WO2015034858A1

WO2015034858A1 - Interferometric sensing systems with polarization noise reduction, and methods of operating the same

Info

Publication number: WO2015034858A1
Application number: PCT/US2014/053782
Authority: WO
Inventors: Peng Li; Eric L. Goldner; David Hall
Original assignee: US Seismic Systems, Inc.
Priority date: 2013-09-03
Filing date: 2014-09-03
Publication date: 2015-03-12

Abstract

An interferometric sensing system is provided. The interferometric sensing system includes: an optical source for providing optical signals; a fiber optic lead cable for receiving the optical signals from the optical source; an array of one or more interferometric sensors optically coupled to the fiber optic lead cable; and a polarization scrambler for scrambling a polarization state of the optical signals.

Description

INTERFEROMETRIC SENSING SYSTEMS WITH POLARIZATION NOISE REDUCTION, AND METHODS OF OPERATING THE SAME

RELATED APPLICATION

This application claims the benefit of priority to U.S. Provisional Patent Application Serial Number 61/873, 188, filed on September 3, 2013, 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 interferometric sensing systems configured to reduce polarization noise, and methods of using the same.

BACKGROUND OF THE INVENTION

Interferometric sensing systems often have long fiber optic lead cables between the interrogation optics and electronics (e.g., including optical source(s), modulation and demodulation optics and/or electronics, etc.) and one or more arrays of interferometric sensors. Such systems often utilize optical sources (e.g., highly coherent fiber lasers, external cavity lasers, etc.) which produce highly polarized light. Changes in the polarization state of the light propagating through the lead cable from the optical source to the sensors in the array can cause changes in the sensor output intensity. These changes in sensor output intensity are output from the system as noise which is indistinguishable from the signal output of interest. Such changes in the polarization state of the light within the lead cable may result, for example, from changes in temperature or mechanical perturbations of the fiber in the cable due to vibration, changing tension, bending or twisting of the cable, etc.

Thus, it would be desirable to provide improved systems and methods for overcoming one or more of the shortcomings described above with respect to conventional interferometric sensing systems. BRIEF SUMMARY OF THE INVENTION

According to an exemplary embodiment of the present invention, an interferometric sensing system is provided. The interferometric sensing system includes: an optical source for providing optical signals; a fiber optic lead cable for receiving the optical signals from the optical source; an array of one or more interferometric sensors optically coupled to the fiber optic lead cable; and a polarization scrambler for scrambling a polarization state of the optical signals.

According to another exemplary embodiment of the present invention, a method of operating an interferometric sensing system is provided. The method includes the steps of: (a) providing optical signals from an optical source; (b) scrambling a polarization state of the optical signals using a polarization scrambler; and (c) receiving the optical signals at an array of one or more interferometric sensors optically coupled to the optical source.

The present invention also includes inventive polarization scrambler configurations. For example, FIGS. 2A-2K illustrate a plurality of polarization scrambler implementations within the scope of the present invention.

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 DRAWINGS

The invention is best understood from the following detailed description when read in connection with the accompanying drawings. It is emphasized that, according to common practice, the various features of the drawings are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawings are the following figures:

FIGS. 1A-1 I are block diagrams illustrating interferometric sensing systems in accordance with various exemplary embodiments of the present invention; FIGS. 2A-2K are block diagrams illustrating polarization scramblers in accordance with various exemplary embodiments of the present invention; and

FIGS. 3A-3C are illustrations of exemplary waveforms that may be applied to a phase modulator of an interferometric sensing system in accordance with various exemplary embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with various exemplary embodiments of the present invention, inventive interferometric sensing systems are disclosed. The interferometric sensing systems reduce the average degree of polarization of the light entering a lead cable optically coupled to an interferometric sensor array. This is accomplished by scrambling the polarization state of the light rapidly compared with the sensing system sample rate. When the scrambling rate is high compared to the sensor system sampling rate (e.g., the rate that the return intensity from each sensor in the array is sampled), perturbations of the fiber within the lead cable have little effect on the time averaged polarization state. For example, when the degree of polarization of the light entering the lead cable is less than 3 degrees, an exemplary reduction in system sensitivity to lead cable noise is on the order of 30dB. In certain exemplary embodiments, the invention utilizes a rapid change in polarization between at least two polarization states.

