Patent Publication Number: US-10771159-B2

Title: Fiber optic patch and voltage conditioning

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of and claims benefit of priority to U.S. patent application Ser. No. 14/814,355, filed Jul. 30, 2015, which claims the benefit under 35 U.S.C. section 119(e) of U.S. Provisional Application No. 62/062,429, filed Oct. 10, 2014, and U.S. Provisional Application No. 62/031,790, filed Jul. 31, 2014, and the entirety of each of the aforementioned provisional and nonprovisional applications is hereby incorporated by reference herein for all purposes consistent herewith. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     The technical feasibility of using claimed technology with an energy harvester for helicopter rotors was demonstrated with government support under Phase I SBIR contract number N68335-13-C-0318, “Energy Harvesting, Wireless Structural Health Monitoring for Helicopter Rotors”. Accordingly, the technology claimed herein existed before entry into the SBIR contract; however, the U.S. Government may have certain rights in other technology, if any, developed pursuant to the terms of such Phase I SBIR contract. 
    
    
     FIELD 
     The following description relates generally to voltage, including signal, conditioning. More particularly, the following description relates to fiber optic voltage conditioning for sensor integration. 
     INTRODUCTION 
     For real-time structural health monitoring, conventional strain measurement instrumentation may be used with conventional voltage or signal conditioners. However, such conventional signal conditioners may not have sufficient performance for some structural health monitoring applications. Along those lines, optical sensors may be used for some structural health monitoring applications involving such performance demands. Use of optical sensors may involve fiber optic voltage conditioning. However, conventional fiber optic voltage conditioning may be too expensive or too heavy for some real-time structural health monitoring applications. 
     BRIEF SUMMARY 
     An apparatus relates to a patch structure for a fiber optic cable. In such an apparatus, a housing has at least one channel or bore for receipt of a portion of the fiber optic cable having at least one fiber optic sensor. An acoustic interface layer is coupled to a surface of the housing to reduce stress wave coupling loss at an interface between the at least one fiber optic sensor and a host structure surface. 
     A system relates to a patch structure for a fiber optic cable and a fiber optic voltage conditioner. In such a system, the patch structure has a housing. The housing has at least one channel or bore for receipt of a portion of the fiber optic cable having at least one fiber optic sensor. An acoustic interface layer is coupled to a surface of the housing to reduce stress wave coupling loss at an interface between the at least one fiber optic sensor and a host structure surface. The fiber optic voltage conditioner is coupled for optical communication to an optical fiber of the fiber optic cable for optical communication. 
     An apparatus relates to a fiber optic voltage conditioner. In such an apparatus, the fiber optic voltage conditioner is coupled for optical communication to a fiber optic cable having a Fiber Bragg Grating sensor. The fiber optic voltage conditioner includes a tunable light source having a broadband light source or a gain medium configured to provide a narrowband light signal from a broadband light signal for providing to the fiber optic cable. 
     Other features will be recognized from consideration of the Detailed Description and Claims, which follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Accompanying drawing(s) show exemplary embodiment(s) in accordance with one or more aspects of the invention; however, the accompanying drawing(s) should not be taken to limit the invention to the embodiment(s) shown, but are for explanation and understanding only. 
         FIG. 1  is a block diagram depicting an exemplary single-channel laser tracking-based fiber optic voltage conditioning (“FOVC”) system. 
         FIGS. 2A and 2B  are block diagrams depicting an exemplary reflection fiber extender system and an exemplary transmission fiber extender system, respectively. 
         FIG. 3  is a block diagram depicting an exemplary multichannel FOVC system. 
         FIG. 4A  is a block diagram depicting an exemplary wavelength-multiplexed multichannel FOVC system in a ring configuration. 
         FIG. 4B  is a block diagram depicting an exemplary wavelength-multiplexed multichannel FOVC system in a star configuration. 
         FIG. 5  is a block diagram depicting an exemplary fiber optic strain voltage conditioner system. 
         FIG. 6  is a flow diagram depicting an exemplary fiber optic voltage conditioning flow. 
         FIG. 7-1  is a block diagram of a cross-sectional view depicting an exemplary patch structure (“patch”) coupled to a host structure to be monitored simultaneously for AE and strain measurements. 
         FIG. 7-2  is a block diagram of a cross-sectional view depicting an exemplary patch structure (“patch”) with another coupling to a host structure to be monitored simultaneously for AE and strain measurements. 
         FIG. 7-3  is a block diagram depicting a cross-sectional view of an exemplary patch with yet another coupling to a host structure to be monitored simultaneously for AE and strain measurements. 
         FIG. 8-1  is a top-down perspective view depicting an exemplary sensor patch system. 
         FIG. 8-2  is an enlarged portion of sensor patch system of  FIG. 8-1 . 
         FIGS. 9-1 through 9-3  are a top-down, a side, and a bottom-down view depicting an exemplary sensor patch system. 
         FIG. 9-4  is a top-down perspective view of  FIG. 9-1 , and  FIG. 9-5  is an enlarged view of a portion of  FIG. 9-4 . 
         FIGS. 10-1 through 10-3  are a top-down, a side, and a bottom-down view depicting another exemplary sensor patch system. 
         FIG. 11  is a bottom-down view depicting another exemplary sensor patch system. 
         FIG. 12  is a perspective view depicting an example of a sensor cover. 
         FIGS. 13-1 through 13-7  are block diagrams depicting respective examples of the FOVC system of  FIG. 1  though with different examples of tunable light sources instead of a laser. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description set forth below in connection with the appended drawings is intended as a description of various embodiments of the invention and is not intended to represent the only embodiments in which the invention may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention may be practiced without these specific details. In some instances, well known structures and components are shown in block diagram form in order to avoid obscuring the concepts of the invention. In other instances, well-known features have not been described in detail so as not to obscure the invention. For ease of illustration, the same number labels are used in different diagrams to refer to the same items; however, in alternative embodiments the items may be different. Furthermore, though particular dimensions, parameters, and other numerical details are described herein for purposes of clarity by way of example, it should be understood that the scope of the description is not limited to these particular numerical examples as other values may be used. 
     Exemplary apparatus(es) and/or method(s) are described herein. It should be understood that the word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any example or feature described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other examples or features. 
     A structural health monitoring system capable of measuring load, vibration, and/or acoustic emission (“AE”) responses corresponding to damages occurring in materials and/or structures is described. Such a sensing system may include a fiber Bragg grating (“FBG”) sensor array interrogated by laser-based detection system. Such a laser-based detection system in an example may be a miniaturized stand-alone laser-based detection system. 
     In an implementation, such a structural health monitoring system may be combined with a multichannel wireless data acquisition node and high-performance energy harvesters, the feasibility of which was demonstrated under the above-identified Phase I SBIR contract. Along those lines, in an example implementation, a load and damage monitoring system for helicopter blades was developed using such a low weight, high-speed structural health monitoring (“SHM”) system, as described in additional detail below. Such additional description of such SHM system with examples of helicopter blades, though wind turbine blades or other rotating objects may be used, using fiber optics (“FO”), including without limitation fiber optic acoustic emission (“FOAE”) monitoring is provided; however, it should be understood that such SHM system is not limited to the application of monitoring helicopter blades, but may be used in other applications. Along those lines, such FOAE monitoring may be used to monitor for corrosion damage, metal fatigue, composite damage, concrete micro-fracture, wire break, and/or pipe damage, among other examples. 
       FIG. 1  is a block diagram depicting an exemplary single-channel laser tracking-based fiber optic voltage conditioning system  100 . In order to provide voltage conditioning for acoustic emissions (“AEs”) measurements, an open/closed loop control  101  of fiber optic voltage conditioner (“FOVC”)  110  of fiber optic voltage conditioning system  100  may be used to actively track a laser to Bragg wavelength of an FBG sensor. 
     A function of laser tracking-based FOVC  110  may be to provide fiber optic voltage conditioning of returned optical signal  103  coupled to FBG sensors (collectively and singly “FBG sensor”)  111  via a fiber optic cable or line, as described below in additional detail. Along those lines, a multifunction fiber optic sensor, such as fiber Bragg grating sensor, may be used. For AE measurements, using a fiber Bragg grating sensor in a fiber optic housing, an entire sensing area can be bonded directly to a surface of a structure (“measurement surface”) such as with a permanent epoxy or other bonding material. The direct bonding of a bare fiber to a measurement surface may provide a high-level of stress wave coupling from such measurement surface to a sensing area. 
