The coupling of Gaussian beam between fibers

There are several means for Gaussian beam to couple between optical fibers.One of these is collimating-focusing system using lenses.To decide the parameters of the coupling system, the Gaussian beam's change through a single lens is studied.It indicates that the waist width passing through the lens is decided by several variables,which is illustrated.By analyzing the illustration, the general principle to select the parameters is found.At last, the conclusion is used to resolve an instance.