Abstract: A fiber Raman laser is configured with a microstructured double clad passive fiber which has an inner cladding receiving and guiding a high intensity pump light. The double-clad passive fiber farther has a eon surrounded by the inner cladding and an outer cladding. An arrangement of air holes is configured to define the inner, waveguiding cladding so that an NA of the latter varies between about 0.25-0.9 allowing this to reduce the diameter of the inner cladding. The passive fiber is characterized by a substantial overlap between the pump light and 1st stokes in the care and further includes an absorber operative to substantially suppress the signal light at the 2nd strokes so that the Ge-doped fiber outputs a SM, bright radiation at up to kW levels.

Abstract: A fiber Raman laser is configured with a microstructured double clad passive fiber which has an inner cladding receiving and guiding a high intensity pump light. The double-clad passive fiber farther has a eon surrounded by the inner cladding and an outer cladding. An arrangement of air holes is configured to define the inner, waveguiding cladding so that an NA of the latter varies between about 0.25-0.9 allowing this to reduce the diameter of the inner cladding. The passive fiber is characterized by a substantial overlap between the pump light and 1st stokes in the care and further includes an absorber operative to substantially suppress the signal light at the 2nd strokes so that the Ge-doped fiber outputs a SM, bright radiation at up to kW levels.

Abstract: The invention provides an optical fiber composition which has a high resistance to aging upon exposure to hydrogen. The inventive optical fiber does not require further treatment steps, such as passivation or hermetic coating, to meet specifications for hydrogen aging. The inventive optical fiber has a core glass composition with a germania doping concentration from 0% to 4% in weight. Decreasing the dopant concentration of germanium increases the resistance of the fiber to hydrogen by reducing the number of germanium defect sites in the core of the fiber. The optical fiber composition is advantageous for rare earth-doped fibers which are particularly sensitive to hydrogen aging.

Abstract: The invention provides an optical fiber composition which has a high resistance to aging upon exposure to hydrogen. The inventive optical fiber does not require further treatment steps, such as passivation or hermetic coating, to meet specifications for hydrogen aging. The inventive optical fiber has a core glass composition with a germania doping concentration from 0% to 4% in weight. Decreasing the dopant concentration of germanium increases the resistance of the fiber to hydrogen by reducing the number of germanium defect sites in the core of the fiber. The optical fiber composition is advantageous for rare earth-doped fibers which are particularly sensitive to hydrogen aging.

Abstract: An optical fiber preform is made by modifying a conventional preform tube having a cladding zone and a core zone, before its thermal collapse normally followed by fiber drawing. The modification is accomplished by depositing, e.g. by MCVD, a thin protective layer of a light-transmissive material, e.g. silica, on the inner surface of the preform tube, over the core zone, before the collapse step. The material of the protective layer has a higher viscosity than the material of the core zone. The protective layer is deposited over the inner (core) zone of the preform tube and its thickness and composition is selected to prevent excessive viscosity drop of the typically alumina-doped core zone during the collapse step. The provision of the protective, viscosity-controlling protective layer is helpful in maintaining good roundness of the collapsed preform tube.