Photonic band gap fibers using a jacket with a depressed softening temperature

A photonic band gap fiber preform, comprising:

• a central structured region comprising a first non-silica based glass, wherein the first glass has a glass viscosity at a fiber draw temperature;
• and
• a jacket comprising a second non-silica based glass, wherein the jacket surrounds the central structured region, and wherein the second glass has a glass viscosity at the fiber draw temperature;
• wherein the base ten logarithm of the glass viscosity in poise of the second glass is at least 0.01 but no more than 2 lower than the base ten logarithm of the glass viscosity in poise of the first glass at the fiber draw temperature.

A method of making a photonic band gap fiber preform, comprising the steps of:

• providing a central structured region comprising a first non-silica based glass, wherein the first glass has a Littleton softening temperature;
• and
• surrounding the central structured region with a jacket comprising a second non-silica based glass, wherein the second glass has a Littleton softening temperature, and wherein the Littleton softening temperature of the second glass is at least one but no more than ten degrees Celsius lower than the Littleton softening temperature of the first glass;
• and
• heating the jacket and central structured region wherein the second glass flows into and fills any voids between the central structured region and the jacket.