Provided is a beam expander, a backlight unit having high optical efficiency and a holographic display device including the backlight unit. The backlight unit includes a light source unit configured to provide a light beam, a first beam expander configured to mix the light beam provided from the light source unit, expand the light beam in a first direction, and output the mixed and expanded lightbeam as white light, and a second beam expander configured to expand the white light output from the first beam expander in a second direction perpendicular to the first direction and output the expanded white light as surface light for illuminating a spatial light modulator.