Shaping an ultrafast laser light sheet for inscription of fiber Bragg gratings.

A beam shaping approach employing a single spatial light modulator to generate uniform light sheets in an optical fiber is presented for the direct fabrication of fiber Bragg gratings using femtosecond laser radiation. A Gaussian beam is transformed into a narrow 700 nm light sheet by uniquely combining astigmatism, a one-dimensional axicon, and higher-order phase corrections to enable aberration-free air focusing into the core of a standard telecommunications fiber for plane-by-plane grating inscription. First-order single-pulse devices targeting 1570 nm and 863 nm wavelengths are reported with minimal insertion losses and reduced cladding mode coupling, achieving a sub-300 nm feature size and low birefringence. Comparison of single-pulse and multi-pulse formation of the grating elements indicates a further reduction in coupling to cladding modes with a 1000-pulse burst envelope.