  A rod-type photonic crystal (PC) with a rectangular lattice shows a
large-angle quasi-self-collimation (quasi-SC) effect by changing the symmetry
of its rectangular lattice to straighten one of the isofrequency contours. To
investigate the straightness of the isofrequency contour as well as the
quasi-SC effect, we propose a straightness factor L based on the method of
least squares. With L smaller than L0 (L0 = 0.01 is the critical value), the
isofrequency contour is sufficiently straight to induce quasi-SC effect with
the beam quasi-collimating in the structure. Furthermore, the efficiency of
light coupling to the quasi-SC PC is studied, and can be greatly improved by
applying a carefully designed antireflection layer.
