'The beam waist (minimal radius).
See Also
w : the beam radius at `1/e^2` intensity
Examples
>>> from sympy.physics.optics import BeamParameter
>>> p = BeamParameter(530e-9, 1, w=1e-3)
>>> p.w_0
0.00100000000000000'

@property
def w_0(self):

    return sqrt(((self.z_r / pi) * self.wavelen))
