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Passive broadband Faraday isolator for hybrid integration to photonic circuits without lens and external magnet

Nature Photonics volume 19, pages 248–257 (2025)Cite this article

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Abstract

Optical isolation based on a non-reciprocal effect is crucial for proper operation of several high-performance photonic devices such as in telecommunications, light detection and ranging, and even quantum platforms. The magneto-optical Faraday rotation is the most commonly used non-reciprocal effect as it offers unique advantages, including broadband operation, wide input optical power range, low insertion losses and high optical isolation, but it is currently not conducive to miniaturization. Two major impediments hinder the direct integration of Faraday isolators into photonic chips: the need for bulky external magnets and the challenging fabrication of low-loss waveguides that would eliminate the need for free-space coupling optics. Here we have addressed both challenges using a new femtosecond laser writing technique to create waveguides within the bulk of latched bismuth-doped iron garnet slabs without altering its magneto-optic functionality. As a result, we have achieved a Faraday rotator waveguide exhibiting <0.15 dB insertion loss with a figure of merit of 346° dB−1. By interposing this Faraday rotator between two 30-μm-thick polarizers, we further demonstrate a miniaturized optical isolator waveguide with >25 dB isolation ratio and <1.5 dB insertion loss over the entire optical telecom C-band for hybrid integration to photonic circuits without lenses and external magnet.

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Fig. 1: Faraday rotator waveguide fabrication without altering the magneto-optic functionality.
Fig. 2: Faraday isolator performances operating without external magnets.
Fig. 3: Preliminary results on the singulation and laser welding of our isolator.

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Data availability

All data supporting the findings of this study are available within the Article and its Supplementary Information. Any additional information can be obtained from the corresponding author on reasonable request.

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Acknowledgements

This research project was funded by the Evolution of Cloud Services in the Quebec-Ontario Corridor for Research and Innovation (ENCQOR) programme under contract reference number IBM-Codev-AEPONYX-1. We acknowledge the contribution of their funding partners IBM CANADA Ltd. and AEPONYX Enterprises Inc. for their valuable and seeding support. Special thanks are addressed to B. A. Janta-Polczynski from IBM for the seminal discussions and ideas, to D. Menard from Polytechnique Montreal for the discussions on magnetism, to S. Subramanian from Coherent-II-VI for the valuable insight and perspective into the BIG material properties, and to S. Gagnon for his valuable technical expertise.

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Authors and Affiliations

  1. Centre d’optique, photonique et laser, Université Laval, Québec, Canada

    Jerome Lapointe, Albert Dupont & Réal Vallée

  2. AEPONYX Enterprises Inc., Montréal, Canada

    Cedrik Coia

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  1. Jerome Lapointe
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  2. Cedrik Coia
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Contributions

J.L., C.C. and R.V. designed the optical isolator. J.L. and R.V. developed the laser writing technique. J.L. performed numerical simulations. J.L. built the experimental set-up. J.L. and A.D. fabricated the waveguides and optical isolators. J.L. and A.D. conducted the optical measurements. All authors contributed to the analysis of the results. J.L., C.C. and R.V. contributed to writing the paper. R.V. and C.C. supervised the project.

Corresponding author

Correspondence to Jerome Lapointe.

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Competing interests

C.C. is employed by AEPONYX Enterprises Inc., an active funder of the project. The work presented in this paper has been filed for provisional US patent application under number 63/363,730 and for Patent Cooperation Treaty application under number CA2023050273 by AEPONYX Enterprises Inc., with J.L., C.C., P. Babin, D. Michel, R.V., M. Bérard and A. Fekecs as inventors. The patent is currently pending national entry phase. The other authors declare no competing interests.

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Supplementary information

Supplementary Information

Supplementary Figs. 1–4 and Sections 1–4.

Supplementary Video 1

Video of a 1,550 nm laser being injected into a waveguide inscribed by a femtosecond laser in a BIG slab using an optical fibre, demonstrating strong optical mode confinement.

Supplementary Video 2

Video of a BIG slab with magnetic domains observed through two polarizers offset by 45°. By moving a magnet around the piece of BIG, the movement of the magnetic domains is observed. When the magnet is brought within a few millimetres above the slab, the BIG is latched, and the magnetic domains are no longer visible.

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Lapointe, J., Coia, C., Dupont, A. et al. Passive broadband Faraday isolator for hybrid integration to photonic circuits without lens and external magnet. Nat. Photon. 19, 248–257 (2025). https://doi.org/10.1038/s41566-024-01601-0

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