Patent Publication Number: US-2015086206-A1

Title: Fiber optic sensing systems and methods of operating the same

Description:
RELATED APPLICATION 
     This application claims the benefit of priority to U.S. Provisional Patent Application Ser. No. 61/642,760, filed on May 4, 2012, the contents of which are incorporated in this application by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to fiber optic sensing systems and, more particularly, to fiber optic sensing systems having particular applicability in high-temperature environments and methods of using the same. 
     BACKGROUND OF THE INVENTION 
     Electronic geophone-based fiber optic sensing systems are well known in the art and have use in a number of applications. For example, fiber optic sensing systems and those including electronic geophones are used to image subsurface structures using vertical seismic profiling and microseismic events during operations such as hydrofracturing for the production of oil, natural gas, and geothermal energy (e.g., enhanced geothermal recovery). Traditional electronic and even some optical tools suffer, however, from the inability to withstand extended periods of time at elevated temperatures above 100° C. 
     Thus, it would be desirable to provide improved fiber optic sensing systems for use in applications with elevated temperatures. 
     BRIEF SUMMARY OF THE INVENTION 
     According to an exemplary embodiment of the present invention, a fiber optic sensing system is provided. The fiber optic sensing system includes an optical source adapted to provide an optical signal at a plurality of wavelengths. The fiber optic sensing system also includes a plurality of wavelength taps for separating the optical signal into signal portions at each of the plurality of wavelengths. The fiber optic sensing system further includes a plurality of optical sensors, each of the optical sensors configured to receive one of the signal portions at a respective one of the plurality of wavelengths. The fiber optic sensing system still further includes a plurality of wavelength combiners for combining signal portions from the plurality of optical sensors (e.g., returned signal portions) into a recombined (e.g., multi-wavelength) signal. Also included in the fiber optic sensing system is an optical receiver (or a plurality of optical receivers) for receiving the recombined signal. One or more optical fiber paths are included in the fiber optic sensing system between the optical source and the optical receiver. 
     The optical receiver (which may be considered return optics) may include optics for separating the multi-wavelength signal received by the optical receiver into individual signals, each at a different wavelength. The optical receiver also may include signal processing for converting the individual signal portions into electronic signals proportional to the physical quantities measured at each of the sensors (e.g., acceleration, velocity, dynamic pressure changes, etc.). 
     According to another exemplary embodiment of the present invention, a method of operating a fiber optic sensing system is provided. The method includes the steps of: (a) transmitting an optical signal from an optical source such that the optical signal includes a plurality of wavelengths; (b) separating the optical signal into signal portions at each of the plurality of wavelengths using a plurality of wavelengths taps; (c) receiving ones of the signal portions at respective ones of a plurality of optical sensors; (d) combining signal portions from the plurality of optical sensors into a recombined signal using a plurality of wavelength combiners; and (e) receiving the recombined signal at an optical receiver. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures: 
         FIG. 1  is a block diagram of a fiber optic sensing system in accordance with an exemplary embodiment of the present invention; 
         FIG. 2  is a block diagram of a fiber optic sensing system in a ladder style configuration in accordance with an exemplary embodiment of the present invention; 
         FIG. 3  is a block diagram of a multiplexed optical source in accordance with an exemplary embodiment of the present invention; 
         FIG. 4  is a block diagram of a Michelson interferometer sensor in accordance with an exemplary embodiment of the present invention; 
         FIG. 5  is a block diagram of a Mach-Zehnder interferometer sensor in accordance with an exemplary embodiment of the present invention; 
         FIG. 6  is a block diagram of a fiber optic accelerometer that may be used in a fiber optic sensing system in accordance with an exemplary embodiment of the present invention; 
         FIG. 7  is a block diagram of a multiplexing configuration of a fiber optic sensing system in accordance with an exemplary embodiment of the present invention; 
         FIG. 8  is a block diagram illustrating a three-sensor module of a fiber optic sensing system in accordance with an exemplary embodiment of the present invention; 
         FIG. 9  is a block diagram illustrating a three-axis module of a fiber optic sensing system in accordance with an exemplary embodiment of the present invention; 
         FIG. 10  is a block diagram illustrating an input signal and return signal wavelength division configuration in accordance with an exemplary embodiment of the present invention; and 
         FIGS. 11-13  are block diagrams illustrating various multiplexing configurations for fiber optic sensing systems in accordance with various exemplary embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As will be explained in greater detail below, according to certain exemplary embodiments of the present invention, improved fiber optic sensing systems are provided that incorporate a sequence of wavelength taps to separate an optical signal into a plurality of signal portions (e.g., narrow line width output signal portions). For example, the wavelength taps are used to strip off individual wavelengths of an optical signal (e.g., wavelength division multiplexing), where each individual wavelength may be used for sensing in an optical sensor (e.g., to sense acceleration to which the optical sensor is subject) (e.g., where the optical sensor may include a fiber optic accelerometer, a transducer, etc.). In one specific example, the optical sensor may be a fiber optic hydrophone (e.g., including an optical fiber coated with a voided elastomer). 
