System and method for resolution enhancement of a distributed sensor
A Brillouin Optical Time-Domain Analysis (BOTDA) distributed sensor system and method use a continuous wave (cw) Stokes wave interrupted with a dark pulse for improved spatial resolution. The cw Stokes wave causes a continuous depletion of the pump wave. The dark pulse causes the depletion to stop for the duration of the pulse. Brillouin interactions are measured during the dark pulse. Very narrow dark pulses can be used because sufficient Stokes wave energy is maintained. The system produces a stronger time-domain signal and narrower linewidth Brillouin spectra than traditional techniques using a bright Stokes pulse. Narrower measurement pulses can be used leading to improved spatial resolution. A quasi-cw Stokes wave can be used to reduce the effect of stimulated Brillouin scattering (SBS) in long measurement fibers. The system can be used for distributed strain or temperature measurements.
1. A Brillouin Optical Time-Domain Analysis (BOTDA) distributed sensor system comprising:
an optical fiber distributed sensor;
a first launching means for launching a first optical radiation in a first direction in the optical fiber;
a second launching means for launching a second optical radiation in an opposing direction in the optical fiber; and
measurement means for measuring the Brillouin interaction of the first and second optical radiation with the acoustic phonon field of the optical fiber,
wherein said first launching means is operable to interrupt said first optical radiation whereby a dark pulse is generated, said dark pulse being a duration of extinction of the first optical radiation, and
wherein said measurement means measures the Brillouin interaction during said dark pulse.
2. A system as claimed in claim 1 , wherein said first optical radiation and said second optical radiation are separated by the frequency of vibration of the acoustic phonon field in the optical fiber.
3. A system as claimed in claim 1 , wherein said dark pulse has a duration shorter than the lifetime of the acoustic phonons in the optical fiber.
4. A system as claimed in claim 1 , wherein said dark pulse has a duration of less than 2 ns.
5. A system as claimed in claim 1 , wherein said dark pulse has a duration of less than 500 ps.
6. A system as claimed in claim 1 , wherein said first optical radiation is a continuous wave radiation.
7. A system as claimed in claim 1 , wherein said first optical radiation is a continuous wave radiation interrupted by a dark pulse.
8. A system as claimed in claim 1 , wherein said first optical radiation is a pulsed radiation interrupted by a dark pulse.
9. A system as claimed in claim 8 , wherein the pulses of said first optical radiation have a duration longer than the acoustic phonon lifetime whereby the said pulses appear as quasi-continuous waves to the acoustic phonon field.
10. A system as claimed in claim 1 , wherein said first launching means comprises a pulse generator operable to control the duration of said dark pulse within said first optical radiation.
11. A system as claimed in claim 1 , wherein said first launching means and said second launching means are connected at opposite ends of said optical fiber.
12. A system as claimed in claim 1 , wherein said first launching means and said second launching means are both connected at a first end of said optical fiber, and said second launching means is adapted to induce said second optical radiation to reflect off of a second end of said optical fiber.
13. A system as claimed in claim 2 , wherein said first optical radiation has a lower frequency than said second optical radiation whereby said system is operable in a Brillouin loss mode.
14. A system as claimed in claim 2 , wherein said first optical radiation has a higher frequency than said second optical radiation whereby said system operates in a Brillouin gain mode.
15. A method for measuring strain or temperature in an optical fiber distributed sensor using a Brillouin Optical Time-Domain Analysis (BOTDA) system, the method comprising steps of:
launching in a first direction in the optical fiber, a first optical radiation;
interrupting said first optical radiation whereby a dark pulse is generated, said dark pulse being a duration of extinction of the first optical radiation;
launching a second optical radiation in an opposing direction in the optical fiber;
measuring the Brillouin interaction of the first and second optical radiation with the phonon field of the optical fiber during said dark pulse; and
calculating a strain or temperature from the Brillouin interaction.
16. A method as claimed in claim 15 , wherein said first optical radiation and said second optical radiation are separated by the frequency of vibration of the acoustic phonon field in the optical fiber.
17. A method as claimed in claim 15 , wherein said dark pulse has a duration shorter than the lifetime of the acoustic phonons in the optical fiber.
18. A method as claimed in claim 15 , wherein said dark pulse has a duration of less than 2 ns.
19. A method as claimed in claim 15 , wherein said dark pulse has a duration of less than 500 ps.
20. A method as claimed in claim 15 , wherein said first optical radiation is a continuous wave radiation.
21. A method as claimed in claim 15 , wherein said first optical radiation is a continuous wave radiation interrupted by a dark pulse.
22. A method as claimed in claim 15 , wherein said first optical radiation is a pulsed radiation interrupted by a dark pulse.
23. A method as claimed in claim 22 , wherein the pulses of said first optical radiation have a duration longer than the acoustic phonon lifetime whereby the said pulses appear as quasi-continuous waves to the acoustic phonon pulses.
24. A method as claimed in claim 15 , wherein said step of launching a first optical radiation, further comprises steps of: generating a pulse; using said pulse to control said launching of said first optical radiation.
25. A method as claimed in claim 15 , wherein said step of launching said second optical radiation comprises launching said second optical radiation from the opposite end of said optical fiber from where said first optical radiation is launched.
26. A method as claimed in claim 15 , wherein said step of launching said second optical radiation further comprises steps of: launching said second optical radiation from the same end as said first optical radiation, and reflecting said second optical radiation from the opposite end of said optical fiber.
27. A method as claimed in claim 16 , wherein said first optical radiation has a lower frequency than said second optical radiation whereby said system operates in a Brillouin loss mode.
28. A method as claimed in claim 16 , wherein said first optical radiation has a higher frequency than said second optical radiation whereby said system operates in a Brillouin gain mode.
29. A system as claimed in claim 1 , wherein said first launching means is operable to reduce said first optical radiation.
30. A method as claimed in claim 15 wherein in the step of interrupting said first optical radiation, said first optical radiation is reduced.