Photodarkening and photobleaching in fiber optic Bragg gratings

A photobleaching effect is reported which reduces the broadband losses induced by the single-pulse writing of fiber Bragg gratings (FBGs). A primary application of the optical FBG reflector is as arrays of wavelength-encoded sensing elements for detecting mechanical strain. Instrumentation is being developed to interrogate >100 point sensors along a single fiber with FBGs and detectors operating in the 790-820 nm band. In this paper, we describe broadband absorptions induced by writing single-pulse FBGs which would impose a 10 dB power penalty with as few as 40 serialized FBGs. We have reduced these photo-induced losses more than seven-fold (measured near 800 nm) using a photobleaching process which minimally affects the FBG's reflectivities. After treatment, optical losses are sufficiently low to allow the interrogation of hundreds of FBGs along a single fiber. Photodarkening and photobleaching are measured in gratings prepared by single laser pulses, and by multiple pulses with and without hydrogen loading. Parasitic loss following photobleaching is lower for single-pulse FBGs operated near 800 nm than for FBGs written with multiple pulses in hydrogen loaded fibers and operated near 1300 nm.