We derive a reciprocity inequality involving the product of the effective size of a statistically stationary, planar, secondary source of any state of coherence and of the angular spread of the far-zone intensity generated by the source. We show that of all possible such sources, the fully spatially coherent lowest-order Hermite–Gaussian laser mode has the smallest possible reciprocity product.
© 2000 Optical Society of America
Sergey A. Ponomarenko and Emil Wolf, "Light beams with minimum phase space product," Opt. Lett. 25, 663-665 (2000)