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1. 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 (May 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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2. A compact and open-source microcontroller-based rapid auto-alignment system

   https://doi.org/10.1063/5.0211005  

   Geng, Yanda; Tsidilkovski, Alan; Weber, Kevin; Mukherjee, Shouvik; Restelli, Alessandro; Subhankar, Sarthak (September 2024, Review of Scientific Instruments)

   Maintaining stable and precise alignment of a laser beam is crucial in many optical setups. In this work, we present a microcontroller-based rapid auto-alignment system that detects and corrects for drifts in a laser beam trajectory using a pair of two-dimensional duo-lateral position sensing detectors (PSDs) and a pair of mirror mounts with piezoelectric actuators. We develop hardware and software for interfacing with the PSDs and for controlling the motion of the piezoelectric mirror mounts. Our auto-alignment strategy—implemented as a state machine on the microcontroller by a real-time operating system kernel from FreeRTOS—is based on a simple linearized geometrical optical model. We benchmark our system using the standard case of coupling laser light efficiently into the guided mode of a single-mode fiber optic patch cable. We can recover the maximum fiber coupling efficiency in ∼10 seconds, even for a laser beam misaligned to the point of zero fiber coupling efficiency. 

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

   https://doi.org/10.1364/OL.44.002314  

   He, Lingyan; Zhang, Mian; Shams-Ansari, Amirhassan; Zhu, Rongrong; Wang, Cheng; Marko, Lončar (April 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 $>10\mathrm{dB}/\text{facet}$) 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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5. Low-loss adiabatic fiber-optic coupler for cryogenic photonics

   https://doi.org/10.1364/AO.502604  

   Zhao, Mengdi; Fang, Kejie (November 2023, Applied Optics)

   Recent developments in quantum light–matter coupled systems and quantum transducers have highlighted the need for cryogenic optical measurements. In this study, we present a packaged fiber-optic coupler with a coupling efficiency of over 50% for telecom wavelength light down to the mK temperature range. Besides the high coupling efficiency, our method enables sensitive photonic device measurements that are immune to mechanical vibrations present in cryogenic setups. 

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