There is provided a beam expander including a first optical element, a second optical element, a composite waveguide including a plurality of waveguide elements, and a dichroic coating provided between the plurality of waveguide elements. The first optical element inputs a collimated incident beam from the outside into the composite waveguide, and the second optical element outputs, from the composite waveguide, collimated incident beam, input to the composite waveguide, wherein the collimated incident beam input into the composite waveguide is divided into multiple wavefronts by the dichroic coating, and the multiple wavefronts are mixed by total internal reflection in the composite waveguide. When the beam expander is used, coherency and collimation of an output beam may be maintained and speckles may be reduced or eliminated while improving the efficiency of an optical system.