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In various embodiments, an optical fiber includes a core having a relatively large area selected so as to raise a threshold of stimulated Raman scattering or stimulated Brillouin scattering, or both, the core having a high aspect ratio elongated cross-section and having a first refractive index. The

In various embodiments, an optical fiber includes a core having a relatively large area selected so as to raise a threshold of stimulated Raman scattering or stimulated Brillouin scattering, or both, the core having a high aspect ratio elongated cross-section and having a first refractive index. The core is narrower in a fast-axis direction and wider in a slow-axis direction, such that the fiber is mechanically flexible in the fast-axis direction and is mechanically rigid in the slow-axis direction.

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1. An optical fiber, comprising: a core having a high aspect ratio elongated cross-section and having a first refractive index;first and second signal claddings positioned in contact with and sandwiching the core, the first and second signal claddings having a second refractive index; anda third cla

1. An optical fiber, comprising: a core having a high aspect ratio elongated cross-section and having a first refractive index;first and second signal claddings positioned in contact with and sandwiching the core, the first and second signal claddings having a second refractive index; anda third cladding substantially surrounding at least slow-axis edges of the core, the third cladding having a third refractive index, the third cladding in contact with the slow-axis edges of the core;wherein the core is narrower in a fast-axis direction and wider in a slow-axis direction such that the fiber is mechanically flexible in the fast-axis direction and is mechanically rigid in the slow-axis direction. 2. The optical fiber of claim 1, wherein a difference between the first refractive index of the core and the third refractive index of the third cladding is less than about 500 ppm to provide a loss for any higher-order transverse electromagnetic modes that is greater than a loss for lower-order transverse electromagnetic modes so as to substantially remove the higher-order transverse electromagnetic modes along the slow-axis direction within the fiber. 3. The optical fiber of claim 1, wherein a width of the core along the slow-axis direction is selected so as to be greater than a width of a laser beam input into the fiber. 4. The optical fiber of claim 1, wherein a width of the core exceeds a width of a diffracted laser beam at an output end of the fiber. 5. An optical fiber, comprising: a core having a high aspect ratio elongated cross-section and having a refractive index;wherein the core is narrower in a fast-axis direction and wider in a slow-axis direction such that the fiber is mechanically flexible in the fast-axis direction and is mechanically rigid in the slow-axis direction; andwherein a width wb of a laser beam input into the fiber is such that a Fresnel length LFr that is proportional to wb2n/λ exceeds a fiber length L, where n is the refractive index of the core and λ is a wavelength of the laser beam input into the fiber. 6. An optical fiber comprising: a core having a high aspect ratio elongated cross-section and having a first refractive index;first and second signal claddings positioned in contact with and sandwiching the core, the first and second signal claddings having a second refractive index; anda third cladding substantially surrounding at least slow-axis edges of the core, the third cladding having a third refractive index;wherein the core is narrower in a fast-axis direction and wider in a slow-axis direction such that the fiber is mechanically flexible in the fast-axis direction and is mechanically rigid in the slow-axis direction; andwherein the third refractive index of the third cladding is selected so that a numerical aperture of the third cladding at an outside boundary surface is less than 0.06 so as to allow laser radiation leaking into the third cladding to leak out of the third cladding. 7. An optical fiber comprising: a core having a high aspect ratio elongated cross-section and having a first refractive index;first and second signal claddings positioned in contact with and sandwiching the core, the first and second signal claddings having a second refractive index;a third cladding substantially surrounding at least slow-axis edges of the core, the third cladding having a third refractive index; anda collar attached to the third cladding, wherein a fourth refractive index of the collar is essentially equal to the third refractive index of the third cladding;wherein the core is narrower in a fast-axis direction and wider in a slow-axis direction such that the fiber is mechanically flexible in the fast-axis direction and is mechanically rigid in the slow-axis direction. 8. An optical fiber comprising: a core having a high aspect ratio elongated cross-section and having a first refractive index;first and second signal claddings positioned in contact with and sandwiching the core, the first and second signal claddings having a second refractive index;a third cladding substantially surrounding at least slow-axis edges of the core, the third cladding having a third refractive index; anda coating covering the third cladding, wherein the coating is configured to be transparent to laser radiation leaking into the third cladding;wherein the core is narrower in a fast-axis direction and wider in a slow-axis direction such that the fiber is mechanically flexible in the fast-axis direction and is mechanically rigid in the slow-axis direction. 