     Multiple sensors may be included at each sensing location for sensing in different directions (e.g., along different axes). The optical signals transmitted to the individual sensors are then recombined using wavelength combiners, and the recombined signal is then transmitted back to the return optics (e.g., to an optical receiver) for demultiplexing into different wavelengths, and signal processing as is known to those skilled in the art. Exemplary wavelength taps and combiners include OADMs (i.e., optical add/drop multiplexers), slope filters, wavelength discriminators, wavelength demultiplexers, and Wavelength Division Multiplexers (WDMs). Various optical sensor configurations may be utilized including but not limited to Michelson interferometer optical sensors, Fabry-Perot interferometer optical sensors, Fiber Bragg Grating (FBG)-based optical sensors, Sagnac interferometer optical sensors, etc. Transducers may also be incorporated into the optical sensors. Exemplary transducers include: a fixed portion (e.g., a fixed mandrel) fixed to a body of interest; a moving portion (a moveable mandrel) that can move with respect to the fixed portion; a spring member disposed between the fixed portion and the moveable portion; and an optical fiber wrapped between the fixed portion and the moveable portion to sense strain in the optical fiber that is proportional to a measurable quantity (e.g., a physical quantity such as acceleration, pressure, etc.). This strain is converted to a change in phase of the light passing through the optical fiber. An interferometer which incorporates the transducer in the sensor converts the change in phase to a change in light/optical intensity. The signal representing this change in optical intensity may be converted to an electrical output signal (e.g., an analog signal, a digital signal) proportional to the original measurable quantity at the interrogation or signal processing electronics. 
     Certain aspects of the present invention provide for fiber optic sensing systems having particular applicability in high-temperature environments (e.g., where the optical sensors included in an optical sensor array may withstand extended periods of time at temperatures exceeding 200° C. without failing or experiencing performance degradation). The electronics (e.g., source optics including a multi-wavelength optical source, return optics, the interrogation system, etc.) may be desirably located remote from the optical sensors to increase the lifetime of the electronics, and to allow for repair or replacement of elements of the electronics without costly retrieval of the sensors from the remote environment being sensed (e.g., a borehole). 
     In accordance with certain exemplary embodiments of the present invention, a high-efficiency fiber optic accelerometer may be utilized that uses a fraction (˜10%) of the fiber in comparison to the conventional fiber optic accelerometers. This reduces the sensitivity to darkening of the fiber due to hydrogen ingress through a pressure housing that is common with other fiber optic sensors. For extreme environments, a pure silica core optical fiber can be used for the lead and connecting cables, and for the sensor fiber, to further reduce the sensitivity of the optical fiber sensor to hydrogen darkening. 
     In accordance with certain exemplary embodiments of the present invention, separate source and return optical fibers are used to minimize potential coherent Rayleigh backscatter noise as it is desirable that return optical signals from the sensors do not interfere with the optical source power. Noise due to coherent Rayleigh backscatter may be particularly problematic when low power optical returns from sensors interfere with high source optical power at locations in a lead cable (e.g., at the many locations where there are small changes in the refractive index of the fiber). 