FIG. 1A illustrates an interferometric sensing system 100. Interferometric sensing system 100 includes an optical source 102 for providing an optical signal (e.g., highly polarized light, for example, linearly polarized light) to a polarization scrambler 104 (where polarization scrambler 104 may also be referred to as a noise reduction unit). Polarization scrambler 104 continually changes the state of polarization of the light coming from optical source 102 (e.g., a laser). Polarization scrambler 104 reduces the degree of polarization of light output from optical source 102 within a sample period of an optical receiver 1 10. Light exiting polarization scrambler 104 passes through a fiber optic lead cable 106 to an interferometric sensor array 108. Interferometric sensor array 108 includes a plurality of fiber optic sensors. The interferometric sensor array includes a plurality of optical sensors, each configured as an interferometer (e.g., a Michelson interferometer, a Mach Zehnder interferometer, etc.). Sensors in array 108 convert physical quantities (e.g., vibration, pressure, temperature, displacement, etc.) to a time-varying optical intensity return signal which passes back through lead cable 106 and is received at optical receiver 1 10. Optical receiver 1 10 includes, for example, demultiplexing and demodulation optics/electronics.

System 100 may have various additional (or more specific) elements, for example, as shown in systems 100a, 100b, 100c, lOOd, and lOOe (see FIGS. I B- IF). Of course, implementations different from those shown in these exemplary drawings are contemplate within the scope of the present invention.

An additional element shown in FIG. IB is a linear polarizer 1 12a. Referring specifically to FIG. IB, an interferometric sensing system 100a includes an optical source 102a for providing an optical signal to a polarization scrambler 104a which continually changes the state of polarization of the light coming from optical source 102a. Light exiting polarization scrambler 104a passes through a fiber optic lead cable 106a, and then passes through linear polarizer 1 12a, on its way to an interferometric sensor array 108a. Linear polarizer 1 12a further reduces polarization noise. Sensors in array 108a convert physical quantities (e.g., vibration, pressure, temperature, displacement, etc.) to a time-varying optical intensity return signal which passes back through lead cable 106a and is received at optical receiver 1 10a. Since the state of polarization within fiber optic lead cable 106a can increase over long lengths (and as a result of perturbation to fiber optic lead cable 106a), linear polarizer 1 12a is used to ensure that only one polarization of light state reaches interferometric sensor array 108a.

An additional element shown in FIG. 1C is a phase modulator 1 14b.

Referring specifically to FIG. 1C, an interferometric sensing system 100b includes an optical source 102b for providing an optical signal to a polarization scrambler 104b which continually changes the state of polarization of the light coming from optical source 102b. Light exiting polarization scrambler 104b is modulated at phase modulator 1 14b, and then passes through a fiber optic lead cable 106b on its way to an interferometric sensor array 108b. Sensors in array 108b convert physical quantities (e.g., vibration, pressure, temperature, displacement, etc.) to a time-varying optical intensity return signal which passes back through lead cable 106b and is received at optical receiver 1 10b.

Additional elements shown in FIG. ID include a phase modulator 1 14c and a linear polarizer 1 12c. Referring specifically to FIG. ID, an interferometric sensing system 100c includes an optical source 102c for providing an optical signal to a polarization scrambler 104c which continually changes the state of polarization of the light coming from optical source 102c. Light exiting polarization scrambler 104c is modulated at phase modulator 1 14c, and then passes through a fiber optic lead cable 106c, and then passes through linear polarizer 1 12a, on its way on its way to an interferometric sensor array 108c. Sensors in array 108c convert physical quantities (e.g., vibration, pressure, temperature, displacement, etc.) to a time-varying optical intensity return signal which passes back through lead cable 106c and is received at optical receiver 1 10c.

FIGS. 1C-1 D illustrate phase modulators 1 14b, 1 14c downstream of the respective polarization scramblers 104b, 104c; however, it is understood depending on other design considerations, the positions of the respective polarization scrambler and phase modulator may be switched. For example, in Frequency Division

Multiplexing (FDM) systems, the phase modulator generally optically precedes (is upstream with respect to) the polarization scrambler since the phase modulator uses highly polarized light because of its high modulation frequency (where the phase modulator(s) may be of a lithium niobate electro-optic type phase modulators). FIGS. 1E-1 F illustrate phase modulators 1 14d, 1 14d upstream of the respective polarization scramblers 104d, 104e.

Referring specifically to FIG. IE, an interferometric sensing system lOOd includes an optical source 102d for providing an optical signal which is modulated at a phase modulator 1 14d, and then passes to a polarization scrambler 104d which continually changes the state of polarization of the light coming from optical source 102b. Light exiting polarization scrambler 104d passes through a fiber optic lead cable 106d on its way to an interferometric sensor array 108d. Sensors in array 108d convert physical quantities (e.g., vibration, pressure, temperature, displacement, etc.) to a time-varying optical intensity return signal which passes back through lead cable 106d and is received at optical receiver 1 lOd.

