'The beam radius at `1/e^2` intensity.
See Also
w_0 : the minimal radius of beam
Examples
>>> from sympy.physics.optics import BeamParameter
>>> p = BeamParameter(530e-9, 1, w=1e-3)
>>> p.w
0.001*sqrt(0.2809/pi**2 + 1)'

@property
def w(self):

    return (self.w_0 * sqrt((1 + ((self.z / self.z_r) ** 2))))
