Composite ultrafast laser inscribed waveguides for high density photonic interconnects

<p dir="ltr">Integrated photonic circuits have established themselves as a powerful and versatile tool for manipulating the properties of light. By their nature, they enable exotic capabilities such as quantum computing and they allow for the miniaturisation & integration of functions in the optical domain which previously required external components or were electronically expensive to implement. In its nascency, the field has become widely recognised for its contributions to astronomy, remote sensing, computer science and especially, telecommunications, where it has supported numerous advancements relating to high-speed internet. Now, as the era of artificial intelligence beckons, it becomes increasingly necessary to interface optical fibre directly with silicon, which is rendered troublesome by the gulf in feature size and refractive index contrast between the respective media. Ultra-fast laser inscribed photonic interposers are a promising solution to this problem, offering broadband and polarisation insensitive operation and true 3D device geometry capable of interfacing with channel-dense transmission media such as multicore fibres. The key challenge of this approach is the low refractive index contrast, typically on the order of 0.6 x 10⁻², from which two design penal- ties arise: poor mode-field matching to silicon waveguides, incurring high coupling loss and susceptibility to bend losses in regions of tight curvature, which limits channel routing and integration density. Improving these characteristics is highly desirable.</p><p dir="ltr">In this work, we address these limitations by optimising the laser material interaction in Corning Eagle XG glass, in order to maximise the peak positive refractive index change attainable via both the thermal and athermal modification regimes. Notably, the physical mechanism giving rise to the refractive index change fundamentally differs between these regimes. On this basis, an overwriting technique was developed in order produce the first demonstration of a novel composite waveguide morphology which additively combines the contributions of both mechanisms, thereby boosting the native thermal waveguide index contrast from 0.6 x 10⁻² to 1.2 x 10⁻². A parameter optimisation study revealed that the minimal achievable mode-field diameter in the thermal regime was 7.2 ��m. By applying the overwriting technique, this was reduced by 25% to 5.3 ��m. To further establish the merit of this technique, a study was performed which compared the bend loss performance of conventional thermal waveguides with the new composite approach. For a cut-off threshold of 1 dB cm⁻¹, it was found that the minimum bend radius was reduced from 11 mm to 4mm. This represents a 2.5x improvement compared to the present state-of-the-art in the literature, which is 10 mm. Based on these learnings, an 8 channel fan-in/fan-out interposer was fabricated. The device was 7 mm in length and remaps a linear array of fibre-matched waveguides with a pitch of 127 ��m at its input, to a pitch of 30 ��m at the output. The composite waveguide overwriting technique was applied at the output facet in order to reduce the output mode-field diameter. At 1550 nm, the interposer achieves an insertion loss of 0.3dB, with 0.15 dB of loss fluctuation across the optical C and L bands.</p><p dir="ltr">Finally, a silicon photonic die was designed containing an assortment of both active and passive test structures, including waveguide loop back arrays, directional couplers, Mach- Zehnder interferometers and switching meshes. These devices are addressed by arrays of inverse taper edge couplers at the chip edge facet, which feature a pitch of 30 ��m. This chip was fabricated externally by a commercial foundry. Finally, the laser written interposer was used to couple to the silicon die and the test structures were characterised. The interposer achieved a coupling loss of 3.6 dB and the interface exhibited broadband operation with ±0.18 dB of loss fluctuation across the optical C and L bands and a peak polarisation dependent loss of 0.8 dB across the same bandwidth. Additionally, the 30 ��m coupler pitch shows the potential for improving integration density, representing a 4.2x increase in the port density as compared to a standard V-groove fibre array.</p>