Stress induced birefringence tuning in femtosecond laser fabricated waveguides in fused silica

Luís A Fernandes,Paulo V S Marques,Jason R Grenier,Peter R Herman,J Stewart Aitchison

doi:10.1364/oe.20.024103

Abstract

Femtosecond laser exposure produces form and stress birefringence in glasses, mainly controlled by laser polarization and pulse energy, which leads to challenges in certain applications where polarization mode dispersion or birefringence splitting is critical for the desired responses from optical devices. In this paper, parallel laser modification tracks with different geometries were applied to preferentially stress the laser-written waveguides and explore the possibility of tuning the waveguide birefringence in devices fabricated in bulk fused silica glass. Polarization splitting in Bragg grating waveguides showed the laser modification tracks to controllably add or subtract stress to the pre-existing waveguide birefringence, demonstrating independence from the nanograting induced form birefringence and the contributions from material stress. Stressing bars are shown that offer tunable birefringence in the range from ~0 up to 4.35 × 10(-4), possibly enabling great flexibility in designing polarization dependent devices, as well as making polarization independent devices.

Highlights

The two major sources of birefringence in waveguides fabricated by femtosecond laser exposure are anisotropic material stress and form birefringence
The femtosecond fabrication platform is very attractive for single step fabrication of threedimensional integrated optical circuits [13]; the birefringence inherent to this process in fused silica may be detrimental for certain application such as integrated quantum entanglement experiments [14, 15] or traditional optical communication systems where a low birefringence is desired to reduce polarization mode dispersion
The devices with different stress geometries were characterized according to: morphology, mode field diameter (MFD) and losses, birefringence and mode effective index

Summary

Introduction

The two major sources of birefringence in waveguides fabricated by femtosecond laser exposure are anisotropic material stress and form birefringence. The alignment of nanogratings with the waveguide propagation vector, that is controlled by the laser polarization direction with respect to the writing direction, offers a strong influence over the waveguide birefringence. Higher birefringence observed for ultrafast laser-written waveguides fabricated in fused silica limit their application in two-photon quantum walk experiments due to polarization dependent coupling [16]. Another aspect of ultrafast laser-written waveguides that has been identified as a challenge towards integrated optical circuits for quantum experiments is the ellipticity of the waveguide modes, even despite the low birefringence provided by materials other than fused silica [17]

Methods

Discussion

Conclusion