Femtosecond laser written waveguides in sapphire for visible light delivery

TL;DR

This paper demonstrates femtosecond laser written waveguides in sapphire for visible light, optimizing parameters to achieve low-loss, single-mode guiding suitable for quantum processor applications.

Contribution

First investigation of curved femtosecond laser written waveguides in sapphire with detailed optimization and loss measurements for visible light.

Findings

Achieved single-mode waveguides with 1.9 dB/cm loss at 728 nm

Curved waveguides with radii below 15 mm show increased loss

Optimized laser parameters enable effective waveguide fabrication in sapphire

Abstract

A promising solution for scalable integrated optics of trapped-ion quantum processors are curved waveguides guiding visible light within sapphire bulk material. To the best of our knowledge, no curved waveguides were investigated in sapphire so far, and no measurements of waveguides with visible light in undoped planar sapphire substrates were reported. Here, we demonstrate femtosecond laser writing of depressed cladding waveguides in sapphire. Laser parameters, such as pulse energy, pulse duration, and repetition rate, as well as waveguide geometry parameters, were optimized to guide 728 nm light. This resulted in single-mode waveguides with a propagation loss of 1.9(3) dB/cm. The investigation of curved waveguides showed a sharp increase in total loss for curvature radii below 15 mm. Our results demonstrate the potential of femtosecond laser writing as a powerful technique for creating integrated optical waveguides in the volume of sapphire substrates. Such waveguides could be a building block for integrated optics in trapped-ion quantum processors.