Researchers led by Erik Jung fromHeidelberg University, a German research university with programs in photonics and nanotechnology, together with collaborators including Wolfram Pernice fromUniversity of Münster, a public research university known for work in nanophotonics and materials science, have demonstrated a plug-and-play fiber interface for photonic integrated circuits (PICs) that achieves 0.78-decibel total coupling loss. Results published inSciencedescribe a passive optical packaging method that uses two-photon polymerization 3D printing directly on chip surfaces to align optical fibers with photonic circuits.

Photonic integrated circuits guide light through nanoscale waveguides to process information, enabling high data throughput and low latency in applications including sensing, optical communication, quantum technologies, and neuromorphic computing. Efficient optical packaging remains a major engineering challenge because optical fibers and on-chip waveguides have different mode field diameters. Fiber-to-chip interfaces must minimize optical loss while maintaining broadband transmission.

The research described in the study introduces a removable interface based on 3D printed alignment pins and polymer couplers fabricated on a silicon nitride (Si₃N₄) photonic platform. A female multifiber termination push-on (MTP) cable connects to these printed structures, allowing passive alignment between the fiber array and the photonic chip. Authors describe this removable architecture as comparable to a USB plug for photonic integrated circuits, enabling repeatable connections without active alignment.

Optical coupling is achieved through polymer out-of-plane couplers based on total internal reflection (TIR). These structures redirect light from the fiber vertically into on-chip waveguides. Couplers include a tapered mode transfer section that transitions optical power from silicon nitride waveguides into polymer waveguides, followed by a mode field widening region, a TIR reflection surface, and an ellipsoidal focusing lens that matches the optical beam to the fiber mode. Finite element frequency-domain simulations were used to optimize coupler geometry.

Measurements show −0.41 dB peak transmission per coupler, corresponding to approximately 91% optical transmission efficiency. Broadband coupling performance remains stable across wavelengths from 1500 to 1600 nanometers, spanning the S-, C-, and L-bands used in optical communication systems. Minimum recorded transmission across this spectral range reached −0.55 dB, with a standard deviation of 0.05 dB across nominally identical couplers.

Fabrication relies on two-photon polymerization (TPP), a nanoscale additive manufacturing method capable of producing complex optical structures directly on semiconductor substrates. Couplers are produced using grayscale TPP with a high-resolution objective, while passive alignment infrastructure is written in a second TPP step using a lower magnification lens capable of printing millimeter-scale structures. Each coupler can be fabricated in under one minute, while the alignment infrastructure requires approximately 25 minutes of writing time.

Experimental testing evaluated reproducibility and alignment tolerance. Transmission measurements across multiple devices showed consistent coupling performance. Intentional offsets between the fiber array and the couplers produced a −1 dB alignment tolerance of approximately 4 micrometers, while misalignment within ±1.5 micrometers introduced less than 0.3 dB additional loss.

Researchers demonstrated the packaging system using a 16 × 1 photonic matrix designed for matrix-vector multiplication (MVM) operations. Phase-change material germanium-antimony-tellurium (GST) integrated on the photonic waveguides acts as programmable weights within the optical network. A 12 × 2 MTP cable interfaces with the chip, addressing a 17-port photonic circuit through the printed coupling structures. Measurements across all ports show an average minimum coupling transmission of −0.78 dB, confirming consistent performance across the device.

Broadband optical compatibility was tested using a superluminescent light-emitting diode (SLED) covering a 100-nanometer optical spectrum. Researchers modulated the broadband signal at 17.6 gigabaud, measuring signal integrity after transmission through two couplers. Broadband operation enables photonic computing architectures that rely on wavelength-division multiplexing or chaotic light sources for probabilistic computing.

Previous passive out-of-plane packaging techniques based on grating couplers typically report total coupling losses between 1.7 and 5.7 dB, depending on fabrication complexity and alignment conditions. The reported interface achieves 0.78 dB total loss, including 0.37 ± 0.11 dB packaging loss, while maintaining broadband optical performance.

Source: 3D Printing Industry