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

Creators: 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.¹; Bowers, John E.

1. California Institute of Technology

Abstract

Photonic integrated circuits are widely used in applications such as telecommunications and data-centre interconnects1,2,3,4,5. However, in optical systems such as microwave synthesizers6, optical gyroscopes7 and atomic clocks8, 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.

Copyright and License

© The Author(s) 2023. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

Acknowledgement

This work is supported by the Defense Advanced Research Projects Agency (DARPA) MTO GRYPHON (HR0011-22-2-0009) and LUMOS (HR0011-20-2-0044) programmes. We thank A. Netherton, M. Li, F. Quinlan and G. Keeler for discussions. O.T. acknowledges support from the Fulbright Scholar Program. A portion of this work was performed in the UCSB Nanofabrication Facility, an open-access laboratory.

Contributions

These authors contributed equally: Chao Xiang, Warren Jin, Osama Terra, Bozhang Dong.

C.X. and W.J. led the 3D PIC device design. C.X. led the heterogeneous integration and device characterization. W.J. designed the SiN devices and 3D couplers. W.J., A.F. and M.P. developed the bilayer SiN fabrication process and handled the SiN wafer processing. C.X. and J.P. fabricated the 3D PIC device with assistance from W.J. O.T. and C.X. characterized and gathered the experimental data from the device, including laser noise, locking ranges, phase tuning and microwave generation, with contributions from J.G., B.D., T.J.M. and Q.-X.J. B.D., O.T. and C.X. performed the feedback sensitivity measurement. H.W. provided theoretical calculations and analysis on the locking dynamics and feedback sensitivity. L.W. performed secondary ion mass spectrometry concentration analysis of the device. E.H. took the focused ion beam scanning electron microscopy and transmission electron microscopy images of the device. C.X. wrote the paper with input from W.J., H.W., B.D., O.T. and J.G. All authors commented on and edited the paper. K.J.V. and J.E.B. supervised the project.

Data Availability

The data used to produce the plots within this paper are available at https://doi.org/10.5281/zenodo.7894620 (ref. 61).

Code Availability

The code used to produce the plots within this paper is available at https://doi.org/10.5281/zenodo.7894620 (ref. 61)

Conflict of Interest

J.E.B. is a co-founder and shareholder of Nexus Photonics and Quintessent, start-ups in silicon photonics.

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