Exploring near-field optical diffraction using a beamlet approach: theory and experiment

We introduce an engaging, hands-on approach for teaching near-field optical diffraction to undergraduate students taking optics or advanced laboratory courses. The approach involves passing a Gaussian laser beam (commonly from a HeNe laser) through a circular aperture and capturing the resulting diffraction pattern using a camera-based beam profiler. We model the truncated Gaussian beam as a sum of smaller Gaussian beamlets, propagate each one to the observation plane via the -parameter formalism, and then recombine the beamlets to construct the diffracted beam. This method allows students to visualize laser beam diffraction in a manner similar to how Huygen's wavelets are used in classical diffraction theory. Additionally, it offers a computationally simpler alternative to traditional diffraction integrals, such as Fresnel-Kirchhoff or Rayleigh-Sommerfeld. While the model provides excellent accuracy in the far-field and most near-field distances, some discrepancies arise at very near-field ranges (with F-numbers > 3). By combining both theory and experiment, we provide a dynamic and interactive way for students to explore Gaussian beam propagation, the -parameter formalism, and diffraction phenomena, enriching their understanding of optics in a laboratory setting.