Optical fiber devices and methods for directing Stimulated Raman Scattering (SRS) light out of a fiber
Optical fiber devices, systems, and methods for coupling Raman spectrum out of an optical fiber selectively over a signal spectrum, which may be propagated in one or more guided modes of a fiber system. A fiber system may include a chirped fiber Bragg grating (CFBG) or a long period fiber grating (LPFG), each to unguide Raman light propagating in a core propagation mode of a fiber completely out of the fiber (through any surrounding cladding layer(s)) selectively over signal spectrum which is to remain in a guided mode of the fiber.
1. An optical fiber device, comprising:
a first and a second length of optical fiber, each of the lengths of optical fiber comprising a core and one or more cladding layers, wherein the core supports at least a first propagation mode for light comprising both signal spectrum and Raman spectrum, wherein the one or more cladding layers of at least the second length of optical fiber further comprise an inner cladding layer and an outer cladding layer; and
a chirped fiber Bragg grating (CFBG) between the first and second lengths of fiber, the CFBG to unguide at least some of the light propagated in the first propagation mode with a greater efficiency over the Raman spectrum than over the signal spectrum, and to unguide at least a portion of the light within the Raman spectrum into a free-space propagation mode beyond the inner cladding layer of the second length of optical fiber and transverse to a direction of signal propagation.
2. The optical fiber device of claim 1 , wherein:
the Raman spectrum comprises one or more first wavelengths that are longer than one or more second wavelengths of the signal spectrum;
the CFBG comprises a third length of optical fiber further comprising a core, an inner cladding layer, and an outer cladding layer in contact with the inner cladding layer, wherein a refractive index of the core alternates between two values as a function of a periodicity of the CFBG that varies over the third length.
3. The optical fiber device of claim 2 , wherein the CFBG has a refractive index that varies azimuthally within the core.
4. The optical fiber device of claim 2 , further comprising an optical oscillator operable to excite at least the signal spectrum, and the optical oscillator coupled to the second length of optical fiber with the CFBG between the optical oscillator and the first length of optical fiber.
5. The optical fiber device of claim 4 , wherein:
the optical oscillator comprises a length of single mode (SM) fiber having a core that supports only the first propagation mode; and
the second length of optical fiber comprises multi-mode (MM) fiber, the core of which supports one or more guided propagation modes in addition to the first propagation mode.
6. The optical fiber device of claim 5 , further comprising an optical amplifier between the optical oscillator and the CFBG, the optical amplifier comprising a length of MM fiber doped with a gain medium operable to excite at least the signal spectrum.
7. An optical fiber device, comprising:
a first and a second length of optical fiber, each of the lengths of optical fiber comprising a core and one or more cladding layers, wherein the core supports at least a first propagation mode for light comprising both signal spectrum and Raman spectrum, wherein the one or more cladding layers of each of the first length of optical fiber and the second length of optical fiber further comprise an inner cladding layer and an outer cladding layer; and
a long period fiber grating (LPFG) between the first and second lengths of fiber, the LPFG having a period greater than half a center wavelength of the Raman spectrum, and to unguide at least some of the light propagated in a first guided mode with a greater efficiency over the Raman spectrum than over the signal spectrum, and wherein the LPFG is to unguide at least a portion of the light within the Raman spectrum into a free-space propagation mode through the inner cladding layer of the first or second lengths of fiber and transverse to a direction of signal propagation.
8. The optical fiber device of claim 7 , wherein:
the Raman spectrum comprises one or more first wavelengths that are longer than one or more second wavelengths of the signal spectrum; and
the LPFG comprises a third length of optical fiber further comprising a core and an inner cladding layer and an outer cladding layer, wherein a refractive index of the core varies between two values over the third length and with a period that exceeds 100 μm.
9. The optical fiber device of claim 8 , wherein the LPFG has a refractive index that varies azimuthally within the core.
10. The optical fiber device of claim 8 , further comprising an optical oscillator to excite at least the signal spectrum, wherein the optical oscillator is coupled to the first length of optical fiber with the LPFG between the optical oscillator and the second length of optical fiber.
11. The optical fiber device of claim 10 , wherein:
the optical oscillator comprises a length of single mode (SM) fiber having a core that supports only the first propagation mode; and
the second length of optical fiber comprises multi-mode (MM) fiber, the core of which supports one or more guided modes in addition to the first propagation mode.
12. The optical fiber device of claim 11 , further comprising an optical amplifier between the optical oscillator and the LPFG, the optical amplifier comprising a length of MM fiber having a core doped with a gain medium to excite at least the signal spectrum.
13. A method of filtering Raman spectrum from an optical fiber system, the method comprising:
propagating light in a core mode of a first length of optical fiber, wherein the first length of optical fiber comprises a core and one or more cladding layers, and the light comprises both signal spectrum and Raman spectrum; and
unguiding with a fiber grating at least some of the light from the core mode into free-space propagation transverse to a direction of signal propagation, the unguiding being selective to light within the Raman spectrum, wherein the fiber grating comprises a chirped fiber Bragg grating (CFBG) or a long period fiber grating (LPFG).
14. The method of claim 13 , wherein:
the one or more cladding layers of both the first and second lengths of fiber further comprise an inner cladding layer and an outer cladding layer; and
unguiding at least some of the light from the core mode further comprises unguiding the light through the inner cladding layer.
15. The method of claim 14 , wherein:
the unguiding further comprises unguiding Raman spectrum energy in a first propagation mode with the CFBG.
16. The method of claim 15 , wherein a first end of the CFBG having a larger period is proximal to an optical oscillator of the fiber system, and second end of the CFBG having a shorter period is proximal to an output of the fiber system.
17. The method of claim 15 , wherein the CFBG has a smallest period that is no more than half a center wavelength of the Raman spectrum.
18. The method of claim 14 , wherein:
the unguiding further comprises unguiding Raman spectrum energy in the first propagation mode with the LPFG.
19. The method of claim 18 , wherein the LPFG has a period that is no less than half a center wavelength of the Raman spectrum.