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.

Transmissive Beam Expander
P/N	Material	Magnification	Dia	Length	Input CA	Output CA	Connect
			(mm)	(mm)	(mm)	(mm)
BEX-2X-10.6	ZnSe	2	22	35	15	17	M22*0.75
EX-2.5-10.6	ZnSe	2.5	37	60	13.5	30	M22*0.75
EX-3X-10.6	ZnSe	3	22	62	10	15	M22*0.75
EX-4X-10.6	ZnSe	4	27	70	10	20	M22*0.75
EX-5X-10.6	ZnSe	5	30	72	10	23	M22*0.75
EX-6X-10.6	ZnSe	6	36	75	10	30	M22*0.75
EX-8X-10.6	ZnSe	8	36	73	10	30	M22*0.75
special size
EX-0.5X-10.6	ZnSe	0.5	25	36.35	10.8	10.8	M22*0.75
EX-0.5X-10.6	ZnSe	1.2	25	30.69	13.1	13.1	M22*0.75
EX-0.5X-10.6	ZnSe	3.5	40	53.26	13.6	33	M22*0.75

Reflective Beam Expander (Collimator)
P/N	Substrate	Dia	Edge Thickness	Radius	Coating
TMC-50-10-1.68MCC	Cu	50mm	10mm	1.68MCC	HEM
TMC-50-10-1.00MCX	Cu	50mm	10mm	1.00MCX	HEM
TMC-50-10-3.25MCC	Cu	50mm	10mm	3.25MCC	HEM
TMC-50-10-2.25MCX	Cu	50mm	10mm	2.25MCX	HEM
TMC-50-25-3.3MCC-WC	Cu	50mm	25mm	3.3MCC	HEM
TMC-50-25-3.0MCX-WC	Cu	50mm	25mm	3.0MCX	HEM
TMC-50-25-1.44MCC-WC	Cu	50mm	25mm	1.44MCC	HEM
TMC-50-25-0.9MCX-WC	Cu	50mm	25mm	0.9MCX	HEM
TMC-50-31.75-1.87MCC-WC	Cu	50mm	31.75mm	1.87MCC	HEM
TMC-50-31.75-3.05MCC-WC	Cu	50mm	31.75mm	3.05MCC	HEM
TMC-50-31.75-3.68MCC-WC	Cu	50mm	31.75mm	3.68MCC	HEM
TMC-50-31.75-3.72MCC-WC	Cu	50mm	31.75mm	3.72MCC	HEM
TMC-50-31.75-1.20MCX-WC	Cu	50mm	31.75mm	1.20MCX	HEM
TMC-50-31.75-2.34MCX-WC	Cu	50mm	31.75mm	2.34MCX	HEM
TMC-50-31.75-3.00MCX-WC	Cu	50mm	31.75mm	3.00MCX	HEM
TMC-57.15-31.75-1.87MCC-WC	Cu	57.15mm	31.75mm	1.87MCC	HEM
TMC-57.15-31.75-2.09MCC-WC	Cu	57.15mm	31.75mm	2.09MCC	HEM
TMC-57.15-31.75-1.33MCX-WC	Cu	57.15mm	31.75mm	1.33MCX	HEM
TMC-57.15-31.75-1.37MCX-WC	Cu	57.15mm	31.75mm	1.37MCX	HEM
★The above are about standard products, they can also be custome made.