Distributed Acoustic Sensing System Based on Delayed Optical Hybrid Phase Demodulator

Abstract

A sensing system adapted to receive backscattered signal from a sensing fiber includes a first Faraday rotator mirror; a second Faraday rotator mirror; an optical hybrid coupled to the Faraday rotator mirrors, wherein one of the mirrors is coupled with an optical path difference; a 3-port optical circulator coupled to the sensing fiber and the optical hybrid; a first photodetector coupled to the circulator; and three photodetectors coupled to the optical hybrid.

Claims

1. A sensing system adapted to receive backscattered signal from a sensing fiber, comprising: a first Faraday rotator mirror; a second Faraday rotator mirror; an optical hybrid coupled to the Faraday rotator mirrors, wherein one of the mirrors is coupled with an optical path difference; a 3-port optical circulator coupled to the sensing fiber and the optical hybrid; a first photodetector coupled to the circulator; and three photodetectors coupled to the optical hybrid.

2. The system of claim 1, comprising a fiber loop to provide the optical path difference.

3. The system of claim 1, comprising three optical attenuators respectively coupled to each of the three photodetectors.

4. The system of claim 1, comprising data converters coupled to each photodetectors.

5. The system of claim 1, wherein the photodetectors generates I.sub.1=A+B cos [φ(t)], I.sub.2=A+B sin [φ(t)], I.sub.3=A−B cos [φ(t)], and I.sub.4=A−B sin [φ(t)], respectively, where A and B are constants and φ(t) is a phase difference between two interfered sections at time t, comprising a processor obtain the time-varying phase information φ(t) from the four photodetector outputs.

6. A sensing system adapted to receive backscattered signal from a sensing fiber, comprising: a first Faraday rotator mirror; a second Faraday rotator mirror; an optical hybrid coupled to the Faraday rotator mirrors, wherein one of the mirrors is coupled with an optical path difference; an optical isolator coupled to the sensing fiber; a coupler receiving signals from the optical isolator and the optical hybrid; a first photodetector coupled to the coupler; and three photodetectors coupled to the optical hybrid.

7. The system of claim 6, wherein the coupler comprises a 2×1 coupler or a 2×2 coupler with one port unused.

8. The system of claim 6, comprising a fiber loop to provide the optical path difference.

9. The system of claim 6, comprising three optical attenuators respectively coupled to each of the three photodetectors.

10. The system of claim 6, comprising data converters coupled to each photodetectors.

11. The system of claim 6, wherein the photodetectors generates I.sub.1=A+B cos [φ(t)], I.sub.2=A+B sin [φ(t)], I.sub.3=A−B cos [φ(t)], and I.sub.4=A−B sin [φ(t)], respectively, where A and B are constants and φ(t) is a phase difference between two interfered sections at time t, comprising a processor obtain the time-varying phase information φ(t) from the four photodetector outputs.

12. A distributed acoustic sensor (DAS) system to receive backscattered signal from a sensing fiber, comprising: a laser; a modulator coupled to the laser; an optical amplifier and filter coupled to the modulator; a backscattered optical amplifier and backscattered filter coupled to the modulator; a circulator coupled to the filter and backscattered optical amplifier and the sensing fiber; and a delayed hybrid phase demodulator coupled to the backscattered filter; and a processor coupled to the delayed hybrid phase demodulator to sense acoustic signals.

13. The system of claim 12, wherein the delayed hybrid phase demodulator further comprises: a first Faraday rotator mirror; a second Faraday rotator mirror; an optical hybrid coupled to the Faraday rotator mirrors, wherein one of the mirrors is coupled with an optical path difference; a 3-port optical circulator coupled to the sensing fiber and the optical hybrid; a first photodetector coupled to the circulator; and three photodetectors coupled to the optical hybrid.

