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1. Graded Index Couplers for Next Generation Chip-to-Chip and Fiber-to-Chip Photonic Packaging

   https://doi.org/10.1088/2515-7647/ae1648  

   Weninger, Drew Michael; Duessel, Christian; Serna-Otalvaro, Samuel F; Kimerling, Lionel C; Agarwal, Anuradha Murthy (October 2025, Journal of Physics: Photonics)

   Abstract The transition towards designs which co-package electronic and photonic die together in data center switch packages has created a scaling path to Petabyte per second (Pbps) input/output (I/O) in such systems. In a co-packaged design, the scaling of bandwidth, cost, and energy will be governed by the number of optical I/O channels and the data rate per channel. While optical communication provide an opportunity to exploit wavelength division multiplexing (WDM) to scale data rate, the limited 127 µm pitch of V-groove based single mode fiber arrays and the use of active alignment and bonding for their packaging present challenges to scaling the number of optical channels. Flip-chip optical couplers which allow for low loss, broadband operation and automated passive assembly represent a solution for continued scaling. In this paper, we propose a novel scheme to vertically couple between silicon based waveguides on separate chips using graded index (GRIN) couplers in combination with an evanescent coupler. Simulation results using a 3D Finite-Difference Time-Domain (FDTD) solver are presented, demonstrating coupling losses as low as 0.35 dB for a chip-to-chip gap of 11 µm; 1-dB vertical and lateral alignment tolerances of approximately 2.45 µm and ± 2.66 µm, respectively; and a possible 1-dB bandwidth of greater than 1500 nm. These results demonstrate the potential of our coupler as a universal interface in future co-packaged optics systems. 

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2. Low-loss fiber-to-chip interface for lithium niobate photonic integrated circuits

   Lingyan He, Mian Zhang (January 2019, Optics letters)

   Integrated lithium niobate (LN) photonic circuits have recently emerged as a promising candidate for advanced photonic functions such as high-speed modulation, nonlinear frequency conversion, and frequency comb generation. For practical applications, optical interfaces that feature low fiber-to-chip coupling losses are essential. So far, the fiber-to-chip loss (commonly >10  dB/facet) has dominated the total insertion losses of typical LN photonic integrated circuits, where on-chip losses can be as low as 0.03–0.1 dB/cm. Here we experimentally demonstrate a low-loss mode size converter for coupling between a standard lensed fiber and sub-micrometer LN rib waveguides. The coupler consists of two inverse tapers that convert the small optical mode of a rib waveguide into a symmetrically guided mode of a LN nanowire, featuring a larger mode area matched to that of a tapered optical fiber. The measured fiber-to-chip coupling loss is lower than 1.7 dB/facet with high fabrication tolerance and repeatability. Our results open the door for practical integrated LN photonic circuits efficiently interfaced with optical fibers. 

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3. Low Loss Chip-to-Chip Couplers for High Density Co-Packaged Optics

   https://doi.org/10.1364/opticaopen.25852735.v1  

   Weninger, Drew; Otálvaro, Samuel Serna; Ranno, Luigi; Kimerling, Lionel; Agarwal, Anuradha (May 2024, Optica Open)

   An experimentally demonstrated, vertical chip-to-chip evanescent coupler between silicon nitride (Si₃N₄) and silicon (Si) is presented with the coupler loss measured to be 0.39 ± 1.06 dB at 1550 nm with a 1-dB bandwidth of 160 nm extending across the C-band, S-band, and L-band (1480-1640 nm). The average coupling loss was determined to be 0.73 dB for the 1480-1640 nm wavelength range with a ± 2σ tolerance of ± 0.92 dB. The 1-dB lateral alignment tolerance was 1.56 ± 0.14 μm at 1550 nm and the average tolerance was 1.38 ± 0.24 μm across the 1480-1640 nm wavelength regime. In addition, the average coupling loss varied by less than ± 0.35 dB and the average 1-dB alignment tolerance varied by less than ± 30 nm for temperatures varying from 23-60°C. Finally, the average coupling loss range was less than 1.5 dB range across four sets of identically packaged die. This is the first experimental demonstration of an inter-chip, passively assembled evanescent coupler using standard CMOS foundry processes for directly coupling between Si and Si₃N₄, overcoming a waveguide refractive index difference of Δn = 1.32 without requiring taper tip widths of less than 100 nm. 

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4. A metasurface-based diamond frequency converter using plasmonic nanogap resonators

   https://doi.org/10.1515/nanoph-2020-0392  

   Shen, Qixin; Shams-Ansari, Amirhassan; Boyce, Andrew M.; Wilson, Nathaniel C.; Cai, Tao; Loncar, Marko; Mikkelsen, Maiken H. (September 2020, Nanophotonics)

   Abstract Diamond has attracted great interest as an appealing material for various applications ranging from classical to quantum optics. To date, Raman lasers, single photon sources, quantum sensing and quantum communication have been demonstrated with integrated diamond devices. However, studies of the nonlinear optical properties of diamond have been limited, especially at the nanoscale. Here, a metasurface consisting of plasmonic nanogap cavities is used to enhance both χ (2) and χ (3) nonlinear optical processes in a wedge-shaped diamond slab with a thickness down to 12 nm. Multiple nonlinear processes were enhanced simultaneously due to the relaxation of phase-matching conditions in subwavelength plasmonic structures by matching two excitation wavelengths with the fundamental and second-order modes of the nanogap cavities. Specifically, third-harmonic generation (THG) and second-harmonic generation (SHG) are both enhanced 1.6 × 10 7 -fold, while four-wave mixing is enhanced 3.0 × 10 5 -fold compared to diamond without the metasurface. Even though diamond lacks a bulk χ (2) due to centrosymmetry, the observed SHG arises from the surface χ (2) of the diamond slab and is enhanced by the metasurface elements. The efficient, deeply subwavelength diamond frequency converter demonstrated in this work suggests an approach for conversion of color center emission to telecom wavelengths directly in diamond. 

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5. Highly efficient fiber to Si waveguide free-form coupler for foundry-scale silicon photonics

   https://doi.org/10.1364/PRJ.514999  

   Ranno, Luigi; Sia, Jia_Xu Brian; Popescu, Cosmin; Weninger, Drew; Serna, Samuel; Yu, Shaoliang; Kimerling, Lionel C; Agarwal, Anuradha; Gu, Tian; Hu, Juejun (January 2024, Photonics Research)

   As silicon photonics transitions from research to commercial deployment, packaging solutions that efficiently couple light into highly compact and functional sub-micrometer silicon waveguides are imperative but remain challenging. The 220 nm silicon-on-insulator (SOI) platform, poised to enable large-scale integration, is the most widely adopted by foundries, resulting in established fabrication processes and extensive photonic component libraries. The development of a highly efficient, scalable, and broadband coupling scheme for this platform is therefore of paramount importance. Leveraging two-photon polymerization (TPP) and a deterministic free-form micro-optics design methodology based on the Fermat’s principle, this work demonstrates an ultra-efficient and broadband 3-D coupler interface between standard SMF-28 single-mode fibers and silicon waveguides on the 220 nm SOI platform. The coupler achieves a low coupling loss of 0.8 dB for the fundamental TE mode, along with 1 dB bandwidth exceeding 180 nm. The broadband operation enables diverse bandwidth-driven applications ranging from communications to spectroscopy. Furthermore, the 3-D free-form coupler also enables large tolerance to fiber misalignments and manufacturing variability, thereby relaxing packaging requirements toward cost reduction capitalizing on standard electronic packaging process flows. 

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