Fig. 2.

Effect of coupling NA, coupling misalignments, and beam pointing instabilities on delivered energy through the 7-μm, air-core PBF. a) Simplified schematic of ultrashort pulse delivery through the PBF with various focusing conditions and measured near-field and far-field profiles at fiber output. b) Beam profiles at the focal point of the coupling lens as measured using the knife-edge test for different NA`s. c) Schematics illustrating the parameters for radial and axial coupling misalignments introduced in Eq. (1). d) Measured maximum input energies successfully coupled into the fiber for different coupling NA’s and estimated maximum values assuming various misalignments during coupling. Error bars represent the standard deviation of three measurements performed at each NA. e) Measured output energies for each coupling NA with the coupling efficiencies above each data point as calculated based on the ratio of observed output energy to the input energy after coupling lens.