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Methods and apparatus relate to optical fibers suitable for use in sensing applications exposed to radiation environments. The fibers include a core of pure silica or chlorine doped silica surrounded by a fluorinated silica cladding. These glasses for the core and cladding utilize dopants that resis

Methods and apparatus relate to optical fibers suitable for use in sensing applications exposed to radiation environments. The fibers include a core of pure silica or chlorine doped silica surrounded by a fluorinated silica cladding. These glasses for the core and cladding utilize dopants that resist radiation-induced attenuation. A two step process for forming the cladding can achieve necessary concentrations of the fluorine by performing a soot deposition process in a different environment from a consolidation process where the soot is sintered into a glass. Concentration of fluorine doped into the cladding layer enables obtaining a numerical aperture that confines a mono-mode of the fiber to resist bend-induced attenuation. Dimensions of the fiber further facilitate bending ability of the fiber.

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What is claimed is: 1. A method of forming an optical waveguide, comprising: depositing at a first temperature a silica soot layer inside a substrate tube using a first gas flow comprising silicon containing a halide and oxygen; consolidating the soot layer at a second temperature higher than the f

What is claimed is: 1. A method of forming an optical waveguide, comprising: depositing at a first temperature a silica soot layer inside a substrate tube using a first gas flow comprising silicon containing a halide and oxygen; consolidating the soot layer at a second temperature higher than the first temperature, wherein the consolidating occurs in an environment of a second gas flow comprising substantially pure fluorine precursor gas to produce a fluorine doped cladding glass; and depositing a chlorine-doped silica layer onto the cladding glass to provide a core glass wherein the core glass forms a solid cylinder surrounded by the cladding glass. 2. The method of claim 1, wherein the consolidating the soot layer and depositing the silica layer are selected to provide the cladding glass with a first refractive index at least 0.008 relatively lower than a second refractive index of the core glass. 3. The method of claim 1, wherein depositing the silica layer includes forming a core soot layer and then consolidating the core soot layer at a different temperature and flow parameters than during forming of the core soot layer. 4. The method of claim 1, further comprising collapsing the substrate tube after forming the cladding and core glasses to provide a preform. 5. The method of claim 4, further comprising drawing the preform to produce a single mode fiber with an outermost glass diameter of about 80 microns. 6. The method of claim 1, further comprising providing the substrate tube that is pure silica. 7. The method of claim 1, further comprising providing the substrate tube that is fluorinated silica with a fluorine concentration less than the cladding glass. 8. The method of claim 1, wherein the halide comprises a metal halide.