Hi Ravil,
The mode filed diameter depends on fiber parameters and the operating wavelength. Usually it is just above the core diameter in the case of SMF, So the MFD is in the micrometer range. It can be find out using one equation (w/a)=0.65+(1.619/v^1.5)+(2.879/v^6). This equation may be valid to the fundamental mode of MMF.
My question is what is the MFD of the fundamental mode of OM2 fiber at 1300nm?