1. Field of the Invention
The present invention relates to an optical fiber, an optical transmission system, and a method of making the optical fiber.
2. Description of the Related Art
Optical fibers, used as an optical transmission line in optical transmission networks with a transmission rate of 100 Gbit/s or higher, preferably have low attenuation and low nonlinearity. When n2 denotes the nonlinear refractive index of an optical fiber and Aeff denotes the effective area thereof, the nonlinearity of the optical fiber is defined by n2/Aeff. As the effective area Aeff is larger, the nonlinearity is lower, because concentration of optical power density on the core is prevented. A general-purpose single-mode optical fiber (SMF) compliant with the ITU-T G. 652 standard has an effective area Aeff of approximately 80 μm2 at a wavelength of 1550 nm. It is however preferable that the effective area Aeff of an optical fiber with low nonlinearity be greater than or equal to 110 μm2 and less than or equal to 180 μm2.
Unfortunately, as the effective area Aeff increases, microbending loss increases, thus leading to increased loss upon cable formation. Additionally, as the effective area Aeff increases, loss in connection with a general-purpose single-mode optical fiber increases. Considering the effect of the effective area Aeff on microbending loss and connection loss, the effective area Aeff is preferably less than or equal to 150 μm2, though this value varies depending on the refractive index profile of an optical fiber and the Young's modulus and thickness of resin coating.
A pure-silica-core optical fiber (PSCF) is known as a low-loss optical fiber. A GeO2-doped core optical fiber (GCF) has been considered as being inferior to the PSCF with respect to the above-described large capacity communication, since the GCF has higher Rayleigh scattering loss than the PSCF due to concentration fluctuations of GeO2. A typical PSCF is expensive. Accordingly, an inexpensive optical fiber with achieved low loss and low nonlinearity is demanded.
S. Sakaguchi et al., Applied Optics, Vol. 37, No. 33, pp. 7708-7711 (1998) and JP2006-58494A disclose technology for reducing loss in the GCF. According to this technology, when an optical fiber preform is drawn to form an optical fiber, the optical fiber is slowly cooled to reduce the fictive temperature of glass constituting the fiber in order to reduce Rayleigh scattering in the optical fiber, thus achieving low loss.