     However, over time such bonding material may result in local stress along such Bragg grating sensor, such as along a grating length for example, and this may lead to creation of an unstable optical cavity formed by multiple gratings, such as along such grating length for example. These optical cavities may negatively affect FOAE sensor performance by changing the slope of a Bragg reflection spectrum and/or increasing Fabry-Perot noise. To address this issue, an overhanging ridge configuration may be used, where a grating under tension may be used for hanging over two stand-offs bonded to a structure&#39;s surface to address the above-described problem, such as for AE and/or strain sensing. Moreover, for an overhanging ridge configuration, a grating may be used for hanging over shear wave-coupling gel, on either or both sides thereof, to provide a flexible bonding to a structure&#39;s surface to address the above-described problem, such as for AE and/or temperature sensing. Optionally, a package sensor multiplexing two or more FBG sensors on the same package with different Bragg wavelengths installed can be multiplexed in serial or parallel. Such a package sensor may be used for AE, strain, and/or temperature sensing. Moreover, optionally a multiplexed-multi-sensing package sensor may be used. For example, two or more different types of sensors (e.g., AE/strain and AE/temperature) can be multiplexed on the same package for multi-sensing. 
     Another function of such laser tracking-based FOVC  110  may be to convert returned optical signal  103  from FBG sensor  111  to an AC voltage analog output  105  that may directly interface with conventional AE instrumentation, which is illustratively depicted as AE data acquisition system  130 . An analog signal output  105  of FOVC  110  may resemble that of a piezo-electric sensor used in conventional AE measurements, facilitating sensor “drop in replacement”. 
     Along those lines, conventional piezo-electric sensors/preamplifier signal conditioners can be completely replaced with a combination of FBG sensor  111  and FOVC  110  as described herein without changing existing AE software and electronics of conventional AE data acquisition systems, such as AE data acquisition system  130  for example, leading to significant cost saving through minimizing additional hardware/software installation. In other words, a high-frequency, high-gain photodetector output  105  carrying a high frequency signal may be interfaced directly to an analog input of a conventional AE data acquisition system  130 , such as a Mistras PCI-2 DAQ board from Mistras Group, Inc. of Princeton Junction, N.J., for example. 
     Light  104 , which may be from a compact, commercially available distributed feedback (“DFB”) laser or other laser  112 , may be passed via an optical circulator  113  to an FBG sensor  111 . A returned optical signal  103  from FBG sensor  111  passing through optical circulator  113  may be routed to a photodetector  114 . Current control  116 , which may be separate from or part of DFB laser  112 , may be initially set at a midrange value between a lasing threshold and a maximum current limit, and a thermoelectric cooler (“TEC”) control  115 , which may be separate from or part of DFB laser  112 , may be tuned to move laser wavelength to a mid-reflection point (“V REF”) of a Bragg wavelength of FBG sensor  111 . 
     With laser current and TEC control voltages settled at initial set points, namely V TEC SET and V CUR SET  121 , laser wavelength may be locked to a mid-reflection point of FBG sensor  111  using simultaneous TEC and current tracking through a closed loop  101  proportional-integral-derivative (“PID”) feedback control. 
     For adjustment for real-time laser tracking control, a chip-based data acquisition (“DAQ”) board  120 , such as an FPGA-based or other System-on-Chip-based (“SoC-based”) circuit board, may be used to record at least one of a low-gain and/or low-frequency of photodetector output signal  106 , namely as associated with an analog input to analog-to-digital converter (“ADC”)  117  and to generate a PID control signal  107 , namely as associated with a digital output of ADC  117 . 
     By “low-gain” and “low-frequency”, it is generally meant an analog output signal being below both a threshold gain and a threshold frequency, respectively, where such thresholds represent an external environmental change and/or perturbation, including without limitation a change in one or more of temperature, strain, pressure, and/or stress of a structure under test, including a structure being monitored, as sensed by one or more FGB sensors coupled to one or more optical fibers. Accordingly, such thresholds may vary from application-to-application depending upon the type of structure being tested, as well as use of such structure. 
     An analog output signal  106  of photodetector  114  may be converted to a digital signal by ADC  117 , and output of ADC  117  may feed an input of a voltage set and store block  119 . An output of voltage set and store block  119  may feed an input of DAC  118  to provide an analog PID control signal  107 . PID control signal  107  may include a PID current error (“CUR ERR”) signal and a PID TEC error (“ERR”) signal. PID control signal  107  may be output from DAC  118 . 
     For purposes of clarity by way of example and not limitation, it shall be assumed that an FPGA is used to set and store voltages; however, in another implementation another type of SoC may be used, including without limitation an ASSP, ASIC, or other IC. Along those lines, voltage set and store block (“FPGA”)  119  may be used to set and store a laser current voltage and a TEC control voltage, namely V TEC SET and V CUR SET  121 , and FPGA  119  may be used to store a mid-reflection point V REF of a Bragg wavelength of FBG sensor  111 . FPGA  119  may be coupled to receive a digital output  123  from ADC  117 , where such digital output  123  is a conversion of an analog photodetector output signal  106 . FPGA  119  may be configured to generate and store TEC and laser current error voltages, namely a V TEC ERR and a V CUR ERR, using such digital output  123  received. FPGA  119  may provide a digital PID control signal  124 , where such digital PID control signal includes a V CUR ERR signal and a V TEC ERR signal, to DAC  118 , and DAC  118  may convert such digital PID control signal  124 , namely a V TEC ERR signal and a V CUR ERR signal, to analog PID control signal  107  having analog PID CUR ERR and PID TEC ERR signals. 
     PID CUR and TEC error signals from PID control  107  may be correspondingly added to CUR and TEC set voltages  121  by adder  122 , and respective sums  108  output from adder  122  may be fed into a controller  150  for adjustment of control information provided to laser  112 . In this example, for purposes of clarity and not limitation controller  150  is broken out into three controllers or modules, namely a laser controller  153  coupled to a current controller  116  and a TEC controller  115 . However, in another implementation, current controller  116  and/or TEC controller  115  may be part of a laser, such as DFB laser  112  for example. Sums  108  may be used together to compensate for drift of a Bragg wavelength, such as due to external environmental changes and/or perturbation including without limitation changes in one or more of temperature, strain, pressure, and/or stress, by actively tuning laser wavelength responsive to such current and TEC control. 
     In this example, both laser TEC and current are used simultaneously to compensate for FBG wavelength drift from DC up to approximately 20 kHz for photodetector output signal  106 . TEC tracking may be provided by changing temperature of DFB laser  112  via TEC control  115  to compensate for FBG sensor  111  wavelength drift caused by environmental changes. While providing large dynamic range, such as for example approximately several thousand microstrains for strain monitoring, TEC compensation may be slow, with a maximum response time in the order of seconds or longer. 
     In this example, fast, such as for example a few Hz to 20 kHz or higher, real time compensation may not be possible with TEC tracking. Along those lines, laser current compensation, as described herein, may be used with a much higher response time, possibly up to approximately 20 kHz or higher, subject to limitations of response time of electronics of laser controller  153 , and such laser current compensation may be used simultaneously with TEC tracking. Tracking by changing laser injection current may cause changes in both laser wavelengths and intensity, although with much more limited dynamic range, such as for example approximately several hundred microstrains for dynamic strain tracking. For larger dynamic strain monitoring, such as more than approximately a thousand microstrains, commercially available distributed Bragg reflector (“DBR”) lasers can be used in place of DFB lasers. However, it should be appreciated that using TEC and current tracking in combination provides extended dynamic range and fast response for laser tracking. 
     Long-distance AE measurement using a laser-based FBG interrogation may be subject to presence of high amounts of optical noise associated with the Fabry-Perot effect generated by an optical cavity created by two or more reflective mirrors. By “interrogation,” it is generally meant providing a light signal to an optical sensor coupled to a material or structure under test and obtaining a light signal in return from such optical sensor to obtain information therefrom regarding such material or structure under test. A Bragg grating itself may be considered a highly reflective mirror. In the presence of another reflective surface from an optical component, such as for example an optical circulator or a scattering center such as a local defect present in a long optical fiber, unstable, unwanted constructive optical interferences can be generated due to laser coherence. Accordingly such interferences may contribute to increased AE background noise, and as a consequence can significantly reduce a signal-to-noise ratio (“SNR”) in AE measurements. 
     To suppress this optical noise, a combination of circulators and optical isolators between reflection and/or scattering surfaces may be used to provide unidirectional optical paths and avoid bidirectional optical paths between any two reflective optical components, such as described below in additional detail. 
       FIG. 2A  is a block diagram depicting an exemplary reflection fiber extender system  200 A, and  FIG. 2B  is a block diagram depicting an exemplary transmission fiber extender system  200 B. One or more instances of each of fiber extender systems  200 A and  200 B may be used separately from one another or a combination of such systems may be used. Accordingly, reflection fiber extender system  200 A and transmission fiber extender system  200 B are generally referred to hereinbelow as “fiber extender system  200 ”, and correspondingly reference to either or both  FIGS. 2A and 2B  hereinbelow is generally to “ FIG. 2 ”. 
     Fiber extender system  200  may include a reflection fiber extender  201  coupled between an FOVC  110 - 1  and a FBG sensor  111 - 1  and/or a transmission fiber extender  202  coupled between an FOVC  110 - 2  and a FBG sensor  111 - 2 . Fiber extender system  200  may be used for long distance AE measurements. 
