Abstract
Integrated photonics has undergone tremendous development in the past few decades. Loss and gain are two fundamental parameters in photonic integrated circuits (PICs) and have direct impacts on nearly all key performance metrics. Surprisingly, the tools to characterize the optical loss and gain inside PICs are very limited. This is because, unlike free-space or fibre optics, integrated circuits cannot be non-destructively disassembled. Here we report a universal method to see inside the PICs and measure loss and gain on the component level non-destructively. The method leverages nonlinear optical devices as optical power discriminators to retrieve the loss and gain information. Our method has a precision better than 0.1 dB and can characterize the loss of individual fibre–chip coupling facets as well as general unknown devices under test. As an application, we measured the true on-chip quantum efficiency of a quantum PIC consisting of heterogeneously integrated balanced photodiodes, a critical building block for integrated quantum technology. Our non-destructive and highly precise method can be implemented on different photonic platforms to understand gain and loss in complex photonic circuits, which is essential to optimize circuit design and to create large-scale systems with predictable, reproducible performance.
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Data availability
The data that support the plots within this Article and other findings of this study are available via Figshare at https://doi.org/10.6084/m9.figshare.30526868 (ref. 45).
Code availability
The codes that support the findings of this study are available from the corresponding authors upon reasonable request.
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Acknowledgements
We acknowledge Ligentec for SiN microresonator fabrication and gratefully acknowledge DARPA INSPIRED (HR0011-24-2-0360), DARPA GRYPHON (HR0011-22-2-0008), DARPA NaPSAC (N660012424000), National Science Foundation (1842641, 2238096), DOE (DE-SC0023337) and QC82 Inc. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing official policies of DARPA, DOE or the US Government.
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X.Y. conceived the concept of the experiments. H.C., R.L. and G.Y.K. performed the measurements. F.T., X.G. and A.B. designed and fabricated the integrated photodiodes. G.Y.K., R.L., H.C., Z.Y., S.S. and B.W. designed the PICs. H.C., R.L., G.Y.K. and X.Y. analysed the experimental results. X.Y. supervised the experiments. All authors participated in preparing the manuscript.
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Nature Photonics thanks Simone Iadanza and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.
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Chen, H., Liu, R., Koehler, G.Y. et al. Universal loss and gain characterization inside photonic integrated circuits. Nat. Photon. (2026). https://doi.org/10.1038/s41566-025-01837-4
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DOI: https://doi.org/10.1038/s41566-025-01837-4
