扩束镜对激光的瑞利长度有何影响？

在Robert. E. Fisher的《Optical System Design》第208页有这么一段关于扩束镜和瑞利长度关系的描述，抄录如下：
    If we use a beam expander at the output of a laser, we can increase the distance of the minimum spreading. If we additionally adjust the beam expander so that the beam waist is located in the middle of the starting Rayleigh range, then the beam will spread to 1.414d0 over a distance twice as long.
      这段话该如何理解？