     Fiber extenders  201  and  202  may respectively be used in a reflection and a transmission mode. Reflection fiber extender  201  may include two long optical fibers  210  and  211  coupled between optical circulators  213 - 1  and  213 - 2 . Transmission fiber extender  202  may include a long optical fiber  220  coupled between an optical circulator  213 - 3  and an optical isolator  231 - 1 , and may include another long optical fiber  221  coupled between an optical isolator  231 - 2  and optical circulator  213 - 3 . 
     Along those lines for a reflection mode, light  104  passed through circulator  113  of FOVC  110 - 1  may be passed through circulator  213 - 1  for optical fiber  210 . Optical fiber  210  may conduct light  104  to circulator  213 - 2  for output therefrom to FBG sensor  111 - 1 . Responsive to wavelengths in such light  104 , including without limitation isolating one or more perturbations in such light, a returned optical signal  103  from FBG sensor  111 - 1  may be provided to circulator  213 - 2 . Along those lines, FBG sensor  111 - 1  may reflect one or more wavelengths in such light  104  for generating returned optical signal  103 , and FBG sensor  111 - 1  may transmit one or more other wavelengths in such light  104  for effectively blocking or filtering out such transmitted wavelengths from being included in returned optical signal  103 . Circulator  213 - 2  may provide returned optical signal  103  to circulator  213 - 1  via optical fiber  211 . Lastly, circulator  213 - 1  may provide such returned optical signal  103  to circulator  113  of FOVC  110 - 1  for processing as previously described herein. 
     For a transmission mode, light  104  passed through circulator  113  of FOVC  110 - 2  may be passed through circulator  213 - 3  for optical fiber  220 . Optical fiber  220  may conduct light  104  to optical isolator  231 - 1  for output therefrom to FBG sensor  111 - 2 . Responsive to wavelengths in such light  104 , including without limitation isolating one or more perturbations in such light, a returned optical signal  103  from FBG sensor  111 - 2  may be provided to optical isolator  231 - 2 . Along those lines, FBG sensor  111 - 2  may reflect or block one or more wavelengths in such light  104  for effectively blocking or filtering out same from a returned optical signal  103 , and FBG sensor  111 - 2  may transmit one or more other wavelengths in such light  104  for generating returned optical signal  103 . Optical isolator  231 - 2  may provide returned optical signal  103  to circulator  213 - 3  via optical fiber  221 . Lastly, circulator  213 - 3  may provide such returned optical signal  103  to circulator  113  of FOVC  110 - 2  for processing as previously described herein. 
       FIG. 3  is a block diagram depicting an exemplary multichannel FOVC system  300 . Multichannel FOVC system  300  is further described with simultaneous reference to  FIGS. 1 through 3 . 
     In multichannel FOVC system  300 , an N-channel FOVC  110 M is respectively coupled to FBG sensors  111 - 1  through  111 -N via corresponding optical circulators  113 - 1  through  113 -N, for N a positive integer greater than one. FBG sensors  111 - 1  through  111 -N may be respective discrete FOAE sensors or an array thereof. 
     DFB1 through DFBN lasers  112 - 1  through  112 -N and corresponding photodetectors (“PD”) PD 1   114 - 1  through PDN  114 -N may be respectively coupled to optical circulators  113 - 1  through  113 -N. Each of DFB lasers  112 - 1  through  112 -N may deliver corresponding laser lights  104 - 1  through  104 -N respectively into Bragg grating sensors  111 - 1  through  111 -N via corresponding circulators  113 - 1  through  113 -N. Circulators  113 - 1  through  113 -N may then be used to pass corresponding returned optical signals  103 - 1  through  103 -N respectively from sensors  111 - 1  through  111 -N on a per channel basis. Returned optical signals  103 - 1  through  103 -N may be respectively provided onto photodetectors  114 - 1  through  114 -N. 
     Each of the outputs of photodetectors  114 - 1  through  114 -N, which may be implemented in an example implementation as photodiodes (“PD”) PD 1  through PDN, may be split into two sections, namely signals  106 - 1  through  106 -N and signals  105 - 1  through  105 -N. One group, namely a low frequency signal output group of signals  106 - 1  through  106 -N, may be input into an analog input interface  301 , such as respective analog input ports for example, of an FPGA-based data acquisition system  120  for laser tracking control generation as previously described herein. Another group, namely a high frequency signal output group of signals  105 - 1  through  105 -N, may be input to a conventional multichannel AE DAQ system  130  for AE measurement. 
     Even though an FPGA  119  is used as described herein for DAQ  120 , another type of SoC, an ASSP, an ASIC, or other VLSI type of integrated circuit device may be used instead of FPGA  119 . However, for purposes of clarity and not limitation, it shall be assumed that an FPGA  119  is used. Furthermore, DAC  120  may exist in a single integrated circuit device, whether such device is a monolithic integrated circuit or an integrated circuit formed of two or more integrated circuit dies packaged together. An FPGA  119  may have sufficient resources for integration of one or more ADCs  117 , one or more DACs  118 , and/or one or more adders  122  therein for providing a multichannel FOVC  110 M. However, an FPGA may lack sufficient analog resources, and so a separate analog chip, such as for providing digital-to-analog conversions, may be used. 
     In this example, FOVC  110 M includes a DAQ  120  having an FPGA  119  configured for inputs 1 through N of an analog input interface  301  (“inputs  301 ”) and outputs 1 through 2N of an analog output interface  302  (“outputs  302 ”), for example separate analog output ports. Inputs  301  may correspond to a group of signals  106 - 1  through  106 -N. Pairs of outputs  108 - 1  through  108 -N of outputs  302  may respectively be provided to laser controllers  153 - 1  through  153 -N. Laser controllers  153 - 1  through  153 -N may provide respective pairs of TEC and current control signals  115 - 1 ,  116 - 1  through  115 -N,  116 -N to DFBs  112 - 1  through  112 -N, respectively. For purposes of clarity by way of example and not limitation, laser controllers  153 - 1  through  153 -N are illustratively depicted as including corresponding pairs of current and TEC controllers, which were illustratively depicted as separate controllers  116  and  115 , respectively, in  FIG. 1  for purposes of clarity. However, it should be understood that controllers  115  and  116  may be incorporated into a laser controller  153 . 
     Accordingly, for purposes of scaling an FOVC  110 , it should be appreciated that a single FPGA  119  may be used by a DAQ  120  configured to support N channels. In this example, FOVC  110 M does not include optical circulators  113 - 1  through  113 -N; however, in another configuration, FOVC  110 M may include optical circulators  113 - 1  through  113 -N. 
     Generally, FPGA  119  generates respective sets, such as pairs for example, of TEC and current control signals  108 - 1  through  108 -N via analog output ports  302  of DAQ  120 , and such respective sets of TEC and current control signals  108 - 1  through  108 -N may be used to provide corresponding pairs of TEC control and current control signals  115 - 1 ,  116 - 1  through  115 -N,  116 -N to respectively lock DFB lasers  112 - 1  through  112 -N to their respective FBG sensors  111 - 1  through  111 -N by adding respective error signals. Such respective error signals may be generated from FPGA  119  generated PID control to provide current and TEC set points via digital summing as previously described herein, though on a per-channel basis in this example of FOVC  110 M. For long distance measurements, N fiber extenders, whether all transmission fiber extenders  202 , all reflection fiber extenders  201 , or a combination of fiber extenders  201  and  202 , as previously described with reference to  FIG. 2  may be used in conjunction with multichannel FOVC  110 M. 
       FIG. 4A  is a block diagram depicting an exemplary wavelength-multiplexed multichannel FOVC system  400  having a ring configuration. Wavelength-multiplexed multichannel FOVC system  400  includes FOVC  110 M of  FIG. 3 , without optical circulators  113 - 1  through  113 -N, coupled to a ring multiplexer  410 . Wavelength-multiplexed multichannel FOVC system  400  may be used for wavelength-multiplexed N-channel fiber optic voltage conditioning for FOAE measurement configured in a ring topology. 
     FBG sensors  111 - 1  through  111 -N are FOAE sensors with Bragg reflection wavelengths for a wavelength range, which may vary from application-to-application. For example, such wavelengths may be in a range of approximately 1510 nm to 1650 nm. For purposes of clarity by way of example and not limitation, it shall be assumed that such wavelengths include 1510 nm and 1650 nm. 
     FOVC  110 M is coupled as previously described to provide laser light  104 - 1  through  104 -N and to receive returned optical signals  103 - 1  through  103 -N. Accordingly, description of FOVC  110 M is generally not repeated for purposes of clarity and not limitation. 
     In a ring topology, FBG sensors  111 - 1  through  111 -N may be connected in series, and a ring multiplexer  410  may be used to provide laser tracking in a transmission mode. In a ring multiplexer  410 , a wavelength division multiplexing multiplexer (“WDM mux”)  401  may be configured to multiplex N DFB laser light outputs  104 - 1  through  104 -N into a single mode multiplexed optical signal  441  for providing to a single-mode telecommunication fiber  440 . 
