Abstract: The selection of starting materials used in the process of forming an MCR is controlled to specifically define the physical properties of the core tube and/or the capillary tubes in the local vicinity of the core tube. The physical properties are considered to include, but are not limited to, the diameter of a given tube/capillary, its wall thickness, and its geometry (e.g., circular, non-circular). A goal is to select starting materials with physical properties that yield a final hollow core optical fiber with a “uniform” core region (for the purposes of the present invention, a “uniform” core region is one where the struts of cladding periodic array surrounding the central core are uniform in length and thickness (with the nodes between the struts thus being uniformly spaced apart), which yields a core wall of essentially uniform thickness and circularity.