在光子晶体光纤典型的空气孔三角形排布图案中,将每个空气孔替换为一对孪生空气孔对,孪生空气孔对之间有确定的间距和固定的轴向,并由其形成包层的基本单元.在端面中心位置缺失一孪生空气孔对,由高折射率的背景材料将光场束缚于此形成纤芯.在包层中所有的孪生空气孔对按照三角形规则均匀排列.在这种新型结构中,由于所有孪生空气孔对都具有相同轴向而使得两个正交方向上的折射率不对称,从而导致双折射效应.本文利用有限差分法进行数值计算,所设计孪生空气孔对光子晶体光纤在两个正交方向上的折射率差△neff可达到10-4.孪生空气孔对结构参量可在一定程度上影响双折射效果,增大空气孔或减小孪生空气孔对内部间距都可在一定程度上增大双折射效应.

In a typical triangular arrangement of photonic crystal fiber (PCF), each air-hole is replaced by a pair of twin air-holes. Two neighboring air holes with the same spacing and fixed axes direction come into being twin air holes pairs and it is the basic cell in the cladding of PCF.In the central of section, not missing one air hole but missing a pair of twin air holes with higher refractive index material can localize optical field and become the core of PCF. All twin air holes are regularly arranged according to triangular arrangement in the cladding. In this novel PCF, an asymmetry character is aroused because of all twin air holes with identical axis direction, and birefringence relevant to axes direction of twin air holes is formed. Finite difference (FD) method is used to analyze modal character of PCF. The results of numerical calculation indicate that, the effective refractive index difference between two orthogonal directions Δneff can reach the magnitude of 10-4. Especially, the structure parameter of twin air holes can change birefringence in slight, such as increasing air hole size or reducing spacing of two air holes within every twin air holes all can increase birefringence effect.