an optical guiding device configured to guide light to propagate along a first direction within the optical guiding device, the light having a spectral bandwidth with a peak wavelength;

an in-coupling diffractive structure configured to diffract the light to propagate in the optical guiding device; and

a plurality of out-coupling diffractive structures arranged downstream the in-coupling diffractive structure along the first direction and configured to diffract at least part of the light out of the optical guiding device along a second direction different from the first direction,

wherein the plurality of out-coupling diffractive structures are configured to have gradually increased diffraction efficiencies for the light along the first direction away from the in-coupling diffractive structure, and

wherein a first diffraction efficiency of the in-coupling diffractive structure for light with the peak wavelength is no smaller than a first threshold, and a second diffraction efficiency of each of the plurality of out-coupling diffractive structures for the light with the peak wavelength is no greater than a second threshold, the first threshold being greater than the second threshold.