Towards a mid-infrared SWIFTS: L-band integrated high resolution spectrometer

This work aims to present the first results towards a mid-infrared (L-band : 3.4 μm - 4.1 μm) high resolution integrated spectrometer, based on the Gabor approach of SWIFTS (Stationary Wave Integrated Fourier Transform Spectrometer). In this configuration, a stationary wave is obtained by injecting the light from the source on both sides of a channel waveguide. The centre of the coherent interferogram is obtained in the middle of the waveguide, allowing for full characterisation of the source, and all differential dispersion effects between the two optical paths. The stationary wave is sampled by nano-scattering centres (= nanogrooves) placed on top of the waveguide. They extract the interferogram, and the spectrum of the source is retrieved through a Fourier Transform. In the mid-infrared, the detection area of the detectors is buried : using only one groove per scattering centre leads to a diffracted signal too wide and causes pixel crosstalk. Several grooves per scattering centres have therefore been implemented, as this configuration creates a small diffraction grating, and reduces the angular divergence of the flux. In addition to avoiding crosstalk, this allows to extract more flux per scattering centre, thus increasing the signal to noise ratio. Our samples are made in Lithium Niobate (LiNbO3), an electro-optic crystal, using two technologies. First, Direct Laser Writing for both the waveguides and the nanogrooves, and secondly Titane diffusion for the waveguides and Focused Ion Beam for the nanogrooves. Because of the electrooptic properties of Lithium Niobate, applying an electric field ramp modulation will change the refractive index of the material, allowing to finely scan the fringe packet under the sampling centres, thus increasing the sampling rate of the interferogram by temporal multiplexing. We demonstrate that our waveguides are fully functional and have a correct transmission rate, and that our antennas are extracting the stationary wave as expected, in both technologies.