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

• Published in: Nature (London) 2023-08, Vol.620 (7972), p.78-85
• Main Authors: Xiang, Chao, Jin, Warren, Terra, Osama, Dong, Bozhang, Wang, Heming, Wu, Lue, Guo, Joel, Morin, Theodore J., Hughes, Eamonn, Peters, Jonathan, Ji, Qing-Xin, Feshali, Avi, Paniccia, Mario, Vahala, Kerry J., Bowers, John E.
• Format: Article
• Language: English
• Subjects: 639/624/1020/1093; 639/624/1075/1079; 639/624/399/1097; 639/624/399/1099; Circuit reliability; Complex systems; Decibels; Design; Humanities and Social Sciences; Integrated circuits; Isolators; Lasers; multidisciplinary; Optics; Phase noise; Photonics; Power consumption; Science; Science (multidisciplinary); Semiconductor lasers; Semiconductors; Silicon; Silicon nitride; Synthesis; Synthesizers; Systems stability; Waveguides
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/s41586-023-06251-w

Photonic integrated circuits are widely used in applications such as telecommunications and data-centre interconnects 1 – 5 . However, in optical systems such as microwave synthesizers 6 , optical gyroscopes 7 and atomic clocks 8 , 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. Three-dimensional integration of distributed-feedback lasers and ultralow-loss silicon nitride waveguides results in ultralow-noise lasers without the need for optical isolators.