Referring specifically to FIG. IF, an interferometric sensing system l OOe includes an optical source 102e for providing an optical signal which is modulated at a phase modulator 1 14e, and then passes to a polarization scrambler 104e which continually changes the state of polarization of the light coming from optical source 102e. Light exiting polarization scrambler 104e passes through a fiber optic lead cable 106e, and then passes through linear polarizer 1 12e, on its way to an interferometric sensor array 108e. Sensors in array 108d convert physical quantities (e.g., vibration, pressure, temperature, displacement, etc.) to a time-varying optical intensity return signal which passes back through lead cable 106e and is received at optical receiver 1 l Oe.

FIG. 1 G illustrates a Wavelength Division Multiplexed (WDM)

interferometric sensing system l OOf including an optical source 102fl (operating at wavelength λΐ ) and an optical source 102f2 (operating at wavelength X2). Of course, while two optical sources 102fl and 102f2 are shown, additional optical sources (operating at different wavelengths) are contemplated. Optical signals from each of sources 102fl and 102f2 are received at polarization maintaining (PM) DWDM (i.e., dense wavelength division multiplexer) 1 16f from which the optical signals are output onto a single optical fiber, but at their respective wavelength. The output optical signal from DWDM 1 16f is provided to a polarization scrambler 104f which continually changes the state of polarization of the light coming from DWDM 1 16f. A cascaded series of polarization maintaining (PM) optical add/drop multiplexers may be substituted for DWDM 1 16f. Light exiting polarization scrambler 104f passes through a fiber optic lead cable 106f on its way to an interferometric sensor array 108f. Sensors in array 108f convert physical quantities (e.g., vibration, pressure, temperature, displacement, etc.) to a time-varying optical intensity return signal which passes back through lead cable 106f and is received at optical receiver 1 l Of.

As compared to FIG. 1G, an additional element shown in FIG. 1 H includes a phase modulator 1 14g. Referring specifically to FIG. 1H, interferometric sensing system l OOg includes an optical source 102gl (operating at wavelength λΐ) and an optical source 102g2 (operating at wavelength XI). Optical signals from each of sources 102gl and 102g2 are received at polarization maintaining (PM) DWDM 1 16g from which the optical signals are output onto a single optical fiber, but at their respective wavelength. The output optical signal from DWDM 1 16g is modulated at a phase modulator 1 14g, and then the signal is provided to a polarization scrambler 104g which continually changes the state of polarization of the light coming from DWDM 1 16g. A cascaded series of polarization maintaining (PM) optical add/drop multiplexers may be substituted for DWDM 1 16g. Light exiting polarization scrambler 104g passes through a fiber optic lead cable 106g on its way to an interferometric sensor array 108g. Sensors in array 108g convert physical quantities (e.g., vibration, pressure, temperature, displacement, etc.) to a time-varying optical intensity return signal which passes back through lead cable 106g and is received at optical receiver 1 l Og.

FIG. I I illustrates an interferometric sensing system l OOh including optical sources 102hl (e.g., a laser operating at wavelength λΐ), 102h2 (e.g., a laser operating at wavelength XI), 102h3 (e.g., a laser operating at wavelength λ3), 102h4 (e.g., a laser operating at wavelength λ4), 102h5 (e.g., a laser operating at wavelength λ5), and 102h6 (e.g., a laser operating at wavelength λ6). The configuration shown in FIG. I I is a wavelength division multiplexed configuration (i.e., an exemplary configuration utilizing multiple polarization scramblers, each connected to a limited wavelength band of lasers); however, other configurations for combining multi- wavelength light (such as a frequency division multiplexed system) are contemplated, in this exemplary configuration, dispersion within a polarization scrambler may only be optimized for a single wavelength of light, such that the degree of polarization varies over that wavelength, potentially degrading the noise reduction at one or more wavelengths. The dispersion is mitigated by use of multiple polarization scramblers, each optimized for a single wavelength, and only operated within a limited wavelength range, dictated by the small number of lasers connected to it.

