is the radial distance from the center axis of the beam, is the axial distance from the beam's focus (or "waist"), is the imaginary unit, is the wave number (in radians per meter) for a free-space wavelength , and is the index of refraction of the medium in which the beam propagates, , the electric field amplitude at the origin (, ), is the radius at which the field amplitudes fall to of their axial values (i.e., where the intensity values fall to of their axial values), at the plane along the beam, is the waist radius, is the radius of curvature of the beam's wavefronts at , and is the Gouy phase at , an extra phase term beyond that attributable to the phase velocity of light.
is the radial distance from the center axis of the beam, is the axial distance from the beam's focus (or "waist"), is the imaginary unit, is the wave number (in radians per meter) for a free-space wavelength , and is the index of refraction of the medium in which the beam propagates, , the electric field amplitude at the origin (, ), is the radius at which the field amplitudes fall to of their axial values (i.e., where the intensity values fall to of their axial values), at the plane along the beam, is the waist radius, is the radius of curvature of the beam's wavefronts at , and is the Gouy phase at , an extra phase term beyond that attributable to the phase velocity of light.