In general, in one aspect, the invention features an apparatus that includes a photonic crystal fiber configured to guide a mode of electromagnetic radiation at a wavelength, .lamda., along a waveguide axis. The fiber includes a core extending along the waveguide axis, and a confinement region extending along the waveguide axis and surrounding the core. The confinement region includes alternating layers of a first and a second dielectric material having thicknesses d.sub.1 and d.sub.2 and different refractive indices n.sub.1 and n.sub.2, respectively. The thickness of at least one of the alternating layers of the first material differs from thickness d.sub.1.sup.QW or at least one of the alternating layers of the second material differs from thickness d.sub.2.sup.QW, where d.sub.1.sup.QW and d.sub.2.sup.QW correspond to a quarter-wave condition for the two dielectric materials given by d.sub.1.sup.QW=.lamda./(4 {square root over (n.sub.1.sup.2-1)}) and d.sub.2.sup.QW=.lamda./(4 {square root over (n.sub.2.sup.2-1)}), respectively. The photonic crystal fiber has an attenuation for the guided mode at the wavelength .lamda. that is reduced by a factor of about two or more relative to an attenuation for a reference fiber that is identical to the photonic crystal fiber except that the reference fiber has alternating layer thicknesses corresponding to the quarter-wave condition.