Systems and methods of collapsing the air lines in the air line-containing region of a nanostructure optical fiber are disclosed. One method includes initiating irradiation of a portion of the nanostructure optical fiber from essentially opposite directions with at least first and second laser beams having substantially equal power and essentially the same mid-infrared wavelength. The method includes continuing the irradiation for an irradiation time t1 so as to bring the optical fiber portion to a softening temperature TS at which the air lines in the optical fiber portion collapse into the adjacent cladding. Exemplary optical systems for carrying out the air- line-collapsing methods of the present invention are also disclosed.