Self-Aligned Fiber-to-Waveguide Configuration for Enhanced Thermal Stability and Cost-Effective Production of Nanoporous Waveguides for Sensing

Nanoporous silica waveguides can be used to improve the low sensitivity of near-infrared spectroscopic gas sensors by allowing the light propagating through the waveguide core to interact directly with molecules entering the pores. Here we report on a self-aligned fiber-to-waveguide configuration that offers a robust and cost-effective solution for coupling fibers to the nanoporous waveguide without the need for active alignment or expensive instrumentation. Our approach uses dedicated fiber-alignment structures next to the waveguide, fabricated at the same time as the waveguide, and made from the same material, thus eliminating the need for separate substrates, additional fabrication steps, and minimizing thermally induced optical misalignment. We present the optimized microfabrication process steps that allow for direct insertion of optical multimode silica fibers next to the multimode porous silica waveguide and provide structural and optical characterization. Gas sensing performance is evaluated using isopropyl alcohol vapor showing excellent sensitivity and detection limit of 1.76 ppm for a 10 mm long waveguide.