Single-mode bending optofluidic waveguides and beam splitters in fused silica enabled by polarization-independent femtosecond-laser-assisted etching

Bending optofluidic waveguides are essential for developing high-performance fluid-based photonic circuits and systems. The combination of femtosecond (fs)-laser-assisted etching of high-precision microchannels and vacuum-assisted liquid-core filling allows the controllable fabrication of low-loss optofluidic waveguides in fused silica. However, to form high-performance bending optofluidic waveguides in fused silica, facile fabrication of long, homogeneous microchannels with arbitrary shapes remains challenging due to the polarization-dependent limitations of conventional fs-laser-assisted etching. Here, we demonstrate the rational fabrication of homogeneous curved microchannels in fused silica using polarization-independent fs-laser-assisted etching enabled by a low-pulse-overlap scheme. An etching rate exceeding 350 μm/h can be reliably achieved at a pulse overlap of 10 pulses μm⁻¹ regardless of the variation of the laser polarization. Highly interconnected nanocracks are observed along the laser writing direction in the laser-modified regions. Using the polarization-independent fs-laser-assisted etching combined with spatial beam shaping and carbon dioxide laser irradiation, uniform and smooth curved microchannels with centimeter lengths, flexible configurations, and nearly circular cross-sections are initially produced. Subsequently, single-mode bending optofluidic waveguides and beam splitters are created by filling tunable refractive index liquid-core solutions into the channels. The proposed method enables efficient processing of arbitrarily oriented homogeneous microchannels, paving the way for the development of large-scale, complex microfluidic photonic circuits.