     Telecommunication fiber  440  may include N FBG sensors  111 - 1  through  111 -N coupled in series. Telecommunication fiber  440  may include optical isolators  231 - 0  and  231 -N respectively bracketing such series of FBG sensors  111 - 1  through  111 -N, and may include optical isolators  231 - 1  through  231 -(N−1) respectively inserted between each adjacent pair of FBG sensors of such series of FBG sensors  111 - 1  through  111 -N. Optionally, an optical isolator  231 -N may be separated out to be more proximate to an input of a WDM demultiplexer (“demux”)  402  of ring multiplexer  410  for providing a returned multiplexed optical signal  442  output from telecommunication fiber  440  after processing such single mode multiplexed optical signal  441 . 
     More generally, an optical isolator  231  may be inserted between each optical component and/or FBG sensor to provide unidirectional optical paths, namely overall a single unidirectional ring path of ring multiplexer  410 , and to suppress cavity-like optical noises. Along those lines, input optical isolators  431 - 1  through  431 -N of ring multiplexer  410  may respectively be coupled to receive laser lights  104 - 1  through  104 -N and to provide optically isolated version thereof as respective inputs to WDM mux  401 , and output optical isolators  432 - 1  through  432 -N of ring multiplexer  410  may respectively be coupled to receive demultiplexed returned optical signals  103 - 1  through  103 -N from WDM demux  402  to generate optically isolated version thereof as respective inputs to photodetectors of FOVC  110 M as previously described. 
     WDM demux  402  of ring multiplexer  410  may be coupled to receive returned multiplexed optical signal  442  and configured to demultiplex such optical signal into transmitted signals passed through FBG sensors and optical isolators onto N separate photodiodes  114 - 1  through  114 -N, as previously described. 
       FIG. 4B  is a block diagram depicting an exemplary wavelength-multiplexed multichannel FOVC system  490  having a star configuration. Wavelength-multiplexed multichannel FOVC system  490  includes FOVC  110 M of  FIG. 3 , without optical circulators  113 - 1  through  113 -N, coupled to a star multiplexer  450 . Wavelength-multiplexed multichannel FOVC system  490  may be used for wavelength-multiplexed N-channel fiber optic voltage conditioning for FOAE measurement configured in a star topology. 
     FBG sensors  111 - 1  through  111 -N may be FOAE sensors with Bragg reflection wavelengths as previously described. FOVC  110 M is coupled as previously described to provide laser light  104 - 1  through  104 -N and to receive returned optical signals  103 - 1  through  103 -N. Accordingly, description of FOVC  110 M is generally not repeated for purposes of clarity and not limitation. FOAE measurement may be performed in a reflection mode with N FBG sensors  111 - 1  through  111 -N connected in parallel using N-channel FOVC  110 M coupled to a star multiplexer  450  of wavelength-multiplexed multichannel FOVC system  490 . 
     A star multiplexer  450  includes an optical mux/demux device  460  coupled to an optical demux/mux device  470  via an optical cable, such as a bidirectional optical fiber  420 . Mux/Demux device  460  includes a WDM mux  401 , a WDM demux  402 , input optical isolators  431 - 1  through  431 -N, and output optical isolators  432 - 1  through  432 -N, as previously described, except that mux/demux device  460  further includes an optical circulator  413 - 1  coupled as described below. 
     WDM mux  401  multiplexes N DFB laser light outputs  104 - 1  through  104 -N into a single-mode multiplexed optical signal  441  for providing to optical circulator  413 - 1  of optical mux/demux device  460 . Such single-mode multiplexed optical signal  441  may be output from optical circulator  413 - 1  into a single-mode telecommunication optical fiber  420 . Optical mux/demux device  460  may be used to multiplex N DFB laser outputs via such an N×1 multiplexer. Optical circulator  413 - 1  may be coupled to optical fiber  420  to provide such single-mode multiplexed optical signal  441  to optical demux/mux device  470  and to receive returned light, namely a returned multiplexed optical signal  442 . Such a returned multiplexed optical signal  442  may be demultiplexed into N respective photodiode inputs via a 1×N demultiplexer, such as WDM demux  402  as previously described. 
     Demux/Mux device  470  may be used to demultiplex multiplexed light, such as multiplexed optical signal  441 , from mux/demux device  460  into N separate outputs for N FBG sensors  111 - 1  through  111 -N coupled for reflection modes. Along those lines, an optical circulator  413 - 2  of demux/mux device  470  may be coupled to optical fiber cable  420  to receive multiplexed optical signal  441  for providing to a WDM demux  472  of demux/mux device  470 . 
     Such a 1×N WDM demux  472  may provide N demultiplexed optical signals respectively to optical circulators  473 - 1  through  473 -N of demux/mux device  470 . Such demultiplexed optical signals may be respectively provided to such separate FBG sensors  111 - 1  through  111 -N coupled for corresponding reflection modes. FBG sensors  111 - 1  through  111 -N may be externally (as illustratively depicted) or internally coupled to star multiplexer  450 . 
     In response to such demultiplexed optical signals, FBG sensors  111 - 1  through  111 -N may respectively generate reflected returned optical signals to optical circulators  473 - 1  through  473 -N. Optical circulators  473 - 1  through  473 -N may be coupled to WDM mux  471  of demux/mux device  470  to respectively provide such returned light signals from FBG sensors  111 - 1  through  111 -N. WDM mux  471  may multiplex such returned light signals to provide a returned multiplexed optical signal  442  to optical circulator  413 - 2 . Such returned multiplexed optical signal  442  may be provide by optical circulator  413 - 2  into optical fiber cable  420  for optical circulator  413 - 1 , and optical circulator  413 - 1  may provide such returned multiplexed optical signal  442  to WDM demux  402  for demultiplexing, as previously described. 
     In another configuration, mux/demux device  460  may be integrated into FOVC  110 M to provide a single optical interface. Along those lines, optical fiber cable  420  and demux/mux device  470  may be used as an external device connected to individual FBG sensors. For long distance measurements, such optical fiber  420  may be replaced by a reflection fiber extender, as previously described herein, to suppress optical noise. 
       FIG. 5  is a block diagram depicting an exemplary fiber optic strain voltage conditioner system  500 . Fiber optic strain voltage conditioning (“FOSVC”) system  500  may be configured to convert optical signals from FBG sensors  111  into analog voltage outputs  502  that may be directly interfaced with conventional strain instrumentation, as described below in additional detail. Strain-induced wavelength shifts experienced by one or more of FBG sensors  111  may be converted to analog voltage signals (“analog output”)  502  that resemble parametric outputs of a conventional strain gauge signal conditioner. This allows FOSVC system  500  to work as a high-performance “drop-in” replacement for a signal conditioner in conventional strain measurement systems, as described below in additional detail. 
     FOSVC system  500  includes an FBG Analyzer (“FBGA”) module  510 , a System-on-Chip (“SoC”) module  520 , FBG sensors  111  in an optical fiber  501 , an optical fiber-to-structure bonding material  504 , and a digital-to-analog (“D/A”) converter  530 . Optionally, FOSVC system  500  may be coupled to a network  590 , which may include the Internet, for cloud storage  591 . Optionally, one or more web-browser enabled devices  592  may be used to communicate with such cloud storage  591  via such network  590 . 
     FBG sensors  111  in an optical fiber  501  housing may be coupled to receive and provide an optical signal via such optical fiber  501 , the former of which is for optical transmission of light from broadband light source  505 . A bonding material  504  may be used to couple optical fiber  501  of FBG sensors  111  to a material or structure under test  503 . A broadband light source  505 , such as an LED light source, of FBGA module  510  may provide light to an optical circulator  506  of FBGA module  510 , and such light may be sent through to optical fiber  501  via passing through optical circulator  506  through to FBG sensors  111 . Responsive to strain-induced wavelength shifts experienced by one or more of FBG sensors  111 , any reflected light from FBG sensors  111  may be provided as optical signals via optical fiber  501  to optical circulator  506  for spectral element  507  of FBGA module  510 . 
     Reflected light may be spectrally dispersed through spectral element  507 , which in this example is a Volume Phase Grating (“VPG”) element  507  of FBGA module  510 . Such dispersed light may be detected by a photodiode array  508  of FBGA module  510 , which in this example is an Indium Gallium Arsenide (InGaAs) photodiode array; however, other types of photodiode arrays may be used in other implementations. Outputs of photodiode array  508  may be digitized using an analog-to-digital converter (“A/D converter”)  509  of FBGA module  510 . Output of A/D converter  509  may be packetized by an on-board integrated circuit packetizer  511  of FBGA module  510 , which in this example is separate from an FPGA of SoC module  520 . However, in another implementation, packetizer  511  and/or A/D converter  509  may be implemented in an FPGA of SoC module  520 . Such packetized information may be forwarded from packetizer  511  to SoC module  520  for post-processing. 
