Cavity-resonator-integrated guided-mode resonance filter with position-shifted grating coupler

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 waveguide, and can provide a narrowband reflection spectrum for an incident free-space beam of a small diameter. The GC is located between the DBRs constructing a waveguide cavity resonator, and acts as an aperture of the CRIGF. The dependence of reflection spectrum on the position of GC relative to the center of the waveguide cavity is investigated theoretically and experimentally. CRIGFs with various position-shifted GCs of about 10-μm aperture size were designed and fabricated for operation at around 1.54 μm wavelength. The maximum reflectance with broadest bandwidth was obtained without the GC position shift. The bandwidth of CRIGF reduced until the GC position shifted to a quarter of GC period, which would mean the reduction of GC coupling coefficient. In particular, a position shift of a quarter of GC period nullifies the guided-mode resonance, and no peak in reflectance was observed. Further position shifts caused the guided-mode resonance and appearance of a resultant reflection peak.