The article reports on observation of nanopores nucleated under a tensile stress in the core of the germanosilicate optical fibers doped with boron. Pores were observed with an atomic-force microscope on the faces of cleaved fiber tips. Under certain experimental conditions, pores form a quasi-periodic structure and their sizes are in a good agreement with predictions of the earlier proposed model based on the theory of phase transitions. The theoretically estimated threshold stress level for effective nucleation of pores corresponds well to the results of experimental observations.