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Design and Optimization of Ultra-Compact Bending Waveguide with Dual Polarization Modes of Scalable Bandwidth
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DOI：10.3788/LOP251553.png)
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Abstract
En 中文
Objective Due to the rapid development of photonic integration technology, photonic integrated circuits have become essential in various fields, including optical communications, biomedicine, data storage, quantum optics, sensing, artificial intelligence, and high-performance computing. As a fundamental component for guiding and coupling optical signals within devices, curved waveguides are indispensable. However, significant beam deflection occurs at the bends of curved waveguides, causing light to escape from the waveguide and resulting in substantial radiation loss. Furthermore, the abrupt change in the curvature radius at the interface between straight and curved waveguides leads to a misalignment of the fundamental mode field centers. This mode mismatch not only excites higher-order modes but also introduces considerable additional loss and inter-modal crosstalk. To mitigate these losses, this paper proposes an ultra-compact curved waveguide structure that supports dual polarization modes and features a scalable width. Methods To reduce the radiation loss as light traverses the bend, a taper structure was introduced to decrease the internal waveguide width of the dual-polarization-mode ultra-compact curved waveguide. Additionally, since the input and output waveguides are relatively wide, the direct binary search (DBS) algorithm was employed to optimize the input waveguide, the taper structure, and the output waveguide, thereby minimizing the transmission loss in these regions. Results and Discussions For the 90 degrees bend waveguide, with a fixed bending radius R-1, increasing the width W-x leads to severe radiation leakage after the sharp turn under both TE0 and TM0 modes. When both R-1 and W-x are fixed, the radiation leakage is more pronounced for the TM0 mode compared to the TE0 mode. Similar observations are made for the 180 degrees bend waveguide: with a fixed R-2, increasing W-y results in radiation leakage at the bend for both polarizations; and with fixed R-2 and W-y, the TM0 mode exhibits more severe leakage than the TE0 mode. Under either TE0 or TM0 mode, and with constant R-1, L-1, and W-1, the number of air holes filling region 1 of the optimized 90 degrees bend waveguide increases with W-x. Similarly, with constant R-2, L-2, and W-2, the number of air holes in region 2 of the 180 degrees bend waveguide increases with W-y. For both 90 degrees and 180 degrees bend waveguides, with a constant bending radius, taper length, and bottom width, increasing the top width (while effectively reducing the waveguide width within the bend via the taper) can mitigate the beam divergence after the sharp turn and reduce radiation leakage in the bending region. Moreover, optimizing region 1 (or region 2) using the DBS algorithm effectively suppresses beam divergence in the wide waveguide sections and enhances the optical field intensity within them. Under TE0 mode, the maximum transmittance for L-bend1, L-bend2, and L-bend3 increased from 0.88, 0.79, and 0.73 to 0.97, 0.95, and 0.93, respectively, while the minimum transmittance increased from 0.86, 0.76, and 0.69 to 0.96, 0.92, and 0.90, respectively. The corresponding losses were significantly reduced from-0.68 dB,-1.2 dB, and-1.64 dB to-0.191 dB,-0.373 dB, and-0.48 dB. Under TM0 mode, the maximum transmittance for L-bend1, L-bend2, and L-bend3 rose from 0.82, 0.76, and 0.72 to 0.91, 0.90, and 0.89, respectively, with the minimum transmittance increasing from 0.67, 0.61, and 0.57 to 0.82, 0.82, and 0.81, respectively. The losses were markedly reduced from-1.76dB,-2.2dB, and-2.49 dB to-0.89 dB,-0.86 dB, and-0.91 dB. For the 180 degrees bend waveguide under TE0 mode, the maximum transmittance for U-bend1, U-bend2, and U-bend3 improved from 0.92, 0.85, and 0.79 to 0.98, 0.98, and 0.96, respectively, and the minimum transmittance from 0.89, 0.82, and 0.76 to 0.96, 0.97, and 0.94, respectively. The losses decreased substantially from-0.54 dB,-0.86dB, and-1.18dB to-0.18dB,-0.15dB, and-0.86dB. Under TM0 mode, the maximum transmittance increased from 0.83, 0.78, and 0.73 to 0.94, 0.93, and 0.93, respectively, and the minimum transmittance from 0.68, 0.62, and 0.57 to 0.89, 0.90, and 0.90, respectively. The corresponding losses were greatly reduced from-1.71 dB,-2.12 dB, and-2.48 dB to-0.49 dB,-0.48dB, and-0.49dB. In comparison with prior works, the curved waveguide presented in this study primarily utilizes a taper structure to narrow the cross-section and the DBS algorithm to optimize specific regions, which together lead to a significant reduction in transmission loss. Furthermore, the designed waveguide features a relatively wide cross-sectional width, a small bending radius, support for dual-mode (TE0/TM0) operation, and a compact device footprint. Conclusions For the designed 90 degrees bend waveguides with widths Wx ranging from 1.2 & micro;m to 1.6 & micro;m, the loss is below-0.48 dB under TE0 mode and below-0.91 dB under TE0 mode across the studied wavelength band. For the 180 degrees bend waveguides with widths W-y between 1.3 & micro;m and 1.7 & micro;m, the loss remains below-0.86 dB under TE0 mode and below-0.49 dB under TM0 mode. The width-scalable, dual-polarization-mode ultra-compact curved waveguide design presented in this work not only reduces transmission loss for beams propagating in photonic integrated circuits but also broadens the compatible width range of connectable waveguide devices, thereby enhancing the generality and applicability of curved waveguides.
Keywords:
inverse design
direct binary search algorithm
90 degrees bend waveguide
180 degrees bend waveguide
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