Referring again to FIG. 1 1, optical signals from each of sources 102hl, 102h2, and 102h3 are received at PM DWDM (polarization maintaining dense wavelength division multiplexer) 1 16hl from which the optical signals are collectively output onto a single optical fiber, and are passed on to polarization scrambler 104hl which continually changes the state of polarization of the light coming from PM DWDM 1 16hl . Optical signals from each of sources 102h4, 102h5, and 102h6 are received at PM DWDM (polarization maintaining dense wavelength division multiplexer) 1 16h2 from which the optical signals are output onto a single optical fiber, but at their respective wavelength, and are passed on to polarization scrambler 104h2 which continually changes the state of polarization of the light coming from PM DWDM 1 16h2. Common modulator drive 1 18h controls the magnitude of the phase modulation in each polarization scrambler 104h l and 104h2. It is understood that while a single drive 1 18h is illustrated, separate drives may be utilized to control the magnitude of the phase modulation in a respective one of each polarization scrambler 104h l and 104h2. Optical signals from each of polarization scramblers 104hl , 104h2 are combined and filtered at red/blue filter 120h. The output optical signal from filter 120h is modulated at phase modulator 1 14h and the light (at the multiple

wavelengths) exiting polarization modulator 1 14h passes through a fiber optic lead cable 106h on its way to an interferometric sensor array 108h. Sensors in array 108h convert physical quantities (e.g., vibration, pressure, temperature, displacement, etc.) to a time-varying optical intensity return signal which passes back through lead cable 106h and is received at optical receiver 1 1 Oh.

While not illustrated in FIG. II, phase modulators may be included upstream or downstream of the polarization scramblers, similar to the configurations illustrated in FIGS. 1 C, I D, IE, I F, and IH.

FIGS. 2A-2K illustrate various exemplary polarization scramblers 204a-204k, each of which desirably reduce the time-averaged degree of polarization of the light exiting the respective polarization scrambler. The scrambling rate is desirably accomplished at a rate higher than the system sampling rate. As will be understood by those skilled in the art, exemplary polarization scramblers 204a-204k may be utilized as various of the polarization scramblers illustrated in FIGS. 1A-1 I.

Referring specifically to FIG. 2A, an optical signal (e.g., such as polarized light from optical source 102 in FIG. 1A) is received by a polarization maintaining (PM) coupler 204a 1 of polarization scrambler 204a. PM coupler 204a 1 preserves the state of polarization of the optical signal and divides it into two paths. One path passes through an optical phase modulator 204a3, for example, which rapidly changes the phase of the light along one polarization. The second path passes through a polarization maintaining (PM) optical attenuator 204a2. For example, the attenuation in this leg may be manually adjustable. A polarizing beam splitter/combiner 204a4 is a coupler which combines the light output from each of the two paths, entering mutually orthogonal polarizations. Attenuator 204a2 may be adjusted to provide input intensities to polarizing beam splitter/combiner 204a4 which are very close to equal. Rapid changes in the phase of light within optical phase modulator 204a3 (e.g., on the order of π radians), when combined with the light entering from the second path along orthogonal polarizations at the polarizing beam splitter/combiner 204a4, reduces the time-average of the state of polarization from -100% at the input to PM coupler 204a 1 input to near zero at the exit (an output signal 204a5 shown exiting polarization scrambler 204a). As will be appreciated by those skilled in the art, because the process described above results in a continual polarization state change over time, it is desirable to operate optical phase modulator 204a3 at a rate higher than the system sampling rate, where the system sampling rate is the frequency with which the analog signals from a photodetector(s) and amplifier(s) are digitized in preparation for demodulation/demultiplexing and other signal processing at the optical receiver. Output signal 204b5 is the output of polarization scrambler 204a, and is passed on to the next element(s) in the interferometric sensing system in which it is utilized (e.g., to a phase modulator, to a lead cable, etc.).

In FIG. 2B, an optical signal (e.g., such as polarized light from optical source 102 in FIG. 1 A) is received by a polarization maintaining (PM) coupler 204b 1 of polarization scrambler 204b. PM coupler 204a 1 preserves the state of polarization of the optical signal and divides it into two paths. One path passes through an optical phase modulator 204b3, for example, which rapidly changes the phase of the light along one polarization. The output of optical phase modulator 204b3 is split at a PM tap coupler 204b5. The second path passes through a polarization maintaining (PM) optical attenuator 204b2 (e.g., an electrically-controllable optical attenuator). The output of PM optical attenuator 204b2 is split at a PM tap coupler 204b4. Most of the intensity from the split signals from each of PM tap coupler 204b4 and 204b5 are received at a polarizing beam splitter/combiner 204b6 and dual optical receiver 204b7. Polarizing beam splitter/combiner 204b6 is a coupler which combines the light output from split signals from each of the two paths. A small portion of the intensity from the split signals from each of PM tap couplers 204b4 and 204b5 are received at a dual optical receiver 207b7. Dual optical receiver 207b7 converts each of the received intensities from the PM tap couplers 204b4 and 204b5 to an electrical voltage output signal(s). The output electrical voltage signals from dual optical receiver 204b7 are used for servo control of PM optical attenuator 204b2 (with the signal amplified in the illustrated example by amplifier 204b8).

