| Abstract | Silicon nitride (Si₃N4₄) waveguides become an appealing choice to realize complex photonic integrated circuits for applications in telecom/datacom transceivers, sensing, and quantum information sciences. However, compared to high-index-contrast silicon-on-insulator platform, the index difference between the Si₃N4₄ waveguide core and its claddings is more moderate, which adversely affects the development of vertical grating-coupled optical interfaces. Si₃N4₄ grating couplers suffer from the reduced strength, therefore it is more challenging to radiate all the waveguide power out of the grating within a beam size that is comparable to the mode field diameter of standard optical fibers. In this work, we present, by design and experiments, a library of low-loss and fabrication-tolerant surface grating couplers, operating at 1.55 μm wavelength range and standard SMF-28 fiber. Our designs are fabricated on 400 nm Si₃N4₄ platform using single-etch fabrication and foundry-compatible low-pressure chemical vapor deposition wafers. Experimentally, the peak coupling loss of − 4.4 dB and − 3.9 dB are measured for uniform couplers, while apodized grating couplers yield fiber-chip coupling loss of − 2.9 dB, without the use of bottom mirrors, additional overlays, and multi-layered grating arrangements. Beside the single-hero demonstrations, over 130 grating couplers were realized and tested, showing an excellent agreement with finite difference time domain designs and fabrication-robust performance. Demonstrated grating couplers are promising for Si₃N4₄ photonic chip prototyping by using standard optical fibers, leveraging low-cost and foundry-compatible fabrication technologies, essential for stable and reproducible large-volume device development. |
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