Abstract

Fiber Bragg gratings (FBGs) are key elements for fiber lasers or sensors to measure strain or temperature. Using femtosecond laser pulses for inscription provides high temperature stable modifications [1]. Point by point inscription enables very flexible control of the Bragg wavelength however at the expense of coupling to cladding modes [2, 3]. In contrast a phase mask technique, where the interference behind the phase mask produces a grating [4], allows to suppress coupling to cladding modes by scanning fiber and phase mask with respect to the beam [5]. This results in high reproducibility however at the cost of a fixed period. Several approaches exist to overcome this limitation, e.g. by tuning the wavefront [6], all having certain drawbacks.

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