The Influence of the MCVD Process Parameters on the Optical Properties of Bismuth-Doped Phosphosilicate Fibers

In this article, we describe the technology for manufacturing of the phosphosilicate bismuth-doped optical fibers providing optical amplification near 1.32
$\mu$
m. The effect of preforming sintering process and fiber drawing on the optical properties of the fabricated fibers were studied. We showed that an increase in the vitrification temperature of the P-doped silica glass soot layers deposed on the inner surface of a fused silica tube from 1850 to
$\mathbf {\sim \text{2000}}$
 °C (while keeping other parameters unchanged) provides a noticeable reduction of the unsaturable losses. We also found that the fiber drawing speed significantly affects the optical properties of bismuth-doped fibers. In particular, an increase in the drawing speed from
$\sim$
1 to
$\sim$
80 m/min led to a decrease of both active absorption and unsaturable losses, while the ratio of active absorption to unsaturable losses became greater. As a result, the laser efficiency of the bismuth-doped fibers grew two-fold from 19.5% to 37%.