Heterogeneous 2D/3D photonic integrated microsystems

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Abstract
The continuing trend of exponential growth in data communications and processing are driving the need for large-scale heterogeneous integration. Similar to the trend we have observed in electronic integrated circuit development, we are witnessing a growing trend in 3D photonic integrated circuits (PICs) development in addition to that in 2D PICs. There are two main methods for fabricating 3D PICs. The first method, which utilizes ultrafast laser inscription (ULI), offers freeform shaping of waveguides in arbitrary contours and formations. The second method, which utilizes multilayer stacking and coupling of planar PICs, exploits relatively mature 2D PIC fabrication processes applied to each layer sequentially. Both the fabrication methods for 3D PICs have advantages and disadvantages such that certain applications may favor one method over the other. However, a joining of 2D PICs with 3D PICs can help develop integrated microsystems with new functionalities such as non-mechanical beam steering, space-division multiplexing (SDM), programmable arbitrary beam shaping, and photonic signal processing. We discuss examples of 3D PICs and 2D/3D integrated PICs in two applications: SDM via orbital-angular-momentum (OAM) multiplexing/demultiplexing and optical beam steering using optical phased arrays. Although a 2D PIC by itself can function as an OAM multiplexer or demultiplexer, it has limitations in supporting both polarizations. Alternatively, a 3D PIC fabricated by ULI can easily support both polarizations with low propagation loss. A combination of a 3D PIC and a 2D PIC designed and fabricated for OAM applications has successfully multiplexed and demultiplexed 15 OAM states to demonstrate polarization-diversified SDM coherent optical communications using multiple OAM states. Coherent excitation of multi-ring OAM states can allow highly scalable SDM utilizing Laguerre–Gaussian modes or linearly polarized (LP) modes. The preliminary fabrication of multi-ring OAM multiplexers and demultiplexers using the multilayer 3D PIC method and the ULI 3D PIC method has also been pursued. Large-scale (for example, 16×16 optical phased array) 3D PICs fabricated with the ULI technique have been demonstrated. Through these examples, we show that heterogeneous 2D/3D photonic integration retains the advantages of 2D PICs and 3D waveguides, which can potentially benefit many other applications.

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