Title of Invention	OPTICAL FIBER AND METHOD OF MEASURING POLARIZATION MODE DIFFUSION FOR OPTICAL FIBER
Abstract	The invention relates to a method of measuring polarization mode dispersion of an optical fiber (1) having a core (2) and a cladding (3) arranged around the core (2), includes estimating polarization mode dispersion when an optical fiber (1) is formed as an optical cable, from a beat length when the optical fiber (1) is wound around a bobbin, and an average coupling length when the optical fiber (1) is formed as the optical cable.

Title of Invention

OPTICAL FIBER AND METHOD OF MEASURING POLARIZATION MODE DIFFUSION FOR OPTICAL FIBER

Abstract

The invention relates to a method of measuring polarization mode dispersion of an optical fiber (1) having a core (2) and a cladding (3) arranged around the core (2), includes estimating polarization mode dispersion when an optical fiber (1) is formed as an optical cable, from a beat length when the optical fiber (1) is wound around a bobbin, and an average coupling length when the optical fiber (1) is formed as the optical cable.

Full Text	The magnitude of local birefringence of the optical fiber can be quantified by using a beat length (hereinafter, abbreviated as "LB")- LB indicates a propagation distance over which optional polarization entering into the optical fiber returns to the incident polarization. Another parameter expressing the local birefringence of the optical fiber is a mode birefringence rate B. The relationship expressed by the following Equation (1) is established between the mode birefringence rate B and LB. In the above Equation (1), X indicates the wavelength of light. When the length of the optical fiber is short, it can be considered that there is no polarization mode coupling; therefore, the PMD is expressed by the following Equation (2) as a function between light velocity C and the length L of the optical fiber. From the above Equation (2), it is understood that the PMD increases in proportion to the length L of the optical fiber. On the other hand, when the length of the optical fiber is long, the PMD is expressed by the following Equation (3). From the above Equation (3), it is understood that the PMD increases in proportion to the square root of the length L of the optical fiber. In the above Equation (3), Lc is referred to as an average coupling length, which is a parameter expressing the magnitude of the polarization mode coupling, and as the polarization mode coupling increases, the magnitude of the average coupling length decreases. The magnitude of the polarization mode coupling is mainly determined by the distortion of the optical fiber, a force applied from the outside, and the like. When the length L of the optical fiber is shorter than Lc, the PMD can be expressed by using Equation (2). On the other hand, when the length L of the optical fiber is longer than Lc, the PMD can be expressed by using Equation (3). It is understood from Equations (2) and (3) that the PMD increases as LB becomes shorter, and Lc becomes longer. Generally, the optical fiber is transferred to an optical cabling process in a state wound around a bobbin, or is shipped and transported just as an optical fiber unit. Therefore, it is preferable that the PMD can be measured in the state with the optical fiber wound around the bobbin. However, by winding the optical fiber around the bobbin, disturbance such as bending, lateral pressure, distortion and the like occurs in the optical fiber, thereby changing LB and Lc, and hence the PMD varies. Therefore, in the same optical fiber, the PMD of the optical fiber wound around the bobbin for transport, and the PMD of the optical fiber after optical cabling, show quite different values (for example, see Non- patent Document 1). As a result, the PMD of the optical fiber after optical cabling increases, and in some cases, exceeds an upper limit of the PMD specified by the standard, thereby causing a problem. Moreover, the optical fiber is shipped to the optical cabling process in a length of about 20 to 100 km, and at the time of optical cabling, the length of the optical fiber is cut to 1 to 10 km. Therefore, if there is a place having locally large PMD, even if the PMD of the whole length of the optical fiber before optical cabling is small, when the optical fiber is cut and formed as a cable, there may be a place where there is large PMD, which has been considered to be a problem. Non-patent Document 1: Scott Grindstaff, Joseph Hill, Omid Daneshvar, "Extrinsic Stress Effects on Polarization Mode Dispersion in Optical Fiber Cables", International Wire & Cable Symposium Proceedings, 1993, pp. 647 to 654. DISCLOSURE OF INVENTION The present invention takes into consideration the above situation, and an object thereof is to provide a method of measuring polarization mode dispersion of an optical fiber, which can estimate the PMD of the optical fiber after optical cabling, in a state with the optical fiber wound around the bobbin for transport, to provide an optical fiber, and to provide an optical fiber cable. In order to achieve the above object, the present invention provides a method of measuring polarization mode dispersion of an