A method including the steps of providing a light-diffusing optical fiber (12a) having a glass core (20), a cladding (40) surrounding the core (20), and a plurality of nano-sized structures in the form of voids (32) situated within said core (20) or at a core-cladding boundary; cleaving the light-diffusing fiber (12a), thereby forming a cleaved end face (66); and applying energy to one or more of 1) the cleaved end face (66) and 2) the light-diffusing fiber (12b) along a portion of the length thereof adjacent the cleaved end face (66), the amount of energy being sufficient to collapse and seal the voids (32) exposed at the cleaved end face (66), leaving a sealed cleaved end face (68). A lens may then be attached to the sealed cleaved end face (68), or the sealed cleaved end face (68) may be softened sufficiently to induce formation of a lensing surface such as a convex lensing surface (60) on the sealed end face (68).