     Referring now to the drawing, in which like reference numbers refer to like elements throughout the various figures that comprise the drawing,  FIG. 1  illustrates a fiber optic sensing system  100  including an optical source  102  configured to provide an optical signal at a plurality of wavelengths. For example, optical source  102  may include light at wavelengths from a plurality of optical light sources (e.g., lasers, etc.) multiplexed onto one or more optical fibers within a fiber optic cable  104 . The optical signal (including light at a plurality of wavelengths) is directed through an optical sensor array  106 , where optical sensors in array  106  are designed to be supported by individual wavelengths included in the optical signal. Return signals from the sensors in array  106  are carried by optical fibers within fiber optic cable  104  that may be separate from (as shown in  FIG. 1 ), or the same as, the optical fibers carrying the multiplexed optical signal from optical source  102 . The return signals (e.g., in the form of a recombined signal) from the sensors in array  106  are separated from one another by wavelength demultiplexing optics  108  and are then directed toward one or more interrogation sub-system(s)  110 . Interrogation sub-system  110  analyzes the return signals using conventional interrogation techniques. 
       FIG. 2  illustrates an exemplary optical sensor array  106   a . In  FIG. 2 , fiber optic cable  104   a  is an example of fiber optic cable  104  from  FIG. 1 , and optical sensor array  106   a  is an example of optical sensor array  106  from  FIG. 1 . The “ladder” style topology illustrated in  FIG. 2  includes an input optical fiber  104   a   1  carrying an optical signal having “n” wavelengths. A plurality of wavelength taps (e.g., optical add/drop multiplexers, or OADMs) strip off individual wavelengths from the optical signal for transmission to respective optical sensors. More specifically, a first wavelength (λ 1 ) is stripped off the n-wavelength optical signal by a wavelength tap  1 . This λ 1  signal is received by a sensor  1 , and then the output signal from sensor  1  is recombined with the other output signals from the other sensors at a wavelength combiner  1 . 
     A second wavelength (λ 2 ) is stripped off the λ 2 -λ n  wavelength optical signal by a wavelength tap  2 . This λ 2  signal is received by a sensor  2 , and then the output signal from sensor  2  is recombined with other output signals from other sensors at a wavelength combiner  2 . Likewise, a third wavelength (λ 3 ) is stripped off the λ 3 -λ n  wavelength optical signal by wavelength tap  3 . This λ 3  signal is received by a sensor  3 , and then the output signal from sensor  3  is recombined with other output signals from other sensors at wavelength combiner  3 . 
     This signal flow continues for each of the n wavelengths and corresponding optical sensors. Thus, the signal flow ends with a wavelength tap n sending the λ n  wavelength to a sensor n, which produces an output signal that is recombined with other output signals from other sensors at a wavelength combiner n. The complete recombined signal is transmitted along an output optical fiber  104   a   2  back to the return optics (e.g., wavelength demultiplexing optics and multiple interrogation sub-systems) for signal processing. 
       FIG. 3  is an example of an n-input wavelength optical source  102   a . Optical source  102   a  is an example of optical source  102  from  FIG. 1 . The output from a light source  300   a   1  (e.g., a laser light source at wavelength λ 1 ) is directed through an optical isolator  302   a   1  to avoid noise and damage due to backscattered light. The output from optical isolator  302   a   1  is then directed through a variable optical attenuator (VOA)  304   a   1 . Variable optical attenuator  304   a   1  is used to balance the outputs of the system so that sensors have outputs that are more nearly matched when received at optical receivers within the interrogation sub-system  110 . 
     Similarly, output light from each of the n light sources are directed through respective optical isolators and variable optical attenuators. The outputs from the n light sources (e.g., laser  1  through laser n), after transmission through respective optical isolators ( 302   a   1  through  302   a   n ) and variable optical attenuators ( 304   a   1  through  304   a   n ), are combined in a DWDM  306  (i.e., a dense wavelength division multiplexer DWDM, cascaded OADMs—one at each of the laser wavelengths, an arrayed waveguide device (AWG), or other multiplexing device), and may then be directed to a phase modulator  310  (after passing through an optical circulator  308 ). After being reflected by a reflector  312 , the modulated signal (now including a phase encoded carrier signal) is boosted to a desired input power level by an optical amplifier  314 . The now combined, modulated, and amplified, optical signal  316  (including wavelengths λ 1 -λ n ) is ready for transmission to optical sensor array  106  as shown in  FIG. 1 . 