optical fiber, including estimating polarization mode dispersion when an optical fiber is formed as an optical cable, from a beat length when the optical fiber is wound around a bobbin, and an average coupling length when the optical fiber is formed as the optical cable. In the abovementioned method of measuring polarization mode dispersion of an optical fiber, preferably the Rayleigh scattered light intensity of the optical fiber is measured by using a P-OTDR. TECHNICAL FIELD The present invention relates to an optical fiber and a method of measuring polarization mode dispersion of an optical fiber. Priority is claimed on Japanese Patent Application No. 2003-361812, filed October 22, 2003, the content of which is incorporated herein by reference. BACKGROUND ART Recently, with progress in speeding up of transmission speed, and lengthening of transmission distance in optical communication, reduction of polarization mode dispersion (hereinafter, abbreviated as "PMD") is required for the optical fiber, being the transmission path. The PMD of the optical fiber is mode dispersion caused by an occurrence of a group velocity difference in two orthogonal intrinsic polarization components, which propagate in the optical fiber, due to non-circularity of the core shape of the optical fiber, asymmetry of stress occurring in the core, etc. There are two parameters for determining the PMD. One parameter is the magnitude of local birefringence of the optical fiber, and the other is polarization mode coupling expressing how the direction of a birefringent axis of the optical fiber changes in the longitudinal direction of the optical fiber. In the abovementioned method of measuring polarization mode dispersion of an optical fiber, preferably the resolution of the P-OTDR is made shorter than the shortest beat length estimated for the optical fiber wound around the bobbin. In the abovementioned method of measuring polarization mode dispersion of an optical fiber, preferably the optical fiber is wound around the bobbin by setting the radius R of the bobbin and a tension at the time of winding the optical fiber around the bobbin, so that the magnitude of birefringence of the optical fiber induced due to winding of the optical fiber around the bobbin becomes smaller than the magnitude of internal birefringence inherently held by the optical fiber. In the abovementioned method of measuring polarization mode dispersion of an optical fiber, preferably the optical fiber is wound around the bobbin by setting the radius R of the bobbin and a tension at the time of winding the optical fiber around the bobbin, so that the magnitude of birefringence of the optical fiber induced due to winding of the optical fiber around the bobbin becomes smaller than the magnitude of internal birefringence allowed in the standard for the optical fiber. In the abovementioned method of measuring polarization mode dispersion of an optical fiber, preferably the optical fiber is wound around the bobbin by setting the radius R of the bobbin, so that the radius R of the bobbin and the magnitude B of internal birefringence allowed in the standard for the optical fiber satisfy the following Equation (4). (where "n" denotes a refractive index of a glass material (normally, silica glass) constituting the optical fiber, p11 and p12 denote Pockels coefficients of the glass material constituting the optical fiber, v denotes a Poisson's ratio of the glass material constituting the optical fiber, and "r" denotes a radius of a glass portion of the optical fiber.) In the abovementioned method of measuring polarization mode dispersion of an optical fiber, at the time of measurement, preferably the measurement of the beat length is performed in a state where the tension applied to the optical fiber is temporarily reduced, because the influence of birefringence due to the lateral pressure resulting from the winding tension, and the influence of the PMD can be removed at the time of measurement. Moreover, after measurement, by returning the tension to the original tension before the measurement, a problem such as that the feed of the optical fiber becomes difficult, can be prevented in the subsequent manufacturing process, which is preferable. In the abovementioned method of measuring polarization mode dispersion of an optical fiber, preferably the bobbin has a structure capable of temporarily reducing the winding tension on the optical fiber. In the abovementioned method of measuring polarization mode dispersion of an optical fiber, it is desirable to measure the PMD in the longitudinal direction of the optical fiber. The present invention further provides an optical fiber including a core and a cladding arranged around the core, wherein the PMD measured by using the above method is equal to or less than 0.1 ps/√km. In the abovementioned optical fiber, preferably a beat length in a state wound around the bobbin is equal to or longer than 15 m, and more preferably, is equal to or longer than 30 m. In the abovementioned optical fiber, preferably a beat length in a state in which a tension due to the bobbin is reduced after wound around the bobbin, is equal to or longer than 15 m, and more preferably, is