OPTICAL FIBER AND METHOD AND APPARATUS FOR MANUFACTURING OPTICAL FIBER BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an optical fiber represented by a silica glass based optical fiber and an optical fiber manufacturing method and in particular, to a technique of reducing the polarization mode dispersion (hereinafter, referred to as "PMD") of an optical fiber. Especially, the invention relates to an optical fiber, in which the amount of increase in PMD is small even if subject to interference, such as lateral pressure or bending, and a method and apparatus for manufacturing optical fiber. Priority is claimed on Japanese Patent Application No. 2011-028387, filed February 14, 2011, the content of which is incorporated herein by reference. Description of Related Art As is well known, a PMD is a phenomenon in which a propagation time difference (delay difference) occurs between two orthogonal polarization mode components in an optical fiber. In addition, if a PMD increases, waveform deterioration occurs in signal light transmitted through the fiber in digital transmission, therefore, it becomes difficult to separate adjacent pulses from each other. As a result, a problem occurs in that problems arise such as the transmission capacity becoming limited. Therefore, suppressing a PMD as much as possible is desired. In addition, a PMD is caused by the optical anisotropy of an optical fiber. The causes of the occurrence of PMD are largely divided into internal factors, in which the optical anisotropy is caused by the internal structure, material, and the like of the optical fiber, and external factors, in which the optical anisotropy is caused by the stress from the outside of the optical fiber and the like. The most high-impacting of the internal factors is the cross-sectional shape of the optical fiber. On the other hand, in the manufacture of optical fibers, it is difficult in practice to realize a completely circular cross-sectional shape including the core of the optical fiber and the cladding around the core regardless of which fiber preform manufacturing method and method of manufacturing a bare optical fiber by drawing (fiber drawing) a fiber preform are selected. Therefore, the actual product has a cross-sectional shape distorted to, for example, a slightly elliptical shape. If the anisotropy of such a cross-sectional shape becomes large, the refractive index distribution in the cross section is no longer a completely concentric circle. Accordingly, birefringence occurs, and this increases PMD. On the other hand, stress applied anisotropically, such as stress caused by bending or lateral pressure applied to the optical fiber from the outside, may be mentioned as the most high-impact of the external factors. The birefringence also occurs due to such anisotropic stress applied from the outside, and this increases PMD. By the way, in order to reduce PMD of the optical fiber, applying torsion to the optical fiber is effective, and methods disclosed in Japanese Unexamined Patent Application Publication No. H8-295528, U. S. Patent. No. 6324872, WO2009/107667, Japanese Unexamined Patent Application Publication No. 2010-122666, and U. S. Patent. No. 7317855 have been proposed. Among them, Japanese Unexamined Patent Application Publication No. H8-295528 and U. S. Patent. No. 6324872 disclose a method of applying torsion before an optical fiber preform is solidified, at the time of drawing of a bare optical fiber, so that the torsion is permanently fixed. The above method is a method of giving a bare optical fiber the torsion as plastic deformation (plastic torsion) so that the torsion is maintained as it is, even if the external force on the optical fiber is removed, that is, a method of maintaining the torsional state as permanent deformation. Hereinafter, such a plastic torsion which remains as permanent deformation may be called a "span". On the other hand, WO2009/107667, Japanese Unexamined Patent Application Publication No. 2010-122666, and U. S. Patent. No. 7317855 disclose a method of applying the torsion to an optical fiber after the optical fiber is drawn and solidified. The torsion in this case occurs due to elastic deformation. That is, torsion in this case is elastic torsion which returns to the state before twisting when the external force is removed and accordingly the optical fiber returns to a free state (external force removal state). In this case, using the optical fiber in an end product, such as a cable, finally in a state where the elastic torsion is held, that is, using the optical fiber in a state where the elastic torsion is held as an optical fiber used in the end product, such as a cable, is assumed. Hereinafter, such elastic torsion may be called a "twist". As described above, the causes of the occurrence of PMD are largely divided into internal factors and external factors, and the method of giving an optical fiber a span (plastic torsion) which is disclosed in Japanese Unexamined Patent Application Publication No. H8-295528 and U. S. Patent. No. 6324872 is effective for PMD caused by the internal factors. However, it is known that such a method of giving an optical fiber a span is not effective for suppressing a PMD increase caused by external factors (for example, refer to WO2009/107667). On the other hand, the method of giving a twist (elastic torsion) as disclosed in WO20097107667, Japanese Unexamined Patent Application Publication No. 2010-122666, and U. S. Patent. No. 7317855 is effective for suppressing PMD increase caused by external factors, such as lateral pressure or bending. However, the above twist returns to the state before twisting due to elastic force when the external force is removed. Here, the external force, such as frictional force, applied to an optical fiber may be removed or the external force, such as frictional force, may become significantly small, for example, in a coloring process, a process of arraying a plurality of optical fibers in a tape form, and an actual mass production process including the process of forming