Direct laser writing is a powerful technique for the development of astrophotonic devices, namely by allowing 3D structuring of waveguides and avoiding in-plane crossings that can induce losses and crosstalk in multi-telescope beam combiners. In this work, a multiplexed device is proposed in order to increase the spectral bandwidth to hundreds of nm, for a central wavelength of 1580nm. Our device is fabricated by ultrafast laser inscription of type I waveguides in bulk IR-grade fused silica glass. A first part of the study was devoted to finding the optical fabrication parameters in terms of depth, speed and number of tracks needed to achieve an optimal waveguide, single mode in the near IR. A second part was focused to the fabrication of different optical lanterns, from one multimode input to several (4 or 16) single mode outputs. The optical chip consists of a multimode input slit-waveguide, that adiabatically converts into an aligned matrix of 4 or 16 single-mode channel waveguides, with a pitch corresponding to the detector pixel size. Two separations (20m and 64m) were studied, in order to avoid crosstalk between parallel waveguides (directional coupling) and from extracted flux into the detector (pixel crosstalk). A final part is dedicated to the spectrometer realization, based on the sampling of a stationary wave inside the waveguide.