An optical device, comprising: a light source assembly configured to generate an image light; and at least one waveguide including an in-coupling element and an out-coupling element configured to transmit, via the at least one waveguide, a plurality of light fields of the image light to an eye-box of the optical device, in a time-multiplexing manner, wherein at least one of the in-coupling element or the out-coupling element comprises N switchable diffraction gratings (1005_1, 1005_2, 1005_3), N being a positive integer and N≥2, and wherein each switchable diffractive optical element comprises: a surface relief grating, SRG, (300, 400, 500, 600, 700, 800, 900) filled with a switchable optically anisotropic material (415, 515, 615, 715, 815, 915) having a first principal refractive index along a groove direction of the SRG and a second principal refractive index along an in-plane direction perpendicular to the groove direction, one of the first and second principal refractive indices substantially matching a refractive index of the SRG, and the other mismatching the refractive index of the SRG, and wherein: during respective time periods, the N number of switchable diffraction gratings sequentially configured to be in a diffraction state to transmit respective light fields of the plurality of light fields, and during one time period, one of the N number of switchable diffraction gratings configured to be in the diffraction state to transmit a light field of the plurality of light fields and the remaining switchable diffraction gratings configured to be in a non-diffraction state; characterised in that the switchable diffraction gratings are partially overlapped with each other at a surface (210-1, 210-2) of the waveguide. a light source assembly configured to generate an image light; and at least one waveguide including an in-coupling element and an out-coupling element configured to transmit, via the at least one waveguide, a plurality of light fields of the image light to an eye-box of the optical device, in a time-multiplexing manner, wherein at least one of the in-coupling element or the out-coupling element comprises N switchable diffraction gratings (1005_1, 1005_2, 1005_3), N being a positive integer and N≥2, and wherein each switchable diffractive optical element comprises: a surface relief grating, SRG, (300, 400, 500, 600, 700, 800, 900) filled with a switchable optically anisotropic material (415, 515, 615, 715, 815, 915) having a first principal refractive index along a groove direction of the SRG and a second principal refractive index along an in-plane direction perpendicular to the groove direction, one of the first and second principal refractive indices substantially matching a refractive index of the SRG, and the other mismatching the refractive index of the SRG, and during respective time periods, the N number of switchable diffraction gratings sequentially configured to be in a diffraction state to transmit respective light fields of the plurality of light fields, and during one time period, one of the N number of switchable diffraction gratings configured to be in the diffraction state to transmit a light field of the plurality of light fields and the remaining switchable diffraction gratings configured to be in a non-diffraction state; characterised in that the switchable diffraction gratings are partially overlapped with each other at a surface (210-1, 210-2) of the waveguide.