Efficient long period fiber gratings inscribed with femtosecond pulses and an amplitude mask.

We present efficient long period fiber gratings written with femtosecond laser pulses at 800 nm and an amplitude mask, to the best of our knowledge, for the first time. The measured transmission spectra depict strong resonances, while the total grating length and polarization-dependent loss could be significantly reduced compared to previous results. Two gratings are exemplarily shown-one in a standard single mode, and one in a large-mode-area fiber revealing a predictable spectrum without intermediate peaks due to the suppression of coupling to asymmetric higher-order cladding modes.