Interferometer with a fiber optic having a high PMD in coupled mode, fiber optic gyroscope (FOG), and inertial navigation system including such a gyroscope
Disclosed is a fiber optic interferometer including: a wideband optical source having a decoherence time τ DC ; a coil including N turns of a fiber optic with length L; an optical element separating the incident beam into first and second beams coupled to first and second ends of the fiber respectively, so the first beam travels through the fiber optic in a first direction and the second beam travels through the fiber optic in a counter propagating direction; and a detector detecting the intensity of the output beam. The fiber optic is a high polarization mode dispersion type, and the length L of the fiber optic coil is more than twice the fiber correlation length, so the fiber operates in a coupled PMD mode, and the propagation differential group delay between two orthogonal polarization states, accumulated over the length of the fiber, is greater than the decoherence time of the source.
1. A fiber-optic interferometer, including:
a wideband optical source ( 1 ) capable of emitting an incident optical beam with a central wavelength λ and a spectral width Δλ FWHM , having a decoherence time τ DC =λ 2 /(c·Δλ FWHM ),
a coil with N turns of a single-mode optical fiber ( 5 ) of total length L, said fiber ( 5 ) having a first and a second ends,
optical means capable of splitting the incident beam into a first and a second beam and of coupling the first beam to the first end and the second beam to the second end of the optical fiber ( 5 ), respectively, so that the first beam travels through the optical fiber ( 5 ) in a first direction and the second beam travels through the optical fiber ( 5 ) in a counter-propagating direction, said optical means being capable of combining into an output beam the two beams emerging respectively from the two ends after propagation through the fiber ( 5 ),
a detector capable of detecting the interference of the output beam,
characterized in that
the optical fiber ( 5 ) is a high polarization mode dispersion (PMD) optical fiber,
the length L of the optical fiber coil ( 5 ) is higher than twice the correlation length of the fiber ( 5 ), L>2·L c , so that the fiber ( 5 ) operates in coupled PMD regime and
the group propagation time differential (DGD) between two orthogonal polarization states cumulated over the length L of the fiber ( 5 ) is higher that the decoherence time of the source:
DGD>τ DC .
2. A fiber-optic gyroscope comprising an interferometer according to claim 1 , characterized in that:
the optical means include a bidirectional optical splitter ( 4 ) capable of spatially splitting the incident beam into a first and a second beam coupled to the first and the second end of the optical fiber ( 5 ), respectively, so that the first beam travels through the optical fiber ( 5 ) in a first direction and the second beam travels through the optical fiber ( 5 ) in a counter-propagating direction, said splitter ( 4 ) being capable of combining into an output beam the two beams emerging respectively from the two ends after propagation through the fiber ( 5 ), and
a spatial single-mode filter and a polarizer ( 3 ) capable of receiving the incident beam from the source and of transmitting said linearly polarized, spatial single-mode incident beam to the optical splitter ( 4 ).
3. A gyroscope according to claim 2 , characterized in that it comprises a splitter ( 2 ) capable of spatially splitting the incident beam coming from the source and the output beam and of transmitting the incident beam to the fiber coil ( 5 ) and the output beam to the detector ( 6 ), respectively.
4. A gyroscope according to claim 2 , characterized in that the optical fiber ( 5 ) has a DGD cumulated over the length L of the fiber ( 5 ) higher than one hundred times the decoherence time of the source
DGD>100·τ DC .
5. A gyroscope according to claim 2 , characterized in that the optical fiber ( 5 ) is a birefringent fiber having a PMD coefficient in coupled mode, PMD c , comprised between 1 and 1000 ps/√km.
6. A gyroscope according to claim 2 , characterized in that the optical fiber ( 5 ) is a birefringent fiber having a group effective index difference between orthogonal polarization modes
(
Δ
Neff
=
Δ
n
eff
-
λ
×
ⅆ
Δ
n
eff
ⅆ
λ
)
at the wavelength λ comprised between 10 −6 and 2.10 −4 .
7. A gyroscope according to claim 2 , characterized in that the length L of the fiber coil ( 5 ) is comprised between 0.1 km and 20 km.
8. A gyroscope according to claim 7 , characterized in that the wavelength λ of the source is comprised between 800 and 1600 nm and in that the spectral width Δλ FWHM is comprised between 0.1% and 10% of λ.
9. An inertial navigation system comprising a gyroscope according to claim 1 .
10. A gyroscope according to claim 3 , characterized in that the optical fiber ( 5 ) has a DGD cumulated over the length L of the fiber ( 5 ) higher than one hundred times the decoherence time of the source
DGD>100·τ DC .
11. A gyroscope according to claim 3 , characterized in that the optical fiber ( 5 ) is a birefringent fiber having a PMD coefficient in coupled mode, PMD c , comprised between 1 and 1000 ps/√km.
12. A gyroscope according to claim 4 , characterized in that the optical fiber ( 5 ) is a birefringent fiber having a PMD coefficient in coupled mode, PMD c , comprised between 1 and 1000 ps/√km.
13. A gyroscope according to claim 3 , characterized in that the optical fiber ( 5 ) is a birefringent fiber having a group effective index difference between orthogonal polarization modes
(
Δ
Neff
=
Δ
n
eff
-
λ
×
ⅆ
Δ
n
eff
ⅆ
λ
)
at the wavelength λ comprised between 10 −6 and 2.10 −4 .
14. A gyroscope according to claim 4 , characterized in that the optical fiber ( 5 ) is a birefringent fiber having a group effective index difference between orthogonal polarization modes
(
Δ
Neff
=
Δ
n
eff
-
λ
×
ⅆ
Δ
n
eff
ⅆ
λ
)
at the wavelength λ comprised between 10 −6 and 2.10 −4 .
15. A gyroscope according to claim 5 , characterized in that the optical fiber ( 5 ) is a birefringent fiber having a group effective index difference between orthogonal polarization modes
(
Δ
Neff
=
Δ
n
eff
-
λ
×
ⅆ
Δ
n
eff
ⅆ
λ
)
at the wavelength λ comprised between 10 −6 and 2.10 −4 .
16. A gyroscope according to claim 3 , characterized in that the length L of the fiber coil ( 5 ) is comprised between 0.1 km and 20 km.
17. A gyroscope according to claim 4 , characterized in that the length L of the fiber coil ( 5 ) is comprised between 0.1 km and 20 km.
18. A gyroscope according to claim 5 , characterized in that the length L of the fiber coil ( 5 ) is comprised between 0.1 km and 20 km.
19. A gyroscope according to claim 6 , characterized in that the length L of the fiber coil ( 5 ) is comprised between 0.1 km and 20 km.
20. An inertial navigation system comprising a gyroscope according to claim 2 .