14. The system of claim 12, wherein the delayed hybrid phase demodulator further comprises: a first Faraday rotator mirror; a second Faraday rotator mirror; an optical hybrid coupled to the Faraday rotator mirrors, wherein one of the mirrors is coupled with an optical path difference; a 3-port optical circulator coupled to the sensing fiber and the optical hybrid; a first photodetector coupled to the circulator; and three photodetectors coupled to the optical hybrid.

15. The system of claim 1, comprising a fiber loop to provide the optical path difference.

16. The system of claim 1, comprising three optical attenuators respectively coupled to each of the three photodetectors.

17. The system of claim 1, comprising data converters coupled to each photodetectors.

18. The system of claim 1, wherein the photodetectors generates I.sub.1=A+B cos [φ(t)], I.sub.2=A+B sin [φ(t)], I.sub.3=A−B cos [φ(t)], and I.sub.4=A−B sin [φ(t)], respectively, where A and B are constants and φ(t) is a phase difference between two interfered sections at time t, comprising a processor obtain the time-varying phase information φ(t) from the four photodetector outputs.

19. The system of claim 1, comprising a data converter coupled to a processor receiving digitized photodetector output and driving a modulator driver.

20. The system of claim 1, wherein the modulator comprises an Acousto-Optic Modulator (AOM), comprising an AOM driver coupled to the AOM.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG. 1 shows an exemplary schematic of a DAS system.

[0025] FIG. 2 shows a conventional extended DAS system.

[0026] FIG. 3A shows a conventional interferometer using MZI with PGC, while FIG. 3B shows an interferometer using MI with PGC.

[0027] FIG. S 4A-4B show various conventional interferometric optics in the (a) 3×3 and (b) 4×4 coupler-based phase demodulator, respectively.

[0028] FIGS. 5A-5B show conventional interferometric optics in Coherent OTDR-based phase demodulator using 90-degree optical hybrid and using 3×3 coupler, respectively.

[0029] FIG. 6 shows an exemplary interferometric optics in the delay hybrid-based phase demodulator

[0030] FIG. 7 shows an exemplary alternative design without optical circulator

[0031] FIG. 8 shows an exemplary alternative design with integrated photodetectors

[0032] FIG. 9 shows an exemplary DAS interrogator with delay hybrid-based phase demodulator.

[0033] FIG. 10 shows an exemplary process for providing DAS sensing.

DESCRIPTION

[0034] FIGS. 6-9 show exemplary phase demodulators for DAS and other distributed fiber optic sensors. The two significant elements in these systems are the interferometric optics design and the associated demodulation control.

[0035] FIG. 6 shows an exemplary interferometric optics design is called “delayed hybrid” (301). The basic schematic is shown in FIG. 6. It has one input and four outputs. The input is the backscattered signal from the sensing fiber (302). It is sent to the Port 1 of a 3-port optical circulator (303), and exits from Port 2. It then enters a 2×4 optical hybrid (304) through one of the 4 “output ports”. (In the proposed method, the light passes through the optical hybrid twice, in opposite directions, therefore strictly speaking there is no pure “input port” or “output port”. However, in accordance with conventional notations, the side with two fiber ports is referred to as the “input ports”, and the side with four fiber ports as the “output ports”).

[0036] The light travels through the optical hybrid in the opposite direction of a regular 2×4 optical hybrid (from the “output end” to the “input end”), and exits the two “input ports” of the hybrid. They are sent to their respective Faraday rotator mirror (305 and 306), and back to the optical hybrid (304). The FRMs ensure that the polarization states of the two beams are maintained when they reenter the optical hybrids. There is an optical path difference (OPD) (307) between the two paths, so that the backscattered light at one section of the fiber interferes with the subsequent section to obtain the phase difference between them. The OPD is essentially a section of fiber, with the length equals to half of the distance between the two fiber sections, since the light travel through it twice.

