We present a method to fabricate fiber Bragg gratings with adjustable refractive index contrast by using the standard phase mask technique. A theoretical analysis of the diffracted field from the phase mask is performed by considering the effect of the spatial coherence of the incident UV beam. The numerical results show that the grating index contrast decreases as the separation between the fiber and the phase mask increases. Strong gratings with various index contrasts have been inscribed in hydrogen-loaded single mode fibers at different writing distances, and the measured index contrast values are in good agreement with the simulation results. Furthermore, thermal decay tests on the gratings demonstrate that the thermal stability of the grating reflectivity is improved for those gratings fabricated at larger separations between the fiber and the phase mask. These results suggest a one-step process to fabricate gratings with an enhanced thermal stability.
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