Researchers at the University of Oxford have used a femtosecond laser to write hundreds of waveguides in sapphire, suggesting that sapphire photonic chips hold the promise of real-world feasibility.

Figure 1: 4cm long sapphire integrated photonic chip.

Building high-quality integrated photonics circuits in sapphire could open up many new possibilities for applications such as communications, sensing or quantum computing.

Femtosecond lasers can write these waveguides into large chunks of material because femtosecond lasers are extremely intense and can be focused down to the micrometer scale. "This leads to nonlinear ionization within the material in the focal volume, which results in a change in refractive index." Wang said, "By relative motion between the femtosecond laser and the sapphire bulk material, which is mounted on a three-dimensional nanoprecision platform, along the designed trajectory, it is possible to write the integrated photonics paths that we designed on the sapphire substrate."

This time they have improved the process and reduced the optical loss of the waveguide compared to the group's previous work on sapphire. This allows them to now write waveguides that are 4cm long, which also means they can write more complex structures such as 1:2 optical splitters (see Figure 1).

Wang explains that changes in refractive index "are critical for designing optimized structures, and this is especially true for crystals because they have a high refractive index and many refractive index measurements are destructive. But writing photonic circuits requires very precise control of the laser-modified profile, so rapid characterization is also desirable."

Julian Fells, the lead researcher on the project, says that because sapphire is a very hard and resilient material, "it can withstand ultra-high temperatures of up to 2,000 degree and high radiation. These properties make it suitable for extreme environments such as aerospace, space and power generation. In addition, sapphire has a very broad spectral window in the mid-infrared region, a window that can be used for medical applications. By increasing the complexity of photonic circuits, higher performance sensors and devices are expected."