[0037] These two light beams then travel through the optical hybrid in the same direction as regular 2×4 optical hybrid (from the “input end” to the “output end”), and exits the four “output ports” of the hybrid. One of the ports is connected to the circulator (303), where the light enters Port 2 and exits Port 3 to go to the photodetector (308). The lights exiting the other three ports go to another three photodetectors (309-311). Since the first output experiences more loss due to the circulator, the lights are the other three outputs are attenuated by optical attenuators (312-314) to maintain signal level balance. The outputs from the four photodetectors (308-311) are then digitized by ADCs (110) and processed by the processor (111).

[0038] Since all the key components, such optical hybrid, FRMs, circulator, optical attenuators, are mature technology and have been commercially available for long time and widely deployed in the field, there is no problem with their availability and quality. Since the interferometric optics is completely passive, there is no issue with electronic stability, symmetry, noise, control, or power consumption. The interferometric optics can be realized in fiber-based technology or free-space technology or a mix of both.

[0039] There are various design alternatives or modifications that can be done. For example, the attenuation can be done inside the optical hybrid. Or the optical attenuators can be removed, and use digital processing to take care of the power difference.

[0040] As shown in FIG. 7, the circulator (303) can be replaced with a 2×1 coupler (401) or a 2×2 coupler with one port unused, an optical isolator (402) is used to prevent light from traveling back to the input port.

[0041] Also, the photodetectors can be integrated with the interferometric optics to build a monolithic device, as shown in FIG. 8.

[0042] Next, a phased demodulation control for delayed hybrid-based phase demodulator is discussed. Based on the interferometric optics design described above, the four photodetectors will produce four outputs:

I.sub.1=A+B cos [φ(t)]  (1)

I.sub.2=A+B cos [φ(t)]  (2)

I.sub.3=A−B cos [φ(t)]  (3)

I.sub.4=A−B cos [φ(t)]  (4)

where A and B are constants determined by the interferometric system design, and φ(t) is the phase difference between the two interfered sections at time t. The sections to be interfered are related to the pre-set OPD length, as well as the pulse width and the propagation speed of light inside the fiber. The target is to obtain the time-varying phase information □(t) from the four photodetector outputs.

[0043] A straightforward way is to obtain the difference between a pair of two outputs, get the ratio, and perform arctangent function, such as:

X(t)=I.sub.1−I.sub.3=2B cos [φ(t)]  (5)

Y(t)=I.sub.2−I.sub.4=2B sin [φ(t)]  (6)

Z(t)=Y(t)/X(t)=tan [φ(t)]  (7)

φ(t)=arctan [Z(t)]  (8)

[0044] However, this method has phase ambiguity problem due to the arctangent function. The phase demodulation method/procedure uses subtraction, differentiation, cross multiplication, normalization, integration, and filter steps to recover the phase information accurately. This process is called the SDCMNIF procedure.

[0045] After obtaining the difference values through subtracting two output pairs (equations (5) and (6) above), the derivatives are calculated:

X′(t)=−2B sin [φ(t)].Math.φ′(t)   (9)

Y′(t)=2B cos [φ(t)].Math.φ′(t)   (10)

Then the cross multiplication is performed:

Y′(t).Math.X(t)−X′(t).Math.Y(t)=4B.sup.2 cos .sup.2[φ(t)].Math.φ′(t)+4B.sup.2 sin .sup.2[φ(t)].Math.φ′(t)=4B.sup.2φ′(t)   (11)

Also, the average power can be calculated as:

X.sup.2(t)+Y.sup.2(t)=4B.sup.2 cos.sup.2 [φ(t)]+4B.sup.2 sin.sup.2 [φ(t)]=4B.sup.2   (12)

Therefore equation (11) can be normalized into:

[Y′(t).Math.X(t)+X′(t).Math.Y(t)]/[X.sup.2(t)+Y.sup.2(t)]=4B.sup.2φ′(t)/4B.sup.2=φ′(t)   (13)

By taking the integration of equation (13), we can recover the signal phase as:

∫{[Y′(t).Math.X(t)+X′(t).Math.Y(t)]/[X.sup.2(t)+Y.sup.2(t)]}=∫φ′(t)=φ(t)+φ(t)   (14)