     SoC module  520  may include CPU complex  521 , a programmable gate array device  522 , and main memory  523 . In this example, such programmable gate array device  522  is an FPGA; however, in other implementations, other types of integrated circuits, whether programmable gate array devices or not, may be used to provide a D/A interface  524  and digital signal processing (“DSP”) hardware  525 . 
     CPU complex  521 , which may be on a same FPGA as D/A interface  524  and DSP hardware  525  in another implementation, in this implementation includes a dual-core CPU  527  running firmware  526 . However, a single core or other types of multi-core CPUs may be used in other implementations. Generally, a signal conversion block  550 , which may be in CPU complex  521 , may include a peak detector  529 , a web socket  536 , firmware stored in memory (“firmware”)  526 , and a spectral power converter  531 . Firmware  526  may receive data from packetizer  511  of FBGA  510  into a ring buffer  528  of SoC module  520 , which may also be of CPU complex  521 . 
     Ring buffer  528  may be used to store a continuous stream of samples from packetizer  511  of FBGA  510  to in effect allow FOSVC system  500  to plot outputs of FBG sensors  111  over a period of time. Firmware  526  may be configured to clean up data from ring buffer  528 . Data from ring buffer  528  may be provided to a peak detector  529 , and detected peaks may be provided from peak detector  529  to spectral power block  531  to quantify spectral power associated with each of such peaks detected. Along those lines, firmware  526  may quantify wavelength shifts, which are directly proportional to the amount of strain experienced and spectral power sensed by each FBG sensor  111 . This post-processed data  532  may be stored in main memory  523 . 
     In this implementation, programmable gate array  522 , which is coupled to main memory  523 , is configured to provide hardware that reads data  532  that firmware  526  has placed in main memory  523  and that performs signal processing tasks on such data  532  using DSP hardware  525 . An example of a signal processing task may be an FFT and/or the like to measure any vibration components in data  532 . 
     Output of DSP hardware  525 , such as an FFT output for example, may be written back to main memory  523  as data  533  for use by CPU complex  521 . D/A interface  524  of programmable gate array device  522  may be used to send FBG sensor data  532  to an external D/A converter  530  to mimic a parametric output of a conventional strain signal conditioner. FOSVC system  500  may work as a high-performance “drop-in” replacement for a signal conditioner in conventional strain measurement systems, and so analog output  502  may be provided to conventional strain measurement instrumentation (not shown). Each output of D/A converter may represent an output of one FBG sensor of FBG sensors  111 . 
     CPU complex  521  via firmware  526  may be configured to read strain data  533  that programmable gate array  522  has placed in main memory  523 . CPU complex  521  may optionally include either or both an Ethernet interface  534  or a USB WiFi interface  535  to forward strain data  533  to one or more remote computers connected over network  590 . Optionally, an external cloud server  591  may take outputs from multiple FOSVC systems  500  and store them in a database for further analysis by software running on such cloud server  591 . In this example, such computers may include multiple HTML5-compliant Web browsers  592  to communicate with cloud server  591  and/or to communicate with one or more FOSVC systems  500  to access strain data  533 , which may allow users to make business decisions and/or configure individual FOSVC systems  500  using corresponding optional Web sockets  536  of CPU complexes  521  of such systems. 
       FIG. 6  is a flow diagram depicting an exemplary fiber optic voltage conditioning flow  600 . Fiber optic voltage conditioning flow  600  is further described in the light of the above-description. 
     At  601 , a light signal may be generated with a laser. At  602 , such light signal may be received by an optical circulator. At  603 , first control information may be generated by a controller coupled to such laser for wavelength-drift control thereof. At  611 , TEC and current controlling of such laser may be responsive to first control information generated at  603 . At  604 , second control information may be generated by a data acquisition module coupled to such controller. Generating second control information at  604  may include generating PID feedback information at  612  by such data acquisition module. Generating PID feedback information at  612  may include recording at  613  by such data acquisition module of an analog output signal of a photodetector responsive to such analog output signal being below at least one of a gain threshold or a frequency threshold therefor. Moreover, PID feedback information may include PID current error information and PID TEC error information. Along those lines, such data acquisition module may add a current set voltage and a TEC set voltage to such PID current error information and such PID TEC error information, respectively, for generating at  604  such second control information. 
     At  605 , such first control information may be adjusted by such controller responsive to such second control information. Such adjusting of such first control information may be to tune wavelength of a laser for wavelength-drift control of such laser responsive to such second control information. Wavelength-drift control of such laser may be to compensate for drift of a Bragg wavelength. 
     At  606 , a returned optical signal may be received by a photodetector coupled to such optical circulator. At  607 , an analog output signal may be generated by such photodetector responsive to such returned optical signal. Such generating by such data acquisition module of such second control information at  604  may be for such analog output signal generated at  607 . 
     Multifunction Fiber Optic Sensor 
     For acoustic emission (“AE”) measurements using a Fiber Bragg Grating (“FBG”) sensor, a patch encompassing an entire sensing area can be bonded directly to a structure, namely a measurement surface of a structure, using a bonding material, such as for example a permanent epoxy. Direct bonding of a bare fiber to a measurement surface may provide enhanced stress wave coupling from such surface to a sensing area. However, over time a bonding material may develop local stress along a grating length, leading to creation of an unstable optical cavity formed by multiple gratings along such grating length. These optical cavities can seriously affect fiber optic AE (“FOAE”) sensor performance by changing the slope of an FBG reflection spectrum and/or increasing Fabry Perot noise. 
     For long term simultaneous AE and strain measurement, an “overhanging ridge” may be used for bonding an FBG sensor to the surface of a structure being monitored.  FIG. 7-1  is a block diagram of a cross-sectional view depicting an exemplary patch structure (“patch”)  715  coupled to a host structure  719  to be monitored simultaneously for AE and strain measurements. 
     A portion of a bare fiber optic cable  711 B is passed through a channel or bore in patch  715  to provide a sensor patch system  710 . Fiber optic cable  711 B may be a fiber optic cable or line as previously described herein having one or more FBG sensors  111 , and such bare fiber optic cable  711 B may be coupled for optical communication to a fiber optic voltage conditioner, as previously described. In this example, bare fiber optic cable  711 B is not jacketed with a sheath. 
     At least one FBG sensor  111 , generally in a sensor area or region  713  (“grating area  713 ”), of fiber optic cable  711 B is under slight tension, such as by pulling and adhering in place opposing portions of bare fiber optic cable  711 B. For purposes of clarity and not limitation, it shall be assumed that only one FBG sensor  111  is present. 
     A lower surface  712  of patch  715  is bonded at bonds  717  to a surface  716  of a host structure  719 . In this example, such bonding may be at only two points, namely bonds  717 , where each of bonds  717  is immediately outside grating area  713 . Bonds  717  may be formed using a permanent epoxy. The thickness of a bonding material used to form bonds  717  may be kept to a small amount, such as less than 100 microns thick, to allow for enhanced stress wave coupling without mode conversion at a bonding interface associated with bonds  717 . 
     By boding at spaced apart bonds  717  outside of a grating area  713  an “overhanging ridge” configuration is provided, namely at least one FBG of a bare fiber optic cable  711 B under tension is hanging over two stand-off bonded bonds  717  to surface  716  of host structure  719 . This overhang or gap between a monitored surface  716  of host structure  719  and a corresponding facing lower surface of a grating area  713  allows for acoustic coupling between such surfaces to effectuate monitoring for AE and strain measurements.  FIG. 7-2  is a block diagram of a cross-sectional view depicting an exemplary patch  715  with another coupling to a host structure  719  to be monitored simultaneously for AE and strain measurements. This coupling may further enhance acoustic coupling between facing surfaces  716  and a lower surface of grating areal  713 . 
     Again, a portion of a bare fiber optic cable  711 B is passed through a channel or bore in patch  715  to provide a sensor patch system  710 . However, in this configuration, there is no overhanging ridge, as an air gap between surfaces  712  and  716  directly below grating area  713  is taken up in whole or mainly by a layer of gel  718 . 
     One or more FBGs, generally in grating area or region  713 , of fiber optic cable  711 B is under slight tension. A lower surface  712  of patch  715  is bonded at an optional bond  717  to a surface  716  of a host structure  719 . In this example, another bond is provided by a layer of a high viscosity gel  718 , such as shear wave couplant gel. 
     Gel  718  is disposed between lower surface  712  and surface  716  directly below grating area  713 . Position of gel  718  is different than position of optional bond  717 , which bond  717  is immediately outside grating area  713 . Gel  718  may optionally be directly applied to a surface of fiber optic cable  711 B in grating area  713  to directly bond or couple one or more FBG sensors of such cable to a monitored surface  716  of a host structure  719 . 
