A backlight unit comprising: a light source unit which outputs coherent light; a frame including: a first reflection unit comprising a parabolic mirror, and light output from the light source unit is directly incident to the first reflection unit, wherein the light source unit is positioned at a focal point of the parabolic mirror of the first reflection unit and the parabolic mirror is curved in a plane in which the focal point is located; a second reflection unit facing the first reflection unit and comprising a flat mirror; two side portions connecting the first reflection unit and the second reflection unit; and a lower plate; and a holographic optical element which opposes the lower plate and changes a path of incident light, wherein a reflection surface of the second reflection unit forms an acute angle with a light incident surface of the holographic optical element, and the light source unit, the first reflection unit, and the second reflection unit are arranged such that the coherent light output from the light source unit is sequentially reflected by the first reflection unit, the second reflection unit, and diffracted by the holographic optical element, and the light source unit is at a corner of the frame where the second reflection unit, the lower plate, and one side portion of the two side portions meet each other. a light source unit which outputs coherent light; a frame including: a first reflection unit comprising a parabolic mirror, and light output from the light source unit is directly incident to the first reflection unit, wherein the light source unit is positioned at a focal point of the parabolic mirror of the first reflection unit and the parabolic mirror is curved in a plane in which the focal point is located; a second reflection unit facing the first reflection unit and comprising a flat mirror; two side portions connecting the first reflection unit and the second reflection unit; and a lower plate; and a holographic optical element which opposes the lower plate and changes a path of incident light, wherein a reflection surface of the second reflection unit forms an acute angle with a light incident surface of the holographic optical element, and the light source unit, the first reflection unit, and the second reflection unit are arranged such that the coherent light output from the light source unit is sequentially reflected by the first reflection unit, the second reflection unit, and diffracted by the holographic optical element, and the light source unit is at a corner of the frame where the second reflection unit, the lower plate, and one side portion of the two side portions meet each other. a first reflection unit comprising a parabolic mirror, and light output from the light source unit is directly incident to the first reflection unit, wherein the light source unit is positioned at a focal point of the parabolic mirror of the first reflection unit and the parabolic mirror is curved in a plane in which the focal point is located; a second reflection unit facing the first reflection unit and comprising a flat mirror; two side portions connecting the first reflection unit and the second reflection unit; and a lower plate; and a holographic optical element which opposes the lower plate and changes a path of incident light, wherein a reflection surface of the second reflection unit forms an acute angle with a light incident surface of the holographic optical element, and the light source unit, the first reflection unit, and the second reflection unit are arranged such that the coherent light output from the light source unit is sequentially reflected by the first reflection unit, the second reflection unit, and diffracted by the holographic optical element, and the light source unit is at a corner of the frame where the second reflection unit, the lower plate, and one side portion of the two side portions meet each other.