Attenuator 204b2 may be adjusted to provide input intensities to polarizing beam splitter/combiner 204b6 which are very close to equal. Rapid changes in the phase of light within optical phase modulator 204b3, when combined with the light entering from the second path (from PM optical attenuator 204b2) along orthogonal polarizations at the polarizing beam splitter/combiner 204b6, reduces the state of polarization at the exit from polarization scrambler 204b (illustrated as an output signal 204bl0).

In the exemplary polarization scrambler configuration shown in FIG. 2C, an optical signal (e.g., such as polarized light from optical source 102 in FIG. 1A) is received by a polarization maintaining (PM) coupler 204c 1 of polarization scrambler 204c. PM coupler 204cl preserves the state of polarization of the optical signal and divides it into two paths. One path passes through an optical phase modulator 204c2, for example, which rapidly changes the phase of the light along one polarization. The second path passes through an optical phase modulator 204c3, for example, which rapidly changes the phase of the light along another polarization preferably, out of synchronism (and uncorrelated) with respect to the phase changes on the one polarization. The two optical phase modulators 204c2, 204c3 may also be operated 180° out of phase relative to each other. A polarizing beam splitter/combiner 204c4 is a coupler which combines the light output from each of the two paths. Combining light output from optical phase modulator 204c2 and optical phase modulator 204c3 (through polarizing beam splitter/combiner 204c4) results in an output signal 204a5 with a substantially reduced state of polarization.

In the exemplary polarization scrambler configuration shown in FIG. 2D, an optical signal (e.g., such as polarized light from optical source 102 in FIG. 1A) is received by a polarizer 204dl of polarization scrambler 204d. The output optical signal from polarizer 204dl is processed by a polarization switch 204d2, where polarization switch 204d2 is driven by a high frequency switch drive 204d3. Light output from polarization switch 204d2 is an output signal 204d4 with a substantially reduced state of polarization when averaged over time.

Rather than a fixed, polarization maintained, manual variable optical attenuator, FIG. 2E includes a servo loop that observes the intensity in each leg (via PM tap couplers) and adjusts an electrically-controlled PM optical attenuator to balance them. In the exemplary polarization scrambler configuration shown in FIG. 2E, an optical signal (e.g., such as polarized light from optical source 102 in FIG. 1 A) is received by a polarization maintaining (PM) coupler 204el of a polarization scrambler 204e. PM coupler 204e l preserves the state of polarization of the optical signal and divides it into two paths. One path passes through a polarization maintaining (PM) optical phase modulator 204e3, for example, which rapidly changes the phase of the light along one polarization. The output of optical phase modulator 204e3 is split at a polarization maintaining (PM) tap coupler 204e6. The second path from PM coupler 204el passes through an electrically controlled polarization maintaining (PM) optical attenuator 204e2 (as opposed to the manual attenuation configuration illustrated, for example, in FIG. 2A). The output of PM optical attenuator 204e2 is split at a polarization maintaining (PM) tap coupler 204e5. The split signals from each of PM tap coupler 204e5 and 204e6 are received at a polarizing beam splitter/combiner 204e8 and dual optical receiver 204e7. Polarizing beam splitter/combiner 204e8 is a coupler which combines the light output from split signals from each of the two paths. The output signal from polarizing beam splitter/combiner 204e8 is split at a tap coupler 204e9. One leg of light output from tap coupler 204e9 is an output signal 204el 1 with a substantially reduced state of polarization. The other leg of light from tap coupler 204e9 is received by a degree of polarization (DOP) monitor 204el0.

Dual optical receiver 204e7 uses the other reduced intensity light output from the split signals from each of the two paths (i.e., from PM tap couplers 204e5, 204e6). Output signals from dual optical receiver 204b7, and from DOP monitor 204el0, are received by processor and control circuits 204e4. More specifically, the electrical output of DOP monitor 204el0 is directed to processor and control circuits 204e4 to adjust the frequency and/or amplitude of the electrical signal sent to PM phase modulator 204a3 to further optimize the output degree of polarization of the polarization scrambler 204e. Processor and control circuits 204e4 are also used for servo control of PM optical attenuator 204e2.