equal to or longer than 30 m. The present invention further provides an optical fiber cable in which a plurality of coated optical fibers having a protective layer around the optical fiber are arranged side by side, and the plurality of coated optical fibers are housed in a sheath. ADVANTAGEOUS EFFECTS OF THE INVENTION According to the method of measuring polarization mode dispersion of an optical fiber of the present invention, the PMD of the optical fiber after optical cabling can be estimated in a state with the optical fiber wound around the bobbin. According to the method of measuring polarization mode dispersion of an optical fiber of the present invention, it can be determined whether the PMD of the optical fiber after optical cabling is within the standard, in a state with the optical fiber wound around the bobbin. According to the method of measuring polarization mode dispersion of an optical fiber of the present invention, since it is not necessary to prepare the optical fiber for measurement in a free state, the optical fiber can be used effectively. According to the method of measuring polarization mode dispersion of an optical fiber of the present invention, since the polarization mode dispersion of the shipped optical fiber itself can be measured, without using the measurement result of the polarization mode dispersion of the optical fiber in the vicinity thereof, an optical fiber having higher quality can be provided. According to the method of measuring polarization mode dispersion of an optical fiber of the present invention, since the PMD value of the optical fiber in the longitudinal direction can be measured, a place having large PMD locally can be found and removed. As a result, an optical fiber having higher quality can be provided. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS FIG. 1 is a graph showing the results of calculating the magnitude of average birefringence, at the time of applying a force from outside to two optical fibers having different internal birefringence, to induce birefringence of a different magnitude. FIG. 2 is a graph showing calculation results of PMD of an optical fiber. FIG. 3 is a graph showing an example of a waveform of Rayleigh scattered light intensity actually measured by using a P-OTDR. FIG. 4 is a graph showing the relationship between the beat length of an optical fiber in a state with the optical fiber wound around a bobbin, and the PMD of the optical fiber after an optical fiber cable is formed by the optical fiber. FIG. 5 is a graph showing the relationship between the PMD of the optical fiber in a state with the optical fiber wound around a bobbin, and the PMD of the optical fiber after the optical fiber cable is formed by the optical fiber. FIG. 6 is a cross-sectional view showing an example of the structure of an optical fiber. FIG. 7 is a cross-sectional view showing an example of the structure of a coated optical fiber. FIG. 8 is a cross-sectional view showing an example of the structure of an optical fiber cable. DESCRIPTION OF THE REFERENCE SYMBOLS 1 ... optical fiber, 2 ... core, 3 ... cladding, 4 ... primary coating (protective layer), 5 ... secondary coating (protective layer), 10 ...coated optical fiber, 11 ... tension member, 12 ... loose tube, 13 ...jelly, 15 ... pressed winding, 16 ... tearing string, 17 ... sheath, 20 ... optical fiber cable BEST MODE FOR CARRYING OUT THE INVENTION The method of measuring polarization mode dispersion of an optical fiber according to the present invention will be described hereunder in detail. At first, an individual study is carried out as to how the beat length LB and the average coupling length Lc change, by winding the optical fiber around a bobbin. LB changes when birefringence is induced from outside, substantially in the radial direction of the bobbin, due to the influence of the bending radius, tension, or lateral pressure at the time of winding the optical fiber around the bobbin. Lc changes due to torsion of the optical fiber, or contact of optical fibers with each other, at the time of winding the optical fiber around the bobbin. Here, the change of LB after the optical fiber is wound around the bobbin is studied from the relationship between the internal birefringence of the optical fiber, and the birefringence of the optical fiber induced from outside. The direction of birefringence induced from outside by winding the optical fiber around the bobbin, is substantially in the radial direction of the bobbin, while the angle of the birefringent axis inside the optical fiber can take any angle. Therefore, the average birefringence of the optical fiber when a lateral pressure is applied, can be considered to be an average when the lateral pressure is applied from various angles. FIG. 1 is a graph showing the results of calculating the magnitude of the average birefringence, at the time of applying a force from outside to two optical fibers having different internal birefringence, to induce birefringence of a different magnitude. From the results shown in FIG. 1, it is understood that when the magnitude of birefringence induced from outside is smaller than the magnitude of internal birefringence, the