     Gel  718  provides for a more accurate acoustic impedance match between surface  716  and grating area  713  for enhancing stress wave energy transfer without creating local stress along a lower surface  712  portion corresponding to and/or near grating area  713 . Because a couplant gel  718  can sufficiently transfer heat but not sufficiently, if at all, transfer strain, one or more FBG sensors bonded or coupled to surface  716  can be used as one or more corresponding strain isolated sensors to simultaneously measure AE and temperature. 
     However, there may not be sufficient protection by patch  715  in a grating area  713  for FBG sensor(s) of bare fiber optic cable  711 B for some applications. Along those lines, a sensor package is described below to provide ease of handling, and shielding protection for a sensing area, such as an FBG sensing area, of a fiber optic cable while facilitating both enhanced pressure waves coupling efficiency and strain transfer. 
       FIG. 7-3  is a block diagram depicting a cross-sectional view of an exemplary patch  715  for yet another coupling to a host structure  719  to be monitored simultaneously for AE and strain measurements.  FIG. 7-3  is the same as  FIG. 7-2 , except with the addition of a bond  717  for having bonds  717  respectively on each end side of couplant gel  718  and outside of sensor grating area  713 . 
       FIG. 8-1  is a top-down perspective view depicting an exemplary sensor patch system  810 .  FIG. 8-2  is an enlarged portion of sensor patch system  810  of  FIG. 8-1 . With simultaneous reference to  FIGS. 7-1 through 7-3, 8-1, and 8-2 , sensor patch system  810  is further described. 
     Sensor patch system  810  includes a patch  715  with a fiber optic cable  711 . Fiber optic cable  711  in this example is a jacketed cable; however, in another example a bare fiber optic cable  711 B may be used. Sensor patch system  810  may be for a multi-sensing sensor package for reducing acoustic impedance interface mismatches, such as for example between a sensor, such an FBG sensor  111  for example, and a measurement surface of a host structure. Sensor patch system  810  may further be for enhanced strain transfer. Different ends of jacketed fiber optic cable  711  may be connected to respective fiber optic connectors  730 . 
     In sensor patch system  810 , a combined strain and AE FBG sensor in a fiber optic cable  711  core is located in a channel or a bore  811  of a body housing  812  of patch  715 , and outside of such channel or bore  811 , fiber optic cable  711  may be a core optical fiber jacketed in an outer sheath  815 . For insertion into channel or bore  811 , a sheath  815  may optionally have at an end thereof a hollow frustoconical portion or tip  813  followed by a hollow tube  817 , as parts of a fiber optic connector. Tube  817  and frustoconical  813  tip may be coupled to one another to protect and allow passage of fiber optic cable  711 . Sheath  815 , frustoconical tip  813 , and/or tube  817  may be formed of a material to resist one or more of various chemicals, ions, and/or moisture. 
     Fiber optic cable  711  may be put under tension and may be sandwiched between a soft, flexible protective housing  812  and a thin acoustic interface layer (“shim”)  814 . Housing may be formed of a deformable plastic or rubber. 
     Another acoustic interface layer, such as gel  718 , may be applied to couple shim  814  to a surface of a structure to be monitored. Shim  814  may cap a channel or a bore in housing  812 . Shim  814  may be a sheet or a laminate, of any of a variety of shapes and/or materials, used as an interface layer or layers for contact with a monitored surface and for acoustic impedance coupling, as described below in additional detail. 
     Protective housing  812  may provide support and maintain tension for a fiber optic sensor of fiber optic cable  711 , such as during sensor alignment. Moreover, tension may be maintained on a fiber optic sensor, including support thereof, by way of protective housing  812  such as for shim  814  to housing  812  bonding, or other assembly operation. Protective housing  812  may be formed of a vulcanized rubber or rubber-like material. 
     Sensor patch system  810  may be hermetically sealed by bonding of shim  814  to housing  812  with fiber optic cable  711  installed. An upper surface of housing  812  may be bonded to a lower surface of shim  814  with fiber optic cable  711  already installed in a channel or bore  811  defined in housing  812 . Optionally, this may create a ridge  816  along an upper surface  823  of shim  814 . However, in another example, bonding of shim  814  to housing  812  may be unaffected by an installed fiber optic cable  711 , and thus shim  814  may have an upper surface  823  without a ridge  816  contour. 
     Shim  814  may provide mechanical support and facilitate enhanced strain transfer and stress wave coupling for when sensor patch system  810  is bonded to a surface of a structure being monitored for strain for example. Along those lines, upper surface  823  of shim  814  may be put in direct contact with or directly bonded to a surface of a host structure being monitored, as described elsewhere herein such as using one or more FBG sensors  111 . In this example, outer perimeters of shim  814  and housing  812  are generally the same. However, in other examples, these outer perimeters may be different. 
     Depending on surface material of a surface of a host structure being monitored, material of shim  814  can be selected so that acoustic impedance of such shim material falls between for example acoustic impedance of glass of an FBG sensor and acoustic impedance of material of a surface of a host structure. This selection of shim  814  material may be used to reduce impedance mismatch between one or more interface layers, including between FBG sensors  111  and a host structure surface coupled in near proximity thereto, for coupling of sensor patch system  810  to a surface of a host structure. 
     Optionally, in addition to selecting an intermediate acoustic impedance material, thickness of shim  814  may be substantially thinner than thickness of housing  812 . A thinner shim may be used to prevent mode conversion which may lead to wave energy coupling loss at a surface boundary of a host structure. However, shim  814  may be sufficiently thick so as to maintain sufficient mechanical integrity to support performance of an FBG sensor, in accordance with the description herein. Generally, thickness of shim  814  may be between 50 microns to 200 microns. 
     Sensor patch system  810  may provide superior AE wave energy coupling and/or strain transfer as compared to conventional commercially available fiber optic strain sensors. Generally, conventional FO strain sensor packages are designed only for enhanced strain transfer, and thus generally have a thicker substrate than 200 microns and suffer from significant acoustic impedance mismatch between interface layers surrounding a sensing area. 
       FIG. 9-1  is a top-down view depicting another exemplary sensor patch system  810 A.  FIG. 9-2  is a side view of sensor patch system  810 A of  FIG. 9-1 , and  FIG. 9-3  is a bottom-down view of sensor patch system  810 A of  FIG. 9-1 .  FIG. 9-4  is a top-down perspective view of  FIG. 9-1 , and  FIG. 9-5  is an enlarged view of a portion of  FIG. 9-4 . With simultaneous reference to  FIGS. 7-1 through 7-3, and 9-1 through 9-5 , sensor patch system  810 A is further described. As sensor patch systems  810  and  810 A are similar, generally only differences are described below for purposes of clarity and not limitation. 
     Sensor patch system  810 A may be for an AE and/or temperature FO sensor system. Shim  814 , as before, may be used as an acoustic impedance matching layer between an FOAE sensor, such as an FBG sensor  111  of a fiber optic cable  711 , and a host structure surface. An acoustic impedance matching layer provided by shim  814  may be used to reduce stress wave coupling loss at an interface between a fiber optic sensor and a measurement surface. By “matching,” as used herein it is generally meant to be closer to acoustic impedance with addition of an acoustic impedance layer than without addition of same. Thus, matching may, but does not require, achieving an exact acoustic impedance match. 
     For simultaneous AE and temperature measurement by sensor patch system  810 A, a rigid housing  812 A, such as may be made of stainless steel or harder material, replaces a flexible housing  812 . Additionally, perimeters of housing  812 A and shim  814  are different, as shim  814  is wider than housing  812 A. Moreover, an FBG sensor  111  of fiber optic cable  711 , may be under zero tension, and such shim  814  may have a portion of a surface thereof bonded to a corresponding surface of housing  812 A. To maintain such absence of tension, openings, such as opening  818 , of a bore or channel  811 , which in this example is a channel, may have retention teeth  822  extending inwardly with respect to such channel or bore  811  at respective openings  818  thereof. In this example, such teeth  822  are formed as part of housing  812 A; however, in another example a rubber or plastic retention ring may be used rather than teeth, such as may be provided for example with feed tube  817 . 
     This bonding of shim  814  to housing  812 A may use shear wave coupling gel in a grating area  713  and stronger epoxy bonding outside such grating area  713 . A rigid housing  812 A may provide thermal conduction from ambient temperature to a fiber optic sensor while isolating such fiber optic sensor from mechanical strain. Using a shear wave couplant gel may be to facilitate effective stress wave coupling from a surface of a host structure to a fiber optic sensor of fiber optic cable  711 . 
     While a strand of a fiber optic cable  711  may have multiple FBG sensors, these sensors may be spaced apart from one another and may otherwise consume some lateral distances. In order to have multiple FBG sensors installed in a patch, without lengthening such patch, more than one fiber optic cable may pass through such patch, as described below in additional detail with reference to  FIG. 11 , and/or a patch may have a fiber optic cable serpentine routed through it using two or more channels or bores  811 . 
