A narrowband retroreflector consisting of a grating coupler and a waveguide cavity integrated on a highly reflective substrate is proposed. A theoretical model based on coupled-mode theories is discussed to provide analytical expression of the reflection and transmission coefficients under oblique incidence. The retroreflector was designed with a 20-µm aperture for 1540-nm-wavelength operation and 8-deg-angle incidence. Finite-difference time-domain simulation showed a retroreflection spectrum with a bandwidth of 2 nm and a maximum retroreflectance of 85% and a minimum specular reflectance of 5%.

A thin-film narrowband retroreflector is proposed for a diverging free-space wave. The retroreflector consists of a focusing grating coupler in a waveguide resonator on a reflective substrate. The retroreflector is designed and fabricated for an incident wave diverging from a 5.25 μm diameter spot. The retroreflector gives a reflection towards the divergence point of the incident wave at the resonance wavelength determined by the waveguide resonator, but otherwise a specular reflection. The retroreflection spectrum of a 4 nm bandwidth with the peak of 75% is demonstrated. This configuration will provide a lens-less external-cavity-mirror diode laser of a lasing wavelength fixed to the resonance wavelength.

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