Overview: We have limited most of our discussion throughout the text up to this point to the lowest-order Gaussian-beam wave and its limiting forms leading to a plane wave or spherical wave. In this chapter we want to briefly examine other beam shapes that may have important propagation characteristics for certain applications. We first discuss beam spreading associated with higher-order Hermite-Gaussian modes and higher-order Laguerre-Gaussian modes. Our analysis of the mean irradiance is limited in each case to a particular lower-order mode. Next, we examine beam spreading and scintillation characteristics of an annular beam, also known as a "doughnut" beam.
We end this chapter with a brief introduction to other beam shapes such as the flattened Gaussian profile and the Bessel beam. Here, however, our treatment is limited to only free-space propagation characteristics.
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