Multicore Cladding-Pumped Fiber Amplifier With Annular Erbium Doping for Low Gain Compression

We report the characterization of a cladding-pumped multicore fiber with annular erbium doping for low gain compression over the C-band (1528.8 nm to 1563.9 nm). The fiber aims to minimize saturation effects by placing the erbium doping in the cladding where the signal intensity is lower. The challenge of ensuring adequate erbium ion solubility in the cladding, without unduly raising its refractive index, was answered by using aluminophosphosilicate for the doped region. Before assembling the eight-core amplifier with fan-in and fan-out, we needed to determine the optimum fiber length to meet the gain compression target when the eight cores are loaded, for a given pump power injected in the cladding. We thus performed a thorough experimental characterization of a single core. This allowed the determination of all the relevant fiber parameters needed to do numerical simulations and predict the multicore fiber amplifier performance under the fully loaded scenario. For the first time, these numerical results are compared to the experimental results of a fully loaded multicore amplifier with cladding pumping and erbium doping in the cladding. We discuss the amplifier performance in terms of gain compression when the input power signal is varied, as would be the case in dynamic optical networks. We also examine the core-to-core gain variations and the sensitivity of the design to fabrication variations. Results show that significant reduction in gain compression of multicore cladding-pump amplifiers can be achieved with the proposed annular doping design in the cladding, although improvement in the fabrication uniformity of the core will be required for practical applications. Lastly, we discuss how this erbium doped fiber design can also lead to improved power conversion efficiency (PCE) in cladding-pumped amplifiers.