Method for making low loss optical fiber
A method of forming an alkali metal oxide-doped optical fiber by diffusing an alkali metal into a surface of a glass article is disclosed. The silica glass article may be in the form of a tube or a rod, or a collection of tubes or rods. The silica glass article containing the alkali metal, and impurities that may have been unintentionally diffused into the glass article, is etched to a depth sufficient to remove the impurities. The silica glass article may be further processed to form a complete optical fiber preform. The preform, when drawn into an optical fiber, exhibits a low attenuation.
1. A method of forming an optical fiber precursor, comprising:
providing a silica glass member;
depositing additional glass onto the silica glass member to form an optical fiber precursor, wherein the silica glass member and the additional glass are essentially chlorine free;
heating an alkali metal material to form an alkali metal vapor;
exposing the optical fiber precursor to the alkali metal vapor, wherein the alkali metal vapor diffuses through a diffusion surface of the optical fiber precursor to a diffusion depth of at least about 100 μm; and
etching the diffusion surface to a depth of at least about 5% of the diffusion depth.
2. The method of claim 1 wherein a chlorine content in the deposited glass is less than 500 ppm.
3. The method of claim 1 wherein the deposited glass further comprises F, Al 2 O 3 , CaO, GeO 2 , or P.
4. The method of claim 1 wherein the deposited glass is substantially free of − OH.
5. The method of claim 4 wherein the deposited glass contains less than 100 ppb − OH.
6. A method of forming an optical fiber precursor, comprising:
providing a soot glass article;
exposing the soot glass article to a fluorine-containing atmosphere;
consolidating the soot glass article to form a core cane;
exposing the core cane to vapor comprising potassium, thereby diffusing potassium through a diffusion surface of the core cane, wherein after the diffusing, a peak potassium concentration in the core cane is at least 0.035 mole %;
etching the diffusion surface to a depth sufficient to remove diffused impurities; and
forming additional glass on the core cane.
7. The method of claim 6 further including chlorine drying the soot glass article prior to exposing the soot glass article to the flourine-containing atmosphere.
8. The method of claim 6 wherein the fluorine-containing atmosphere comprises CF 4 or SiF 4 .
9. The method of claim 6 wherein the step of exposing the soot glass article to a flourine-containing atmosphere occurs at a temperature of less than 1100° C.
10. The method of claim 6 wherein the forming additional glass comprises collapsing a glass tube onto the core cane.
11. The method of claim 6 wherein the forming additional class comprises vapor deposition.