       FIG. 4  illustrates a Michelson interferometer sensor  400 . Such a sensor  400  is one of many possible configurations of sensors included in optical sensor array  106  shown in  FIG. 1 . An input light beam  402  (light at a specific wavelength stripped off the multi-wavelength optical signal from optical source  102  shown in  FIG. 1 ) is split into two light beams at a fiber optic coupler  404 . The first light beam passes through a fiber leg including a transducer  406  and terminating at a reflector  408 . The second light beam passes through a fiber leg including a reference coil  410  and terminating at a reflector  412 . Reflectors  408 ,  412  may be, for example, a mirror (e.g., a dielectric mirror, a metallic mirror, etc.), a fiber grating, a Faraday rotator mirror, etc. The optical signals that reflect back from respective reflectors  408 ,  412  are then directed back to fiber optic coupler  404 , where the optical signals that are reflected back from reflectors  408 ,  412  are combined coherently to create a time-varying intensity proportional to the time-varying relative phase change between the two fiber legs. The output of fiber optic coupler  404  is then directed along an output optical fiber  414  to the return optics (i.e., not shown, but where the return optics include wavelength demultiplexing optics and an interrogation sub-system for analyzing the optical signals returned from sensor  400  and other sensors in the sensor array). 
       FIG. 5  is a diagram of a Mach-Zehnder interferometer sensor  500 . Such a sensor  500  is one of many possible configurations that may be included in the sensors in optical sensor array  106  shown in  FIG. 1 . An input light beam  502  (light at a specific wavelength stripped off the multi-wavelength optical signal from optical source  102  shown in  FIG. 1 ) is split into two light beams at a fiber optic coupler  504 . The first light beam passes through a fiber leg including a transducer  508  and terminating at a fiber optic coupler  512 . The second light beam passes through a fiber leg including a reference coil  510  and also terminating at fiber optic coupler  512 . Thus, the two light beams are then recombined coherently at fiber optic coupler  512  to create a time-varying intensity proportional to the time-varying relative phase change between the two fiber legs. The output from fiber optic coupler  512  is then directed along an output optical fiber  514  to the return optics (i.e., not shown, but where the return optics include an interrogation sub-system for analyzing the optical signals returned from sensor  500  and other sensors in the sensor array). 
       FIG. 6  is a cross-sectional view of a fiber optic accelerometer transducer  600 . Such a transducer  600  may be included in a fiber optic sensor of an optical sensor array (such as array  106  shown in  FIG. 1 ) to maximize the sensitivity of the sensor to linear acceleration in a particular direction. Transducer  600  includes a single-piece, machined spring assembly  604  and a moveable mass  606 . Spring assembly  604  includes a fixed mandrel  604   a , spring member  604   b , and moveable mandrel  604   c . Fixed mandrel  604   a  is attached to a body  602  of interest (e.g., a pressure-sealed vessel clamped to a borehole casing, etc.). A fiber  608  is continuous with (or connected to) an optical fiber included in the respective fiber optic sensor. Fiber  608  is coiled around spring assembly  604  such that, when coiled fiber  608  is subject to acceleration, spring assembly  604  causes fiber  608  to elongate and contract in coordination and in proportion to the received linear acceleration in direction y. 
     The optical path length change caused by this elongation and contraction may then be interferometrically compared to a fiber comprising a reference coil through the action of an interferometer (e.g., a Michelson interferometer, a Mach-Zehnder interferometer, etc.) associated with transducer  600  within a sensor,  400  or  500 , for example. Output of such an interferometer may then be directed along an output optical fiber to the return optics (including an interrogation sub-system). Moveable mass  606  may be formed of a high-density material (e.g., brass, copper, tungsten, etc.) and is shaped to wrap around (or envelope) spring assembly  604 , thereby providing a high mass value within a small volume to greatly increase the sensitivity of the transducer to acceleration. Transducer  600  is one example of a transducer that may be included in fiber optic sensors in accordance with the present invention. It is understood that alternative transducer configurations are contemplated. Transducers of the type illustrated in  FIG. 6 , as well as other exemplary configurations, are described in PCT Publication WO 2011/050227. 
       FIG. 7  illustrates elements of a fiber optic sensing system  700 . In this exemplary configuration, a single input optical fiber  702  carries an input optical signal (including “n” wavelengths) transmitted from source optics of fiber optic sensing system  700 . System  700  is configured to support “n” fiber optic sensors, one sensor for each of the “n” wavelengths. Each of the “n” wavelengths is divided/stripped off the input optical fiber by a wavelength tap (e.g., an OADM element). For example, an OADM 1    704  strips wavelength 1 (λ 1 ) from the input optical signal. The optical signal carrying the remaining wavelengths (λ 2  . . . λ n ) is transmitted to an OADM 2    716 . Referring back to the wavelength 1 (λ 1 ) signal, this signal is divided by an optical coupler  706  (e.g., a 2×2 fused biconical taper coupler) and is directed toward two sensors (sensor  708  and sensor  710 ) at wavelength 1 (λ 1 ). The output signals from each of sensor  708  and sensor  710  are in turn directed along distinct optical fibers to respective ones of OADM 1    714 ,  712  (i.e., wavelength combiners  714 ,  712 ). 
