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1. Cryogenic packaging of nanophotonic devices with a low coupling loss < 1 dB

   https://doi.org/10.1063/5.0170324  

   Zeng, Beibei; De-Eknamkul, Chawina; Assumpcao, Daniel; Renaud, Dylan; Wang, Zhuoxian; Riedel, Daniel; Ha, Jeonghoon; Robens, Carsten; Levonian, David; Lukin, Mikhail; et al (October 2023, Applied Physics Letters)

   Robust, low-loss photonic packaging of on-chip nanophotonic circuits is a key enabling technology for the deployment of integrated photonics in a variety of classical and quantum technologies including optical communications and quantum communications, sensing, and transduction. To date, no process has been established that enables permanent, broadband, and cryogenically compatible coupling with sub-dB losses from optical fibers to nanophotonic circuits. Here, we report a technique for reproducibly generating a permanently packaged interface between a tapered optical fiber and nanophotonic devices on diamond with a record-low coupling loss <1 dB per facet at near-infrared wavelengths (∼730 nm) that remains stable from 300 K to 30 mK. We further demonstrate the compatibility of this technique with etched lithium niobate on insulator waveguides. The technique lifts performance limitations imposed by scattering as light transfers between photonic devices and optical fibers, paving the way for scalable integration of photonic technologies at both room and cryogenic temperatures. 

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2. Fiber array to chip attach using laser fusion splicing for low loss

   https://doi.org/10.1364/OE.492752  

   Nauriyal, Juniyali; Song, Meiting; Zhang, Yi; Granados-Baez, Marissa; Cardenas, Jaime (January 2023, Optics Express)

   With the ever-increasing need for higher data rates, datacom and telecom industries are now migrating to silicon photonics to achieve higher data rates with reduced manufacturing costs. However, the optical packaging of integrated photonic devices with multiple I/O ports remains a slow and expensive process. We introduce an optical packaging technique to attach fiber arrays to a photonic chip in a single shot using CO₂laser fusion splicing. We show a minimum coupling loss of 1.1 dB, 1.5 dB, and 1.4 dB per-facet for 2, 4, and 8-fiber arrays (respectively) fused to the oxide mode converters using a single shot from the CO₂laser. 

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3. Design of GaSb-based monolithic passive photonic devices at wavelengths above 2 µm

   https://doi.org/10.1088/2515-7647/ace509  

   Sumon, Md Saiful; Sankar, Shrivatch; You, Weicheng; Faruque, Imad I; Dwivedi, Sarvagya; Arafin, Shamsul (July 2023, Journal of Physics: Photonics)

   Abstract In this paper, we report, for the first time, a theoretical study on passive photonic devices including optical power splitters/combiners and grating couplers (GCs) operating at non-telecom wavelengths above 2 µ m in a monolithic GaSb platform. Passive components were designed to operate, in particular, at around 2.6 µ m for monolithic integration with active photonic devices on the III–V gallium antimonide material platform. The three popular types of splitters/combiners such as directional couplers, multimode interferometer-, and Y-branch-couplers were theoretically investigated. Based on our optimized design and rigorous analysis, fabrication-compatible 1 × 2 optical power splitters with less than 0.12 dB excess losses, large spectral bandwidth, and a 50:50 splitting ratio are achieved. For fiber-to-chip coupling, we also report the design of GCs with an outcoupling efficiency of ∼29% at 2.56 μ m and a 3 dB bandwidth of 80 nm. The results represent a significant step towards developing a complete functional photonic integrated circuits at mid-wave infrared wavelengths. 

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4. 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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5. Low-noise frequency-agile photonic integrated lasers for coherent ranging

   https://doi.org/10.1038/s41467-022-30911-6  

   Lihachev, Grigory; Riemensberger, Johann; Weng, Wenle; Liu, Junqiu; Tian, Hao; Siddharth, Anat; Snigirev, Viacheslav; Shadymov, Vladimir; Voloshin, Andrey; Wang, Rui Ning; et al (December 2022, Nature Communications)

   Abstract Frequency modulated continuous wave laser ranging (FMCW LiDAR) enables distance mapping with simultaneous position and velocity information, is immune to stray light, can achieve long range, operate in the eye-safe region of 1550 nm and achieve high sensitivity. Despite its advantages, it is compounded by the simultaneous requirement of both narrow linewidth low noise lasers that can be precisely chirped. While integrated silicon-based lasers, compatible with wafer scale manufacturing in large volumes at low cost, have experienced major advances and are now employed on a commercial scale in data centers, and impressive progress has led to integrated lasers with (ultra) narrow sub-100 Hz-level intrinsic linewidth based on optical feedback from photonic circuits, these lasers presently lack fast nonthermal tuning, i.e. frequency agility as required for coherent ranging. Here, we demonstrate a hybrid photonic integrated laser that exhibits very narrow intrinsic linewidth of 25 Hz while offering linear, hysteresis-free, and mode-hop-free-tuning beyond 1 GHz with up to megahertz actuation bandwidth constituting 1.6 × 10¹⁵Hz/s tuning speed. Our approach uses foundry-based technologies - ultralow-loss (1 dB/m) Si₃N₄photonic microresonators, combined with aluminium nitride (AlN) or lead zirconium titanate (PZT) microelectromechanical systems (MEMS) based stress-optic actuation. Electrically driven low-phase-noise lasing is attained by self-injection locking of an Indium Phosphide (InP) laser chip and only limited by fundamental thermo-refractive noise at mid-range offsets. By utilizing difference-drive and apodization of the photonic chip to suppress mechanical vibrations of the chip, a flat actuation response up to 10 MHz is achieved. We leverage this capability to demonstrate a compact coherent LiDAR engine that can generate up to 800 kHz FMCW triangular optical chirp signals, requiring neither any active linearization nor predistortion compensation, and perform a 10 m optical ranging experiment, with a resolution of 12.5 cm. Our results constitute a photonic integrated laser system for scenarios where high compactness, fast frequency actuation, and high spectral purity are required. 

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