Through-the-jacket inscription of fiber Bragg gratings using femtosecond infrared radiation for sensor applications

Abstract

Silica-based fiber Bragg gratings (FBG) sensors are versatile devices that are typically fabricated using UV laser exposure. As most standard optical fiber polymer coatings are highly absorbing in the UV, grating inscription typically requires the removal and reapplication of the protective coating by either chemical or mechanical means. Optical fiber stripping and recoating are time-consuming processes that can seriously degrade the mechanical integrity of the fiber. For high temperature sensor applications (> 200 °C), the optical fiber is coated in polyimide which is resistant to chemical attack. Invasive and hazardous techniques for its removal are required such as hot sulphuric acid stripping. In this paper, results of FBG inscription directly through the polymer coating of standard optical fiber with a femtosecond infrared laser and a phase mask are reviewed. Significant grating reflectivities are achieved along with improved mechanical reliability and performance at elevated temperatures. The only example of direct FBG inscription through polyimide coatings for high temperature stable grating sensors will also be presented.