A Modified Structure for All-Glass Photonic Bandgap Fibers: Dispersion Characteristics and Confinement Loss Analysis

This paper investigates a modified structure for all-glass photonic bandgap fiber (AGPBF) having up-doped silica rods in the cladding region instead of air holes using plane wave expansion (PWE) and finite difference time domain (FDTD) methods. The proposed AGPBF structure exhibits tunable dispersion properties and improved confinement loss. It is observed that the confinement loss can be reduced simply by using a higher doping concentration in silica rods in the cladding. Also, it is possible to achieve flattened dispersion of the order of 1？ps/nm/km over a wide wavelength range. 1. Introduction Photonic bandgap fibers (PBFs) have the most basic and original structure in the large family of photonic crystal fibers (PCFs) [1, 2], as the light in these structures is confined in a low index core compared to all the other fibers. In PBF, light is confined to the core by the photonic bandgap effect (PBG), instead of total internal reflection. Hollow-core PBF (HC-PBF) with air holes in a glass background allows low-loss propagation in an empty core, giving the fibers several interesting properties and applications [3]. Since much of the optical power in PBFs can travel in air and not in the glass, they do not suffer from the same limitations to loss as conventional fibers and can exhibit radically reduced optical nonlinearity, making them promising candidates for future ultralow loss transmission fibers [4]. The fabrication of the hollow core photonic bandgap fiber with required specification is difficult due to air-hole deformation and measured losses are more. In order to decrease the confinement loss, we need to accommodate more number of air holes in the given cladding region. Further, to satisfy this condition, the interhole spacing should be minimum, which causes the merger of the adjacent air holes. Confinement loss in triangular air-guiding PBFs has been discussed in [5], and it is concluded that, with ( and are the air-hole diameter and interhole spacing, resp.), at least 20 rings of air holes that are totally 1380 air holes are required to reduce the confinement loss to a level of 0.01？dB/km. Confinement loss in air-core PBF having 8 rings of (totally 264) circular air holes has been analyzed, and the confinement loss can be reduced to lower than 0.01？dB/km with [6]. However, it is very difficult to make air-core PBF with so many air holes in the former case and so high for circular air holes in the latter case, respectively. Confinement loss can be reduced by increasing the air-filling fraction of PBF; the air-filling fractions in [5] is 73%,