Mode Field Diameter (MFD)
The mode field diameter(MFD) is the width of the fundamental (most strongly guided) mode in a single mode fiber above its cutoff wavelength.
Any significant difference in MFD at the splice/joint will cause unacceptably high losses.
Whilst the MFD characteristics are frozen at the time of manufacture and as such are relatively fixed for the life of the fiber, it is a parameter that is influenced by various properties within the fiber that do vary along its length. It is therefore is useful and considered good practice to measure the MFD even within a cable manufacturing environment to ensure the consistency of the product.
The 'Production/Lab/R&D' range is suitable for manufacturers of fiber and cables. It is considered unlikely that this measurement would be required in a field environment.