Analysis of Partial Crystallization in Yb3+ Doped Aluminophosphosilicate Fiber Preforms Prepared with Organic Chelate Precursor Doping Technique

Abstract

Yb3+ doped aluminophosphosilicate fiber preforms have been prepared by modified chemical vapor deposition (MCVD) with organic chelate precursor doping technique (OCD). It was found that the flow ratio of Yb(thd)3/POCl3 had an important influence on partial crystallization in the preform cores. The wholly transparent core was obtained when the flow ratio of Yb(thd)3/POCl3 ranged in 0.71:1–0.82:1. Crystallization occurred during etching process when the Yb(thd)3/POCl3 flow ratio was over 0.83:1, corresponding to a doping ratio of Yb2O3/P2O5 larger than 1.01:1, as characterized by electron probe microanalysis (EPMA). X-ray photoelectric spectroscopy (XPS) analysis indicated exsistence of P in the quasi-metaphosphate [PO4] bond structures in the crystallized area. Raman spectra displayed characteristic peaks of silica glasses except 1330 cm−1 Raman peaks, which can be assigned to P=O bond due to the presence of quasi-metaphosphate [PO4] units. When the content ratio of Yb2O3/P2O5 exceeded 1.01:1, Al was depleted and P was enriched, and the Yb3+ ions was mainly connected with the quasi-metaphosphate [PO4] units, which weakened the role of P and Al preventing Yb3+ ions clustering. As a result, crystallization occurred. And this schematic glass structural model was proposed for the partially crystallized preform core.