Gouy phase

Summary

The Gouy phase is the extra phase shift that a focused beam picks up as it passes through its focus. It is measured against a plane wave of the same frequency. For a Gaussian beam it equals $\arctan(z / z_R)$, so it changes by $\pi$ in total from far before the focus to far after it. Higher-order laser modes pick up a multiple of this shift, which is why modes of different order resonate at different frequencies in a laser cavity.