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Register a new userConstraining continuous topology optimizations to discrete solutions for photonic applications
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Photonic topology optimization is a technique used to find the electric permittivity distribution of a device that optimizes an electromagnetic figure-of-merit. Two common techniques are used: continuous density-based optimizations that optimize a grey-scale permittivity defined over a grid, and discrete level-set optimizations that optimize the shape of the material boundary of a device. More recently, continuous optimizations have been used to find an initial seed for a concluding level-set optimization since level-set techniques tend to benefit from a well-performing initial structure. However, continuous optimizations are not guaranteed to yield sufficient initial seeds for subsequent level-set optimizations, particularly for high-contrast structures, since they are not guaranteed to converge to solutions that resemble only two discrete materials. In this work, we present a method for constraining a continuous optimization such that it converges to a discrete solution. This is done by inserting a constrained sub-optimization at each iteration of an overall gradient-based optimization. This technique can be used purely on its own to optimize a device, or it can be used to provide a nearly discrete starting point for a level-set optimization.
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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) dominates the total insertion losses of typical LN photonic integrated circuits, where on-chip propagation 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 symmetric 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 door for practical integrated LN photonic circuits efficiently interfaced with optical fibers.
We demonstrate an ultra-compact waveguide taper in Silicon Nitride platform. The proposed taper provides a coupling-efficiency of 95% at a length of 19.5 um in comparison to the standard linear taper of length 50 um that connects a 10 um wide waveguide to a 1 um wide photonic wire. The taper has a spectral response > 75% spanning over 800 nm and resilience to fabrication variations; >200 nm change in taper and end waveguide width varies transmission by <5%. We experimentally demonstrate taper insertion loss of <0.1 dB/transition for a taper as short as 19.5 um, and reduces the footprint of the photonic device by 50.8% compared to the standard adiabatic taper. To the best of our knowledge, the proposed taper is the shortest waveguide taper ever reported in Silicon Nitride.
We experimentally demonstrate a broadband, fabrication tolerant, CMOS compatible compact silicon waveguide taper (34.2 um) in silicon-on-insulator wire waveguides. The taper works on multi-mode interference along the length of the taper. A single taper design has a broadband operation with coupling efficiency >70% over 700 nm that can be used in O, C and L-band. The compact taper is highly tolerant to fabrication variations; >100 nm change in the taper and end waveguide width varies the taper transmission by <5%. The footprint of the device i.e. taper along with the linear gratings is ~ 442 m2; i.e. 11.5X smaller than the adiabatic taper. The taper with linear gratings provides comparable coupling efficiency as standardly used focusing gratings. We have also compared the translational and rotational alignment tolerance of the focusing grating with the linear grating.
Drawing inspiration from bilayer graphene, this paper introduces its photonic analog comprising two stacked graphene-like photonic crystals, that are coupled in the near-field through spoof surface plasmons. Beyond the twist degree of freedom that can radically alter the band structure of the bilayer photonic graphene, the photonic dispersion can be also tailored via the interlayer coupling which exhibits an exponential dependence on the distance between the two photonic crystals. We theoretically, numerically, and experimentally characterize the band structures of AA- and AB-stacked bilayer photonic graphene, as well as for twisted bilayer photonic graphene with even and odd sublattice exchange symmetries. Furthermore, we numerically predict the existence of magic angles in bilayer photonic graphene, which are associated with ultra-flat bands resulted from interlayer hybridization. Finally, we demonstrate that the bilayer photonic graphene at a particular twist angle satisfying even sublattice exchange symmetry is a high-order photonic topological insulator. The proposed bilayer photonic graphene could constitute a useful platform for identifying new quantum materials and inspiring next-generation photonic devices with new degrees of freedom and emerging functionality.
Mode-division multiplexing (MDM) is becoming an enabling technique for large-capacity data communications via encoding the information on orthogonal guiding modes. However, the on-chip routing of a multimode waveguide occupies too large chip area due to the constraints on inter-mode cross talk and mode leakage. Very recently, many efforts have been made to shrink the footprint of individual element like bending and crossing, but the devices still occupy >10x10 um2 footprint for three-mode multiplexed signals and the high-speed signal transmission has not been demonstrated yet. In this work, we demonstrate the first MDM circuits based on digitized meta-structures which have extremely compact footprints. The radius for a three-mode bending is only 3.9 {mu}m and the footprint of a crossing is only 8x8um2. The 3x100 Gbit/s mode-multiplexed signals are arbitrarily routed through the circuits consists of many sharp bends and compact crossing with a bit error rate under forward error correction limit. This work is a significant step towards the large-scale and dense integration of MDM photonic integrated circuits.
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