magnitude of the average birefringence hardly changes. On the other hand, when the magnitude of birefringence induced from outside is larger than that of the internal birefringence, the magnitude of the average birefringence becomes substantially equal to that of the magnitude of the birefringence induced from outside, and finally, the magnitude of the average birefringence becomes larger than that of the magnitude of the birefringence induced from outside. However, the magnitude of the average birefringence after birefringence is induced from outside due to lateral pressure does not become smaller than that of the internal birefringence. Therefore, by winding the optical fiber around the bobbin, the average LB becomes shorter inevitably, which causes an increase in the PMD. On the other hand, Lc after the optical fiber is wound around the bobbin becomes very short, due to torsion in the optical fiber or due to contact of the optical fibers with each other, when the optical fiber is wound around the bobbin. Therefore, a change in Lc due to winding of the optical fiber around the bobbin causes a decrease in the PMD. Accordingly, when the influence of the change in Lc is larger than the influence of the change in LB, the PMD decreases when the optical fiber is wound around the bobbin, and when the optical fiber is formed as an optical fiber cable and put in a free state, the PMD increases, thereby causing a problem. A calculation example of the PMD is shown below. For example, when the length of the optical fiber is 1000 m, and this optical fiber is divided into fine sections of 0.1 mm, it is assumed that in the fine sections, the optical fiber has only linear birefringence. A change in the PMD of the optical fiber is calculated by simulation, using the Jones matrix method (refer to IEEE Photonics Technology Letters, Sept. 1992, vol. 4, No. 9, pp. 1066 to 1069). In a free state, when optical fibers having LB of 10 m and 20 m and Lc of 30 m are wound around the bobbin, Lc decreases, and birefringence is induced in the radial direction of the bobbin, and from this, how the PMD of the optical fiber changes is calculated. Both optical fibers are set so that when the optical fibers are wound around the bobbin, Lc becomes 3 m. The average coupling length used for the calculation is a typical value for when the optical fiber is put in a free state, and for when the optical fiber is wound around the bobbin. The calculation results of the PMD of the optical fiber are shown in FIG. 2. From the results shown in FIG. 2, it is understood that when the optical fiber is wound around the bobbin, and when the magnitude of birefringence induced from outside is small, the PMD of the optical fiber wound around the bobbin becomes smaller than that of the optical fiber in the free state. It is also understood that when LB of the optical fibers in the free state is different, the magnitude of the change in the PMD with respect to the magnitude of birefringence induced from outside becomes different. Therefore, when the optical fiber is put in the free state, though the PMD is different, the PMD of both optical fibers may take the same value, according to the magnitude of birefringence induced from outside. If birefringence larger than that is induced, the magnitude correlation of the PMD of the two optical fibers is reversed to that of optical fibers in the free state. That is, the PMD of the optical fiber in the free state cannot be estimated only by measuring the PMD of the optical fiber wound around the bobbin. Here, what the inventors want to know is the magnitude of the PMD of the optical fiber after optical cabling. Since the optical fiber cable has such a structure that an external force applied to the optical fiber is made as small as possible, the PMD of the optical fiber after optical cabling is substantially the same as that of the optical fiber in the free state. Therefore, the PMD of the optical fiber after optical cabling cannot be estimated only by measuring the PMD of the optical fiber wound around the bobbin. However, it has been found that if LB of the optical fiber wound around the bobbin is obtained, LB of the optical fiber after optical cabling does not become shorter than that. Therefore, by obtaining Lc of the optical fiber after optical cabling, the PMD of the optical fiber after optical cabling can be estimated by using the following Equation (2). Lc of the optical fiber cable is determined mainly by external factors such as the structure of the optical fiber cable, and the material and the surface properties of the resin used for the optical fiber and the optical fiber cable. Therefore, by measuring Lc beforehand by using the same type of optical fiber as in other optical fiber cables formed from a resin of the same material, having the same structure, and the same surface properties, Lc of the optical fiber after optical cabling can be obtained, without using the optical fiber for measurement of Lc separately. Next a specific example of the measurement method of Lc of the optical fiber after optical cabling will be described below. At first, the PMD of the optical fiber for the whole length of the optical fiber cable is measured, and