optical fiber cables and the inter-processes, which are all processes for making twisted optical fibers end products such as optical cables. In above case, since the torsion is removed or the torsion becomes significantly small, the effect of suppressing a PMD increase caused by external factors disappears. Therefore, there has been a problem in that it is difficult to reliably and stably suppress a PMD increase caused by external factors in end products such as cables. As described above, in the related art, it has been difficult to reliably and stably suppress a PMD increase, which is caused by external factors such as anisotropic external forces as exemplified by lateral pressure or bending stress applied to optical fibers, in end products. SUMMARY OF THE INVENTION The invention has been made in view of the above situation, and it is an object of the invention to provide an optical fiber capable of reliably and stably suppressing a PMD increase caused by external factors such as anisotropic external forces as exemplified by lateral pressure or bending stress, in end products such as cables, and a method and apparatus for manufacturing the optical fiber. The inventors conducted various kinds of experiments and analyses in order to solve the above-described problems. As a result, the inventors found that elastic torsion was fixed (held) by cured coating resin by giving elastic torsion to a bare optical fiber until the coating resin was cured after the bare optical fiber was solidified when the bare optical fiber is coated with curable resin in a liquid state (non-cured), which was drawn from the optical fiber preform melted by heating, and curing the coating resin. In addition, the inventors found that elastic torsion (twist) could be held when the above-described optical fiber was used in end products, such as optical cables, and this could suppress a PMD increases caused by external factors. Here, the cured coating resin also has elasticity, and generally the Young's modulus of the cured coating resin is smaller than that of glass. For this reason, even if elastic torsion is applied to the bare optical fiber after the bare optical fiber is solidified until the coating resin is cured as described above, it is difficult to fix the torsion by the coating resin as it is, that is, it is difficult to completely prevent an operation (untwisting), in which the elastic torsion returns to the state before twisting due to resilience, using the coating resin. In addition, when the external force is removed after torsion is applied and the state changes to a free state, untwisting of a bare optical fiber portion to some extent cannot be avoided. When the bare optical fiber portion is untwisted (restored to the state before being twisted), however, torsion in the return direction (direction in which the bare optical fiber portion is restored to the state before twisted) is applied to the coating resin layer commensurate with the untwisting of the bare optical fiber portion. As a result, the untwisting of the bare optical fiber portion is stopped in a state where the elastic repulsive force of the coating resin against the torsion in the return direction applied to this coating resin layer and the return force of the torsion of the bare optical fiber portion (resilience of the elastic torsion trying to return to the state before twisting) are in balance. Therefore, the elastic torsion of the bare optical fiber portion when the external force is removed after the torsion is applied is not eliminated 100%, and the torsion of the bare optical fiber portion is necessarily maintained at a certain level due to elastic repulsion of the coating resin. Then, the torsion maintained in this way is held by the coating resin even in the external force removal state, and functions as elastic torsion (twist). It was confirmed that typically at least approximately 20% to 30% of applied elastic torsion remained and was held by coating resin, as will be described later. Accordingly, even if the external force is removed when making an end product through processes, such as a process of arraying a plurality of optical fibers in a tape form and a process of forming an optical fiber cable, the elastic torsion (twist) held and fixed by the coating resin in this way is reliably achieved. As a result, PMD increase caused by external factors can be stably and effectively suppressed. According to a first aspect of the invention, an optical fiber includes: a bare optical fiber portion to which elastic torsion is applied; and a coating layer which coats the bare optical fiber portion, is formed of curable resin, and causes elastic repulsion against resilience occurring in the bare optical fiber portion so that the elastic torsion applied to the bare optical fiber portion is held. By using the optical fiber described above, elastic torsion (twist) of the bare optical fiber portion is held by elastic repulsion of the coating layer against the force in the return (restoration) direction of the torsion. Accordingly, also in a state of an optical cable which is an end-use form, elastic torsion of the bare optical fiber portion can be reliably and stably held. Therefore, it is possible to reliably and stably suppress a PMD increase caused by external factors. Moreover, in the optical fiber according to the first aspect of the invention, preferably, first torsion and second torsion, the second torsion occurring in an opposite direction to a direction in which the first torsion occurs, are alternately applied to the bare optical fiber portion as elastic torsion applied to the bare optical fiber portion every predetermined length in a longitudinal direction of the optical fiber. By using the optical fiber described above, the PMD increase caused by external factors when the first torsion and the second torsion, which occurs in the opposite direction to the direction in which the first torsion occurs, are alternately applied to the bare optical fiber portion as elastic torsion every applied length in the longitudinal direction of