While lasers have a number of “wow!” factors that make them popular in the press, perhaps the most unusual - and least understood property is that of Gaussian beam propagation. While the mathematics is straightforward, it is difficult to physically explain why, for example, a beam waist placed at the front focal length of a lens images to. . .the back focal length of the lens!
This counter-intuitive result is not obvious only because our intuition is usually developed using conventional objects, where an incoherent source placed at the front focal length of a lens images to infinity, producing collimated beams. In this chapter, we will take a look at the unique properties of Gaussian beams - some of which can be traced back to the wavefront curvature incident on the lens - but we first need to understand the general concepts of Gaussian beam propagation (Section 3.1) and their real-world imperfections (Section 3.2).
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