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

TL;DR

This paper presents the development and characterization of composite multimode waveguides fabricated by ultrafast laser inscription, demonstrating their low loss and potential for efficient light reformatting in astronomical instruments.

Contribution

It introduces novel composite multimode waveguide structures with high efficiency and low loss, suitable for reformatting telescope focal plane light for spectrographs.

Findings

Negligible propagation losses for focal ratios >8

Preservation of focal ratio near cutoff

Potential application in telescope light reformatting

Abstract

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