there are myriad ways in which movable layer  404  can be supported/suspended above stationary layer  402  and that the support/suspension means will depend on the measurand being sensed by the FP sensor and/or the desired sensor sensitivity. Examples of optically resonant cavities having a movable layer are disclosed in U.S. Pat. Nos. 7,355,723, 7,583,390, 7,359,067, 7,551,295, and 7,626,707, each of which is incorporated herein in its entirety. Further, it should be noted that the design of FP sensor  306 - i  is merely exemplary and that alternative design characteristics, such as R 0 , d, λ, etc., are within the scope of the present invention. 
     Although the illustrative embodiment includes an FP sensor whose cavity length is sensitive to pressure, it will be clear to one skilled in the art, after reading this Specification, how to specify, make, and use alternative embodiments of the present invention wherein cavity length, d, is sensitive to any of a plurality of measurands that includes acceleration, motion, temperature, strain, force, density, angle, a chemical or biological component, light, radiation, gas flow, mass flow, nuclear energy, magnetic field, gravity, humidity, moisture, vibration, electrical fields, sound, or any other physical aspect of an environment. 
       FIG. 5  depicts a typical theoretical response function of an FP sensor in accordance with the illustrative embodiment of the present invention. Plot  500  shows the intensity of light signal  312 - i  for an exemplary FP sensor  306 - i  having a cavity length of 200 microns and mirror reflectance of 50%. 
     At operation  204 , sensor  106 - i  couples light signal  118 - i  into fiber bus  104 . 
     Light signal  118 - i  includes light signal  312 - i  as well as light signal  118 - i +1, which is received from upstream sensor  106 - i +1 and combined with light signal  312 - i  at wavelength filter  304 - i.    
     It should be noted that, as light signal  118  travels along the downstream direction through the series of sensors  106 , it contains more of the wavelength bands originally contained in light signal  116 - 1  since, at each sensor  106 - i , a reflected portion  312 - i  is added to light signal  118 - i +1. For example, light signal  118 - 3  includes all of wavelength bands λ 3  through λN when it arrives at sensor  106 - 2 . After passing through sensor  106 - 2 , however, light signal  312 - 2 , which contains wavelength band λ 2 , is combined with light signal  118 - 3  to form light signal  118 - 2 . As a result, light signal  118 - 2  includes all of wavelength bands λ 2  through λN when it arrives at sensor  106 - 1 , where wavelength band λ 1  is added to it to form light signal  118 - 1 . As a result, light signal  118 - 1  includes all of the wavelength bands of contained in light signal  116 - 1 , as originally provided by source  102 ; however, each wavelength band is now dependent upon the measurand sensed by each of the sensors. 
     At operation  205 , light signal  118 - 1  is provided to receiver  108 . 
       FIG. 6  depicts a typical theoretical response function of a sensor in accordance with the present invention. Plot  600  shows the intensity of light signal  118 - 1  for a network that includes eight sensors  106 , where all of the FP sensors  306  have substantially the same response but wavelength filters  304  have a spectral bandwidth of approximately 15 nm and are sequentially offset in wavelength by approximately 20 nm. It should be noted that in reality, some deviation from the ideal would be expected due to losses in the fiber and connectors, etc. 
     At operation  206 , processor  110  processes the output of receiver  108  to measure the wavelength-response function at each of sensors  106  and determine a value for the pressure at each of the sensors. 
     In order to determine the pressure at each sensor, processor  110  calculates a real-time estimate of the fluctuations in the FP optical cavity length. It should be noted that in order to do this, the spectral bandwidth of each of wavelength filters  304  must be at least the size of one free-spectral range its respective FP sensor  306 . One skilled in the art will recognize that the free-spectral range is given by the distance between neighboring reflectance minima in the WRF. A spectral bandwidth of at least one FSR ensures that there will always be one or more minima within the filtered region of each sensor. If changes in the FP sensor result in a minima moving beyond the range of the filter, then another minima will appear on the other side of the filter window. As such, it is possible to always use the positions of these peaks to generate an estimate of the sensor cavity length. 
     Processor  110  measures the fluctuations in the cavity lengths of the FP sensors by detecting the minima in their respective reflectance functions. The positions of these can be determined by any of a number of methods that are used to detect peaks or valleys in spectral functions. The positions of these minima are determined by the relation: 
               λ   n     =       2   ⁢   d     n           
where the integer n is called the “order” of the given minima. In the proposed approach, there is one or more minima within each filter window. Once the positions of the minima are determined, these positions are tracked over time to provide a measure of the cavity length. As long as the position of a given order does not vary by more than ½ of a FSR between samples, the individual orders can easily be tracked.
 
       FIGS. 7A-C  depict plots of minima in the wavelength-response function of a sensor detected at three different times in accordance with the illustrative embodiment of the present invention. Plots  700 ,  702 , and  704  show three different time samples wherein the wavelength of the respective orders has changed due to changes in the optical cavity length. It can be seen that, as long as the change is less than one half of one free-spectral range, the orders can be readily tracked as they come and go within a given filter window. 
     In some embodiments of the present invention, sensors  106  are combined with a plurality of fiber-Bragg grating-based sensors in series. It should be noted, however, that in such embodiments, the FBG sensors must be active for wavelengths that are outside the passband of any of wavelength filters  304 . 
     It is to be understood that the disclosure teaches just one example of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.