Less than one month ago, we spotlighted the development of a high-performance visible-range tuneable laser integrated in Si₃N₄ platform (https://www.osapublishing.org/spotlight/summary.cfm?id=459900). We concluded our comment by saying that, thanks to the wide transparency range of Si₃N₄, other laser solutions covering different portions of the optical spectrum were likely to appear soon. Probably we didn’t expect it to be so soon. Only a few weeks later, the Si₃N₄ platform is back with an integrated laser working on a completely different side of the optical spectrum. That is the 2-μm band, where small-form-factor and low-cost laser sources are still missing, but they are becoming of great interest for many applications, such as communications, diagnostics, spectroscopy, and sensing.

In this wavelength range, Kiani and coworkers were able to develop a compact chip-scale laser that is monolithically integrated on a low-loss Si₃N₄ platform. As an active material, their device exploits thulium-doped tellurium oxide (TeO₂) that is used as an upper cladding of a Si₃N₄ microring resonator with a quality factor of 4.8×10⁵. The high refractive index of TeO₂ enables a high confinement in the active region for both pump and laser radiation. With a resonant pumping  at 1610 nm, emission at any wavelength from 1815 nm to 1895 nm can be achieved by controlling the coupling between the microring laser cavity and the input waveguide. This microlaser exhibits on-chip output powers with up to 4.5 mW with a threshold as low as 11 mW and a slope efficiency as high as 17%.

Also, this time, we can conclude that the transparency wavelength range of Si₃N₄ and TeO₂ may offer the possibility to extend even more the spectral range of chip-scale laser sources, going to longer and longer wavelengths. We just have to wait, probably not so long. So… who’s next?

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