Abstract: A 1.5 .mu.m zero dispersion single mode optical fiber comprising a center core, a side core disposed on an outer side of the center core and having a refractive index lower than that of the center core, and a cladding portion disposed on an outer side of the side core. Each of the refractive indices of the center core and the side core has a step-like profile in the direction of the radius of the optical fiber. The bending loss is low. The mode field diameter can be increased without deteriorating the bending loss, and the splice loss can be reduced. The chromatic dispersion varies only to a small extent with respect to changes in the core diameter, so that the zero dispersion wavelength can be well controlled.

Abstract: An optical fiber of a silica-based material includes a coating of cured silicone rubber covering an outer surface thereof. The cured silicone rubber coating, when subjected to an infrared spectroscopic measurement, has a transmissivity of not less than 70% at a wave number of 2100 to 2200 cm.sup.-1 at a coating thickness of 0.1 mm due to the infrared absorption due to the stretching and vibration of SiH. The coating of cured silicone rubber contains a neglibible amount of SiH so that the transmission loss of the optical fiber can be prevented from increasing over a long period of time.