Ultrafast laser inscription of mid-IR directional couplers for stellar interferometry

TL;DR

This paper demonstrates the fabrication of mid-infrared directional couplers using ultrafast laser inscription in gallium lanthanum sulphide glass, achieving high-quality power splitting suitable for stellar interferometry.

Contribution

It introduces a novel ultrafast laser fabrication process for mid-IR directional couplers with adjustable splitting ratios in a specific glass substrate.

Findings

Propagation losses of 0.8 dB/cm in fabricated waveguides

Power splitting ratios between 8% and 99% at 3.39 microns

Successful demonstration of ultrafast laser inscribed mid-IR couplers

Abstract

We report the ultrafast laser fabrication and mid-IR characterization (3.39 microns) of four-port evanescent field directional couplers. The couplers were fabricated in a commercial gallium lanthanum sulphide glass substrate using sub-picosecond laser pulses of 1030 nm light. Straight waveguides inscribed using optimal fabrication parameters were found to exhibit propagation losses of 0.8 dB/cm. A series of couplers were inscribed with different interaction lengths, and we demonstrate power splitting ratios of between 8% and 99% for mid-IR light with a wavelength of 3.39 microns. These results clearly demonstrate that ultrafast laser inscription can be used to fabricate high quality evanescent field couplers for future applications in astronomical interferometry.