Next generation of 3D printed photonic circuits for scalability
and high-performance hybrid integration
Adrià Grabulosa1 , Johnny Moughames1 , Xavier Porte1 and Daniel Brunner1
1. Institut FEMTO-ST, Université Franche-Comté, CNRS UMR6174, Besançon, France.

Summary.
Using additive and CMOS compatible one- and two-photon polymerization, i.e. flash-TPP printing, we create air- and polymer-cladded 3D photonic waveguides and single-mode splitters for scalable,
dense and parallel interconnects. Our 3D technology represents a breakthrough towards the highlyinterconnection required in optical neural networks, which removes the high energy dissipation ofelectronics and where 3D integration enables scalability that is challenging to realize in 2D.

1Introduction

In electronics, when realizing a direct connection of multiple channels, the capacity of long and dense
electronic wires results in high energy dissipation and bandwidth limitations. Optical routing is advantageous in terms of speed, latency and energy dissipation, since the energy associated with charging thecapacity of the electronic wires is removed. Likewise, combining the strength of multiple photonic andelectronics concepts in one hybrid and multi-chip platform is a promising solution for the diversificationof chips for specific computing tasks to boost performance [1]. We have demonstrated a new routing
strategy for integrating photonic neural networks via additive 3D fabrication, showing parallel and largescale interconnection of 225 in- and 529 output channels [2], see Fig. 1 (a). We further demonstrated
flash-TPP [3], a novel CMOS compatible lithographic methodology that enables ultra-fast fabricationof high-performance 3D photonic circuits based on additive fabrication via one- (OPP) and two-photon
polymerization (TPP) combined with direct-laser writing (DLW), see Fig. 1 (b).
(a)

(b)

(i)

(ii)

Mechanical supports
Waveguidecore(iii)

200 μmTPP OPP

50 μm

Figure 1: 3D printed photonic circuits. (a) SEM micrograph depicting a 3D printed array of opticalsplitters interconnecting in parallel 225 in- and 529 output waveguides. (b) Flash-TPP concept forphotonic integration: (i) the waveguides cores (mechanical supports) are printed with high(low)-resolution
via DLW-TPP; (ii) the cladding is polymerized under UV blanket exposure via OPP. (iii) SEM micrographof a cross-section where the red (blue) region represents the sections polymerized via TPP (OPP).

2

Experimental results

Using DLW-TPP additive fabrication, we demonstrated multi-mode optical splitters based on air-cladded
waveguides with a 1 to 4, 1 to 9 and 1 to 16 configuration [4] as well as efficient scalability of opticalinterconnects by cascading a double-layer of 1 to 9 splitters, resulting in 81 outputs, see Fig. 1 (a-b). For

large-scale networks, we fabricated 3D parallel interconnects by spatially multiplexing 1 to 9 splitters,
resulting in an array of 3x3 input waveguides with 25 outputs, see Fig. 1 (c).
Via flash-TPP, we fabricated polymer-cladded waveguides with a refractive index contrast between
core and cladding in the order of ∆n ≈ 5 × 10−3 , enabling single-mode propagation over large (6 mm)
distances, and with only 1.3 dB/mm (0.26 dB) propagation (injection) losses. We further showed ageneral tapering strategy of single-mode optical splitters leveraging adiabatic transfer from one input toup to 4 outputs in a single component.
In a last investigation, we showed the concept’s CMOS compatibility by successfully printing a cascaded 1 to 16 splitters on top of GaAs quantum dot micropillar laser array [5], as shown in the SEM
micrograph from Fig. 2 (d). Preliminary optical characterization of photonic waveguides printed on topof such semiconductor device showed good performance in terms of optical losses and stability over time.
(a)

(b)

(c)

(d)
Polymer

GaAs micro laser array

25 μm

25 μm

Figure 2: 3D printed optical splitter and CMOS compatibility. (a) SEM micrographs depicting 3D printed
air-cladded 1 to 4 optical splitter, (b) Double layer of 1 to 9 splitters resulting in 81 optical outputs, and(c) 3x3 input array of large-scale interconnects after spatially multiplexing 1 to 9 splitters. (d) SEM
micrograph of a 3D cuboid integrating a 1 to 16 adiabatic coupler printed on top of a GaAs quantum
dot micropillar laser array [5].

3Conclusions

We have demonstrated the 3D additive fabrication of multi-mode and air-cladded optical splitters with
1 to 4, 1 to 9 and 1 to 16 optical outputs [4] as well as highly-connected photonic circuits with 225
in- and 529 output waveguides that only occupies 460x460x300 µm3 [2]. These architectures are prime
candidate for highly-dense photonic packaging, specially towards photonic neural networks schemes. We
further developed flash-TPP [3], a simple lithographic configuration that combines DLW-TPP and OPP
for the ultra-fast fabrication of polymer-cladded single-mode photonic waveguides and adiabatic splitters.
Finally, we have demonstrated the reliability of flash-TPP with CMOS technology by printing a cascaded
1 to 16 adiabatic couplers on top of micro-laser arrays [5]. With this, we lay a promising foundation forscalable integration of hybrid photonic and electronic platforms [1]. Such scalability is essential for
efficient parallel communication throughout a densely-connected network, which is challenging in 2D.

References[1] N. Lindenmann, G. Balthasar, D. Hillerkuss, R. Schmogrow, W. Freude, and C. Koos, ”Photonicwire bonding: a novel concept for chip-scale interconnects,” Opt. Express 20, 17667-17677 (2012).
[2] J. Moughames, X. Porte, M. Thiel, G. Ulliac, L. Larger, M. Jacquot, M. Kadic, and D. Brunner,“Three-dimensional waveguide interconnects for scalable integration of photonic neural networks,”
Optica 7, 640 (2020).
[3] A. Grabulosa, J. Moughames, X. Porte, and D. Brunner. ”Combining one and two photon polymerization for accelerated high performance (3 + 1)D photonic integration” Nanophotonics, vol. 11, no.
8, 2022, pp. 1591-1601 (2022).
[4] J. Moughames, X. Porte, L. Larger, M. Jacquot, M. Kadic, and D. Brunner, “3d printed multimodesplitters for photonic interconnects,” Opt. Mater. Express 10, 2952–2961 (2020).
[5] S. Reitzenstein and A. Forchel, “Quantum-dot micropillars,” J. Phys. D: Appl. Phys. 43 033001(2010).