Double-cladding structure dependence of guiding characteristics in six-fold symmetric photonic quasi-crystal fiber

In this work, the double-cladding (DC) structure dependence of guiding characteristics in six-fold symmetric photonic quasi-crystal fiber (PQF) is investigated. Three PQFs featuring one, two and three rings of inner air holes are considered. Numerical results show that with the increase of inner air-filling fraction to around 0.65, the confinement loss begins hopping. The confinement loss varies very slightly when the outer air-filling fraction is less than 0.3, and then presents an exponential decrease with the fraction increasing. In addition, the dispersion of DC-PQF can be made flattened and near-zero with the inner air-filling fraction about 0.3-0.4 despite what the ring number is. Three distinct bands of effective area and nonlinear coefficient can be achieved for three types of inner cladding and there exists the same hopping phenomenon as the confinement loss. This investigation is helpful for balancing fiber performance parameters and providing a guidance for engineering specific fiber.