Abstract

Fiber Bragg gratings (FBGs) are essential optical components, which due to their design flexibility offer numerous prospects for a wide range of applications in fiber laser, sensor, and telecommunication technologies. Here we demonstrate that a phase mask interferometer driven by a deep-ultraviolet femtosecond laser enables inscription of FBGs with top-hat spectral reflection bandwidth. FBGs with the bandwidth of 2 nm have been achieved with inscription of several superimposed narrow FBGs with bandwidth less than 0.2 nm. The induced refractive index modulation profile of the superimposed gratings has been investigated with two measurement methods, namely, optical frequency domain reflectometry and the layer-peeling method. The analysis has shown that the spatial modulation index profile follows a sinc profile and has a very narrow central peak of less than 0.4 mm. Importantly, the FBGs provide low group delay values in comparison to their chirped counterparts. Additionally, the small center structure makes such gratings ideal for fiber sensing with high local resolution. The demonstrated FBG inscription method, developed initially to fabricate optical reflectors for infrared laser systems, can be translated to other applications, such as biophotonics, telecommunications, sensing and astrophysics.

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