Dense and efficient circuits with component sizes approaching the physical limit is the hallmark of high performance integration. Ultimately, these features and their pursuit enabled the multi-decade lasting exponential increase of components on integrated electronic chips according to Moore’s law, which culminated with the high performance electronics we know today. However, current fabrication technology is mostly constrained to 2D lithography, and thermal energy dissipation induced by switching electronic signal lines presents a fundamental challenge for truly 3D electronic integration. Photonics reduces this problem, and 3D photonic integration is therefore a highly sought after technology that strongly gains in relevance due to the need for scalable application-specific integrated circuits for neural networks. Direct laser writing of a photoresin is a promising high-resolution and complementary metal-oxide-semiconductor (CMOS) compatible tool for 3D photonic integration. Here, we combine one and two-photon polymerization (TPP) for waveguide integration for the first time, dramatically accelerating the fabrication process and increasing optical confinement. 3D additive printing is based on femtosecond TPP, while blanket irradiation with a UV lamp induces one-photon polymerization (OPP) throughout the entire 3D chip. We locally and dynamically adjust writing conditions to implement (3 + 1)D flash-TPP: waveguide cores are printed with a small distance between neighboring writing voxels to ensure smooth interfaces, mechanical support structures are printed at maximal distance between the voxels to speed up the process. Finally, the entire chip’s passive volume not part of waveguide cores or mechanical support is polymerized in a single instance by UV blanket irradiation. This decouples fabrication time from the passive volume’s size. We succeed in printing vertical single-mode waveguides of 6 mm length that reach numerical apertures up to NA = 0.16. Noteworthy, we achieve exceptionally low −0.26 dB injection losses and very low propagation losses of −1.36 dB/mm at λ 0 = 660 nm, which is within one order of magnitude of standard integrated silicon photonics. Finally, the optical performance of our waveguides does not deteriorate for at least ∼3000 h after printing, and remains stable during ∼600 h of continuous operation with 0.25 mW injected light.
We experimentally demonstrate, based on a generic concept for creating 1-to-M couplers, single-mode 3D optical splitters leveraging adiabatic power transfer towards up to 4 output ports. We use the CMOS compatible additive (3+1)D flash-two-photon polymerization (TPP) printing for fast and scalable fabrication. Optical coupling losses of our splitters are reduced below our measurement sensitivity of 0.06 dB by tailoring the coupling and waveguides geometry, and we demonstrate almost octave-spanning broadband functionality from 520 nm to 980 nm during which losses remain below 2 dB. Finally, based on a fractal, hence self-similar topology of cascaded splitters, we show the efficient scalability of optical interconnects up to 16 single-mode outputs with optical coupling losses of only 1 dB.
Low-loss single-mode optical coupling is a fundamental tool for most photonic networks, in both, classical and quantum settings. Adiabatic coupling can achieve highly efficient and broadband single-mode coupling using tapered waveguides and it is a widespread design in current 2D photonic integrated circuits technology. Optical power transfer between a tapered input and the inversely tapered output waveguides is achieved through evanescent coupling, and the optical mode leaks adiabatically from the input core through the cladding into the output waveguides cores. We have recently shown that for advantageous scaling of photonic networks, unlocking the third dimension for integration is essential. Two-photon polymerization (TPP) is a promising tool allowing dynamic and precise 3D-printing of submicrometric optical components. Here, we leverage rapid fabrication by constructing the entire 3D photonic chip combining one (OPP) and TPP with the (3+1)D flash-TPP lithography configuration, saving up to ≈ 90 % of the printing time compared to full TPP-fabrication. This additional photo-polymerization step provides auxiliary matrix stability for complex structures and sufficient refractive index contrast ∆n ≈ 5 × 10 −3 between core-cladding waveguides and propagation losses of 1.3 dB/mm for single-mode propagation. Overall, we confront different tapering strategies and reduce total losses below ∼ 0.2 dB by tailoring coupling and waveguides geometry. Furthermore, we demonstrate adiabatic broadband functionality from 520 nm to 980 nm and adiabatic couplers with one input and up to 4 outputs. The scalability of output ports here addressed can only be achieved by using the three-spatial dimensions, being such adiabatic implementation impossible in 2D.
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