9. The optical fiber of claim 1, wherein the third cladding substantially surrounds the core and the first and second signal claddings. 10. The optical fiber of claim 1, wherein the fiber is configured to transmit a laser beam having a power greater than or equal to about 10 kW. 11. The optical fiber of claim 1, wherein the core has a substantially rectangular shape elongated in the slow-axis direction. 12. The optical fiber of claim 1, wherein the fiber has a substantially rectangular external shape elongated in the slow-axis direction so as to allow mechanical flexing in the fast-axis direction and to resist mechanical flexing in the slow-axis direction. 13. An optical fiber, comprising: a core having a high aspect ratio elongated cross-section and having a refractive index; anda twisted portion occurring over a fiber length near an output end of the fiber, wherein the fiber length near the output end of the fiber is shorter than a characteristic distortion length that is related to a twist-induced focal length;wherein the core is narrower in a fast-axis direction and wider in a slow-axis direction such that the fiber is mechanically flexible in the fast-axis direction and is mechanically rigid in the slow-axis direction. 14. The optical fiber of claim 13, wherein: the characteristic distortion length Ldist is proportional to wcore/δθF;δθF=(wb/2F) is a twist-induced divergence angle;wcore is a width of the core;wb is a width of an input laser beam; andF=δL/φ2 is the twist-induced focal length associated with a twisting angle φ. 15. The optical fiber of claim 1, wherein a beam divergence is conserved from an input end of the fiber to an output end of the fiber. 16. An optical fiber, comprising: a core having a high aspect ratio elongated cross-section and having a refractive index, wherein the core is narrower in a fast-axis direction and wider in a slow-axis direction such that the fiber is mechanically flexible in the fast-axis direction and is mechanically rigid in the slow-axis direction; anda bent portion bent in the fast-axis direction and a twisted portion, wherein the twisted portion is distinct from the bent portion so as to maintain substantially a same beam quality throughout the fiber. 17. The optical fiber of claim 16, wherein the optical fiber further comprises: first and second signal claddings positioned in contact with and sandwiching the core, the first and second signal claddings having a second refractive index; anda third cladding substantially surrounding at least slow-axis edges of the core, the third cladding having a third refractive index. 18. An optical fiber, comprising: a core having a high aspect ratio elongated cross-section and having a refractive index, wherein the core is narrower in a fast-axis direction and wider in a slow-axis direction such that the fiber is mechanically flexible in the fast-axis direction and is mechanically rigid in the slow-axis direction; anda twisted portion, wherein the twisted portion of the fiber induces an optical distortion within the fiber, and wherein the twisted portion is configured to compensate for the optical distortion, thereby yielding a transmitted beam in which no net optical distortion remains after passing through the twisted portion. 19. The optical fiber of claim 18, wherein: the optical distortion is a negative lens within the fiber; andthe negative lens is compensated by building into the core of the twisted portion a positive lens having opposite and substantially equal lens power to a power of the negative lens. 20. The optical fiber of claim 18, wherein: the optical distortion is a polarization distortion within the fiber; andthe polarization distortion is compensated using at least one polarization component. 21. The optical fiber of claim 19, wherein the positive lens comprises at least one of: a slow-axis refractive index variation in the core or a profiled thickness of the core along the slow-axis direction so as to make the core thicker at a middle along a fiber axis and narrower near edges of the core. 22. The optical fiber of claim 19, wherein a magnitude of the negative lens is tuned to cancel out the positive lens, thereby yielding the transmitted beam in which no net beam-quality distortion remains after passing through the twisted portion. 23. The optical fiber of claim 22, wherein the negative lens is tuned by varying a length of the twisted portion. 24. The optical fiber of claim 1, wherein onset of stimulated Raman scattering and onset of stimulated Brillouin scattering are controlled by selecting at least one of: a length of the fiber, an area of the core of the fiber, a power of laser radiation propagating within the fiber, or a medium dependent Raman gain. 25. An optical fiber, comprising: a core having a high aspect ratio elongated cross-section and having a refractive index;wherein the core is narrower in a fast-axis direction and wider in a slow-axis direction such that the fiber is mechanically flexible in the fast-axis direction and is mechanically rigid in the slow-axis direction; andwherein, for a given power of laser radiation and for a given Raman gain, the core is configured with a sufficiently large area such that at least one of stimulated Raman scattering or stimulated Brillouin scattering is prevented from occurring at fiber lengths greater than approximately 2 meters. 