A novel dielectric-metal-dielectric sandwich structure based on the theory of sub-wavelength diffraction grating was presented for efficient side-coupling of high power double-clad fiber (DCF) lasers. The structure design considerations and simulation results with high coupling efficiency which can be over 80% were also presented. It can be used for high power laser diodes bar to side-pump DCF lasers without any beam-shaping. Besides, this side-coupling technique was scalable by multi-point pumping with several laser diodes bars.