There are two kinds of Gaussian beam definition in lumerical: fully vectorial beam and scalar beam.

In my caculation, the overlap between a waveguide mode and a gaussian beam with MFD=6um has different values using two different definitions of gaussian beam.

For the scalar beam, the parameters are set as. The overlap is 93.12%.

However, for the vectorial beam, the overlap is 77.64% which is smaller.

The parameters are

Here, NA=2*1.55/(6*Pi)=0.16646

Would anyone like to explain the differences between these two definitions? When to use the scalar beam or the vectorial beam?