An optical fibre (figure 1, 100) according to an embodiment has a structure suitable for enabling determination of improvement in transmission loss at a preform stage. The optical fibre comprises a core 10 containing chlorine (Cl) and having an average refractive index lower than a refractive index of pure silica glass; a first cladding 20 containing fluorine (F); a second cladding 30; and a resin coating (figure 1, 40 and 50), in which an effective area at a wavelength of 1550 nm is 135μm2 or more and 170 μm2 or less, a ratio of the effective area to a cut off wavelength λC is 85.0 μm or more, a bending loss of an LP01 mode at a wavelength of 1550 nm and at a bending radius of R15 mm is less than 4.9 dB per 10 turns, and the resin coating (figure 1, 40 and 50) includes a primary resin layer (figure 1, 40) having a Young's modulus of 0.3 MPa or less. The refractive index profile may be described as a W-type profile.
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