Lasers operating near 2 µm wavelength have recently attracted significant interest for applications in communications, medicine, sensing and spectroscopy. Thulium-doped waveguides are particularly promising for such applications because of thulium’s broad emission around 1.7-2.1 µm and efficient optical pumping at a wavelength of 790 or 1600 nm, the former with low-cost pump diodes and the latter allowing for commercial pump sources in the standard communications L band. Thulium lasers can reach efficiencies > 80%, in a wavelength region where semiconductor lasers are well-known to drop off in efficiency and increase in cost, which enhances thulium lasers’ prospects as light sources in photonic microsystems. Integrating such lasers on a silicon chip, as opposed to more well-developed bulk glass or fiber-optic platforms, adds the advantage of compact size and allows for co-integration with other well-established silicon-based integrated photonic components.

In this work, A. S. K. Tong and colleagues present on low-losses, spectroscopic measurements, and demonstration of lasing around 1860 nm in thulium-doped tantalum pentoxide waveguides on silicon. Tantalum pentoxide is a highly prospective CMOS-compatible material which has received much recent attention in the integrated photonics community, but prior to this work had been less explored as a rare-earth host material for thulium. Here, the authors have fabricated thulium-doped ridge waveguides using similar methods to those which their research group has used to demonstrate erbium-, neodymium- and ytterbium-doped devices. They characterize the fluorescent lifetime, showing a factor of ~5 increase in the thulium excited state ³F₄ lifetime (up to 477 µs) with annealing at temperatures up to 650 °C. They also characterize the absorption and emission spectra and cross-sections for the Tm³⁺:Ta₂O₅ system, providing the groundwork for future device design and development on this promising platform. Importantly, they show laser emission at 1238, 1855 and 1858 nm in a 1-cm-long waveguide with polished end facets. From the laser results they deduce a very high gain in the material of 9 dB/cm, which is also promising for compact integrated amplifiers. These results add to many other recent advances on tantalum pentoxide photonic integrated circuits, including the development of low-loss waveguides and high-speed all-optical nonlinear devices. Thulium lasers are an important step for such circuits because they can provide efficient, high power light sources to power near to mid-infrared all-optical processes on the same chip, for many emerging applications including AI and quantum optics.

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