Robust On-Chip Polymer Coupler for All-Optical Ultrasound Detection

Fiber-chip couplers play an important role in the field of on-chip all-optical ultrasound detection; however, they have received limited attention in research. Here, we present an on-chip photoresin coupler fabricated via two-photon lithography, combining the benefits of compact size, wide bandwidth, low loss, and robust coupling. Utilizing a high-refractive-index photoresin medium, we achieved transmission efficiencies better than −1.3 dB in water environments within a 1528–1567 nm bandwidth. Alignment errors were constrained to ±2.5 μm laterally and 20 μm axially, with angular deviations exceeding ±3° at a −1 dB loss. Its sturdy structure facilitates 5–30 MHz ultrasound detection in liquid environments through phase-shifted Bragg grating.