Optical fiber devices and methods for directing stimulated raman scattering (SRS) light out of a fiber core and into a cladding

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.

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.