Abstract
We introduce the fabrication and use of microcracks embedded in glass as an optical element for manipulating light propagation, in particular for enhancing waveguide performance in silica integrated optics. By using a femtosecond laser to induce a strong asymmetric stress pattern in silica, uniform cracks with set dimensions can be created within the substrate and propagated along a fixed path. The smoothness of the resulting cleave interface and large index contrast can be exploited to enhance waveguide modal confinement. As a demonstration, we tackle the longstanding high bend-loss issue in femtosecond laser written silica waveguides by using this technique to cleave the outer edge of laser written waveguide bends, to suppress radiative bend loss. The microcrack cross section is estimated to be 15 μm in height and 30 nm in width, for the 10 times 10 μm waveguides. At 1550 nm wavelength, losses down to 1 dB/cm at 10 mm bend radius were achieved, without introducing additional scattering. Both the cleave stress pattern and waveguide are fabricated with the same multiscan writing procedure, without requiring additional steps, and re-characterisation of the waveguides after 1 year confirm excellent long term performance stability.
Highlights
LED light source simpler yet highly effective, as the waveguide inscription simultaneously induces the stress pattern for crack formation, and benefits compact multi-waveguide designs as the crack does not occupy additional volume
The cracks could be adopted for glass microstructuring, for example, by pre-cleaving glass to define and more selectively etch microchannels used in optofluidic chips
The waveguide core was modelled as a 10 × 10 μm square cross section with a uniform step-index profile and corecladding index difference n = n1 − n2 of 4.1 × 10−3 , based on previous work comparing the experimental and simulated mode field diameter of waveguides
Summary
LED light source simpler yet highly effective, as the waveguide inscription simultaneously induces the stress pattern for crack formation, and benefits compact multi-waveguide designs as the crack does not occupy additional volume. To illustrate the mechanism by which the microcrack reduces bend loss, we simulated the = 1.55 μm fundamental mode field profiles of a curved waveguide as shown, for a 10 × 10 μm core silica waveguide of bend radius rb = 7 mm, with and without a crack on the outside bend edge of the core.
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