A guided-mode-resonance device for both narrowband retroreflection and waveguide input coupling was designed and fabricated for operation wavelength of 1550 nm. It consists of a focusing grating coupler (FGC) integrated in a waveguide cavity resonator constructed by a pair of distributed Bragg reflectors (DBRs) on a reflection substrate. The size of the FGC was 31 × 20 µm 2 . The device was used as a laser mirror of an external cavity laser. The oscillation wavelength was fixed to the retroreflection wavelength, and the laser power was coupled to a waveguide. The coupled power depends on the length of one DBR.The optimal length for the maximum coupling was also discussed theoretically and experimentally.

A guided-mode resonance mirror was designed for reflecting a vertically incident Gaussian beam of 3.6-µm beam waist to a backpropagating Gaussian beam. A grating coupler (GC) is integrated in a waveguide resonance cavity consisting of a pair of distributed Bragg reflectors (DBRs) on a reflection substrate. An incident free-space wave is coupled by the GC into the waveguide, and the guided wave is resonated in the waveguide cavity and coupled out by the same GC to a free-space wave simultaneously in resonance condition. The reflection phase can vary by 2π rad, according to wavelength in a wavelength band of resonance. The grating fill factors of the GC were apodized to have a Gaussian profile in its coupling strength and resultantly maximize a Gaussian reflectance defined by the power ratio of backpropagating Gaussian beam to the incident Gaussian beam. The fill factors of the DBR were also apodized in the boundary zone to the GC in order to avoid discontinuity in equivalent refractive index distribution and resultant scattering loss. Guided-mode resonance mirrors were fabricated and characterized. The Gaussian reflectance of the mirror with the grating apodization was measured to be 90%, higher by 10% than that of the mirror without apodization. It is also demonstrated that the reflection phase changes more than π rad within wavelength band of 1 nm. The fill factor apodization narrows the resonance band.

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