Hybrid multi-chip assembly of optical communication engines via 3-D nanolithography

Three-dimensional (3-D) nanoprinting of freeform optical waveguides also known as photonic wire bonding can efficiently couple between photonic chips to greatly simplify optical system assembly. The shape and trajectory of photonic wire bonds offers a key advantage as an alternative to conventional optical assembly techniques that rely on technically complex and expensive high-precision alignment. In a new study now published on Nature: Light, Science & Applications, Matthias Blaicher, Muhammed Rodlin Billah and a research team in photonics, quantum electronics and microstructure technology in Germany, demonstrated optical communication engines. The device relied on photonic wire bonding to connect arrays of silicon photonic modulators to lasers and single-mode fibers. They engineered the photonic wire bonds onto the chips in the lab using advanced 3-D lithography to efficiently connect a variety of photonic integration platforms. The scientists simplified the assembly of advanced photonic multistep modules to transform a variety of applications ranging from high-speed communications to ultra-fast signal processing, optical sensing, and quantum information processing.

Photonic integration is a key method to transform a variety of quantum technologies. Most commercial products in the field rely on the discrete assembly of photonic chips that require coupling elements such as on-chip adaptors and bulky microlenses, or redirecting mirrors. Assembling such systems require technically complex active alignment, to continuously monitor the coupling efficiency during device development. Such techniques are classified as high cost and low throughput methods, and as a result they set back any advantages of wafer-scale mass production of photonic integrated circuits (PIC). In this study, Blaicher et al. combined the performance and flexibility of conventional systems with the compactness and scalability for monolithic integration using advanced additive nanofabrication techniques. To engineer freeform polymer waveguides on photonic devices, the team relied on direct-write-two-photon-lithography. The method is also known as photonic wire bonding to allow highly efficient optical coupling in a fully automated process.

During the experiments Blaicher et al. engineered 100 densely spaced photonic wire bonds (PWBs). The experimental results formed the basis for the simplified assembly of advanced photonic multi-chip systems. The experimental module contained multiple photonic dies based on different material systems including indium phosphide (InP) or silicon-on-insulator (SOI). The experimental steps of assembly did not require high-precision alignment and the scientists achieved chip-to-chip and fibre-to-chip connections using 3-D freeform photonic wire bonds. Before fabricating PWBs, Blaicher et al. detected on-chip alignment markers using 3-D imaging and computer vision techniques. Thereafter, they used two-photon lithography to fabricate the PWBs, allowing resolution at the sub-micron scale. The team placed optical clips side by side in the setup to prevent thermal bottlenecks for efficient thermal connection. The hybrid multichip module (MCM) relied on efficient connections of the silicon photonic (SiP) chip to the InP light source and to the output transmission fiber. The team realized the light sources as horizontal cavity surface emitting lasers (HCSELs) and when they combined the PWBs with microlenses, they could facilitate optical out-of-plane connections to the chip surface.

In the first experiment, using test chips fabricated via deep-UV lithography, the team showed that PWBs provided low-loss optical connections. Each test chip contained 100 test structures to separate the PWB loss from fibre-chip coupling loss. The in-lab fabrication of PWB was fully automated, taking approximately 30 seconds per connection and the process could be further accelerated. The team obtained comparable results by replicating the experiments on other test chips to clearly demonstrate the excellent reproducibility of the process. The scientists then exposed the sample to multiple temperature cycles varying from -40°C to 85°C to prove reliability of the structures under technically relevant environmental conditions. The samples did not undergo performance degradation or deformation during the experiments. To understand high-power handling capability of the PWB structures, they subjected the samples to continuous laser irradiation at 1550 nm, with increasing optical power levels. The experiments showed the possibility of using PWBs for high performance in industrially relevant environments and under realistic power levels.

To then demonstrate the technical feasibility of the PWB approach, Blaicher et al. realized a functional eight-channel photonic multi-chip transmitter (Tx) engine that combined InP-based laser arrays and SiP (silicon photonic chip) modulator arrays to modulate intensity. The complete assembly contained two arrays of four horizontal cavity surface emitting lasers, connected via PWBs to an array of travelling-wave-depletion-type Mach Zehnder modulations. The demonstration was a proof-of-principle, leaving room for optimization.

During the second series of experiments, the team formed a four-channel multi-step transmitter module for coherent communication. In this module, they combined hybrid multi-chip integration containing PWBs with hybrid on-chip integration of electro-optic modulators, to combine the SiP nanowire waveguides with highly efficient electro-optic materials. The setup resulted in highly efficient devices with low power consumption.

In this way, Matthias Blaicher, Muhammed Rodlin Billah and colleagues conducted 3-D nanofabrication of photonic wire bonds (PWBs) to overcome the existing limits of hybrid photonic integration approaches. The team demonstrated the viability of the experimental setup using two key protocols to realize two different hybrid multi-chip transmitter engines. While the team focused on transmitter modules for high speed optical communication during this work, the technology may unlock a wide range of novel applications that benefit from the advantages of hybrid photonic integration.

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