Three-Dimensional Integrated Optical Waveguide Devices Based on Multi-Material Systems(Invited)
Significance With the advent of big data and artificial intelligence eras,key enabling technologies such as high-capacity optical communication,high-performance computing,and high-sensitivity sensing and detection have experienced rapid development.Photonic integrated systems have attracted increasing attention,elevating demands on both the performance of basic building blocks and the overall integration and scalability of the system.Optical waveguides,fundamental units of optical interconnects in photonic integrated systems,offer notable advantages over traditional electrical wires,including higher data capacity,lower loss,and improved resistance to electromagnetic interference.Traditional optical waveguide structures are primarily fabricated using two-dimensional(2D)semiconductor fabrication processes,constraining light propagation to a single plane.Given that basic photonic building blocks face diffraction limits,achieving device miniaturization through advanced fabrication technology nodes,unlike those in microelectronics,is challenging.Therefore,the number of integrated devices on a single chip is limited by the chip's footprint.To overcome these constraints of traditional 2D integration technology,there is an escalating need for three-dimensional(3D)integration techniques,which promise superior performance and increased integration density.The silicon photonic platform,compatible with complementary metal oxide semiconductor processes,is vital for photonic integrated devices.Currently,various photonic devices have been realized on the silicon-on-insulator platform,encompassing low-loss optical waveguides,passive optical waveguide devices,high-speed modulators,and detectors.As integrated systems become more complex,routing waveguides on a single device layer becomes more challenging.Efficient interlayer coupler structures capable of routing the light into another device layer facilitate waveguide routing in 3D stacked photonic structures.These interlayer couplers not only enable vertical light transmission between different device layers but also substantially reduce waveguide crossings,associated crosstalk and loss.This method of 3D photonic integration enhances intensity density and system scalability.Beyond integrating different silicon-based materials like silicon,silicon nitride,and silicon oxide waveguides,silicon-based materials can be combined with other heterogeneous materials exhibiting superior electro-optical and light-emitting properties,such as thin-film lithium niobate(TFLN),Ⅲ-Ⅴ group materials,and various two-dimensional materials.By leveraging the exceptional optical or electrical properties of these diverse materials,the performance of individual optical chips is significantly improved.Moreover,the 3D stacking of optical chips and electrical chips via advanced packaging techniques achieves photonic-electronic convergence,improving bandwidth and power consumption.Femtosecond laser direct writing(FLDW)technology plays a crucial role in manufacturing photonic-integrated devices.This technology harnesses the ultra-short duration and high peak power of femtosecond laser pulses to make precise modifications in transparent dielectric materials.FLDW's key advantage lies in its ability to accurately control refractive index changes in optical waveguides,facilitating low-loss optical transmission while preserving high mechanical strength and chemical stability.These attributes make FLDW essential in modern optical communication systems and extended transmission distances.Compared to traditional optical waveguide fabrication methods,FLDW offers a more flexible and controllable 3D processing approach.It allows for the direct writing of intricate photonic structures on various material platforms,including glass,crystals,and polymers,broadening application opportunities in fields like optical communications and signal processing.Progress We focus on two main types of multi-material system 3D integrated optical waveguide technologies:3D stacking technology and femtosecond laser fabrication technology.Firstly,we introduce 3D optical coupling technology based on interlayer couplers and 3D integrated optical waveguide devices.In 3D integrated optical waveguides,the interlayer coupler is the key component connecting different device layers and significantly affects the system loss.Light transmitted in one waveguide layer couples to another layer through these interlayer couplers,enabling optical path switching within the 3D structures.Depending on their material applications,interlayer couplers are categorized into silicon/silicon nitride interlayer couplers and silicon-based heterogeneous integrated interlayer couplers.We then discuss photonic-electronic co-integrated devices based on 3D stacking technology.Early data communication systems using discrete photon-integrated chips and electrical control modules have limited performance in terms of energy efficiency,bandwidth,and latency.To enhance optical module performance,efforts have been focused not only on developing 3D integration technology for passive optical waveguide devices but also on heterogeneous integration schemes for devices and corresponding electrical control modules in active optical chips.These schemes are mainly classified into four categories:monolithic integration,2D integration,2.5D integration,and 3D integration.Currently,2.5D and 3D integrated technologies are applied in high-performance optical transmitters,receivers,wavelength-division multiplexing transceivers,and optical interconnect modules.We further explore 3D integrated optical waveguide devices based on FLDW technology.Passive devices,such as polarization multiplexing devices,mode multiplexing devices,and fan-in/fan-out devices,play crucial roles in optical communication systems.Polarization multiplexing devices utilize the birefringence effect of waveguides to multiplex and demultiplex optical signals of different polarization states,thereby increasing system transmission capacity.Mode multiplexing devices enhance communication capacity by multiplexing signals of different spatial modes in multimode optical fibers.Fan-in and fan-out devices address the efficient coupling between multi-core fibers and single-mode fibers or photonic integrated circuits,facilitating the construction of high-density integrated optoelectronic systems.Additionally,topological quantum devices based on FLDW are widely used to explore interactions between topological effects and particle interactions in depth.Conclusions and Prospects Overall,compared to existing optoelectronic integrated devices,3D integrated waveguide devices based on multi-material system can significantly enhance integration levels by leveraging the expanded space dimension.However,this also increases fabrication and packaging complexity.Future development in multi-material system 3D integrated optical waveguide devices will require a careful trade-off between complexity,system performance,cost,and yield.With ongoing improvements in device design,fabrication processes,and wafer-level testing,these devices hold promising potential in various fields,including high-speed large-capacity optical communications,data center optical interconnects,high-performance optical computing,quantum information processing,and intelligent microsystems.
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