     OADM 2    716  strips wavelength 2 (λ 2 ) from the optical signal, and the optical signal  728  carrying the remaining wavelengths (λ 3  . . . λ n ) is next transmitted to subsequent OADMs (not shown). Referring back to the wavelength 2 (λ 2 ) signal, this signal is divided by an optical coupler  718  (e.g., a 2×2 fused biconical taper coupler) and is directed toward two sensors (sensor  720  and sensor  722 ) at wavelength 2 (λ 2 ). The output signals from each of sensors  720 ,  722  are in turn directed along distinct optical fibers to respective ones of OADM 2    726 ,  724  (i.e., wavelength combiners  726 ,  724 ). Each of wavelength combiners  726 ,  724  also receives another optical signal for recombining from a respective one of optical fibers  730 ,  732  (e.g., from downstream wavelength combiners, not shown). The combined output optical signal from OADM 2    726  is combined with the output of sensor  710  at OADM 1    714 , and the output of OADM 1    714  is transmitted along an optical fiber  703  to return optics of fiber optic sensing system  700 . Similarly, the combined output optical signal from OADM 2    724  is combined with the output of sensor  708  at OADM 1    712 , and the output of OADM 1    712  is transmitted along an optical fiber  705  to return optics of fiber optic sensing system  700 . 
     Although details of only two wavelengths are shown in  FIG. 7 , it is clear that such a configuration could be used to support many more wavelengths. In one specific example, this architecture could be used to support  12  laser wavelengths and 24 sensors. Further, this architecture may be extended to “n” lasers on one input optical fiber with 3n optical fibers out, supporting 3n sensors. In general, one input optical fiber in with n laser wavelengths can support Mn sensors with M output optical fibers. 
     In certain fiber optic sensing applications, it may be desirable to provide sensors for sensing along multiple axes at a given location. For example, a module (e.g., a tri-axial accelerometer module) may be provided at such a location where the module houses sensors for sensing acceleration along each of an x-axis, y-axis, and z-axis.  FIG. 8  illustrates such an arrangement including elements of a fiber optic sensing system  800 . In this exemplary configuration, a single input optical fiber  802  carries an input optical signal (including “n” wavelengths) transmitted from source optics of fiber optic sensing system  800 . In  FIG. 8 , an OADM 1    804  strips wavelength 1 (λ 1 ) from the input optical signal. The optical signal carrying the remaining wavelengths (λ 2  . . . λ n ) is next transmitted to an OADM 2    816 . Referring back to the wavelength 1 (λ 1 ) signal, this signal is divided by an optical coupler  806  (e.g., a 2×2 beam splitter) and is directed toward two sensors  808 ,  810  (e.g., optical sensors  808 ,  810 ) for sensing acceleration along the x-axis and y-axis. The output signals from each of sensors  808 ,  810  are in turn directed along distinct optical fibers to respective ones of OADM 1    814 ,  812  (i.e., wavelength combiners  814 ,  812 ). 
     OADM 2  816 strips wavelength 2 (λ 2 ) from the optical signal, and the optical signal  824  carrying the remaining wavelengths (λ 3  . . . λ n ) is transmitted to subsequent OADMs (not shown). Referring back to the wavelength 2 (λ 2 ) signal, this signal is directed toward a sensor  820  for sensing acceleration along the z-axis at wavelength 2 (λ 2 ). The output signal from sensor  820  is directed to OADM 2    828  (i.e., wavelength combiner  828 ). An OADM 2    828  also receives another optical signal for recombining from an optical fiber  826  (e.g., from downstream wavelength combiners, not shown). The combined output optical signal from OADM 2    828  is combined with the output of sensor  810  at OADM 1    814 , and the output of OADM 1    814  is transmitted along an optical fiber  803  to return optics of fiber optic sensing system  800 . Similarly, the optical signal transmitted from sensor  808  is combined with another optical signal from an optical fiber  830  at OADM 1    812 , and the output of OADM 1    812  is transmitted along an optical fiber  805  to return optics of fiber optic sensing system  800 . 
