Optical fiber devices and methods for directing stimulated raman scattering (SRS) light out of a fiber core and into a cladding
Optical fiber devices, systems, and methods for separating Raman spectrum from signal spectrum. Once separated, the Raman spectrum may be suppressed (e.g., as a result of a reduction in gain from the signal spectrum, and/or through dissipation of the Raman spectrum energy), while the signal spectrum may be propagated in one or more guided modes of a fiber system. In some embodiments, a fiber system may include a chirped fiber Bragg grating (CFBG) or a long period fiber grating (LPFG), each configured to couple a core propagation mode into a cladding propagation mode with an efficiency that is higher for Raman spectrum than for signal spectrum. A fiber system further may include a cladding light stripper (CLS) configured to preferentially remove cladding modes containing the Raman component.
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 two or more cladding layers, wherein the core supports at least a first propagation mode for light comprising both signal spectrum and Raman spectrum, the Raman spectrum comprises one or more first wavelengths that are longer than one or more second wavelengths of the signal spectrum, the two or more cladding layers of at least the first length of optical fiber comprise an inner cladding layer and an outer cladding layer in contact with the inner cladding layer; and
a chirped fiber Bragg grating (CFBG) between the first and second lengths of fiber, the CFBG to couple at least some of the light propagated in the first propagation mode into one or more cladding modes of the first or second length of fiber with a greater coupling efficiency over the Raman spectrum than over 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 of optical fiber;
an optical oscillator to excite at least the signal spectrum, and the optical oscillator coupled to the first length of optical fiber with the CFBG between the optical oscillator and the second length of optical fiber, wherein the CFBG is to couple the first propagation mode into one or more counter-propagating cladding modes supported by the inner cladding layer of the first length of optical fiber; and
a cladding light stripper CLS to dissipate light from the cladding mode, the CLS comprising a fourth length of optical fiber having one or more cladding features to perturb light propagating in the cladding mode, the fourth length of optical fiber being located between the optical oscillator and the CFBG to exclude the light from the cladding mode from reaching the optical oscillator.
2. The optical fiber device of claim 1 , wherein the CFBG has a refractive index that varies azimuthally within the core.
3. The optical fiber device of claim 1 , wherein the CFBG has a direction of chirp relative to the optical oscillator to reflect at least a portion of the light within the Raman spectrum that propagates toward the optical oscillator in the first propagation mode of the first fiber length into light that counter-propagates away from the optical oscillator in the first propagation mode of the first fiber length.
4. The optical fiber device of claim 1 , 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 fiber comprises multi-mode (MM) fiber, the core of which supports one or more guided propagation modes in addition to the first propagation mode.
5. The optical fiber device of claim 4 , further comprising an optical power amplifier between the optical oscillator and the CFBG, the optical power amplifier comprising a length of MM fiber having a core doped with a gain medium to excite at least the signal spectrum.
6. An optical fiber device, comprising:
a first and a second length of optical fiber, each of the lengths of fiber comprising a core and two or more cladding layers, wherein the core supports at least a first propagation mode for light comprising both signal spectrum and Raman spectrum, the Raman spectrum comprises one or more first wavelengths that are longer than one or more second wavelengths of the signal spectrum, the two or more cladding layers of each of the first and second lengths of optical fiber 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 the LPFG to couple at least some of the light propagated in a first guided mode into a cladding mode with a greater coupling efficiency over the Raman spectrum than over the signal spectrum, wherein the LPFG comprises a third length of optical fiber comprising a core and an inner cladding layer and an outer cladding layer, wherein a refractive index of the core varies between at least two values over the third length, and with a period that exceeds 100 um;
an optical oscillator to excite at least the signal spectrum, and the optical oscillator coupled to the first length of optical fiber with the LPFG between the optical oscillator and the second length of optical fiber, wherein the LPFG is to couple the first propagation mode into one or more co-propagating cladding modes supported by the inner cladding layer of each of the first and second lengths of optical fiber; and
a cladding light stripper CLS to dissipate light from the cladding mode, wherein the CLS comprises a fourth length of optical fiber having one or more cladding features to perturb light propagating in the cladding mode, the fourth length of optical fiber is located between the optical oscillator and the LPFG to exclude the light from the cladding mode from reaching the optical oscillator.
7. The optical fiber device of claim 6 , wherein the LPFG has a refractive index that varies azimuthally within the core.
8. The optical fiber device of claim 6 , 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 fiber comprises multi-mode (MM) fiber, the core of which supports one or more guided modes in addition to the first propagation mode.
9. The optical fiber device of claim 8 , further comprising an optical power amplifier between the optical oscillator and the LPFG, the optical power amplifier comprising a length of MM fiber having a core doped with a gain medium to excite at least the signal spectrum.
10. The optical fiber device of claim 6 , wherein:
the one or more cladding layers of at least the second length of fiber further comprise an inner cladding layer and an outer cladding layer; and
the grating is to couple the first guided mode into a cladding mode supported by the inner cladding layer.
11. 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 fiber comprises a core and one or more cladding layers, and the light comprises both signal spectrum and Raman spectrum;
coupling at least some of the light from the core mode into a cladding mode with a fiber grating that has greater coupling efficiency over the Raman spectrum than over the signal spectrum, wherein the fiber grating comprises a chirped fiber Bragg grating (CFBG) or a long period fiber grating (LPFG); and
propagating the cladding mode in a second length of optical fiber in the case of the LPFG or the first length of optical fiber in the case of the CFBG, wherein the second length of fiber comprises a core and one or more cladding layers; and
stripping, with a cladding light stripper CLS, the light from the cladding mode to exclude light from the cladding mode from reaching an optical oscillator located between the first and second lengths of optical fiber,
wherein the CLS is disposed between the optical oscillator and the CFBG or LPFG.
12. The method of claim 11 , 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
coupling at least some of the light from the core mode into the cladding mode further comprises coupling a first guided mode into a cladding mode supported by the inner cladding layer.
13. The method of claim 12 , wherein:
coupling at least some of the light from the core mode into the cladding mode further comprises coupling a first propagation mode with the CFBG into one or more counter-propagating cladding modes supported by the inner cladding layer of the second first length of fiber.
14. The method of claim 12 , wherein coupling at least some of the light from the core mode into the cladding mode further comprises coupling a first propagation mode with the LPFG into one or more co-propagating cladding modes supported by the inner cladding layer of the second length of fiber.
15. The method of claim 12 , further comprising stripping the cladding mode from the inner cladding layer over a third length of fiber.