Backlight unit and holographic display apparatus including the same

A backlight unit comprising:

• a light source configured to provide coherent light;
• a light guide plate comprising a light entrance surface on which the light is incident from the light source and a light exit surface through which the light is output;
• a first input grating provided on the light guide plate and configured to diffract the light from the light source in a first direction, such that the light diffracted by the first input grating propagates in the first direction while being totally reflected in the light guide plate;
• a second input grating provided on the light guide plate and configured to diffract the light that is diffracted by the first input grating in a second direction orthogonal to the first direction, such that the light diffracted by the second input grating propagates in the second direction different from the first direction while being totally reflected in the light guide plate, wherein beam width of the light propagating in the second direction increases as the light propagates in the light guide in the second direction;
• a third input grating provided on the light guide plate and configured to diffract the light diffracted by the second input grating in a third direction orthogonal to the second direction, and parallel to the first direction, such that the light diffracted by the third input grating propagates in the third direction different from the second direction, wherein beam width of the light propagating in the third direction increases as the light propagates in the light guide in the third direction;
• and
• an output grating provided on the light guide plate and configured to diffract the light diffracted by the third input grating in a fourth direction orthogonal to the first direction, the second direction and the third direction, such that the light is output in a fourth direction toward an outside of the light guide plate.