Abstract: In one aspect, the invention relates to optical fibers and systems that include such fibers. In another aspect, the invention provides an optical fiber that includes a core, a cladding surrounding the core, a layer surrounding the cladding, and a region between the layer and the cladding. The region can comprise an index of refraction that is different than an index of refraction comprised by the cladding. In one embodiment, the region can include a void containing air or a liquid. The void can be evacuated. The region can include a solid, such as, for example, a polymer. The layer can contact the cladding. The fiber can comprise rare earth ions.

Abstract: The invention features a fiber that includes an optical fiber (110) containing a gain medium having a Raman active material with a Raman gain spectrum. The optical fiber is configured to receive energy at a wavelength ?p. The fiber also includes at least three pairs of reflectors (120, 125, 150, 130, 160) disposed in the optical fiber. Each pair of reflectors forms a resonance cavity (140) with a resonance frequency.

Abstract: Fiber laser systems. One system comprises a pump energy source, a first fiber and a WDM for transferring pump energy from the first fiber to a second fiber that comprises a grating for reflecting energy at a predetermined wavelength. The first fiber is devoid of a grating substantially reflecting energy of the predetermined wavelength. In another embodiment, a system includes a pump energy source and a fiber comprising a loop-shaped portion, a first non loop-shaped portion comprising a grating for reflecting pump energy, and a second non loop-shaped portion comprising a grating for reflecting a Stoke shifted wavelength.

Abstract: Fibers, including fiber lasers and fiber amplifiers, and systems containing such fibers are disclosed.

Abstract: In one aspect, the invention relates to optical fibers and systems that include such fibers. In another aspect, the invention provides an optical fiber that includes a core, a cladding surrounding the core, a layer surrounding the cladding, and a region between the layer and the cladding. The region can comprise an index of refraction that is different than an index of refraction comprised by the cladding. In one embodiment, the region can include a void containing air or a liquid. The void can be evacuated. The region can include a solid, such as, for example, a polymer. The layer can contact the cladding. The fiber can comprise rare earth ions.