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.
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.
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.
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:
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,
A second wavelength (λ2) is stripped off the λ2-λn wavelength optical signal by a wavelength tap 2. This λ2 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 (λ3) is stripped off the λ3-λn wavelength optical signal by wavelength tap 3. This λ3 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 λn 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 104a2 back to the return optics (e.g., wavelength demultiplexing optics and multiple interrogation sub-systems) for signal processing.
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 (302a1 through 302an) and variable optical attenuators (304a1 through 304an), 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 λ1-λn) is ready for transmission to optical sensor array 106 as shown in
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
OADM2 716 strips wavelength 2 (λ2) from the optical signal, and the optical signal 728 carrying the remaining wavelengths (λ3 . . . λn) is next transmitted to subsequent OADMs (not shown). Referring back to the wavelength 2 (λ2) 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 (λ2). The output signals from each of sensors 720, 722 are in turn directed along distinct optical fibers to respective ones of OADM2 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 OADM2 726 is combined with the output of sensor 710 at OADM1 714, and the output of OADM1 714 is transmitted along an optical fiber 703 to return optics of fiber optic sensing system 700. Similarly, the combined output optical signal from OADM2 724 is combined with the output of sensor 708 at OADM1 712, and the output of OADM1 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
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.
OADM2 816 strips wavelength 2 (λ2) from the optical signal, and the optical signal 824 carrying the remaining wavelengths (λ3 . . . λn) is transmitted to subsequent OADMs (not shown). Referring back to the wavelength 2 (λ2) signal, this signal is directed toward a sensor 820 for sensing acceleration along the z-axis at wavelength 2 (λ2). The output signal from sensor 820 is directed to OADM2 828 (i.e., wavelength combiner 828). An OADM2 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 OADM2 828 is combined with the output of sensor 810 at OADM1 814, and the output of OADM1 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 OADM1 812, and the output of OADM1 812 is transmitted along an optical fiber 805 to return optics of fiber optic sensing system 800.
The optical signal carrying the remaining wavelengths (λ2 . . . λn) 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 (λ2) 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 (λ3 . . . λn) is next transmitted to an optical coupler 1120, and an optical signal 1122 carrying the remaining wavelengths (λ4 . . . λn) 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
The optical signal carrying the remaining wavelengths (λ2 . . . λn) 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 (λ2) 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 (λ3 . . . λn) is next transmitted to an optical coupler 1216, and an optical signal 1218 carrying the remaining wavelengths (λ4 . . . λn) 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.
The optical signal carrying the remaining wavelengths (λ2 . . . λn) is transmitted to an OADM2 1314. OADM2 1314 (a wavelength tap) strips off wavelength 2 (λ2) 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 OADM2 1338 to be recombined with the return optical signal from an OADM3 1340.
The optical signal carrying the remaining wavelengths (λ3 . . . λn) is next transmitted to an OADM3 1324. OADM3 1324 (a wavelength tap) strips off wavelength 3 (λ3) 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 OADM3 1340 to be coherently recombined with a return optical signal 1342 (e.g., from downstream OADMs). An optical signal 1334 carrying the remaining wavelengths (λ4 . . . λn) 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 OADM1 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.
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.
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/US2013/039498 | 5/3/2013 | WO | 00 |
Number | Date | Country | |
---|---|---|---|
61642760 | May 2012 | US |