Hybrid Raman and Brillouin scattering in few-mode fibers
Methods and systems of sensing conditions in a fiber includes launching a light beam into a fiber. A first branch of scattered light is set to a mode other than a fundamental mode. A second branch of scattered light is optically filtered to remove unscattered input light. Brillouin scattered light is coherently detected on the first branch to produce a combined temperature/strain profile of the fiber. Raman scattered light on the second branch is directly detected to produce a temperature profile of the fiber. A strain profile of the fiber is determined, using a processor, based on the combined temperature/strain profile and the temperature profile.
1. A hybrid Brillouin/Raman sensor, comprising:
a mode converter configured to convert Brillouin backscattered light from a fiber to a spatial mode other than a fundamental spatial mode;
an optical filter configured to separate Raman forward-scattered light from a fiber to remove unscattered input light;
a Brillouin receiver configured to coherently detect the Brillouin backscattered light output by the mode converter and to produce a combined temperature/strain profile of the fiber;
a Raman receiver configured to directly detect the Raman forward-scattered light output by the optical filter and to produce a temperature profile of the fiber; and
a processor configured to determine a strain profile of the fiber based on the combined temperature/strain profile and the temperature profile.
2. The sensor of claim 1 , wherein the Raman receiver comprises:
a photodiode;
a trans-impedance amplifier; and
an analog to digital converter.
3. The sensor of claim 1 , wherein the Brillouin receiver comprises:
a P-I-N photodiode;
a trans-impedance amplifier;
an electrical spectrum analyzer configured to produce a Brillouin scattered trace; and
an analog to digital converter.
4. The sensor of claim 1 , further comprising a polarization controller configured to set light input to the fiber to a single polarization.
5. The sensor of claim 1 , wherein the mode converter comprises one of the group consisting of a long-period fiber grating, a spatial light modulator, free-space phase plates, and a fused spatial mode coupler.
6. A hybrid Brillouin/Raman sensor, comprising:
a coupler configured to split backscattered light from a fiber into two paths;
a mode converter configured to convert backscattered light on a first path to a spatial mode other than a fundamental spatial mode;
an optical filter configured to separate backscattered light on a second path to remove unscattered input light;
a Brillouin receiver configured to coherently detect the Brillouin backscattered light output by the mode converter and to produce a combined temperature/strain profile of the fiber;
a Raman receiver configured to directly detect the Raman forward-scattered light output by the optical filter and to produce a temperature profile of the fiber; and
a processor configured to determine a strain profile of the fiber based on the combined temperature/strain profile and the temperature profile.
7. The sensor of claim 6 , wherein the Raman receiver comprises:
a photodiode;
a trans-impedance amplifier; and
an analog to digital converter.
8. The sensor of claim 6 , wherein the Brillouin receiver comprises:
a P-I-N photodiode;
a trans-impedance amplifier;
an electrical spectrum analyzer configured to produce a Brillouin scattered trace; and
an analog to digital converter.
9. The sensor of claim 6 , further comprising a polarization controller configured to set light input to the fiber to a single polarization.
10. The sensor of claim 6 , wherein the mode converter comprises one of the group consisting of a long-period fiber grating, a spatial light modulator, free-space phase plates, and a fused spatial mode coupler.
11. A method for sensing conditions in a fiber, comprising:
launching a light beam into a fiber;
mode controlling a first branch of scattered light to a spatial mode other than a fundamental spatial mode;
optically filtering a second branch of scattered light to remove unscattered input light;
coherently detecting Brillouin scattered light on the first branch to produce a combined temperature/strain profile of the fiber;
directly detecting Raman scattered light on the second branch to produce a temperature profile of the fiber; and
determining a strain profile of the fiber, using a processor, based on the combined temperature/strain profile and the temperature profile.
12. The method of claim 11 , wherein the first branch comprises light that is Brillouin backscattered from the fiber.
13. The method of claim 12 , wherein the second branch comprises light that is Raman forward-scattered from the fiber.
14. The method of claim 12 , wherein the second branch comprises light that is Raman backscattered from the fiber.
15. The method of claim 11 , further comprising controlling a polarization of the light beam to set the light beam to a single polarization before launching the light beam into the fiber.
16. The method of claim 11 , wherein mode controlling the first branch of scattered light comprises passing the first branch of scattered light through a mode converter that comprises one of the group consisting of a long-period fiber grating, a spatial light modulator, free-space phase plates, and a fused spatial mode coupler.