In the exemplary polarization scrambler configuration shown in FIG. 2F, an optical signal (e.g., such as polarized light from optical source 102 in FIG. 1A) is received by a polarization maintaining (PM) coupler 204f 1 of polarization scrambler 204f. PM coupler 204fl preserves the state of polarization of the optical signal and divides it into two paths, where each path/leg includes a phase modulator followed by a variable optical attenuator. More specifically, one path passes through a polarization maintaining (PM) phase modulator 204f2, for example, which rapidly changes the phase of the light along one polarization. The second path passes through another polarization maintaining (PM) phase modulator 204f3, which operates to rapidly change the phase of the light passing through it asynchronously with respect to phase modulator 204f2. The output of PM phase modulators 204f2, 204f3 are directed to a respective one of polarization maintaining (PM) attenuators 204f4, 204f5. The attenuated optical signals are directed from attenuators 204f4, 204 f5 to polarizing beam splitter/combiner 204f6, which acts as a coupler combining the light output from each of the two paths. Combining light output from attenuators 204 4, 204f5 (through polarizing beam splitter/combiner 204f6) results in an output signal 20417 with a substantially reduced state of polarization.

FIG. 2G illustrates a polarization scrambler 204g including a 45° aligned phase modulator 204g2. An optical signal (e.g., such as polarized light from optical source 102 in FIG. 1A) is received by phase modulator 204g2 with its input oriented at 45 degrees with respect to the fast and slow axes of the optical fiber of the polarized optical source. An optional linear polarizer 204gl is shown upstream of phase modulator 204g2, with its input axis aligned with respect to the polarization orientation of the polarized optical source. Because the polarizer 204gl and phase modulator 204g2 are intentionally misaligned, the phase of half of the light is changing much differently (e.g., more rapidly) than the other half of the light. Thus, the state of polarization of the light is attenuated differently, resulting in a reduced state of polarization.

In the exemplary polarization scrambler configuration shown in FIG. 2H, an optical signal (e.g., such as polarized light from optical source 102 in FIG. 1 A) is received by a polarization maintaining (PM) optical switch 204hl of polarization scrambler 204h. Optical switch 204hl, which is driven by high speed switch driver 204h5, alternates power between its two output ports. Polarization maintaining (PM) optical attenuators 204h2, 204h3 balance the power in each arm to balance the intensity of the light in the two orthogonal output states of polarization scrambler 204h. The output signals from optical attenuators 204h2, 204h3 are received by polarizing beam splitter/combiner 204h4 which ensures that the two legs are combined as two orthogonal polarization states to provide an output signal 204h6 with a reduced state of polarization. An additional linear polarizer (not shown in FIG. 2H) may be installed upstream, of an interferometric sensor array.

In the exemplary polarization scrambler configuration shown in FIG. 21, an optical signal (e.g., such as polarized light from optical source 102 in FIG. 1A) is received by a polarization maintaining (PM) coupler 204i l of a polarization scrambler 204i. PM coupler 204i l preserves the state of polarization of the optical signal and divides it into two paths. One path passes through a polarization maintaining (PM) phase modulator 204Ϊ3, for example, which rapidly changes the phase of the light along one polarization. The second path passes through a polarization maintaining (PM) optical attenuator 204i2. The output of PM phase modulator 204i3 is split at a polarization maintaining (PM) tap coupler 204i5. The output of PM optical attenuator 204i2 is split at a polarization maintaining (PM) tap coupler 204i4. The split signals from each of PM tap couplers 204i4 and 204Ϊ5 are received at a polarizing beam splitter/combiner 204i6 and at a 2 channel receiver 204i7. Polarizing beam splitter/combiner 204b6 is a coupler which combines the light output from split signals from each of the two paths, where the combined light output results in an output signal 204i9 with a substantially reduced state of polarization. 2 channel receiver 204i7 (or alternatively a pair of optical receivers as opposed to the illustrated 2 channel receiver 204i7) uses the other light output from split signals from each of the two paths, where the portion of the light split off and sent to 2 channel receiver 204i7 may be a small portion of the light from each leg. This small portion of light provides a means for actively inferring the degree of polarization (DOP) resulting at the output 204i9. The output signals from receiver 204i7 are fed through a differential amplifier 204i8, whose output is then fed to an input of PM optical attenuator 204Ϊ2 for controlling attenuator 204i2 (e.g., an electrically-controllable, PM VOA). As will be understood by those skilled in the art, dithering an additional input to differential amplifier 20418 allows use of a hill-climbing scheme, or similar algorithm, to search for the minimum DOP, corresponding to maximum noise reduction due to the lead cable. The arrangement of FIG. 21 allows for a reduction of the thermal sensitivity of polarization scrambler 204i and avoid problems such as VOA over temperature.

