An apparatus for use in replicating an image associated with an input-pupil to an output-pupil, the apparatus comprising: an optical waveguide (700, 800, 800, 900, 1100, 1200) including input-coupler (712, 812, 912, 1112, 1212), one or more intermediate-components and an output-coupler, which is separate from the one or more intermediate-components (714, 814, 914, 1114a-d, 1214a-c), the optical waveguide includes a first major surface and a second major surface opposite to the first major surface; the input-coupler (716, 816, 916, 1116, 1216) configured to couple light corresponding to the image associated with the input pupil into the optical waveguide and diffract the light corresponding to the image in at least two different directions so that light corresponding to the image is diffracted toward each of the one or more intermediate-components, wherein the input-coupler comprises a first grating that is located in or on one of the first and second major surfaces of the optical waveguide and is configured to diffract the light corresponding to the image in a first direction toward a said intermediate-component, and a second grating that is located in or on the other one of the first and second major surfaces of the optical waveguide and is configured to diffract the light corresponding to the image in a second direction toward a said intermediate-component; the first grating includes a first set of linear grating lines; and the second grating includes a second set of linear grating lines that are rotated at an angle relative to first set of linear grating lines of the first grating; the one or more intermediate-components configured to individually or collectively perform odd-order pupil expansion on a first portion of the light corresponding to the image that travels from the input-coupler to the one or more intermediate-components by way of total internal reflection, to perform even-order pupil expansion on a second portion of said light, and to diffract the first and second portions of the light corresponding to the image towards the output-coupler; and the output-coupler (716, 816, 916, 1116, 1216) configured to couple the first and second portions of the light corresponding to the image, that has travelled from the one or more intermediate-components to the output-coupler by way of TIR, out of the optical waveguide so that the light corresponding to the image is viewable from the output-pupil. an optical waveguide (700, 800, 800, 900, 1100, 1200) including input-coupler (712, 812, 912, 1112, 1212), one or more intermediate-components and an output-coupler, which is separate from the one or more intermediate-components (714, 814, 914, 1114a-d, 1214a-c), the optical waveguide includes a first major surface and a second major surface opposite to the first major surface; the input-coupler (716, 816, 916, 1116, 1216) configured to couple light corresponding to the image associated with the input pupil into the optical waveguide and diffract the light corresponding to the image in at least two different directions so that light corresponding to the image is diffracted toward each of the one or more intermediate-components, wherein the input-coupler comprises a first grating that is located in or on one of the first and second major surfaces of the optical waveguide and is configured to diffract the light corresponding to the image in a first direction toward a said intermediate-component, and a second grating that is located in or on the other one of the first and second major surfaces of the optical waveguide and is configured to diffract the light corresponding to the image in a second direction toward a said intermediate-component; the first grating includes a first set of linear grating lines; and the second grating includes a second set of linear grating lines that are rotated at an angle relative to first set of linear grating lines of the first grating; a first grating that is located in or on one of the first and second major surfaces of the optical waveguide and is configured to diffract the light corresponding to the image in a first direction toward a said intermediate-component, and a second grating that is located in or on the other one of the first and second major surfaces of the optical waveguide and is configured to diffract the light corresponding to the image in a second direction toward a said intermediate-component; the first grating includes a first set of linear grating lines; and the second grating includes a second set of linear grating lines that are rotated at an angle relative to first set of linear grating lines of the first grating; the one or more intermediate-components configured to individually or collectively perform odd-order pupil expansion on a first portion of the light corresponding to the image that travels from the input-coupler to the one or more intermediate-components by way of total internal reflection, to perform even-order pupil expansion on a second portion of said light, and to diffract the first and second portions of the light corresponding to the image towards the output-coupler; and the output-coupler (716, 816, 916, 1116, 1216) configured to couple the first and second portions of the light corresponding to the image, that has travelled from the one or more intermediate-components to the output-coupler by way of TIR, out of the optical waveguide so that the light corresponding to the image is viewable from the output-pupil.