Distributed acoustic sensing system based on delayed optical hybrid phase demodulator
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.
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
second, third, and fourth 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 1 =A+B cos[ϕ(t)], I 2 =A+B sin[ϕ(t)], I 3 =A−B cos[ϕ(t)], and I 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, and I 1-4 comprise an intensity of detected light generated by the respective photodetectors.
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
a second, third, and fourth 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 1 =A+B cos[ϕ(t)], I 2 =A+B sin[ϕ(t)], I 3 =A−B cos[ϕ(t)], and I 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, and I 1-4 comprise an intensity of detected light generated by the respective photodetectors.
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
second, third, and fourth 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
second, third, and fourth photodetectors coupled to the optical hybrid.
15. The system of claim 14 , comprising a fiber loop to provide the optical path difference.
16. The system of claim 14 , comprising three optical attenuators respectively coupled to each of the three photodetectors.
17. The system of claim 14 , comprising data converters coupled to each photodetectors.
18. The system of claim 14 , wherein the photodetectors generates I 1 =A+B cos[ϕ(t)], I 2 =A+B sin[ϕ(t)], I 3 =A−B cos[ϕ(t)], and I 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, and I 1-4 comprise an intensity of detected light generated by the respective photodetectors.
19. The system of claim 14 , comprising a data converter coupled to a processor receiving digitized photodetector output and driving a modulator driver.
20. The system of claim 14 , wherein the modulator comprises an Acousto-Optic Modulator (AOM), comprising an AOM driver coupled to the AOM.