Reciprocating reflective double gratings based LCOS spectral filter with sharp response

Optical filter based on liquid crystal on silicon (LCOS) is widely used in wavelength selective switch (WSS) to allocate the optical spectrum flexibly. Constrained by the achievable spectral resolution of single grating used in the WSS, the roll-off bandwidth of optical filter is challenging to improve for the purpose of precise and efficient spectrum allocation. In this work, we demonstrate a novel double gratings configuration to realize higher spectral resolution by repeatedly enlarging and accumulating the dispersion, under the condition of the maintenance of the final light spot size. After examining the filtering performance with respect to the diffraction number, we are able to achieve a spectral resolution of 1.63 GHz, indicating of almost tenfold enhancement in comparison with traditional single grating configuration. In particular, an LCOS-based optical filter with a spectral roll-off bandwidth of 1.26 GHz and addressable resolution of 187 MHz is demonstrated. Meanwhile, the insertion loss and the range of operation wavelength are investigated to determine the optimal number of diffractions for various applications. The proposed LCOS-based filter enables spectral manipulation of GHz roll-off bandwidth, fulfilling the performance requirements of future flexible optical networking.