Femtosecond laser written diffractive optical devices

Abstract

This thesis presents the development of optical diffractive and scattering-based devices fabricated by femtosecond laser direct writing. Structures investigated include waveguides, Fresnel lenses, diffractive gratings and scattering chips. Firstly, the laser induced refractive index change was estimated as 7×10⁻⁴ to 6×10⁻³ for writing powers of P_w=35 mW to 55 mW in silica. It was determined by fabricating waveguides and analysing their mode field diameter. A new writing method for controlling laser induced stress distribution was developed for improving the uniformity of inscribed structures and reducing laser-induced damage. A 36% diffraction efficiency improvement for a 50/50 grating was achieved compared to the standard multiscan writing technique. A two-layer Fresnel lens with a focal efficiency of 55% was also demonstrated. The writing technique was applied to create an integrated waveguide structure within multi-core fibre, thus forming a directional bend sensor, for which a sensitivity of -1.4 nm/degree was achieved. For a diffractive Fresnel lens, the design and the propagation simulation were demonstrated. For the high aspect ratio structure, Bessel beam writing has been utilised for a faster fabrication. A new method for refractive index estimation has been tested with the blazed diffractive grating. The diffraction efficiency of the grating has been investigated in both silica and lead silicate, a 57% diffraction efficiency was achieved with Gaussian beam writing. Finally, a laser-fabricated scattering chip based on induced voids was presented in this thesis, with applications as a wavemeter, spectrometer, and interferometric distance sensor. The algorithm based on the correlation and singular value decomposition has been demonstrated.

More information

Identifiers

ORCID for Qi Sun: orcid.org/0000-0002-0850-5463

ORCID for Gilberto Brambilla: orcid.org/0000-0002-5730-0499

Catalogue record

Export record