then, a section of the optical fiber of about 5 m is taken out and the PMD is measured. In a short optical fiber cable of about 5 m, it can be considered that there is no polarization mode coupling, and hence LB can be obtained by using Equation (2). Lc can be determined by using the following Equation (3), from LB obtained here, and the PMD of the optical fiber for the whole length of the optical fiber cable. Accordingly, from the above Equations (2) and (3), the PMD after optical cabling can be estimated from LB of the optical fiber wound around the bobbin, and the previously calculated Lc. Next, a method of estimating the PMD by using a P-OTDR (Polarization Optical Time Domain Reflectometry) after an optical fiber cable is formed by using an optical fiber to be measured, will be explained. In this method, by using the P-OTDR, LB and Lc can be independently measured with the optical fiber to be measured wound around the bobbin. Specifically, methods described in Reference document A (F. Corsi, A Galtarossa, and L. Palmieri, "Polarization Mode Dispersion Characterization of Single-Mode Optical Fiber Using Backscattering Technique", Journal of Lightwave Technology, Vol. 16, No. 10, Oct. 1998, pp. 1832 to 1843), and Reference document B (M. Wuilpart, G. Ravet, P. Megret, and M. Blondel, "PMD measurement with a polarization-OTDR", ECOC 2002) can be used. FIG. 3 is a graph showing an example of a waveform of Rayleigh scattered light intensity actually measured by using the P-OTDR. According to the Reference documents A and B, as one calculation method of LB, when there are N extreme values of the waveform in a section length L, from which the waveform of the Rayleigh scattered light intensity measured by using the P-OTDR is obtained, there is a method of calculating LB by using the following Equation (5). In the case of an optical fiber having a waveform as shown in FIG. 3, since there are 19 extreme values in a 100 m section, it is understood that LB is 21 m. Thus, by measuring LB of the optical fiber wound around the bobbin by using the P-OTDR, and separately measuring Lc of the optical fiber cable in the free state, the PMD of the optical fiber formed as an optical fiber cable can be estimated. Here, as the method for calculating LB, the above Equation (5) is used. However, the calculation method of LB is not limited thereto, and other methods may be used. In the Reference documents A and B, LB and Lc of the optical fiber to be measured in the free state are measured by using the P-OTDR. On the other hand, in the present invention, the calculation method is different from the conventional method in that LB is measured by winding the optical fiber around the bobbin, to estimate the PMD of the optical fiber after optical cabling. Next the relationship between LB of the optical fiber to be measured, and the resolution of the OTDR used for the measurement of the PMD will be described. In a single-mode fiber excluding a polarization maintaining fiber, since LB is normally 10 cm or longer, if the resolution of the OTDR is shorter than 10 cm, LB and Lc can be measured independently in any single-mode fiber. In such an application, resolution equal to or shorter than 1 cm can be obtained, for example, by using a technique referred to as photon counting OTDR. Therefore, it can be applied to any single-mode fiber other than the polarization maintaining fiber. The measurement method of the PMD using the P-OTDR can also be applied to a multi-mode fiber, by using a device which selectively excites and receives only a mode to be measured. Next an optical fiber in which the PMD after the optical fiber is formed as an optical fiber cable, can be estimated in a state with the optical fiber wound around the bobbin, will be described. When the magnitude of birefringence of the optical fiber induced due to winding of the optical fiber around the bobbin is smaller than that of internal birefringence, then as described above, the magnitude of the birefringence of the optical fiber wound around the bobbin is substantially the same as the magnitude of the birefringence of the optical fiber after optical cabling. Therefore, LB of the optical fiber measured in the state wound around the bobbin is substantially equal to LB of the optical fiber after optical cabling. Accordingly, an optical fiber can be provided, in which the PMD of the optical fiber after optical cabling can be estimated by measuring Lc of the optical fiber cable beforehand using the same type of optical fiber. Lc after cabling is mainly determined by the internal birefringence of the optical fiber, and the external force induced due to the structure of the optical fiber cable. In many cases, there is no problem in considering that Lc is constant regardless of LB. However, when LB is short, that is, when the internal birefringence of the optical fiber is large, then even if an external force is applied to the optical fiber, polarization mode coupling is hardly induced. Hence there is a case in which Lc cannot be regarded as constant. Even in this case, by obtaining Lc as a function of LB, the present method can be applied. Japanese Unexamined Patent Application, First Publication