In the exemplary polarization scrambler configuration shown in FIG. 2J, an optical signal (e.g., such as polarized light from optical source 102 in FIG. 1A) is received by a push-pull phase modulator 204j 1 of a polarization scrambler 204j. For example, push-pull phase modulator 204j l may be a Y junction push-pull 50/50 output LiNb0₃ integrated optic phase modulator. Push-pull phase modulator 204j l changes the phase of the light in one path with respect to the other, which when combined at the polarizing beam splitter, yields one polarization state with respect to other polarization states at a rate higher than the system sampling rate. The output of push-pull phase modulator 204j l is sent along polarization maintaining (PM) fiber 204j2 and is received by legs X and Y of polarization beam splitter 204j3, the output of which is optical signal 204j4 having a substantially reduced state of polarization.

In the exemplary polarization scrambler configuration shown in FIG. 2K, an optical signal (e.g., such as polarized light from optical source 102 in FIG. 1A) is received by a push-pull phase modulator 204kl of a polarization scrambler 204k. For example, push-pull phase modulator 204kl may be a Y junction push-pull 50/50 output LiNb0₃ phase modulator. Push-pull phase modulator 204k 1 changes the phase of the light in one path with respect to the other, which when combined at the polarizing beam splitter, yields one polarization state with respect to other polarization states at a rate higher than the system sampling rate. The output of push- pull phase modulator 204kl is sent along polarization maintaining (PM) fiber 204k2 and is received by an optical fiber on common fiber wrapped mandrel 204k3. By wrapping the fiber on mandrel 204k3, minor tuning of the optical signal is performed to achieve, for example, a 50/50 split ratio (e.g., to within 0.01 dB = 2%). That is, the power in each leg (i.e., legs X and Y) received by polarization beam splitter 204k4 is substantially balanced. The output of polarization beam splitter 204k4 is optical signal 204k5, which has a substantially reduced state of polarization. Thus, the configuration of FIG. 2K incorporates a fixed attenuation (e.g., using common fiber wrapped mandrel 204k3) into either one or both legs to balance the power in each leg.

FIGS. 3A-3C illustrate exemplary waveforms that may be applied to a phase modulator of a interferometric sensing system such as those illustrated and described herein including phase modulation. The vertical scale of each of FIGS. 3A-3C is a modulation phase angle (in radians of phase shift, which is dependent upon the optical wavelength), and the horizontal axis is time. The period of each waveform is desirably higher than the sampling rate of the received intensity signals from the sensors of the sensor array.

As provided herein, certain of the inventive interferometric sensing systems include at least one array of interferometric sensors. For example, such interferometric sensors may include Fabry-Perot interferometers, Michelson interferometers, Mach Zehnder interferometers, etc. The interferometric sensors may also include transducers including those for sensing characteristics such as acceleration, dynamic pressure, etc. Exemplary transducers and accelerometers (and related fiber optic sensing systems) are disclosed in U.S. Patent Application

Publication No. 20120257208, entitled "FIBER OPTIC TRANSDUCERS, FIBER OPTIC ACCELEROMETERS AND FIBER OPTIC SENSING SYSTEMS", which is herein incorporated by reference in its entirety.

The interferometric sensing systems disclosed herein (and the inventive polarization scramblers) will find use in many different fiber optic sensing applications, for example: vertical seismic profiling (VSP), three dimensional subsurface mapping, microseismic monitoring, machine vibration monitoring, civil structure (e.g., dams, bridges, levees, buildings, etc.) monitoring, tunnel detection, perimeter/border security, earthquake monitoring, borehole leak detection, roadbed erosion, railbed erosion, marine detection (e.g., sonar), amongst others.

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.

Claims

What is Claimed:

1. An interferometric sensing system comprising: an optical source for providing optical signals; a fiber optic lead cable for receiving the optical signals from the optical source; an array of one or more interferometric sensors optically coupled to the fiber optic lead cable; and a polarization scrambler for scrambling a polarization state of the optical signals.

2. The interferometric sensing system of claim 1 wherein the polarization scrambler is optically positioned between the optical source and the fiber optic lead cable.