       FIG. 10-1  is a top-down view depicting another exemplary sensor patch system  810 .  FIG. 10-2  is a side view of sensor patch system  810  of  FIG. 10-1 , and  FIG. 10-3  is a bottom-down view of sensor patch system  810  of  FIG. 10-1 . With simultaneous reference to  FIGS. 7-1 through 7-3, 8-1, 8-2, and 10-1 through 10-3 , sensor patch system  810  is further described. As sensor patch systems  810  of  FIGS. 8-1 and 10-1  are similar, generally only differences are described below for purposes of clarity and not limitation. 
     Sensor patch system  810  of  FIG. 10-1  may be used for multiplexing two or more FBG sensors  111  on the same patch  715 . FBG sensors  111 - 1  through  111 - 3  with different Bragg wavelengths may be installed in the same patch  715  and can be multiplexed in a serial or parallel. For a parallel configuration, an optical demux and an optical mux can be used on both ends of sensor patch system  810  to combine multiple fibers into a single fiber without reducing optical power. Two or more different types of fiber optic sensors (e.g., AE/strain and AE/temperature) of FBG sensors  111 - 1  through  111 - 3  can be multiplexed on the same sensor patch system  810  for multi-sensing. 
     In this example, fiber optic cable  711  has optional feed tubes  817  for insertion respectively into corresponding ends of channels  811 . Feed tubes  817  may abut sides of channels  811  to provide a friction hold for maintaining a slight tension, or no tension, on FBG sensors  111  of fiber optic cable  711 . Fiber optic cable  711  serpentines from one channel  811  to the next channel  811  in housing  812 . 
     Perimeters of shim  814  and housing  812  may be co-terminus. Having a wide housing  812  allows for more than one channel  811  defined in housing  812 . While parallel channels  811  are depicted, channels  811  need not be parallel with respect to one another. Furthermore, though bends  824  in fiber optic cable  711  are external to housing  812  in this example, in another example one or more of bends  824  may be internal with respect to housing  812 . Radius of curvature of bends  824  may be limited so as not to damage optical fiber  827  of fiber optic cable  711 . 
       FIG. 11  is a bottom-down view depicting another exemplary sensor patch system  810 A. With simultaneous reference to  FIGS. 7-1 through 7-3, 9-1 through 9-5 , and  11 , sensor patch system  810 A is further described. As sensor patch systems  810 A of  FIGS. 9-1 and 11  are similar, generally only differences are described below for purposes of clarity and not limitation. 
     Sensor patch system  810 A of  FIG. 11  may be used for multiplexing two or more FBG sensors  111  on the same patch  715  or for having two or more separate FBG sensors  111  on the same patch  715 . FBG sensors  111 - 1  and  111 - 2  with different or same Bragg wavelengths respectively of optical fibers  827  may be installed in the same patch  715 . These FBG sensors  111 - 1  and  111 - 2  can be multiplexed in serial or parallel. For a parallel configuration, an optical demux and an optical mux can be used on both ends of sensor patch system  810 A to combine multiple fibers into a single fiber without reducing optical power. Two different types of fiber optic sensors (e.g., AE/strain and AE/temperature) of FBG sensors  111 - 1  and  111 - 2  can be multiplexed on the same sensor patch system  810 A for multi-sensing. 
     In this example, fiber optic cables  711  have optional feed tubes  817  for insertion respectively into corresponding ends of channels  811 . Feed tubes  817  may abut sides of channels  811  to provide a friction hold for maintaining a slight tension, or no tension, on FBG sensors  111  of fiber optic cables  711 . 
     Perimeters of shim  814  and housing  812 A may be co-terminus at ends; however, in this example width of shim  814  is wider than housing  812 A, and so perimeters of shim  814  and housing  812 A are not co-terminus lengthwise along sides of patch  715 . Having a slightly wider housing  812 A allows for more than one channel  811  defined in housing  812 A. While parallel channels  811  are depicted, channels  811  need not be parallel with respect to one another. 
       FIG. 12  is a perspective view depicting an example of a sensor cover  900 . Because fiber optic connectors  730  of  FIG. 7-1  for example may be bulky, such connectors may be attached to fiber optic cable  711  after bonding of such fiber. However, if a fiber optic cable sensor breaks during attachment of such connectors, a fiber optic cable  711  and a package thereof may be discarded. To reduce likelihood of such failures, a sensor cover  900  may be used. Sensor cover  900  may be positioned over a grating area or areas of a fiber optic cable  711  to protect one or more FBG sensors thereof. 
     Sensor cover  900  may be one integral piece of material having four bends or corners. A lowermost side flap portion  903  may be connected to a tall wall  906 , and an uppermost side flap portion  902  may be connected to a short wall  904 . Walls  904  and  906  are connected to a cover portion  905  at opposite sides. A difference in height between walls  904  and  906  provides a gap  901  for side entry and exit to and from a covered volume  907 . Gap  901 , which may be an elevational difference between side flap portions  902  and  903 , may be greater than 150 microns tall, namely at least enough to allow a fiber optic cable to pass through. 
     Sensor cover  900  facilitates a simple low cost sensor bonding strategy for high volume manufacturing by providing a patch or base with an integrated sensor cover  900  with unequal side walls. Next, a fabricated fiber optic cable with one or more FBG sensors and fiber optic connectors for a predetermined length can be inserted under sensor cover  900  including into covered volume  907  through gap  901 . 
     Such fiber optic cable may then be bonded to such base or patch at appropriate ends outside a grating area under a predetermined fiber tension. Lastly, an epoxy can be applied to seal gap  901  between an underside of side flap portion  902  with such base or patch with a fiber optic cable and sensor installed. For strain and AE sensing, a flexible thin base may be used for efficient strain, temperature, and AE transfer. For temperature sensing, a rigid thick base with good thermal conductivity may be used for efficient thermal transfer. 
       FIGS. 13-1 through 13-7  are block diagrams depicting respective examples of FOVC system  100  of  FIG. 1  though with different examples of tunable light sources  999  instead of a DFB laser  112 . For purposes of clarity and not limitation, much of the description regarding FOVC system  100  is not repeated. Furthermore, optionally an analog output  502  of  FIG. 5  may be provided to an optional AE data acquisition system  130  in order to trigger operation of such a system for receiving and processing AE data. 
     With reference to  FIG. 13-1 , a broadband light source  910  and a tunable optical filter (“tunable filter”)  912  provide a tunable light source  999 , which replaces DFB laser  112  in FO voltage conditioner  110 . Broadband light source  910  produces a broadband light beam  911  which is provided to an input port of tunable filter  912 , and tunable filter  912  filters such a broadband light beam  911  into a narrowband light beam  913  for input to optical circulator  113 . An example of a broadband light source  910  that may be used is a superluminescent-light-emitting diode (SLD or SLED). 
     Tunable filter  912  may be tuned by a controller  914 . Controller  914  may include voltage/current controller  116  and TEC controller  115 . As previously described, PID CUR and TEC error signals from PID control  107  may be correspondingly added to CUR and TEC set voltages  121  by adder  122 . Respective sums  108  output from adder  122  may be fed into a controller  914  for adjustment of control information provided to tunable filter  912 . In this example, for purposes of clarity and not limitation controller  914  is broken out into two controllers or modules, namely a current controller  116  and a TEC controller  115 . Current controller  116  and TEC controller  115  may each receive sums  108  for providing current control signaling  908  and TEC control signaling  909 , respectively to tunable filter  912 . In response to such control signaling feedback, tunable filter  912  may tune broadband light beam  911  down to a narrowband light beam  913  which includes a Bragg wavelength for FBG  111 . Because DFB laser  112  is omitted, a laser controller  153  may likewise be omitted. 
     With reference to  FIG. 13-2 , in this example tunable light source  999  includes a gain medium  920 , a tunable filter  912 , and an N-to-1 optical coupler  921 . An example of a gain medium  920  that may be used is a semiconductor optical amplifier (SOA). In an SOA, such a gain medium is a semiconductor. A broadband incoming optical signal  919  to gain medium  920  may be amplified for a particular wavelength range to provide a broadband light beam  911 G to tunable filter  912 . Tunable filter  912 , as well as controller  914 , are as previously described with reference to  FIG. 13-1 , and thus such description is not repeated. Incoming optical signal  919  may be provided from a broadband LED (not shown in this figure) as a light source. 
     However, tuned optical output  922  of tunable filter  912  is not narrowband light beam  913  in this example. Rather, tuned optical output  922  is provided to an N-to-1 optical coupler  921 , and output of optical coupler  921  is narrowband light beam  913 , which may be provided to optical circulator  113 . 
     In this example, optical coupler  921  is a 99-to-1 optical coupler; however, other optical coupling ratios may be used in other examples. However, generally optical feedback signal  923  is orders of magnitude greater than a tuned optical output  922 . Accordingly, in this example, approximately 99% of tuned optical output  922  is fed back as optical feedback signal  923  to gain medium  920 , and approximately 1% of tuned optical output  922  is fed forward as narrowband light beam  913  to optical circulator  113 . 