No. H11-208998 discloses a method of winding an optical fiber so that the PMD of the optical fiber wound around the bobbin matches with the PMD of the optical fiber in the free state. As described above, however, the way of change of the PMD of the optical fiber wound around the bobbin changes due to the internal birefringence of the optical fiber in the free state. Therefore, in this method, the winding method cannot be determined unless the internal birefringence of the optical fiber in the free state is known beforehand. Therefore, when LB is different for each optical fiber, that is, in the actual manufacturing process, the effect thereof is smaller than that obtained by the present invention. Next an optical fiber in which it can be determined whether the PMD standard of the optical fiber is satisfied after optical cabling, in a state wound around the bobbin, will be described. If it is found that the magnitude of the birefringence induced by winding the optical fiber around the bobbin is smaller than the internal birefringence allowed in the standard for the optical fiber, it is understood that the birefringence after optical cabling is smaller than the internal birefringence allowed in the standard for the optical fiber. Therefore, by measuring the magnitude of the induced birefringence, and the magnitude of the internal birefringence allowed in the standard for the optical fiber, in a state with the optical fiber wound around the bobbin, it can be determined whether the optical fiber satisfies the PMD standard after optical cabling. The maximum value of the internal birefringence allowed in the standard for the optical fiber can be calculated from a predetermined upper limit of the PMD, and the average coupling length of the optical fiber cable, by using the above Equations (1) and (3). Next the radius of a bobbin, which can determine whether the optical fiber satisfies the PMD standard of the optical fiber in an optical fiber cable after optical cabling, in a state with the optical fiber wound around the bobbin will be described here. According to Reference document C (R. Ulrich, S. C. Rashleigh, and W. Eickhoff, "Bending-induced birefringence in single-mode fibers", Optics Letters, Vol. 5, No. 6, June 1980, pp. 273 to 275), birefringence B induced by bending at the time of winding the optical fiber around the bobbin is expressed by the following Equation (6), from a refractive index "n" of the optical fiber, Pockels coefficients p11 and p12, Poisson's ratio v, a radius "r" of the optical fiber, and a radius R of the bobbin. From the above Equation (6), if the magnitude B of the birefringence induced by bending of the optical fiber is smaller than the magnitude of the internal birefringence allowed in the standard for the optical fiber, it can be determined whether the optical fiber satisfies the PMD standard after optical cabling, in the measurement with the optical fiber wound around the bobbin. Accordingly, it is preferable that the radius R of the bobbin satisfies the relationship in the following Equation (4). Here, B' denotes the maximum value of the magnitude of the internal birefringence allowed in the standard for the optical fiber, and can be calculated from the predetermined upper limit of the PMD, and the average coupling length of the optical fiber cable, by using the above Equations (1) and (3). Here, if the bobbin has a structure capable of temporarily reducing the tension applied to the optical fiber, then this is preferable because the influence of the birefringence generated inside of the optical fiber due to the tension can be removed. Here it is preferable to estimate the distribution of PMD in the longitudinal direction of the optical fiber. Prior to the present invention, there was no measurement method for performing measurement of the beat length, without cutting the optical fiber. Accordingly, a fiber in which the PMD after optical cabling can be known before the optical cabling process, can be provided only by using the method of the present invention. Moreover, in the present invention, since the beat length can be measured in the longitudinal direction of the optical fiber, then even if there is a place where the beat length is short partially, the place can be determined. Then, by using the method of the present invention, and first determining the place where the beat length of the optical fiber is short partially, the cause of shortage can be specified and the manufacturing process can be improved. As a result, even after the optical fiber is divided in the optical cabling process, an optical fiber which can obtain a favorable PMD in the whole optical fiber cable can be provided. In 40 Gb/s transmission, preferably the PMD of the optical fiber is equal to or less than 0.1 ps/√km. Moreover, since the transmission path is formed of a plurality of optical fibers, it is required at least as the whole transmission path, that the PMD be equal to or less than 0.1 ps/√km. The present inventor has executed investigations by using combinations of optical fibers and optical fiber cables having various structures, to find that when optical fibers having a beat length of 15 m or longer after optical cabling are