Silicon photonics based on the silicon-on-insulator (SOI) platform, offering a high index-contrast between the silicon core and its surroundings for strong mode field confinement and light-matter interaction, is a promising technology for realizing ultracompact devices in the high-density photonic integrated circuits (PICs).
As fundamental and indispensable building elements serving the purpose of power distribution (1-to-N channels), optical power splitters (OPS) have been broadly utilized in light manipulation and/or forming ultra-density PIC and other complex on-chip devices, e.g., optical modulators, sensor, optical phased arrays, switches, and logic circuits, which are promising for integrated photonics.
Currently, directional couplers, Y-branches, and multimode interference (MMI) couplers are three main approaches to realize on-chip power splitting. However, as the number N of output splitting channels/ports increases, the following issues are highlighted for conventional power splitting schemes. (1) Only one operating polarization is supported (transverse electric TE mode or transverse-magnetic TM mode), indicating that the power splitter is sensitive to the polarization state. Under this scenario, the cascade of polarization controlling devices is necessary to ensure good performance of the power splitting. (2) Relatively larger splitting lengths are required. For example, 1 × 4 power splitters by using MMI couplers normally have several tens of micrometers, which is not beneficial for building ultra-dense PICs. (3) The power splitting scheme is not universal and thus cannot be expanded to apply in other cases of different numbers for N, i.e., rather low scalability and flexibility, which means a redesign or a cascaded strategy is required as the number N increases.
To address the abovementioned issues, a joint research group led by Jinbiao Xiao at School of Electronic Science and Engineering, Southeast University in collaboration with Shengbao Wu from College of Physics Science and Technology, Hebei University has designed and experimentally demonstrated a ultracompact, scalable, and polarization-independent power splitting model by using fan-out bending metamaterials, where an ultracompact device length of < 4.3 ?m can be maintained as the output splitting channels are increased from 2 to 5. Based on this model, the shortest 1×3, 1×4, and 1×5 polarization-independent power splitters are realized. The relevant research results were published in Photonics Research, Volume 10, No. 11, 2022 (Zhenzhao Guo, Jinbiao Xiao, Shengbao Wu. Ultracompact, polarization-independent, and highly scalable optical power splitting model employing fan-out bending metamaterials[J]. Photonics Research, 2022, 10(11): 2448).
In this model, fanout bending subwavelength gratings (FBSWGs) metamaterials instead of classical straight SWGs are leveraged to expand the input TE/TM mode in an ultracompact region and further bend its wavefronts. By using N angled tapers to match bending wavefronts, the light expanded by FBSWGs can be collected and evenly distributed into N output channels, corresponding to the 1 × N power splitting. The schematic configuration and light propagation profiles are given in Figure 1. Notably, TE and TM modes are both supported in this model, showing polarization-independent operations.
Fig.1 (a) Schematic of proposed power splitting model. (b) Mode propagation profiles for the 1×2, 1×3, 1×4, and 1×5 power splitters.
Based on such a model, three optical power splitters are designed and experimentally demonstrated, including the 1 × 3, 1 × 4, and 1 × 5 power splitters, and the corresponding scanning electron microscopy images of devices are shown in Figure 2(a). Experimental results show that such a power splitting model using FBSWG metamaterials can offer insertion losses < 1.2 dB, < 1.35 dB, and < 1.65 dB (power uniformities < 0.9 dB, < 1 dB, and < 1 dB) over bandwidths of 54 nm, 49 nm, and 38 nm for the 1 × 3, 1 × 4, and 1 × 5 power splitters, respectively, for both TE and TM modes. To the best of our knowledge, this is the first power splitting model that can maintain an ultracompact device length of < 4.3 ?m and a polarization-independent operating state simultaneously as the output channel number N is increased from 2 to 5.
Fig.2 (a) Scanning electron microscopy images of the 1×3, 1×4, and 1×5 power splitters. (b) Measured and normalized transmittance of the 1×3, 1×4, and 1×5 power splitters, for both TE and TM modes.
The article reports ultracompact, polarization-independent, and highly scalable optical power splitting model for flexible on-chip power distributions. Furthermore, by further extending the working bandwidth and splitting channel N, and manipulating the phase of output modes, such a power splitting model can be applied for building ultrahigh-density PICs and complicated optical devices, which has a huge implication for the power distributions in silicon photonics.
