Midfocus matching: a method for calculating lens positions for a Gaussian beam in a two-lens system.

For the situation in which a specific laser beam waist is required at a certain position, we develop a practical method of selecting focal lengths for a combination of two lenses and of calculating their positions. The method provides, in an easy graphical way, the solution of this two-lens focusing problem of Gaussian beams by finding the crossing point of two parametric curves.