Manufacture of three-dimensional optofluidic spot-size con-verters in fused silica using hybrid laser microfabrication

We propose a hybrid laser microfabrication approach for the manufacture of three-dimensional (3D) optofluidic spot-size converters in fused silica glass by a combination of femtosecond (fs) laser microfabrication and carbon dioxide laser irradiation. Spatially shaped fs laser-assisted chemical etching is first performed to form 3D hollow microchannels in glass, which are composed of embedded straight channels, tapered channels, and vertical channels connected with the glass surface. Then, carbon dioxide laser-induced thermal reflow is carried out for internal polishing of the whole microchannels and sealing parts of vertical channels. Finally, 3D optofluidic spot-size converters (SSC) are formed by filling a liquid-core waveguide solution into laser-polished microchannels. With a fabricated SSC structure, the mode spot size of the optofluidic waveguide can be expanded from ~8 μm to ~23 μm with a conversion efficiency of ~84.1%. Further measurement of the waveguide-to-waveguide coupling devices in the glass showed that the total insertion loss of two symmetric SSC structures through two ~50 μm diameter coupling ports was only ~6.73 dB at 1310 nm, which is about half of non-SSC structures with diameters of ~9 μm at the same coupling distance. The proposed approach holds great potential for developing 3D novel fluid-based photonic devices for mode conversion, optical manipulation, and lab-on-a-chip sensing.