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Planar lightwave circuits (PLCs) provide economical, high-capacity solutions for systems using wavelength-division multiplexing. To accommodate a higher volume of optical integration in a smaller footprint, PLC technology has trended towards higher refractive index contrast platforms resulting in tighter optical confinement. Further progress in the densification of photonic functionality, especially for multi-stage interferometric configurations, must rely on the development of advanced architectures to increase the density of functional units. We present a breakthrough approach to the synthesis of ultra-dense interferometric chains, reaching packing density of waveguides close to theoretical limits. The proposed framework is well suited for mixed parallel and sequential interferometric structures in low- or high-refractive index contrast platforms. The new methodology allows the addition of stages to an interferometric chain without appreciable increase in device footprint, thus creating a highly-optimized ultra-dense waveguide layout. To validate this approach, we designed and fabricated a 4-λ LAN multiplexer that comprises 7 interferometric stages in a silica-on-silicon platform with a refractive index contract of Δn = 2.0%. Despite the relatively low refractive index contrast, the device was realized in a footprint of only 0.15 cm2. The multiplexer exhibits exceptional optical performance, including on-chip loss of 0.2 dB, negligible polarization-dependent loss, and a remarkably flat single-mode spectral response with no insertion loss penalty. This ultra-compact implementation, combined with the state-of-the-art optical performance characteristics, led to a wide deployment of the multiplexer in data center applications, and provided a rapidly-advancing roadmap for unprecedented densification of optical functionality in PLCs in any refractive index platform.
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