An optical device comprising: a light guide comprising an input region and an output region, wherein the light guide is adapted to receive light at the input region and output at least some of the received light at the output region; and at least first and second diffraction gratings, wherein: the first diffraction grating is disposed on a first surface of the light guide, the first surface facing away from a light input, the second diffraction grating is disposed on a second surface of the light guide, the second surface facing towards the light input, the second diffraction grating is disposed at the input region of the light guide and adapted to diffract at least some of the light input into the light guide, the first diffraction grating is adapted to diffract at least some light transmitted through the light guide back into the light guide, the first diffraction grating is formed by recording first interference fringes in a first interference fringe forming area, the second diffraction grating is formed by recording second interference fringes in a second interference fringe forming area, and alignment marks A1 and B1 that are formed by recording interference fringes that are identical to the interference fringes provided to the first interference fringe forming area of the first diffraction grating and alignment marks A2 and B2 that are formed by recording interference fringes that are identical to the interference fringes provided to the second interference fringe forming area of the second diffraction grating wherein: the alignment mark A1 does not overlap with the alignment mark A2; the alignment mark B1 does not overlap with the alignment mark B2; and a pitch or an angle of the first interference fringes is different from a pitch or an angle of the second interference fringes. a light guide comprising an input region and an output region, wherein the light guide is adapted to receive light at the input region and output at least some of the received light at the output region; and at least first and second diffraction gratings, wherein: the first diffraction grating is disposed on a first surface of the light guide, the first surface facing away from a light input, the second diffraction grating is disposed on a second surface of the light guide, the second surface facing towards the light input, the second diffraction grating is disposed at the input region of the light guide and adapted to diffract at least some of the light input into the light guide, the first diffraction grating is adapted to diffract at least some light transmitted through the light guide back into the light guide, the first diffraction grating is formed by recording first interference fringes in a first interference fringe forming area, the second diffraction grating is formed by recording second interference fringes in a second interference fringe forming area, and alignment marks A1 and B1 that are formed by recording interference fringes that are identical to the interference fringes provided to the first interference fringe forming area of the first diffraction grating and alignment marks A2 and B2 that are formed by recording interference fringes that are identical to the interference fringes provided to the second interference fringe forming area of the second diffraction grating wherein: the alignment mark A1 does not overlap with the alignment mark A2; the alignment mark B1 does not overlap with the alignment mark B2; and a pitch or an angle of the first interference fringes is different from a pitch or an angle of the second interference fringes. the first diffraction grating is disposed on a first surface of the light guide, the first surface facing away from a light input, the second diffraction grating is disposed on a second surface of the light guide, the second surface facing towards the light input, the second diffraction grating is disposed at the input region of the light guide and adapted to diffract at least some of the light input into the light guide, the first diffraction grating is adapted to diffract at least some light transmitted through the light guide back into the light guide, the first diffraction grating is formed by recording first interference fringes in a first interference fringe forming area, the second diffraction grating is formed by recording second interference fringes in a second interference fringe forming area, and alignment marks A1 and B1 that are formed by recording interference fringes that are identical to the interference fringes provided to the first interference fringe forming area of the first diffraction grating and alignment marks A2 and B2 that are formed by recording interference fringes that are identical to the interference fringes provided to the second interference fringe forming area of the second diffraction grating wherein: the alignment mark A1 does not overlap with the alignment mark A2; the alignment mark B1 does not overlap with the alignment mark B2; and a pitch or an angle of the first interference fringes is different from a pitch or an angle of the second interference fringes. the alignment mark A1 does not overlap with the alignment mark A2; the alignment mark B1 does not overlap with the alignment mark B2; and a pitch or an angle of the first interference fringes is different from a pitch or an angle of the second interference fringes.