Enhanced temperature sensitivity of thermally regenerated direct ultraviolet written gratings in germanium doped core fiber

Abstract

The use of thermally regenerated gratings for monitoring harsh environments is becoming increasingly attractive due to their thermal resilience and high precision. They are a unique type of Bragg grating created through annealing UV-laser written Fiber Bragg Gratings (FBGs) at high temperatures (above 600°C) and they have been demonstrated to operate at temperatures over 1000 °C in oxygen free environments. In this work we report two new observations. Firstly, a second phase thermal regeneration process at temperatures above 650°C and secondly an enhanced thermal response for these second phase regenerated gratings of over 140%. Uniquely, fabrication of the gratings was made through a small spot Direct Ultraviolet Writing (DUW) process. This utilizes a high precision four-axis stage system and a frequency double argon-ion continuous wave 244 nm laser that is split and recombined at a focus to form a ~7 µm diameter interferometric spot. A series of 46 different FBGs, from 1400 to 1620 nm, were written into a single non-hydrogen loaded double clad germanium doped 4 µm core fiber (Nufern GF4A). Gratings in the series were written with fluences ranging from 0.05 kJ/cm² up to 90 kJ/cm².&more...

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Identifiers

ORCID for Peter Smith: orcid.org/0000-0003-0319-718X

ORCID for Christopher Holmes: orcid.org/0000-0001-9021-3760

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