Fabrication of polarization-independent waveguides deeply buried in crystal using aberration-corrected femtosecond laser direct writing

TL;DR

This paper demonstrates a method to inscribe deeply buried, polarization-independent optical waveguides in lithium niobate crystals using aberration-corrected femtosecond laser writing, enabling advanced 3D photonic circuits.

Contribution

It introduces a spherical aberration correction technique that allows for the fabrication of deeply buried waveguides with high fidelity in nonlinear crystals.

Findings

Waveguides inscribed up to 1.4 mm deep in lithium niobate.

Aberration correction significantly improves axial resolution.

Technique enables multi-layer 3D photonic integration.

Abstract

Writing optical waveguides with femtosecond laser pulses provides the capability of forming three-dimensional photonic circuits for manipulating light fields in both linear and nonlinear manners. To fully explore this potential, large depths of the buried waveguides in transparent substrates are often desirable to facilitate achieving vertical integration of waveguides in a multi-layer configuration, which, however, is hampered by rapidly degraded axial resolution caused by optical aberration. Here, we show that with the correction of the spherical aberration, polarization-independent waveguides can be inscribed in a nonlinear optical crystal lithium niobate (LN) at depths up to 1.4 mm, which is more than one order of magnitude deeper than the waveguides written with aberration uncorrected femtosecond laser pulses. Our technique is beneficial for applications ranging from miniaturized nonlinear light sources to quantum information processing.