Mid-infrared photonic devices for stellar interferometry
Multi-scan ultrafast laser inscription at a wavelength of = 1026 nm induces a positive refractive index change gallium lanthanum sulfide (GLS). It can be used to produce single mode waveguides for mid-infrared wavelengths. Simulations of the GLS structure show GaS4 tetrahedra and LaS8 polyhedra. The direct comparison of Raman spectra reveals a structural change of the waveguide from the bulk. The laser parameters have a crucial influence on the spectra. This indicates a change in La coordination and a reorientation of the structural units, which results in a local densification and partial crystallization. Structural investigations with mode field diameter measurements, SEM and polarimetry proved these assumptions. Annealing of GLS aiming at the relaxation of stress induced birefringence showed that a material relaxation is accompanied by a smaller difference between bulk and achievable waveguide refractive index. Stresses induced by the laser writing have an influence on the properties of waveguides written within their range. Birefringence control and loss reduction by changing the propagation constant of closely spaced waveguides can improve the performance of integrated optics components for interferometric beam combination. A symmetrization of a 7-waveguide and a 23-waveguide zig-zag array has been achieved for waveguide arrays up to 22 mm length. A new 4-telescope discrete beam combiner with 23 parallel waveguides with zig-zag geometry was manufactured and interferometrically characterized for 6 baselines. It turns out to be comparable to the performance of already used interferometer instruments. Also the ABCD beam combination scheme was tested in a 2-telescope configuration. The Raman response function of a waveguide in GLS was measured by fitting the spectral broadening and Raman signal heights for different input laser powers and compared to pulse propagation simulations.