A backlight unit comprising: a light source (121, 122) configured to emit light; and a light guide structure configured to guide the light emitted from the light source (121, 122), the light guide structure comprising: a first coupler layer; and a second coupler layer facing the first coupler layer, wherein the first coupler layer comprises: a first output coupler configured to expand light traveling inside the light guide structure in a first direction and output the expanded light in the first direction to the outside of the light guide structure; and a first expansion coupler configured to expand the light traveling inside the light guide structure in a second direction perpendicular to the first direction and provide the expanded light in the second direction to the first output coupler, and wherein the second coupler layer comprises: a second output coupler disposed to face the first output coupler and configured to expand light traveling inside the light guide structure in the first direction and output the expanded light to the outside of the light guide structure; and a second expansion coupler disposed to face the first expansion coupler and configured to expand light traveling inside the light guide structure in the second direction and provide the expanded light to the second output coupler, wherein the light guide structure further comprises: a first input coupler disposed at a first side of the first and second expansion coupler (MC1, MC2) in the second direction and configured to provide light to the first and second expansion coupler (MC1, MC2); and a second input coupler disposed at a second side of the first and second expansion coupler (MC1, MC2) in the second direction, the second side being opposite the first side, wherein the second input coupler is configured to provide light to the first and second expansion coupler (MC1, MC2). a light source (121, 122) configured to emit light; and a light guide structure configured to guide the light emitted from the light source (121, 122), the light guide structure comprising: a first coupler layer; and a second coupler layer facing the first coupler layer, a first coupler layer; and a second coupler layer facing the first coupler layer, wherein the first coupler layer comprises: a first output coupler configured to expand light traveling inside the light guide structure in a first direction and output the expanded light in the first direction to the outside of the light guide structure; and a first expansion coupler configured to expand the light traveling inside the light guide structure in a second direction perpendicular to the first direction and provide the expanded light in the second direction to the first output coupler, and a first output coupler configured to expand light traveling inside the light guide structure in a first direction and output the expanded light in the first direction to the outside of the light guide structure; and a first expansion coupler configured to expand the light traveling inside the light guide structure in a second direction perpendicular to the first direction and provide the expanded light in the second direction to the first output coupler, and wherein the second coupler layer comprises: a second output coupler disposed to face the first output coupler and configured to expand light traveling inside the light guide structure in the first direction and output the expanded light to the outside of the light guide structure; and a second expansion coupler disposed to face the first expansion coupler and configured to expand light traveling inside the light guide structure in the second direction and provide the expanded light to the second output coupler, a second output coupler disposed to face the first output coupler and configured to expand light traveling inside the light guide structure in the first direction and output the expanded light to the outside of the light guide structure; and a second expansion coupler disposed to face the first expansion coupler and configured to expand light traveling inside the light guide structure in the second direction and provide the expanded light to the second output coupler, wherein the light guide structure further comprises: a first input coupler disposed at a first side of the first and second expansion coupler (MC1, MC2) in the second direction and configured to provide light to the first and second expansion coupler (MC1, MC2); and a second input coupler disposed at a second side of the first and second expansion coupler (MC1, MC2) in the second direction, the second side being opposite the first side, wherein the second input coupler is configured to provide light to the first and second expansion coupler (MC1, MC2). a first input coupler disposed at a first side of the first and second expansion coupler (MC1, MC2) in the second direction and configured to provide light to the first and second expansion coupler (MC1, MC2); and a second input coupler disposed at a second side of the first and second expansion coupler (MC1, MC2) in the second direction, the second side being opposite the first side, wherein the second input coupler is configured to provide light to the first and second expansion coupler (MC1, MC2).