FIRST 5T 3D: A laser written device for FIRST/SUBARU reducing crosstalk and propagation losses

FIRST (Fibered Imager foR a Single Telescope instrument) is an instrument that enables high contrast imaging and spectroscopy, thanks to a unique combination of sparse aperture masking, spatial filtering by single-mode waveguides and cross-dispersion in the visible. In order to increase the instrument's stability and sensitivity, we have designed and fabricated a 3D laser-written optical chip 5T beam combiner. The multi-aperture beam combiner consists of 5 input waveguides spaced by 250um. Each input is split into 4 waveguides. A pairwise recombination scheme with Yjunctions produces/leads to 10 outputs (127um separation, compatible with commercial V-grooves). In this work, we present the experimental characterization of the chip: transmission performance, polarization issues and single mode spectral range. The targeted single mode spectral range must cover 550-800nm. Different optical powers for laser writing are used to finely tune the ideal single-mode behavior of the waveguides, ranging from 220 to 270mW. A straight waveguide was used as a reference, imprinted close to the first interferometric channel. Using different lasers (635nm, 780nm and 980nm) as well as wideband sources, we have been able to characterize the spectral transmission, the polarization behavior (TE/TM) and the interferometric contrast. The chip was inserted in the FIRST/SUBARU optical bench simulator at LESIA, in order to inject the 5 inputs simultaneously and scan the fringes using 4 independent MEMS, inducing a relative OPD modulation. Preliminary results show very good transmission for such a complex chip: all input channels are above 45% at 635nm (comparing the injected single mode with the sum of the 4 corresponding outputs), with two inputs reaching 80%. A huge advantage of this technology is to avoid the crosstalk due to in-plane waveguide crossings, and we show that no crosstalk is indeed observed. Both polarizations are transmitted, without noticeable birefringence. However, perfect vertical alignment of the outputs is difficult to obtain, and must be optimized prior to any connectorization to fiber bundles.