Femtosecond inscribed mode modulators in large mode area fibers -experimental and theoretical analysis

摘要

We present the experimental and theoretical analysis of a mode modulator in a few mode LMA fiber. The mode modulator consists of a section with a modified refractive index alongside the fiber core in the cladding, disturbing the guidance of the modes in the core. The extent of excitation of these disturbed modes depends on the overlap of the excited undisturbed and disturbed modes. At the end of the modulator, undisturbed modes will be excited again in the fiber core, in dependency of the spatial field distribution of the disturbed modes at the boundary. In the mode modulator disturbed higher order modes lead to modal interference, causing a dependency of the spatial distribution of the light in the mode modulator on the propagation length of the disturbed modes. Hence, the modal output field depends on the length of the mode modulator. For the experiments, the mode modulator was inscribed directly into the LMA fiber with ultrashort laser pulses. During the inscription process the modal content at the end of the fiber was measured using a computer generated hologram as a correlation filter. In dependency of the length of the modulator strong oscillations between the content of the fundamental and the higher order modes are observable. In the case of an initially excited fundamental mode, its content could be reduced to below 5%, whereat the content of the LP11 modes was up to 90%. While measurement and simulation show qualitative agreement, differences are caused by inhomogeneities of the refractive index modifications.