We propose a microelectromechanical system (MEMS) mirror design, which is suitable for use in a flexible-grid wavelength selective switch (WSS). In the flexible grid operation with a conventional MEMS-based WSS, the diffraction loss at the inter-mirror gap regions caused a large ripple in the transmission spectra. On the other hand, the newly introduced slot structures in each mirror can reduce the ripples by intentionally attenuating the light on the mirrors to equalize the reflectance of the entire mirrors. We optimize the slot structure aiming for the flexible-grid WSS operation with a bandwidth-granularity of 12.5 GHz. The ripples are successfully reduced to below 0.004 dB with a loss penalty of only 1.05 dB.
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