A route to an all direct laser written integrated FTS (SWIFTS) in the 3.39–4.1 <i>μ</i>m range

This work aims to present the building blocks for an all direct laser written integrated spectrometer in the mid-infrared (3.39–4.1 μm range) based on the SWIFTS (Stationary Wave Integrated Fourier Transform Spectrometer) principle. In a SWIFTS-Gabor configuration, the light from the source interferes with itself in the middle of a channel waveguide, creating a stationary wave. This interferogram is then sampled through scattering centers placed on top of the waveguide that radiate the light they extract onto a detector placed directly on the sample. Finally, the spectrum of the source is retrieved through a Fourier transform. To implement a SWIFTS, two main photonic functions are required: the waveguide and the scattering centers. In this work, both functions have been created using Direct Laser Writing (DLW), a versatile technique, allowing to easily access 3D configurations and to reduce fabrication time. DLW focuses a femtosecond laser onto a sample so as to locally change the crystal lattice, resulting in its structural modification. The waveguides presented here are surface half-circular cladding structures made through type II modifications in the sample, and the scattering centers are surface damage tracks (also referred to as grooves). These surface tracks are creating dielectric discontinuities in the evanescent part of the stationary wave, resulting in the light being radiated outside the waveguide. All of these are made in a z-cut lithium niobate substrate for future implementation of the electro-optic effect. We demonstrate that we have functional waveguides in the mid-IR and that our grooves are extracting the stationary wave as expected, showing promising results for future implementation of a complete mid-IR SWIFTS using DLW.