connected to form one transmission path, the PMD can be set to 0.1 ps/√km or less as the whole transmission path. For optical fibers having a beat length of 30 m or longer after optical cabling, it has been found that the PMD is 0.1 ps/√km or less in all individual optical fibers. Accordingly, it is preferred that the beat length measured by using the method of the present invention is equal to or longer than 15 m, and more preferably, equal to or longer than 30 m. Hereunder the present invention will be described more specifically by way of examples, but the present invention is by no means limited to the examples below. Example 1 An optical fiber to be measured before cabling, was wound in a length of 3000 m around a bobbin having a diameter of 300 mm at a tension of 20 gf. When the magnitude of the internal birefringence of the optical fiber to be measured was measured, LB was 30 m. On the other hand, when an optical fiber cable having a length of 3000 m was formed by using the same type of optical fiber, separately from the optical fiber to be measured, the PMD of the optical fiber constituting the optical fiber cable was 0.05 ps/√km, and LB was 30 m. Consequently, it could be estimated that Lc after forming an optical fiber cable by using the optical fiber to be measured was about 85 m, from the above Equation (3). From the above result, it could be estimated that the PMD after forming a cable by the optical fiber to be measured was 0.05 ps/√km. When the optical fiber to be measured was actually used to form an optical fiber cable, the PMD was 0.05 ps/√km. Example 2 An optical fiber to be measured before cabling was wound in a length of 3000 m around a bobbin having a diameter of 300 mm at a tension of 20 gf. When LB was measured by a measurement using the P-OTDR, LB was 25 m. On the other hand, when an optical fiber cable having a length of 3000 m was formed by using the same type of optical fiber, separately from the optical fiber to be measured, the PMD of the optical fiber constituting the optical fiber cable was 0.07 ps/√km, and LB was 20 m. Consequently, it could be found that Lc after forming an optical fiber cable by using the optical fiber to be measured was about 73 m, from the above Equation (3). From the above result, it could be estimated that the PMD after forming a cable by the optical fiber to be measured was 0.06 ps/√km. When the optical fiber to be measured was actually used to form an optical fiber cable, the PMD was 0.06 ps/√km. Example 3 An optical fiber to be measured was wound in a length of 3000 m around a bobbin having a diameter of 300 mm at a tension of 20 gf. When the P-OTDR measurement was carried out by using an OTDR having a resolution of 2 m, the waveform as shown in FIG. 3 required for measuring LB could not be observed. On the other hand, when the P-OTDR measurement was carried out by using an OTDR having a resolution of 1 cm, the waveform as shown in FIG. 3 could be observed, and it was found that LB was 1.2 m. On the other hand, when an optical fiber cable having a length of 3000 m was formed by using the same type of optical fiber, separately from the optical fiber to be measured, the PMD was 1.0 ps/√km, and LB was 2.0 m. Consequently, it could be found that Lc after forming an optical fiber cable by using the optical fiber to be measured was about 150 m, from the above Equation (3). From the above result, it could be estimated that the PMD after forming a cable by the optical fiber to be measured was 1.67 ps/√km. When the optical fiber to be measured was actually used to form an optical fiber cable, it was found that the PMD was 1.60 ps/√km, and that the PMD could be estimated accurately. Example 4 3000 m of an optical fiber was prepared, which was the same type as that used in Example 1, and had LB of 30 m in a state where birefringence was not induced by an external force, and this optical fiber was wound around a bobbin having a diameter of 300 mm at a tension of 20 gf. After winding the optical fiber around the bobbin, when LB was measured by the P-OTDR, LB was 30 m. When the optical fiber was wound around the same bobbin at a tension of 70 gf, LB was 16 m, which was shorter than that for in a state where birefringence was not induced by an external force. When the same optical fiber cable as in Example 1 was formed by this optical fiber, Lc was 85 m. Therefore, it could be estimated that the PMD of the optical fiber after optical cabling was 0.05 ps/√km, when the optical fiber was wound around the bobbin at a tension of 20 gf. Moreover, it could be estimated that the PMD of the optical fiber after optical cabling was 0.1 ps/√km, when the optical fiber was wound around the bobbin at a tension of 70 gf. The PMD after forming cables, respectively, by these two optical fibers were 0.05 ps/√km in both cases, and the PMD could be accurately estimated from the beat length when the optical fiber was wound around the bobbin at a tension of 20 gf. However, the PMD could not be estimated from the beat length when the optical fiber was wound around the bobbin at a tension of 70 gf. Example 5 In 40 Gb/s transmission, it is preferable that the PMD of an optical fiber of an optical fiber cable be