Low loss: the lifeline of photonic chips
Optical fibres underpin the global internet because light can travel through them over long distances with extremely low loss. Researchers now hope to bring similar capabilities onto photonic chips, where light can be guided, controlled and stored at the micrometre scale for communications, AI computing, precision measurement and quantum information.
However, as optical circuits shrink, light becomes more sensitive to fabrication defects, surface roughness and material absorption. Every lost photon means higher power consumption, more noise and lower coherence. A long-standing question is whether fibre-level low loss can be brought onto a photonic chip.
Flame hydrolysis deposition: from fibre manufacturing to integrated photonics
In a new paper published in Light: Science & Applications , a team led by Professor Kerry Vahala at the California Institute of Technology, in collaboration with Professor James Gates at the University of Southampton and co-workers, has demonstrated ultrahigh Q integrated germano-silicate microresonators on silicon using flame hydrolysis deposition.
Flame hydrolysis deposition is widely used in optical-fibre manufacturing. Gaseous precursors react in an oxygen-hydrogen flame to form high-purity glass soot, which is then consolidated into transparent glass. This work extends this deposition from fibre-manufacturing process to integrated photonics.
The device material is germano-silicate, or germanium-doped silica, a mainstream fibre-core glass. In standard fibres, a small amount of GeO 2 doping (~3.5 mol%) increases the core refractive index and confines light. In a recent related work [ Nature 649, 338–344 (2026)], the team demonstrated integrated devices based on 25 mol%. Here, they increased the concentration to about 50 mol%, which increases refractive-index contrast for compact circuits and lowers the glass softening temperature, opening a window for thermal reflow in a standard furnace.
Large fabrication tolerance enabled by wafer-scale reflow
For ultrahigh Q microresonators, sidewall roughness left by nanofabrication is a major source of loss. Even a small etching defect can scatter light repeatedly as it circulates inside the microcavity.
The wafer-scale thermal reflow enabled by high GeO 2 doping provides a way to reduce this sensitivity. Under heat treatment, the etched germano-silicate waveguide undergoes controlled viscous flow, and its initially sharp and rough cross-section becomes rounded and smooth under surface tension. As a result, etch-induced defects can be effectively repaired after fabrication.
The researchers directly tested this repair capability. Even when the initial etch quality was poor and the microresonator Q was low, device performance could recover by nearly two orders of magnitude after thermal reflow. Experimentally, the platform achieved an intrinsic Q of 566 million at 1064 nm, corresponding to a propagation loss as low as 0.07 dB per metre. Q factors above 100 million were also demonstrated from the telecommunications band to the violet spectrum, showing a broadband, low-loss and high-coherence integrated photonic platform.
Outlook
This work is a first step toward transferring optical-fibre deposition concepts to ultra-low loss photonic chips. Compared with commercial optical fibres, whose loss can be around 0.2 dB/km, current on-chip devices still have more than two orders of magnitude of room for improvement. With further optimization of related processes, chip-scale optical paths may continue to approach fibre-level performance and integrate kilometre-scale optical waveguides onto palm-sized wafers, fully unleashing the advantages of large-scale photonic integrated circuits.
At the same time, wafer-scale thermal reflow greatly improves the fabrication tolerance of micro-photonic devices. Traditionally, integrated microresonators with Q factors above 100 million have required demanding etching conditions and have been accessible to only a limited number of laboratories. In this platform, post-processing reflow can smooth sidewalls and repair etch-induced defects, suggesting a route toward more repeatable and scalable wafer-level manufacturing of ultrahigh Q microresonators.
From fibre to chip, the goal is not merely to make optical devices smaller. The deeper significance is to bring the low-loss optical capability that once enabled the global internet into integrated, scalable and deployable chip systems, providing a new optical infrastructure for next-generation AI data-centre interconnects, quantum technologies and precision measurement.
Light: Science & Applications
Ultrahigh-Q integrated flame-hydrolysis-deposited germano-silicate resonators on silicon