The size of the beam is, well. the size of the beam on the specified surface.
The waist is the waist (minimum size) of the beam, and it is located at the location (position) specified as measured from that surface .
In your example they are the same since the surface is at the waist in any case.
You compare the position of the waist to the Rayleigh range to determine whether the beam on the surface is inside the slowly-varying part of the wavefront propagation or is in the linearly varying part of the wavefront propagation.
Hi Mark,
Yes,size and waist are the same when the surface is at the waist.
I am confused when the M2 = 1,waist size,size,waist are the same. when M2=16,waist=SQRT(M2)*waist size.
How do they calculate and when I simulate multimode laser, should I use waist size or waist?