Numerical analyses of a mid-IR metasurface spectral beam combiner with high efficiency and low phase distortion

Mid-infrared lasers have been widely used in chem/bio-sensing, medical diagnostics, environmental monitoring, and defense. These applications frequently require broad spectral coverage, high brightness, and compact size. Spectral beam combining (SBC) technology has been proven to be efficient in scaling laser power and broadening spectral range. Although the blazing grating has been successfully applied as the SBC component in mid and far infrared, its performance is limited by strong dispersion and polarization dependence. In this paper, we proposed a spectral beam combiner based on the dual-band anomalous reflective metasurface. The combiner had ultra-high, polarization-independent anomalous reflectivity in two spectral bands (4.0 µm and 4.6 µm). In both bands, the bandwidth with over 95% reflectivity was 100 nm. These properties made both the dense SBC and two-band SBC possible. Moreover, the impacts of the input laser properties on the combining efficiency and beam quality were thoroughly studied. The metasurface beam combiner exhibited high tolerance to the incident angle deviation and input laser spectral linewidth. Even when the laser has a broad linewidth of 70 nm incident on the beam combiner with a large angle deviation of 10°, the beam combiner still showed a higher than 90% efficiency and good reflective beam quality M²<1.5. In addition, in the dense SBC situation, when 7 individual lasers with a central wavelength interval of 25 nm were combined the beam propagation parameter M² was better than 1.5. We believe that the metasurface spectral beam combiner was an effective alternative to the commercial diffractive gratings and was capable of combining multiple beams from broadband laser sources.