Integrated photonic building blocks for next-generation astronomical instrumentation I: the multimode waveguide

We report on the fabrication and characterization of composite multimode structures that consist of a stack of single-mode waveguides by ultrafast laser inscription. We explore 2 types of composite ; those that consist of overlapping single-mode waveguides which the maximum effective index contrast and non overlapped structures which multiple modes via strong evanescent coupling. We demonstrate that both of waveguides have negligible propagation losses (to within experimental ) for light injected with focal ratios >8, which corresponds to the of the waveguides. We also show that right below cutoff, there is a region where the injected focal ratio is preserved (to within uncertainty) at the output. Finally, we outline the major of these highly efficient waveguides; in a device that is used to the light in the focal plane of a telescope to a slit, in order to a diffraction-limited spectrograph.