Lens-Less Coupling of Cavity-Resonator-Integrated Guided-Mode Resonance Filter

A cavity-resonator-integrated guided-mode resonance filter (CRIGF) consists of a grating coupler (GC) and a pair of distributed-Bragg reflectors (DBRs) on a thin-film dielectric waveguide, and provides a narrowband reflection spectrum for an incident small-diameter beam. A lens-less direct coupling of CRIGF to a single-mode optical fiber is theoretically and experimentally studied. A field-profile matching was discussed with alignment tolerance between CRIGF and the fiber. Design, fabrication, and characterization of CRIGF were described for an operation wavelength of 1540 nm. The aperture size and a nonuniform fill factors of the GC were investigated in order to realize high-efficiency coupling with an incident Gaussian beam of a 10.5-μm diameter. GeO₂:SiO₂ guiding core and Si-N cladding layers were deposited on a SiO₂ glass substrate, and GC and DBRs were formed in the cladding layer by the electron-beam direct writing lithography. The lens-less direct coupling was demonstrated with wavelength selective coupling of the maximum efficiency of 50% and full-width at half-maximum of 1.3 nm.