26. An optical apparatus comprising: a laser;an optical fiber having an input end and an output end; andan optical coupler configured to input radiation from the laser through the input end of the optical fiber;the optical fiber comprising: a core having a high aspect ratio elongated cross-section and having a first refractive index;first and second signal claddings positioned in contact with and sandwiching the core, the first and second signal claddings having a second refractive index; anda third cladding substantially surrounding at least slow-axis edges of the core, the third cladding having a third refractive index, the third cladding in contact with the slow-axis edges of the core;wherein the core is narrower in a fast-axis direction and wider in a slow-axis direction such that the fiber is mechanically flexible in the fast-axis direction and is mechanically rigid in the slow-axis direction. 27. The optical apparatus of claim 26, wherein a difference between the first refractive index of the core and the third refractive index of the third cladding is less than about 500 ppm to provide a loss for any higher-order transverse electromagnetic modes that is greater than a loss for lower-order transverse electromagnetic modes so as to substantially remove the higher-order transverse electromagnetic modes along the slow-axis direction within the fiber. 28. An optical apparatus comprising: a laser;an optical fiber having an input end and an output end; andan optical coupler configured to input radiation from the laser through the input end of the optical fiber;the optical fiber comprising a core having a high aspect ratio elongated cross-section and having a refractive index;wherein the core is narrower in a fast-axis direction and wider in a slow-axis direction such that the fiber is mechanically flexible in the fast-axis direction and is mechanically rigid in the slow-axis direction; andwherein the laser includes a fiber laser and the optical coupler includes a planar gradient index (GRIN) lens having an index gradient, a plane of the index gradient substantially parallel to the slow-axis direction of the fiber, the optical coupler configured to couple the fiber laser to the input end of the optical fiber. 29. The optical apparatus of claim 28, wherein a length of the optical coupler is selected to be approximately equal to a quarter-pitch length of the GRIN lens. 30. The optical apparatus of claim 26, wherein the laser and the optical fiber are attached to opposite ends of the optical coupler to form a monolithic device. 31. The optical apparatus of claim 26, wherein the laser and the optical fiber are attached to opposite ends of the optical coupler using fusion splicing. 32. The optical apparatus of claim 26, wherein the optical coupler is configured to reformat a beam of the laser into a collimated high-aspect ratio elliptical beam for input to the optical fiber. 33. The optical apparatus of claim 26, wherein the optical coupler comprises at least one free-space lens configured to direct the radiation from the laser into the optical fiber. 34. An optical apparatus comprising: an optical fiber having an input end and an output end; anda first end cap connected to the output end of the optical fiber;the optical fiber comprising: a core having a high aspect ratio elongated cross-section and having a first refractive index;first and second signal claddings positioned in contact with and sandwiching the core, the first and second signal claddings having a second refractive index; anda third cladding substantially surrounding at least slow-axis edges of the core, the third cladding having a third refractive index, the third cladding in contact with the slow-axis edges of the core;wherein the core is narrower in a fast-axis direction and wider in a slow-axis direction such that the fiber is mechanically flexible in the fast-axis direction and is mechanically rigid in the slow-axis direction. 35. An optical apparatus comprising: an optical fiber having an input end and an output end; andan end cap connected to the output end of the optical fiber;the optical fiber comprising: a core having a high aspect ratio elongated cross-section and having a refractive index,wherein the core is narrower in a fast-axis direction and wider in a slow-axis direction such that the fiber is mechanically flexible in the fast-axis direction and is mechanically rigid in the slow-axis direction; andwherein the end cap comprises an exit surface through which a laser beam input through the input end exits the end cap, the exit surface positioned at a location relative to the output end of the optical fiber where a laser beam size in the fast-axis direction is substantially equal to a laser beam size in the slow-axis direction. 36. The optical apparatus of claim 35, wherein the exit surface of the end cap is shaped so as to reduce a divergence of the laser beam along the fast-axis direction to a divergence of the laser beam along the slow-axis direction without changing the divergence of the laser beam along the slow-axis direction. 37. The optical apparatus of claim 36, wherein the divergence of the laser beam along the fast-axis direction is substantially equal to the divergence of the laser beam along the slow-axis direction. 