First-order type A waveguide Bragg gratings with under-diffraction-limited periodicities and sub-millimeter length for NIR applications

Since its demonstration in 2017, laser-induced Type A modifications in silver-containing glasses have been exploited for fabricating waveguides, couplers, volume Bragg Gratings and, more recently, waveguides Bragg Gratings. Laser-induced silver-supported structures exhibit sub-diffraction characteristics being always associated with a positive refractive index change resulting from multi-photon energy deposition under multiple-pulse femtosecond laser irradiation. These properties have been exploited in this work for the fabrication of efficient first-order helicoidal Type A waveguides Bragg gratings in the near-infrared (NIR) range, with periodicities well below the diffraction limit and with typical length L = 500 μ m, showing narrowband mirror behaviours. Moreover, this work demonstrates that such helicoidal Type A waveguides Bragg gratings allow for modulating the refractive index of the waveguide at half the periodicity of the helix. Such a peculiarity has been exploited to fabricate efficient first-order Type A waveguides Bragg gratings in the NIR range, as well as in the far red range of the visible spectrum, showing a remarkably strong coupling coefficient, up to one order of magnitude higher than what is reported in the literature.