Analysis of the fluorescence method of profiling single-mode optical fiber preforms

A fluorescence method of measuring the index of refraction of single-mode optical fiber preforms has recently been reported based on germania fluorescence under UV excitation.1 The germania glass UV absorption spectrum consists of three distinct absorption bands.2 A band at 185 nm is due to the GeO2 species, while bands at 242 and 325 nm are assigned to GeO.2,3 The sources used for this technique excite primarily the absorption band at 242 nm; hence trace amounts of GeO determine the fluorescence intensity. Since the total germanium concentration determines the index, the ratio of GeO to GeO2 must be constant for the technique to work. However, GeO may be produced as a result of chemical reduction or thermal decomposition of GeO2, which implies that the ratio of GeO to GeO2 may be process dependent. Several experiments were undertaken to test this hypothesis.