(a) A diffractive optical element (DOE) alters the beam profile of an incoming optical beam (with uniform intensity and phase). The DOE is positioned in the front focal plane of the Fourier lens (the DOE plane), which collects the first-order diffracted beam, so that the the complex amplitude in the back focal plane of the Fourier lens (the focal plane) is the Fourier transform of the complex amplitude in the DOE plane. Intensity (top) and phase (bottom) profiles of (b) the incoming beam, (c) the beam after the DOE and (d) the focused beam at the focal plane.
Fig. 11.2 — HOT working principle