38. The optical apparatus of claim 34, further comprising a second end cap connected to the input end of the optical fiber, wherein the second end cap is configured to receive a laser beam. 39. A method for fiber delivery of high power laser beams, the method comprising: launching a laser beam into a fiber including: a core having a high aspect ratio elongated cross-section and having a first refractive index;first and second signal claddings positioned in contact with and sandwiching the core, the first and second signal claddings having a second refractive index; anda third cladding substantially surrounding at least slow-axis edges of the core, the third cladding having a third refractive index, the third cladding in contact with the slow-axis edges of the core;wherein the core is narrower in a fast-axis direction and wider in a slow-axis direction such that the fiber is mechanically flexible in the fast-axis direction and is mechanically rigid in the slow-axis direction; andwherein the laser beam has a width smaller than a width of the core along the slow-axis direction; andpropagating the laser beam through the fiber while avoiding onset of at least one of: stimulated Raman Scattering or stimulated Brillouin Scattering. 40. The method of claim 39, further comprising delivering the laser beam to a desired location by at least one of bending or twisting the fiber along at least a portion of the fiber. 41. The method of claim 40, wherein at least one of bending or twisting the fiber comprises bending a first portion of the fiber and twisting a second portion of the fiber distinct from the first portion so as to maintain substantially a same beam quality throughout the fiber. 42. The method of claim 41, further comprising compensating an optical distortion within the second portion of the fiber induced by the twisting of the second portion of the fiber, thereby yielding a transmitted beam in which no net optical distortion remains after passing through the twisted portion. 43. The method of claim 42, wherein compensating the optical distortion comprises compensating a negative lens within the fiber by building, into the core of the twisted portion, a positive lens having opposite and substantially equal lens power as that of a lens power of the negative lens. 44. The method of claim 43, further comprising tuning a magnitude of the negative lens to cancel out the positive lens, thereby yielding the transmitted beam in which essentially no net beam-quality distortion remains after passing through the twisted second portion. 45. The method of claim 44, wherein tuning the negative lens comprises varying a length of the twisted second portion. 46. A method for fiber delivery of high power laser beams, the method comprising: launching a laser beam into a fiber including: a core having a high aspect ratio elongated cross-section and having a first refractive index;first and second signal claddings positioned in contact with and sandwiching the core, the first and second signal claddings having a second refractive index; anda third cladding substantially surrounding at least slow-axis edges of the core, the third cladding having a third refractive index, the third cladding in contact with the slow-axis edges of the core;wherein the core is narrower in a fast-axis direction and wider in a slow-axis direction such that the fiber is mechanically flexible in the fast-axis direction and is mechanically rigid in the slow-axis direction; andwherein the laser beam has a width smaller than a width of the core along the slow-axis direction; andpropagating the laser beam through the fiber to provide a loss for any higher-order transverse electromagnetic modes that is greater than a loss for lower-order transverse electromagnetic modes so as to substantially remove the higher-order transverse electromagnetic modes along the slow-axis direction within the fiber while reducing onset of at least one of stimulated Raman Scattering or stimulated Brillouin Scattering. 47. The method of claim 46, further comprising delivering the laser beam to a desired location by at least one of bending or twisting the fiber along at least a portion of the fiber. 48. The method of claim 47, wherein at least one of bending or twisting the fiber comprises bending a first portion of the fiber and twisting a second portion of the fiber distinct from the first portion so as to maintain substantially a same beam quality throughout the fiber. 49. The method of claim 48, further comprising compensating an optical distortion within the second portion of the fiber induced by the twisting of the second portion of the fiber, thereby yielding a transmitted beam in which no net optical distortion remains after passing through the twisted portion. 50. The method of claim 49, wherein compensating the optical distortion comprises compensating a negative lens within the fiber by building, into the core of the twisted portion, a positive lens having opposite and substantially equal lens power as that of a lens power of the negative lens. 51. The method of claim 50, further comprising tuning a magnitude of the negative lens to cancel out the positive lens, thereby yielding the transmitted beam in which essentially no net beam-quality distortion remains after passing through the twisted second portion. 52. The method of claim 51, wherein tuning the negative lens comprises varying a length of the twisted second portion. 53. The optical apparatus of claim 26, wherein the third cladding is in contact with substantially all of the slow-axis edges of the core. 54. The optical fiber of claim 1, wherein the third cladding is in contact with substantially all of the slow-axis edges of the core.