As a widely studied fundamental block in photonic integrated circuits, multimode interferometer (MMI) is excellent in coupling of multiple light sources with equal intensity. However, unacceptable excess loss occurs if phase-matching is not satisfied at any input port. In this paper, we proceed direct binary search (DBS) algorithm to optimize an inverse designed 3 × 1 MMI coupler with nano-pixel structure and realize high-efficiency coupling of equal input (intensity and phase) sources of 1550 nm fundamental TE mode, with a compact footprint of 2.5 × 2.5 μm2 and low excess loss of 0.04dB. We also investigated the possibility of inverse design method to handle the coupling of multiple input sources with arbitrary phase difference among different ports.
We propose and experimentally demonstrate the ultra-compact on-chip silicon mode (de)multiplexers [(De)MUXs] based on a tapered bent asymmetric direction coupler. By combining the advantages of tapered asymmetric directional coupler and bent directional coupler, low loss, broad bandwidth, and good robustness against fabrication variations can be obtained within ultra-compact coupling length. The coupling length of the proposed TE1-TE0 and TE2-TE0mode (de)multiplexers is less than 8.7 µm. Experimental results reveal that, over the wavelength range from 1.534 μm to 1.6 μm, the insertion loss is lower than 1.4 dB and 1.55 dB for the TE1-TE0 and TE2-TE0 mode multiplexing, respectively, and the crosstalk is lower than -10dB.
We designed an ultra-broadband, compact, CMOS compatible, arbitrary ratio power splitter based on 220-nm-thick silicon-on-insulator (SOI) platform. The geometry of power splitter was digitalized into 20 parameters. For each different power splitting ratio (PSR), these 20 parameters were optimized to achieve low excess loss, using variational finite difference time domain (varFDTD) simulation and adjoint shape optimization. After many iterations of optimization, the structure was finally determined. The simulated excess loss was optimized to a low value, which was below 0.13dB. The PSR variation was limited to less than 0.459dB over 500 nm, across the O band and C band, showing that the PSR of the device was wavelength independent. An order of magnitude smaller than other kind of typical power splitters, the footprint of the proposed device is only about 1.2 μm × 2 μm, ensuring the compactness of the photonic integrated circuits (PICs). Simultaneously, it is easy to fabrication since the boundaries are smooth with a fairly large feature size.
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