Beam quality of photonic-crystal fibers

In this paper, we investigate the beam quality of photonic-crystal fibers (PCFs) with one, three, and seven missing airholes. The beam quality factor M-2 of a PCF is generally larger than unity, even if it is single mode. The results are especially important for the design and measurements of single-mode PCF lasers. A comparison between an M2 measurement of the laser beam and the reported numbers can establish whether a PCF laser is operating as single mode or is contaminated by higher order modes.