A new fundamental mode field-radius definition usable for non-Gaussian and noncircular field distributions

A new definition for the mode field radius of fundamental mode fibers is proposed which can be applied to circular as well as to noncircular and non-Gaussian field distributions. The mode field radius according to this definition can be strictly determined by the well-established transverse offset technique. Moreover, an alternative test method is presented offering the advantage of using one standard launching fiber for each measurement.