In the rapidly evolving field of integrated photonics, the challenge of efficiently coupling light between optical fibers and on-chip waveguides remains a critical bottleneck in the development and commercialization of photonic devices. Traditional methods of fiber-to-waveguide alignment often rely on complex and time-consuming active alignment techniques, which significantly increase production costs and limit scalability. We developed a nanoporous silica waveguide, on a silicon substrate, where the open pore structure enables improved interaction of the propagating light with the environment, particularly suitable for sensing applications of volatile organic compounds. The waveguide is transparent in the near-infrared spectral region and can be used for selective spectroscopic detection of gaseous compounds between 1 and to 2.4 um wavelength. Temperature modulation of the waveguide allows for precise control of gas adsorption and desorption, improved detection sensitivity, and ability to perform frequent sensor calibrations. The overall system includes the waveguide chip, a Peltier element, a heat sink, two optical fibers butt-coupled to the waveguide, actively aligned, and secured using UV epoxy on a metallic plate next to the waveguide chip. However, only after a few heat-cool cycles, the system stops working due to optical misalignment between the fiber and the waveguide and light transmission completely disappears. This is probably caused by small thermally-induced permanent movements of components and the epoxy. To address this issue, we developed a self-aligned fiber-to-waveguide configuration that addresses this issue, offering a robust and cost-effective solution. Our approach uses dedicated fiber-alignment structures, fabricated at the same time as the waveguide, thus eliminating the need for separate substrates, additional fabrication steps, and minimizing thermally induced optical misalignment. The distance between the alignment structures is carefully tailored to match the outer diameter of the fibers, ensuring a snug fit and precise alignment with the waveguide core without the need for active alignment. The fabrication is as follows: boron-doped p-type silicon samples of 25×25 mm with a resistivity of 0.01–0.02 Ω cm and orientation (100) were electrochemically etched in a 1:1 mixture of 48% hydrofluoric acid and ethanol. To create a porous core layer on top of a porous cladding layer, two different current densities were used. The cladding was fabricated to have a slightly higher porosity than the core using higher current density (higher porosity = lower refractive index). The resulting double-layered porous silicon area had a diameter of 1.5 cm. Following that, standard lithography process and reactive ion etching were used to remove parts of the porous material, leaving a slab of 200 µm wide, 200 µm high, and a 10 mm long waveguide, and alignment structures on both sides of the waveguide. Finally, to convert the porous silicon to porous silica, the sample was thermally oxidized in a furnace at 900oC for 9 hours. The distance between the alignment structures has been designed to accommodate for the volume expansion due to oxidation from silicon to silica, so that the final structure fits the silica fiber perfectly. The optical fibers with a core diameter of 200 µm and a cladding dimeter of 220 µm, are cleaved, and once the fibers are positioned in place, a small amount of epoxy can be applied to permanently secure the fibers. The porous silica waveguide, the alignment structures, and the silica optical fiber, experience similar thermal expansion upon temperature modulation, thus this co-integration significantly reduces the risk of optical misalignment due to temperature modulation required for device operation. Moreover, this design facilitates easy insertion of fibers which can be done manually under a microscope, thus eliminating the need for active alignment.
Hutter et al. (Mon,) studied this question.