We simulated the mode structure of photonic crystal fibers with a single ring of elements hexagonally and octagonally arranged around either a solid core or a central defect with either lower or higher optical density, counting upon the refractive indices of AgCl1-xBrx system, where x = 0.75, 0.82, and 0.84. Mode comparison revealed the advantages of the active-core fiber, i.e. with a higher refractive index of the central rod element. We also predicted such a fiber to be capable of guiding more optical power, comparing with a standard double-layered one with similar dimensions.