Analysis and Design of Bragg Fibers Using a Novel Confinement Loss Diagram Approach

In this paper, a confinement loss diagram (CLD) is proposed as a novel design tool for Bragg fibers. The CLD is a 2-D pseudocolor diagram, which presents the loss properties of a Bragg fiber in a coordinate of normalized frequency and the effective refractive index (ω_r, n_eff), to quantitatively characterize the Bragg cladding without the parameters of the core. The CLD is derived from a ray reflection model, which describes the propagation in Bragg fibers as reflection and ray trajectory. Confinement loss is first determined from the reflection process and then developed into a 2-D CLD. In this paper, the confinement loss calculation with the ray reflection model is first shown to be accurate for Bragg fibers with a core radius as small as 0.8 wavelength, both in the TE modes and the TM modes. Then, the CLD is exploited to characterize the level core Bragg fiber and the depressed core Bragg fiber. It is found that the level core fiber is suitable for wide band designs, while the depressed core fiber is good for narrow band designs. Lastly, the CLD is applied to characterize the loss properties of different order bandgaps, so that the propagation region can be located for optimum Bragg fiber design.