The mode field diameter of a Ge-doped silica single-mode optical fiber is expanded from 9.02㎛ to 22.62㎛ by thermal treatment for 30 hours at 1250℃. Fibers were put into a silica tube after stripping off the protective coating layer. After evacuating the tube, Ar gas(0.2 atm at room temperature) was filled to prevent it from collapse. Fibers were heat treated at temperatures ranging from 1200 to 1250℃(at the center of the furnace) for 30 hr. The properties of TEC fibers are investigated. We can estimate the spot size of the mode from the far-field patterns. Fig. 17 shows the far-field patterns at output ends of fiber before and after the heat treatment, where we approximate the modal intensity distribution as Gaussian. The maximum spot size of the fiber heated at 1250℃ for 30hr is 2.5 times larger than that of the original size. Agreement between measurements and calaulations is satisfactory. Change of the pattern reflects broadening of the refractive-index distribution. The diffraction of the modal intensity distributions have reduced due to heat treatment. The spot size gradually changes along the fiber axis. The spot size along the fiber axis at λ=1.3㎛ has been obtained from the far-field patterns. Measured spot sizes along the fiber axis are shown in Figs. 18 and 19. Tapered regions(~13mm) are longer than ~3mm, so the access loss is expected to be sufficiently small. We measured coupling losses between SMF's. Heat-treated fibers were cut at the point where the propagating mode has the maximum spot size. Measured coupling losses as a function of spacing are shown in Fig. 20. Coupling losses decrease dramatically with increasing the spot size. The TEC fiber has specific features that the fiber can change the spot size of a propagating mode arbitrily without changing the normalized frequency. TEC fiber is expected to reduce fiber-to-fiber connection loss. The insertion loss in optical connectors for coupling small core fibers or dissimilar fibers can be reduced by using TEC fibers.