As we have mentioned multiple times, the rate of divergence of a Gaussian beam is highly dependent on the laser aperture radius $w_0$. See [[Ideal laser beam divergence]] for more details. The larger the aperture $w_0$, the slower the laser beam diverges, so it is advantageous to use a large aperture. However, a large laser beam is not particularly easy to create for a variety of reasons. Thus, it is necessary to use a **beam expander** and other optical equipment to be able to fine-tune a small laser beam to a larger aperture: - For infrared-range lasers, an optical beam expander is sufficient: see [[Ideal laser beam divergence]] for calculations on this - For microwave-range lasers, lenses and mirrors must be replaced with waveguides instead, but the fundamental principles are mostly the same