[0046] After passing through a high pass filter, the phase information 0 (t) can be recovered. [0047] Next, a DAS based on delayed hybrid phase demodulator is detailed in FIG. 9. A distributed acoustic sensor (DAS) system to receive backscattered signal from a sensing fiber includes [0048] a laser; [0049] a modulator coupled to the laser; [0050] an optical amplifier and filter coupled to the modulator; [0051] a backscattered optical amplifier and backscattered filter coupled to the modulator; [0052] a circulator coupled to the filter and backscattered optical amplifier and the sensing fiber; and [0053] a delayed hybrid phase demodulator coupled to the backscattered filter; and [0054] a processor coupled to the delayed hybrid phase demodulator to sense acoustic signals.

[0055] The DAS interrogator preferably works with a delay hybrid-based phase demodulator. The system can have as its delayed hybrid phase demodulator the following: [0056] a first Faraday rotator mirror; [0057] a second Faraday rotator mirror; [0058] an optical hybrid connected to the Faraday rotator mirrors, wherein one of the mirrors is coupled with an optical path difference; [0059] a 3-port optical circulator coupled to the sensing fiber and the optical hybrid; [0060] a first photodetector coupled to the circulator; and [0061] three photodetectors coupled to the optical hybrid.

[0062] The delayed hybrid phase demodulator further comprises can alternatively have the following: [0063] a first Faraday rotator mirror; [0064] a second Faraday rotator mirror; [0065] an optical hybrid coupled to the Faraday rotator mirrors, wherein one of the mirrors is coupled with an optical path difference; [0066] a 3-port optical circulator coupled to the sensing fiber and the optical hybrid; [0067] a first photodetector coupled to the circulator; and [0068] three photodetectors coupled to the optical hybrid.

[0069] A fiber loop to provide the optical path difference. Three optical attenuators respectively can be connected to each of the three photodetectors. Data converters can be connected to each photodetectors. The photodetectors generates I.sub.1=A+B cos[φ(t)], I.sub.2=A+B sin[φ(t)], I.sub.3=A−B cos[φ(t)], and I.sub.4=A−B sin[φ(t)], respectively, where A and B are constants and φ(t) is a phase difference between two interfered sections at time t. A processor obtains the time-varying phase information φ(t) from the four photodetector outputs. A data converter can digitize the acoustic phase data to the processor receiving digitized photodetector output and the output of the data converter in turn drives a modulator driver. The modulator can be an Acousto-Optic Modulator (AOM), comprising an AOM driver coupled to the AOM.

[0070] FIG. 10 shows the key features of the instant system. As shown on FIG. 10, the system is delayed hybrid-based phase demodulator, which is used in DAS interrogator. The two main elements for the invention (new and different) are (1) the delayed hybrid-based interferometer (hardware) and (2) the associated SDCMNIF phase demodulation procedure (control).

[0071] The instant phase demodulator overcomes the disadvantages of the existing solutions, such as polarization sensitivity, limited dynamic range, electronic symmetry requirement, frequency stability requirement, splitter ratio inequality, large optical power difference, laser phase instability, signal fading, strict modulation depth requirement, high computation complexity, among others, therefore offers better stability and sensitivity. Also, since its interferometric optics is totally passive (does not require active phase control or carrier modulation), it is more compact, low cost, and stable. Therefore, by using this phase demodulator, the DAS and other fiber optic sensors can achieve better sensing performance with lower cost and real-time operation.

[0072] To sum up, the description of the above-mentioned preferred embodiments is for providing a better understanding on the strengths and spirits of this present invention, not for limiting the domain of the invention. Moreover, it aims to include various modification and arrangement parallel in form into the domain of the patent applied by this present invention. Due to the above mentioned, the domain of the patent applied by the invention should be explained in a macro view to cover all kinds of possible modification and arrangement of equal form.