3D integration enables ultralow-noise isolator-free lasers in silicon photonics

Abstract

<p>Photonic integrated circuits are widely used in applications such as telecommunications and data-centre interconnects<sup><a title="Margalit, N. et al. Perspective on the future of silicon photonics and electronics. Appl. Phys. Lett. 118, 220501 (2021)." href="https://www.nature.com/articles/s41586-023-06251-w#ref-CR1">1</a>,<a title="Doerr, C. Silicon photonic integration in telecommunications. Front. Phys. https://doi.org/10.3389/fphy.2015.00037

(2015)." href="https://www.nature.com/articles/s41586-023-06251-w#ref-CR2">2</a>,<a title="Thomson, D. et al. Roadmap on silicon photonics. J. Optics 18, 073003 (2016)." href="https://www.nature.com/articles/s41586-023-06251-w#ref-CR3">3</a>,<a title="Cheng, Q., Bahadori, M., Glick, M., Rumley, S. & Bergman, K. Recent advances in optical technologies for data centers: a review. Optica 5, 1354–1370 (2018)." href="https://www.nature.com/articles/s41586-023-06251-w#ref-CR4">4</a>,<a title="Soref, R. The past, present, and future of silicon photonics. IEEE J. Sel. Top. Quantum Electron. 12, 1678–1687 (2006)." href="https://www.nature.com/articles/s41586-023-06251-w#ref-CR5">5</a></sup>. However, in optical systems such as microwave synthesizers^{<a title="Marpaung, D., Yao, J. & Capmany, J. Integrated microwave photonics. Nat. Photon. 13, 80–90 (2019)." href="https://www.nature.com/articles/s41586-023-06251-w#ref-CR6">6</a>}, optical gyroscopes^{<a title="Lefevre, H. C. The Fiber-optic Gyroscope (Artech House, 2014)." href="https://www.nature.com/articles/s41586-023-06251-w#ref-CR7">7</a>} and atomic clocks^{<a title="Ludlow, A. D., Boyd, M. M., Ye, J., Peik, E. & Schmidt, P. O. Optical atomic clocks. Rev. Mod. Phys. 87, 637 (2015)." href="https://www.nature.com/articles/s41586-023-06251-w#ref-CR8">8</a>}, photonic integrated circuits are still considered inferior solutions despite their advantages in size, weight, power consumption and cost. Such high-precision and highly coherent applications favour ultralow-noise laser sources to be integrated with other photonic components in a compact and robustly aligned format—that is, on a single chip—for photonic integrated circuits to replace bulk optics and fibres. There are two major issues preventing the realization of such envisioned photonic integrated circuits: the high phase noise of semiconductor lasers and the difficulty of integrating optical isolators directly on-chip. Here we challenge this convention by leveraging three-dimensional integration that results in ultralow-noise lasers with isolator-free operation for silicon photonics. Through multiple monolithic and heterogeneous processing sequences, direct on-chip integration of III–V gain medium and ultralow-loss silicon nitride waveguides with optical loss around 0.5 decibels per metre are demonstrated. Consequently, the demonstrated photonic integrated circuit enters a regime that gives rise to ultralow-noise lasers and microwave synthesizers without the need for optical isolators, owing to the ultrahigh-quality-factor cavity. Such photonic integrated circuits also offer superior scalability for complex functionalities and volume production, as well as improved stability and reliability over time. The three-dimensional integration on ultralow-loss photonic integrated circuits thus marks a critical step towards complex systems and networks on silicon.</p>