We achieved the lowest-ever transmission losses of 0.1419 dB/km at 1560 nm wavelength and 0.1424 dB/km at 1550 nm in a Ge-free silica-core optical fiber. It was an improvement by 4 mdB/km from the previous record realized in 2015. The Ge-free silica core included fluorine co-doping, which helps to reduce disorder in the microscopic glass network structure that causes Rayleigh scattering loss without a significant increase in waveguide imperfection loss. A two-layered polymer coating with an inner layer having lower elastic modulus than before also contributed to the ultralow loss without influence of microbending loss increase even with an enlarged effective area of 147 μm2. The present fiber with ultralow loss and a large effective area benefits an ultralong haul optical transmission system including transoceanic submarine cable systems. We estimate system performance based on the fiber figure of merit theory that the present fiber enables a 0.10 bit/s/Hz increase in spectral efficiency or 7% reduction in the number of repeaters, compared to the previous record-loss fiber.
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