In this paper, the authors measure the quality of the delivered beam from hollow core photonic crystal fibers (HC-PCFs). The$M^2$parameter is determined, and the near- to far-field transition is examined. The influence on these properties due to the presence of a core surround mode is evaluated. The applicability of the International Standards Organization 11146 : 1999 standard for$M^2$measurement of the beam quality of HC-PCFs is discussed. Because they are dependent on the measurement parameters, such as choice of aperturing scheme and the axis of measurement,$M^2$values could vary from 1.32 to 3.17 for the same output beam. This highlights the need for careful consideration when measuring and describing the beam quality delivered by these novel photonic fibers.
Journal of Lightwave Technology, 2006, Vol 24, Issue 10, p. 3761-3769