Abstract
Type I fiber Bragg gratings (FBG) written through the coating of various off-the-shelf silica fibers with a femtosecond laser and the phase-mask technique are reported. Inscription through most of the common coating compositions (acrylate, silicone and polyimide) is reported as well as writing through the polyimide coating of various fiber cladding diameters, down to 50 µm. The long term annealing behavior of type I gratings written in a pure silica core fiber is also reported as well as a comparison of the mechanical resistance of type I and II FBG. The high mechanical resistance of the resulting type I FBG is shown to be useful for the fabrication of various distributed FBG arrays written using a single period phase-mask. The strain sensing response of such distributed arrays is also presented.
Highlights
Fiber Bragg grating (FBG) has been an active research domain for the last 40 years [1].Sensor applications of FBG were first proposed 30 years ago [2] and continue nowadays to be an active research field [3]
FBG writing can be achieved with UV radiation [4,5,6] or with infrared (IR) femtosecond pulses [7,8]
The index change related to type I FBG is generally resulting from a combination of glass densification and color centers formation [6], while type II FBG is resulting from a damage process in the glass matrix [9]
Summary
Fiber Bragg grating (FBG) has been an active research domain for the last 40 years [1].Sensor applications of FBG were first proposed 30 years ago [2] and continue nowadays to be an active research field [3]. UV-induced modifications of the refractive index rely on single- and multi-photon absorption [5,6,10,11] while writing with IR fs pulses rely exclusively on a multi-photon absorption process [8,9]. Such process has some significant advantages for sensing applications as it makes writing in non-UV photosensitive glasses possible. The devices reported in this paper have all been written through the coating (WTC)
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