Generally, in the section of photonic crystal fiber, all air holes are arranged to a triangular regulation, when the size of air holes, the pitch of between neighboring air holes, and refractive index of background material are mapped optimally, one missing air hole in the central of section can localize optical field and form a single mode fiber. Here, each air hole is replaced by twin air holes with fixed distance and axis direction. Accordingly, we can think the central of
section where twin air holes missing is the core of fiber, and optical field is guided in here. In the novel photonic crystal
fiber, all twin air holes arranged according to identical axis direction in the cladding of PCF bring an asymmetry structure of section, and birefringence can come into being in this novel PCFs. After some parameters are selected optimally, the effective refractive index difference between two orthogonal directions Δneff can reach the magnitude of 10-4. From the result of numerical calculation, we also can see that the birefringence parameter Δneff can increase
slightly when the distance between twin air holes is shortened a little but keeping each air holes size and the pitch of
neighboring cell composed by twin air holes.