       FIG. 9  illustrates elements of a fiber optic sensing system  900 . Elements of system  900  having the same reference numerals as elements of system  800  shown in  FIG. 8  (but with the first digit of such numerals being a “9” instead of “8”) are the same as those shown in, and described with respect to,  FIG. 8 . One difference of note between  FIGS. 8 and 9  is that in  FIG. 9  the output optical signal from OADM 2    916  is split at an optical coupler  918  such that half of the light is transmitted to a sensor  920  (for sensing acceleration along the z-axis), while the other half of the light is recombined with the primary optical signal at an OADM 2    922  such that this half of the light at wavelength 2 (λ 2 ) may be used in connection with sensing acceleration at a downstream sensor/interferometer. 
       FIG. 10  illustrates an exemplary use of three optical fibers  1002 ,  1004 , and  1006  of a fiber optic cable  1000  in a fiber optic sensing system in accordance with an exemplary embodiment of the present invention. The light source inputs and the sensor outputs are shared among the three optical fibers  1002 ,  1004 , and  1006 . For example, optical fiber  1002  carries an input optical signal for wavelengths λ 1  λ 2  λ 3  λ 4 , but also an output optical signal for wavelengths λ 9  λ 10  λ 11  λ 12  λ 5  λ 6  λ 7  λ 8 . This configuration has the advantage that the optical power is more evenly divided among the optical fibers, and losses associated with OADMs may be more evenly balanced as the drop and add losses are often unequal. In  FIG. 10  there are twelve input light source wavelengths and each optical fiber carries four of them. Each of the three fibers carries eight optical return signals. This topology can be extended to “m” optical fibers with an exemplary configuration being that the number of light source wavelengths is a multiple of the number of optical fibers utilized. 
       FIG. 11  illustrates elements of a fiber optic sensing system  1100  in a Michelson interferometric configuration. An input optical fiber  1102  carries an input optical signal (including “n” wavelengths) transmitted from the source optics of fiber optic sensing system  1100 . An optical coupler  1104  divides the optical signal between two fiber legs included in a sensor S1, where sensor S1 includes coupler  1104 , a reference leg (including a FBG  1108 ), and a sensing leg (including a FBG  1110 ). The optical signal at wavelength 1 (λ 1 ) reflects back to optical coupler  1104  from each of FBGs  1108 ,  1110  where the optical signals that are reflected back from FBGs  1108 ,  1110  are combined coherently to create a time-varying intensity proportional to the time-varying relative phase change between the two fiber legs. 
     The optical signal carrying the remaining wavelengths (λ 2  . . . λ n ) is next transmitted to an optical coupler  1112 . Optical coupler  1112  divides the optical signal between two fiber legs included in a sensor S2, where sensor S2 includes optical coupler  1112 , a reference leg (including a FBG  1116 ), and a sensing leg (including a FBG  1118 ). The optical signal at wavelength 2 (λ 2 ) reflects back to optical coupler  1112  where the optical signals that are reflected back from FBGs  1116 ,  1118  are combined coherently to create a time-varying intensity proportional to the time-varying relative phase change between the two fiber legs (and eventually to optical coupler  1104 ). The optical signal carrying the remaining wavelengths (λ 3  . . . λ n ) is next transmitted to an optical coupler  1120 , and an optical signal  1122  carrying the remaining wavelengths (λ 4  . . . λ n ) is transmitted downstream to subsequent sensors in the optical sensor array. All of the return optical signals from each of the sensors (i.e., sensors S1, S2, etc.) are recombined at optical coupler  1104 , and this recombined optical signal  1101  is transmitted along optical fiber  1102  to the return optics for wavelength demultiplexing, interrogation, and analysis. 
     In  FIG. 11 , the input and return optical signals are transmitted along the same optical fiber. Input and return transmission along the same optical fiber(s) may result in certain undesirable circumstances (e.g., coherent Rayleigh backscatter, etc.). In such a circumstance, it may be desirable to separate the input and return optical signals.  FIG. 12  illustrates an example of such a configuration.  FIG. 12  illustrates elements of a fiber optic sensing system  1200 . An input optical fiber  1202  carries an input optical signal (including “n” wavelengths) transmitted from the source optics of fiber optic sensing system  1200 . An optical coupler  1204  divides the optical signal between two fiber legs of a sensor S1 including coupler  1204 , a reference leg (including a FBG  1206 ), and a sensing leg (including a FBG  1208 ). The optical signal at wavelength 1 (λ 1 ) reflects back to optical coupler  1204  from each of FBGs  1206 ,  1208 , and then on to an OADM  1220  to be coherently recombined with the optical signal from an OADM  1222 . 
