Ultrafast Laser-Fabricated Fluoride Glass Waveguides with Exceptionally High Positive Refractive Index Change for Mid-Infrared Integrated Optics

This study presents the successful fabrication of waveguides with a high positive refractive index change exceeding 0.02 in rare earth-doped fluoride glass, marking a major advancement in integrated optical components for visible to mid-infrared applications. By overcoming persistent challenges in mid-infrared direct-write photonics, this research enables the development of waveguides with high refractive index contrast and mode tailoring in optical substrates, supporting the miniaturization of optical devices. The investigation reveals that the exceptionally high index change results from material densification, driven primarily by the migration of barium within the glass composition. With low propagation losses ( 0.21 dB/cm) and a highly customizable V-number over a broad wavelength range from visible to mid-infrared, these waveguides hold significant promise for chip laser technologies and the development of advanced optical devices for sensing and spectroscopy.