Which constraint primarily limits the focal spot tightness achievable with a spatially coherent laser beam focused by a high numerical aperture lens?

💡 Explanation

Focal spot tightness is limited by diffraction because spatial coherence enforces wave-like propagation. The mechanism is diffraction, where waves interfere based on path length rather than straight-line trajectories. Therefore diffraction dominates, rather than material properties affecting performance under ideal focusing conditions.

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