We report a strongly anisotropic photonic crystal fiber. Twofold rotational
symmetry was introduced into a single-mode fiber structure by creation of
a regular array of airholes of two sizes disposed about a pure-silica core.
Based on spectral measurements of the polarization mode beating, we estima
te that the fiber has a beat length of approximately 0.4 mm at a wavelength
of 1540 nm, in good agreement with the results of modeling. (C) 2000 Optic
al Society of America.