PHOTODARKENING AND PHOTOBLEACHING IN FIBER OPTIC BRAGG GRATINGS

A photobleaching effect is reported which reduces the broadband lasses induced by the single-pulse writing of fiber Bragg gratings (FBG's). A primary application of the optical FBG reflector is as arrays of wav elength-encoded sensing elements for detecting mechanical strain. Inst rumentation is being developed to interrogate >100 point sensors along a single fiber,vith FBG's and detectors operating in the 790-820 nm b and. In this paper, we describe broadband absorptions induced by writi ng single-pulse FBG's which would impose a 10 dB power penalty with as few as 40 serialized FBG's. We have reduced these photo-induced losse s more than seven-fold (measured near 800 nm) using a photobleaching p rocess which minimally affects the FBG's reflectivities. After treatme nt, optical losses are sufficiently low to allow the interrogation of hundreds of FBG's along a single fiber. Photodarkening and photobleach ing are measured in gratings prepared by single laser pulses, and by m ultiple pulses with and without hydrogen loading. Parasitic loss follo wing photobleaching is lower for single-pulse FBG's operated near 800 mm than for FBG's written with multiple pulses in hydrogen loaded fibe rs and operated near 1300 nm.