Before revision: Far from the waist (z>>zR) , q can be calculated according to where z is the location at which q is calculated, relative to the location of the beam waist, z0 is the Rayleigh range, and i is the imaginary unit.
After revision: Alternatively , q can be calculated according to where z is the location at which q is calculated, relative to the location of the beam waist, z0 is the Rayleigh range, and i is the imaginary unit.