Fast, precise, high contrast laser writing for photonic chips with phase aberrations

Integrated photonic chips have significant potential in telecommunications, classic computing, quantum systems, and topological photonics. Direct laser writing offers unique capability for creating three-dimensional photonic devices in an optical glass chip with quick prototyping. However, it is a challenge for existing laser writing schemes to create index-modified structures in glass that precisely match the laser focal shape while also achieving high refractive index contrasts and high scanning speeds. Here, we introduce a refractive index modification scheme that combines the advantages of non-thermal and thermal regime fabrication methods. We also propose a waveguide formation model that is verified through a thorough study on the effects of phase aberrations. The presented new photonic chip fabrication scheme uses a novel focal intensity distribution, where pulse energy is relocated to the bottom of a laser focus by manipulating primary and higher order spherical aberrations. The technique can produce index modifications with high scanning speed (can be 20 mm/s or higher), high index contrast (ranging from 0.009 to 0.021), and high precision to fabricate with arbitrary cross-sections. This method has potential to expand the capabilities of photonic chips in applications that require small-scale, high precision, or high contrast refractive index control.