Physical origin of the Gouy phase shift

We show explicitly that the well-known Gouy phase shift of any focused beam originates from transverse spatial confinement, which, through the uncerta inty principle, introduces a spread in the transverse momenta and hence a s hift in the expectation value of the axial propagation constant. A general expression is given for the Gouy phase shift in terms of expectation values of the squares of the transverse momenta. Our result also explains the pha se shift in front of the Kirchhoff diffraction integral. (C) 2001 Optical S ociety of America.