     The optical signal carrying the remaining wavelengths (λ 2  . . . λ n ) is next transmitted to an optical coupler  1210 . Optical coupler  1210  divides the optical signal between two fiber legs of sensor S2 including optical coupler  1210 , a reference leg (including a FBG  1212 ), and a sensing leg (including a FBG  1214 ). The optical signal at wavelength 2 (λ 2 ) reflects back to optical coupler  1210  (and on to OADM  1222 ) from each of FBGs  1212 ,  1214 , and then on to OADM  1222  to be coherently recombined with the optical signal from an OADM  1224 . The optical signal carrying the remaining wavelengths (λ 3  . . . λ n ) is next transmitted to an optical coupler  1216 , and an optical signal  1218  carrying the remaining wavelengths (λ 4  . . . λ n ) is transmitted downstream to subsequent sensors in the optical sensor array. All of the return optical signals from each of the sensors (i.e., sensors S1, S2, etc.) are recombined at respective OADMs, and are eventually recombined at OADM  1220 , and this recombined optical signal is transmitted along an optical fiber  1226  to the return optics for wavelength demultiplexing, interrogation, and analysis. 
       FIG. 13  illustrates elements of a fiber optic sensing system  1300  in a Mach-Zehnder interferometric configuration. An input optical fiber  1302  carries an input optical signal (including “n” wavelengths) transmitted from source optics of fiber optic sensing system  1300 . An OADM  1304  (a wavelength tap) strips off wavelength 1 (λ1) for transmission to an optical coupler  1306 . Input optical coupler  1306  divides the wavelength 1 optical signal between two fiber legs (i.e., a reference leg  1308  and a sensing leg  1310 ) included in a sensor S1. The output of sensor S1 is transmitted from its output optical coupler  1312 , and then on to an OADM 1    1336  to be recombined with the return optical signal from an OADM 2    1338 . 
     The optical signal carrying the remaining wavelengths (λ 2  . . . λ n ) is transmitted to an OADM 2    1314 . OADM 2    1314  (a wavelength tap) strips off wavelength 2 (λ 2 ) for transmission to an input optical coupler  1316 . Input optical coupler  1316  divides the wavelength 2 optical signal between two fiber legs (i.e., a reference leg  1318  and a sensing leg  1320 ) included in a sensor S2. The output of sensor S2 is transmitted from its optical coupler  1322 , and then on to OADM 2    1338  to be recombined with the return optical signal from an OADM 3    1340 . 
     The optical signal carrying the remaining wavelengths (λ 3  . . . λ n ) is next transmitted to an OADM 3    1324 . OADM 3    1324  (a wavelength tap) strips off wavelength 3 (λ 3 ) for transmission to an input optical coupler  1326 . Input optical coupler  1326  divides the wavelength 3 optical signal between two fiber legs (i.e., a reference leg  1328  and a sensing leg  1330 ) included in a sensor S3. The output of sensor S3 is transmitted to an optical coupler  1332 , and then on to OADM 3    1340  to be coherently recombined with a return optical signal  1342  (e.g., from downstream OADMs). An optical signal  1334  carrying the remaining wavelengths (λ 4  . . . λ n ) is transmitted downstream to subsequent sensors in the optical sensor array. All of the return optical signals from each of the sensors (i.e., sensors S1, S2, S3, etc.) are eventually recombined at OADM 1    1336 , and this recombined optical signal is transmitted along an optical fiber  1344  to the return optics for wavelength demultiplexing, interrogation, and analysis. 
     Although the wavelength taps and wavelength combiners utilized in connection with the present invention have largely been described in connection with OADMs, it is understood that different or additional optical elements (e.g., FBGs as wavelength taps, among others) may be utilized. 
     Although the present invention has particular applicability in high-temperature environments, it is understood that the invention has broad applicability in fiber optic sensing applications. 
     Although illustrated and described above with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention.