An interesting characteristic of the borate glass is that, there are
variations in its structural properties when RE cations are introduced.
The structure of the borate glasses is not a random distribution
of BO3 triangles and BO4 tetrahedral, but a gathering of these
units to form well-defined and stable borate groups such as
diborate, triborate, tetraborate, etc., that constitute the random
three-dimensional network [1]. These things make the borate
glasses as one of the best choices for RE doping. Borate glass is a
suitable optical material for RE ions with high transparency, low
melting point, high thermal stability, good RE ion solubility [5,6]
and show more clear relationship between glass structure and
physical properties.