3. The interferometric sensing system of claim 1 further comprising a phase modulator for modulating the optical signals prior to the optical signals being received by the fiber optic lead cable.

4. The interferometric sensing system of claim 3 wherein the polarization scrambler is optically positioned between the optical source and the phase modulator.

5. The interferometric sensing system of claim 1 further comprising an optical receiver for receiving return optical signals from the array of one or more interferometric sensors.

6. The interferometric sensing system of claim 5 wherein the optical receiver includes demodulation electronics for demodulating the return optical signals.

7. The interferometric sensing system of claim 1 further comprising a linear polarizer optically positioned between the fiber optic lead cable and the array of one or more interferometric sensors.

8. The interferometric sensing system of claim 1 further comprising a linear polarizer positioned upstream of each of the interferometric sensors included in the array.

9. The interferometric sensing system of claim 1 wherein the polarization scrambler includes a polarizer and a polarization switch operated by a high frequency switch drive.

10. The interferometric sensing system of claim 1 wherein the polarization scrambler continuously changes a state of polarization of the optical signals.

1 1. The interferometric sensing system of claim 10 further comprising demodulation electronics for sampling return optical signals from the array at a sampling rate.

12. The interferometric sensing system of claim 1 1 wherein a scrambling rate of the polarization scrambler is higher than the sampling rate.

13. The interferometric sensing system of claim 1 wherein the polarization scrambler continuously changes a state of polarization of the optical signals such that an output of the polarization scrambler has a reduced time average degree of polarization as compared to the optical signals input to the polarization scrambler.

14. The interferometric sensing system of claim 1 wherein the optical source includes a plurality of optical sources, each of the optical sources operating at one of a plurality of optical wavelengths.

15. The interferometric sensing system of claim 14 further comprising a polarization maintaining dense wavelength division multiplexer upstream of the polarization scrambler, the polarization maintaining dense wavelength division multiplexer receiving optical signals from the plurality of optical sources.

16. The interferometric sensing system of claim 1 wherein the polarization scrambler includes a polarization maintaining fiber optic coupler having (a) a first output leg connected to a polarization maintaining input lead of a phase modulator and (b) a second output leg connected to a polarization maintaining input lead of a polarization maintaining variable optical attenuator.

17. The interferometric sensing system of claim 16 wherein the polarization scrambler includes a polarizing beam splitter whose input leads are connected to (a) a polarization maintaining output lead of the phase modulator and (b) a polarization maintaining output lead of the polarization maintaining variable optical attenuator.

18. A method of operating an interferometric sensing system, the method comprising the steps of:

(a) providing optical signals from an optical source;

(b) scrambling a polarization state of the optical signals using a polarization scrambler; and

(c) receiving the optical signals at an array of one or more interferometric sensors optically coupled to the optical source.

19. The method of claim 18 wherein step (b) includes:

(bl ) splitting the optical signals into two independent paths, each of the split optical signals carrying one of two orthogonal polarization states;

(b2) periodically varying a phase between the two orthogonal polarization states at a rate higher than a sampling rate of the interferometric sensing system;

(b3) recombining the split optical signals from the two independent paths; and

(b4) inputting the resultant, randomly polarized optical signals, into a lead cable upstream of the array of one or more interferometric sensors.

20. The method of claim 18 wherein step (b) includes scrambling the polarization state of the optical signals using the polarization scrambler where the polarization scrambler is optically positioned between the optical source and a lead cable, the lead cable being optically coupled to the array of one or more

interferometric sensors.

21. The method of claim 18 further comprising the step of modulating the optical signals using a phase modulator prior to step (b).

22. The method of claim 18 further comprising the step of modulating the optical signals using a phase modulator after step (b).

23. The method of claim 18 further comprising the step of receiving return optical signals from the array of one or more interferometric sensors at an optical receiver, the optical receiver including demodulation electronics for demodulating the return optical signals.

24. The method of claim 18 wherein step (b) comprises continuously changing a state of polarization of the optical signals using the polarization scrambler.

25. The method of claim 18 wherein step (b) includes scrambling the polarization state of the optical signals at a scrambling rate that is higher than a sampling rate for sampling return optical signals from the array of one or more interferometric sensors.

26. The method of claim 18 wherein step (b) comprises continuously changing a state of polarization of the optical signals using the polarization scrambler such that an output of the polarization scrambler has a reduced time average degree of polarization as compared to the optical signals input to the polarization scrambler.