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Characterization of surface crystallization in Ge-doped graded-index silica glass.

Surface crystallization was induced in Ge-doped silica glass samples from a graded-index optical fiber preform by re-heating them at 1100 degrees C for several hours. X-ray diffraction and second-harmonic generation (SHG) microscopy have been utilized to investigate the crystalline phases formed. Experimental results indicate that the predominant crystalline phase is alpha-cristobalite. The cross-sectional distribution of the crystal particles has also been measured with the SHG microscopy and the result is in good agreement with that from traditional bright field light microscopy.