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Ultra-low loss (ULL) fibers are being widely deployed in optical networks due to their high transmission capacities. Existing studies on ULL fiber deployment have assumed to completely replace old standard single-mode fibers (SSMFs) when deploying new ULL fibers. This may not be practical since many carriers would prefer to continue utilizing the existing SSMFs for their remaining lifespans. This article investigates the problem of ULL fiber deployment while allowing for the utilization of old SSMFs. In the context of an elastic optical network (EON) with fixed and variable topologies, we formulate the problem of ULL fiber deployment as two mixed integer linear programming (MILP) models. Four strategies for selecting links deployed with ULL fibers are considered, which include physical length (PL), shortest route traversed (SRT), traffic demand (TD), and maximum network performance (MNP) strategies. Efficient …