Nanoimprint lithography for the fabrication of multimode waveguide-based out-of-plane optical interconnect

This paper reports the fabrication and optical characterization of an out-of-plane hybrid polymer–based multimode optical interconnect. A multi-lithography fabrication strategy is demonstrated, integrating two-photon polymerization direct laser writing (2PP-DLW), UV lithography, and nanoimprint lithography (NIL). Building upon earlier work that established the fabrication of NIL masters incorporating waveguide and micro-mirror patterns using 2PP-DLW and UV lithography, this study focuses on the realization of a nanoimprinted planar multimode waveguide with integrated micro-optical mirrors at both termini. The optical interconnect is fabricated using hybrid-polymer OrmoCer® resin and is imprinted onto a transparent, flexible polyethylene naphthalate (PEN) substrate with precise control of the residual layer thickness. To mitigate residual layer formation, a compliant polydimethylsiloxane (PDMS) element is employed during imprinting to facilitate efficient extraction of excess resin following channel filling, thereby improving resin distribution and pattern fidelity. The combined use of the three lithography techniques yields high structural quality with surface roughness below 0.1λ, while remaining compatible with cost-effective, high-throughput manufacturing. The demonstrated structure is well suited for fast and energy-efficient optical signal transfer between on-board optical transceivers. Optical performance was evaluated using cut-back measurements at a wavelength of 850 nm with multimode fiber coupling, enabling accurate determination of waveguide insertion loss.