     With reference to  FIG. 13-3 , another example of a tunable light source  999  in an FO voltage conditioner  110  is depicted. In this example, a tunable light source  999  includes a broadband light source  910 , an optical circulator  925  and a tunable reflection filter  912 R. Broadband light source  910  and controller  914  are as previously described with reference to  FIG. 13-1 , and so such description is not repeated. 
     A broadband light beam  911  from broadband light source  910  is provided to an input port of optical circulator  925 . Another input port of optical circulator  925  is coupled for optical communication with tunable reflection filter  912 R. Along those lines, tunable reflection filter  912 R reflects back into optical circulator  925  a tuned narrowband of broadband light beam  911 . This tuned narrowband light is output from an output port of optical circulator  925  as a narrowband light beam  913 , such as to include a Bragg wavelength for FBG  111 , for optical circulator  113 . 
     With reference to  FIG. 13-4 , another example of a tunable light source  999  in an FO voltage conditioner  110  is depicted. In this example, a tunable light source  999  includes a gain medium  920 , an optical circulator  925  and a tunable reflection filter  912 R. Gain medium  920  and controller  914  are as previously described with reference to  FIG. 13-2 , and so such description is not repeated. 
     A broadband light beam  911  from gain medium  920  is provided to an input port of optical circulator  925 . Another input port of optical circulator  925  is coupled for optical communication with tunable reflection filter  912 R. Along those lines, tunable reflection filter  912 R reflects back into optical circulator  925  a tuned narrowband of broadband light beam  911 . This tuned narrowband light is output from an output port of optical circulator  925  as a narrowband light beam  913 , such as to include a Bragg wavelength for FBG  111 , for optical circulator  113 . 
     With reference to  FIG. 13-5 , another example of a tunable light source  999  in an FO voltage conditioner  110  is depicted. In this example, tunable light source  999  includes a broadband light source  910 , an optical demultiplexer (“demux”)  928 , a first tunable filter  912 - 1 , a second tunable filter  912 - 2 , and an optical multiplexer (“mux”)  929 . 
     Tunable filter  912 - 1  is coupled to a first controller  914 - 1 , and tunable filter  912 - 2  is coupled to a second controller  914 - 2 . Controllers  914 - 1  and  914 - 2  are separate instances of controller  914 , as previously described. Furthermore, tunable filters  912 - 1  and  912 - 2  are separate instances of tunable filter  912 , as previously described. 
     In this example, broadband light beam  911  from broadband light source  910  is provided as an input to optical demux  928 . Optical demux  928  provides broadband light beam  911  to either tunable filter  912 - 1  as broadband light beam  931  or to tunable filter  912 - 2  as broadband light beam  933 . Optical demux  928  may receive a select signal  930  to select as between providing broadband light beam  911  to either tunable filter  912 - 1  or  912 - 2 . 
     Responsive to receipt of a broadband light beam  911  via light signal  931  or  933 , output of tunable filter  912 - 1  is a first wavelength light signal  932 , and output of tunable filter  912 - 2  is a second wavelength light signal  934 . First and second wavelengths of light signals  932  and  934  are different from one another. Light signals  932  and  934  are provided as inputs to optical mux  929 . Light signal  932  or  934  may be selected for output from optical mux  929  responsive to select signal  930 . Output from optical mux  929  may be narrowband light beam  913 . 
     With reference to  FIG. 13-6 , another example of a tunable light source  999  in an FO voltage conditioner  110  is depicted. In this example, tunable light source  999  includes a gain medium  920 , an optical circulator  925 , a first tunable reflection filter  912 R- 1 , a second tunable reflection filter  912 R- 2 , and an optical coupler  921 . 
     Tunable reflection filter  912 R- 1  is coupled to a first controller  914 - 1 , and tunable reflection filter  912 R- 2  is coupled to a second controller  914 - 2 . Controllers  914 - 1  and  914 - 2  are separate instances of controller  914 , as previously described. Furthermore, tunable reflection filters  912 R- 1  and  912 R- 2  are separate instances of tunable reflection filter  912 R, as previously described. 
     In this example, broadband light beam  911  from gain medium  920  is provided as an input to optical circulator  925 . Optical circulator  925  provides an amplified light beam  926  to N:1 optical coupler  921 . A feedback output of optical coupler  921  is provided to gain medium  920 , as previously described. A feed forward output of optical coupler  921  is provided as a narrowband light beam  913  to optical circulator  113 , as previously described. 
     Tunable reflection filter  912 R- 1  is coupled for optical communication to optical circulator  925  to receive a broadband light signal to reflect back a first wavelength light signal  932 . Tunable reflection filter  912 R- 2  is coupled for optical communication to tunable reflection filter  912 R- 2  to receive a broadband light signal to reflect back a second wavelength light signal  934 . Second wavelength light signal  934  is provided to optical circulator  925  through tunable reflection filter  912 R- 1 . 
     With reference to  FIG. 13-7 , another example of a tunable light source  999  in an FO voltage conditioner  110  is depicted. In this example, tunable light source  999  includes a gain medium  920 , an optical demux  928 , a first tunable filter  912 - 1 , a second tunable filter  912 - 2 , and an optical coupler  921 . 
     Tunable filter  912 - 1  is coupled to a first controller  914 - 1 , and tunable filter  912 - 2  is coupled to a second controller  914 - 2 . Controllers  914 - 1  and  914 - 2  are separate instances of controller  914 , as previously described. Furthermore, tunable filters  912 - 1  and  912 - 2  are separate instances of tunable filter  912 , as previously described. 
     In this example, broadband light beam  911  from broadband light source  910  is provided as an input to optical demux  928 . Optical demux  928  provides broadband light beam  911  to either tunable filter  912 - 1  as broadband light beam  931  or tunable filter  912 - 2  as broadband light beam  933 . Optical demux  928  may receive a select signal  930  to select as between providing broadband light beam  911  to either tunable filter  912 - 1  or  912 - 2 . 
     Responsive to receipt of a broadband light beam  911  via light signal  931  or  933 , output of tunable filter  912 - 1  is a first wavelength light signal  932 , and output of tunable filter  912 - 2  is a second wavelength light signal  934 . First and second wavelengths of light signals  932  and  934  are different from one another. Light signals  932  and  934  are provided as inputs to optical mux  929 . Light signal  932  or  934  may be selected for output from optical mux  929  responsive to select signal  930 . Output from optical mux  929  may be a light beam  938  provided as an input to N: 1  coupler  921 . A feedback output of optical coupler  921  is provided to gain medium  920 , as previously described. A feed forward output of optical coupler  921  is provided as a narrowband light beam  913  to optical circulator  113 , as previously described. 
     The above description relates to monitoring using sensors. More particularly, the following description relates to monitoring using fiber optic sensors for sensor integration. Providing real-time sensor monitoring to predict and/or detect status of a monitored device or structure may be useful in many applications. Structures, including without limitation pipes, may be monitored for corroding, cracking, and/or other structural defects or degradations. With respect to pipes, AE monitoring may be used for leak detection. 
     Those of skill in the art would understand that information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof. 
     The foregoing method descriptions and the process flow diagrams are provided merely as illustrative examples and are not intended to require or imply that the steps of the various embodiments must be performed in the order presented. As will be appreciated by one of skill in the art the order of steps in the foregoing embodiments may be performed in any order. Words such as “thereafter,” “then,” “next,” etc. are not intended to limit the order of the steps; these words are simply used to guide the reader through the description of the methods. Further, any reference to claim elements in the singular, for example, using the articles “a,” “an” or “the” is not to be construed as limiting the element to the singular. 
     The various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope hereof. 
     The hardware used to implement the various illustrative logics, logical blocks, modules, and circuits described in connection with the aspects disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but, in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. Alternatively, some steps or methods may be performed by circuitry that is specific to a given function. 
     In one or more exemplary aspects, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium. The steps of a method or algorithm disclosed herein may be embodied in a processor-executable software module executed which may reside on a non-transitory computer-readable medium. Non-transitory computer-readable media includes any available media that may be accessed by a computer. By way of example, and not limitation, such non-transitory computer-readable media may comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that may be used to carry or store desired program code in the form of instructions or data structures and that may be accessed by a computer. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk, and Blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. Additionally, the operations of a method or algorithm may reside as one or any combination or set of codes and/or instructions on a non-transitory processor-readable readable medium and/or non-transitory computer-readable medium, which may be incorporated into a computer program product. 
     The preceding description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the following claims and the principles and novel features disclosed herein. Thus, while the foregoing describes exemplary embodiment(s) in accordance with one or more aspects of the invention, other and further embodiment(s) in accordance with the one or more aspects of the invention may be devised without departing from the scope thereof, which is determined by the claim(s) that follow and equivalents thereof. Any trademarks are the property of their respective owners.