A transmissive beam expander combines two or more elements into one optical system which changes the beam size and divergence. Because the beam expander can improve a beam’s collimation, the smaller focal spot size can be achieved through expanding a beam prior to focusing. 
The reflective beam expanders are popularly used in high power laser systems, to ensure laser beam collimation.  Generally it will use 2 pieces OFHC copper mirrors: one is convex copper mirror, the other is concave copper mirror.