the bare optical fiber portion, can be suppressed more reliably and stably than that in the case where the elastic torsion is applied continuously in only one direction. Moreover, in the optical fiber according to the first aspect of the invention, preferably, the coating layer is configured to include a first coating layer, the first coating layer formed of a resin with a relatively low Young's modulus, and a second coating layer, the second coating layer formed of a resin with a relatively high Young's modulus. By using the optical fiber described above, it is possible to improve the adhesion of the coating layer to the bare optical fiber using resin with a low Young's modulus as the first coating layer being in contact with the peripheral surface of the bare optical fiber and also to increase elastic repulsion using resin with a high Young's modulus as the second coating layer located at the outer side. Since this is advantageous in holding the elastic torsion (twist) of the bare optical fiber by the coating layer, the PMD increase caused by external factors can be suppressed more reliably and stably. Moreover, in the optical fiber according to the first aspect of the invention, preferably, an inversion period T is in the range of 5 to 30 m in the longitudinal direction of the optical fiber and a maximum amplitude of an accumulated torsion angle of the bare optical fiber portion in an inverted torsion profile is 100xT (°) to 1200xT (°) under the condition where a remaining elastic torsion is applied to the bare optical fiber portion in a state where the elastic torsion applied to the bare optical fiber portion is held by the elastic repulsion caused by the coating layer. By using the optical fiber described above, because the remaining the elastic torsion is sufficient amount by the inversion period T of the remaining elastic torsion being within the above range and the maximum amplitude of the accumulated torsion angle of the bare optical fiber portion being within the above range, a PMD increase caused by external factors can be reliably and stably suppressed. In addition, according to a second aspect of the invention, there is provided a method for manufacturing the above-described optical fiber according to the first aspect of the invention. That is, the optical fiber manufacturing method according to the second aspect of the invention includes: melting an optical fiber preform by heating; drawing a bare optical fiber with a predetermined diameter from the melted optical fiber preform; solidifying the drawn bare optical fiber; applying elastic torsion to the bare optical fiber after solidification by transmitting elastic torsion to the bare optical fiber toward an upstream side in a drawing direction of the bare optical fiber; forming a coating layer before curing by coating an outer periphery of the solidified bare optical fiber with curable resin in a liquid state; forming an optical fiber, to which torsion is applied so that the elastic torsion of the bare optical fiber is held, by curing the coating layer formed on the outer periphery of the bare optical fiber to which the elastic torsion is applied; and drawing the optical fiber to which the torsion is applied. By using the optical fiber manufacturing method described above, it is possible to manufacture the optical fiber in which elastic torsion (twist) applied to the solidified bare optical fiber is held by the cured coating layer, that is, the optical fiber in which elastic torsion remains in the bare optical fiber even after the external force is removed. Moreover, in the optical fiber manufacturing method according to the second aspect of the invention, preferably, a twisting device is used to apply torsion to the bare optical fiber, and torsion is applied to the bare optical fiber in a state where a member, the member preventing transmission of the torsion of the bare optical fiber, is not present at an upstream side of the twisting device. When the optical fiber manufacturing method described above is used, torsion is smoothly transmitted from the twisting device to the upstream side of the twisting device. As a result, elastic torsion can be reliably and stably applied to the bare optical fiber. Moreover, in the optical fiber manufacturing method according to the second aspect of the invention, it is preferable that when applying torsion to the optical fiber, a direction of the torsion applied to the bare optical fiber be periodically reversed. By using the optical fiber manufacturing method described above, it is possible to obtain the optical fiber applied with torsion whose direction is periodically reversed. As a result, the PMD increase caused by external factors can be suppressed more effectively. Moreover, in the optical fiber manufacturing method according to the second aspect of the invention, it is preferable that when the bare optical fiber is coated with curable resin, a viscosity of the liquid-state curable resin at the time of coating be 0.1 to 3 Pa- sec. By using the optical fiber manufacturing method described above, the viscosity of the liquid-state resin at the time of coating becomes equal to or larger than 0.1 Pa-sec. Since this suppresses a change in the external diameter of a coat of an optical fiber, it is possible to obtain an optical fiber with a coat having a uniform external diameter. In addition, the viscosity of the liquid-state resin at the time of coating is equal to or smaller than 3 Pa-sec, and this prevents the liquid-state resin from becoming resistant to the transmission of torsion. In particular, in the case of reversing the torsional direction periodically, the transmission of torsion and the direction of torsion can be reliably reversed. Moreover, in the optical fiber manufacturing method according to the second aspect of the invention, preferably, an inversion period T of the torsion applied to the optical fiber in a longitudinal direction of the optical fiber is 5 to 30 m, and a maximum amplitude of an accumulated torsion angle in an inverted torsion profile is 500>