In OptiFDTD, transverse Gaussian Beam is expressed as

FDTD - Equation 57

where x0 is the center position and T is being called as half width.

In general, Gaussian Beam radius is the radius at which the field amplitude and
intensity drop to 1/e and 1 / e2, respectively. Gaussian Beam Size or Gaussian
Beam Spot size is the beam diameter which is two times of Beam Radius. Suppose
the beam size is denoted as a, which means

FDTD - Equation

This means

FDTD - Equation

Please use the above formula to set the half width in OptiFDTD for your desired
Gaussian Beam size.