A diffractive optical waveguide (100, 200'), comprising a waveguide substrate (100a, 200a) and a grating structure (10, 30) formed on a surface of the waveguide substrate, wherein, the grating structure (10, 30) comprises a plurality of grating lines (11, 31) arranged in a plane, the grating lines (11, 31) extend along a first direction (x) in the plane and are arranged at a predetermined interval in a second direction (y) perpendicular to the first direction (x); each of the grating lines (11, 31) has a first sidewall 11a, 31a) and a second sidewall (11b, 31b) which are opposite to each other in the second direction (y); the grating structure (10, 30) is configured to diffract light, which is incident thereon at a non-zero angle with respect to the plane, out of the plane through a predetermined diffraction order; and the diffractive optical waveguide (100, 200') comprises a coupling-in grating (110, 210') and a coupling-out grating (120, 220') formed on the waveguide substrate, wherein the grating structure is configured as the coupling-in grating (110, 210'), and the coupling-in grating couples a light beam irradiated on the waveguide substrate at an incident angle within a predetermined range into the waveguide substrate through the diffraction of the predetermined diffraction order so that it propagates in the waveguide substrate through total reflection, and the coupling-out grating (120, 220') is configured to couple at least a portion of the light propagating from the coupling-in grating (110, 210') into it through total reflection out of the waveguide substrate by diffraction, characterized in that: at least one of the first sidewall (11a, 31a) and the second sidewall (11b, 31b) has a periodic structure (12, 32) along the first direction (x), and the periodic structure (12, 32) makes the grating structure (10, 30) as a whole asymmetrical on both sides in the second direction (y); the first sidewall or the second sidewall formed with the periodic structure in the coupling-in grating (110, 210') faces in a direction, in which light of the predetermined diffraction order propagates to the coupling-out grating (120, 220'), and the periodic structures on the first sidewalls of different grating lines in the coupling-in grating (110, 210') are aligned with each other in the second direction (y). the grating structure (10, 30) comprises a plurality of grating lines (11, 31) arranged in a plane, the grating lines (11, 31) extend along a first direction (x) in the plane and are arranged at a predetermined interval in a second direction (y) perpendicular to the first direction (x); each of the grating lines (11, 31) has a first sidewall 11a, 31a) and a second sidewall (11b, 31b) which are opposite to each other in the second direction (y); the grating structure (10, 30) is configured to diffract light, which is incident thereon at a non-zero angle with respect to the plane, out of the plane through a predetermined diffraction order; and the diffractive optical waveguide (100, 200') comprises a coupling-in grating (110, 210') and a coupling-out grating (120, 220') formed on the waveguide substrate, wherein the grating structure is configured as the coupling-in grating (110, 210'), and the coupling-in grating couples a light beam irradiated on the waveguide substrate at an incident angle within a predetermined range into the waveguide substrate through the diffraction of the predetermined diffraction order so that it propagates in the waveguide substrate through total reflection, and the coupling-out grating (120, 220') is configured to couple at least a portion of the light propagating from the coupling-in grating (110, 210') into it through total reflection out of the waveguide substrate by diffraction, characterized in that: at least one of the first sidewall (11a, 31a) and the second sidewall (11b, 31b) has a periodic structure (12, 32) along the first direction (x), and the periodic structure (12, 32) makes the grating structure (10, 30) as a whole asymmetrical on both sides in the second direction (y); the first sidewall or the second sidewall formed with the periodic structure in the coupling-in grating (110, 210') faces in a direction, in which light of the predetermined diffraction order propagates to the coupling-out grating (120, 220'), and the periodic structures on the first sidewalls of different grating lines in the coupling-in grating (110, 210') are aligned with each other in the second direction (y). at least one of the first sidewall (11a, 31a) and the second sidewall (11b, 31b) has a periodic structure (12, 32) along the first direction (x), and the periodic structure (12, 32) makes the grating structure (10, 30) as a whole asymmetrical on both sides in the second direction (y); the first sidewall or the second sidewall formed with the periodic structure in the coupling-in grating (110, 210') faces in a direction, in which light of the predetermined diffraction order propagates to the coupling-out grating (120, 220'), and the periodic structures on the first sidewalls of different grating lines in the coupling-in grating (110, 210') are aligned with each other in the second direction (y).