Efficient point-by-point Bragg gratings fabricated in embedded laser-written silica waveguides using ultrafast Bessel beams.

We demonstrate highly efficient Bragg gratings written point-by-point by sequential single-pulse ultrashort Bessel laser beams in laser photoinscribed single-mode waveguides in bulk fused silica. The use of chirped non-diffractive Bessel beams determines a strong Bragg resonance in a weak-to-strong transitional regime, augmenting to a record value of 40  dB/cm at 1550 nm in the third order. The Bessel-induced refractive index modulation is negative and localized to sub-micrometer (200 nm) transverse scales. The strong light confinement in Bessel beams ensuring uniform one-dimensional void conditions thus allows for enhanced precision in the Bragg grating waveguide design. We demonstrate flexible fabrication of multiplexed waveguide gratings for multiple and tunable spectral resonances.