The most common type of beam expander is derived from the Galilean telescope which usually has one negative input lens and one positive output lens, as shown in Figure 1. The input lens presents a virtual beam focus at the output. For low expansion ratios (1.3-20´), the Galilean telescope is most often employed due to

its simplicity, small package size, and low cost.

Figure 1: Diagram of a beam expander

As shown in Figure 1, the lens M3 focuses the laser beam onto the front focus plane and the new beam waist w¢0 and divergence angle q¢ can be represented as

and

(1)

(2)

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