The measurement technique consists of: The cut should be made to retain 1 meter or more of the fiber, in order to establish equilibrium mode distribution conditions for the second measurement.
In a multimode fiber, the lack of an equilibrium mode distribution could introduce errors in the measurement due to output coupling effects.
The benefit of this technique is that it allows measurement of the fiber characteristics without introducing errors due to variation in the launch conditions.
For example, the coupling efficiency of the light source is kept consistent between the initial and the cutback measurements.
A variation of the cutback technique is the substitution method, in which measurements are made on a full-length of fiber, and then on a short length of fiber having the same characteristics (core size, numerical aperture), with the results from the short length being subtracted to give the results for the full length.
This article incorporates public domain material from Federal Standard 1037C.