equal to or less than 0.1 ps/√km. When the same type of optical fiber cable as in Example 1 is formed by using the same type of optical fiber as in Example 1, since Lc of the optical fiber cable is about 85 m, then according to Equation (3), if LB at the time of optical cabling is longer than 15 m, a PMD equal to or less than 0.1 ps/√km can be achieved. An optical fiber of the same type as that mentioned in Example 1 and having a length of 3000 m was prepared. Then when this optical fiber was wound around a bobbin having a diameter of 300 mm at a tension of 20 gf, and LB was measured by using the P-OTDR, it was found that LB was 20 m. Next, when this optical fiber was wound around a bobbin having a diameter of 150 mm at a tension of 70 gf, and LB was measured by using the P-OTDR, it was found that LB was 10 m. Moreover, when the optical fiber was used to form optical fiber cables, the PMD was 0.06 ps/√km for both cases, which was less than 0.1 ps/√km. In other words, when the optical fiber was wound around the bobbin having a diameter of 300 mm at a tension of 20 gf, the magnitude of the birefringence induced by winding the optical fiber around the bobbin was smaller than the magnitude of the internal birefringence allowed in the standard for the optical fiber. Therefore it could be confirmed that the PMD standard could be satisfied after optical cabling, in the state where the optical fiber was wound around the bobbin. However, when the optical fiber was wound around the bobbin having a diameter of 150 mm at a tension of 70 gf, it could not be confirmed that the PMD standard could be satisfied after optical cabling, in the state where the optical fiber was wound around the bobbin. Example 6 If the same type of optical fiber cable as in Example 1 is formed by using the same type of optical fiber as in Example 1, since Lc of the optical fiber cable is about 85 m, then according to Equation (3), it is found that if LB at the time of winding the optical fiber around the bobbin is longer than 15 m, a PMD equal to or less than 0.1 ps/√km can be achieved, at the time of optical cabling. Here, according to Equations (1) and (6), the radius ofthe bobbin making LB 15 mis about 0.07 m for a wavelength of 1.55 µm. To calculate this value, as values for the wavelength of 1.55 µm, n = 1.444, p11 = 0.1486, p12 = 0.2959, and v = 0.186 were used. These values are those of synthetic quartz for the wavelength of 1.55 \|xm, and since members constituting the optical fiber are mainly synthetic quartz, these values can be used. Moreover, it is assumed that r = 62.5 x 10-6 m, and the influence of a fiber coating is not taken into consideration. Accordingly, when the optical fiber was wound around a bobbin having a radius of 0.07 m or larger, it could be confirmed that the PMD of the optical fiber after optical cabling became equal to or less than 0.1 ps/√km, if LB at that time was equal to or longer than 15 m. On the other hand, when the optical fiber was wound around a bobbin having a radius smaller than 0.07 m, LB became shorter than 15 m under the influence of birefringence induced by bending. Therefore, in the state where the optical fiber was wound around the bobbin, it could not be confirmed whether the PMD after optical cabling became 0.1 ps/√km or less. An optical fiber of the same type as that mentioned in Example 1 and having a length of 3000 m was prepared. Then when this optical fiber was wound around a bobbin having a diameter of 300 mm at a tension of 20 gf, and LB was measured by using the P-OTDR, it was found that LB was 20 m. Next, when this optical fiber was wound around a bobbin having a diameter of 100 mm at a tension of 20 gf, and LB was measured by using the P-OTDR, it was found that LB was 7 m. Moreover, when the optical fiber was used to form optical fiber cables, the PMD was 0.06 ps/√km in both cases, which was less than 0.1 ps/√km. In other words, when the optical fiber was wound around the bobbin having a diameter of 300 mm, it could be confirmed that the PMD standard could be satisfied after optical cabling, in the state where the optical fiber was wound around the bobbin, but it could not be confirmed, when the optical fiber was wound around the bobbin having a diameter of 100 mm. An optical fiber of the same type as that mentioned in Example 1 and having a length of 10000 m was prepared. Then when this optical fiber was wound around a bobbin having a diameter of 300 mm at a tension of 100 gf, and LB was measured by using the P-OTDR, it was found that LB was 10 m. The bobbin around which the optical fiber was wound had a structure capable of reducing the tension. Therefore, when the tension was temporarily reduced, to perform the same measurement, it was found that LB was 25 m in a section of from 0 m to 9000 m. However LB was 10 m in a section of from 9000 m to 10000 m. Moreover, when the optical fiber was cut into ten pieces of 1000 m, and bundled to form an optical fiber cable of ten cores, the PMD of the coated optical fiber formed by using the section of 0 m to 9000 m was within the range of from 0.03 to 0.06 ps/√km, but in the core line using the section of 9000 to 10000 m, the PMD exceeded 0.1 ps/√km. In other words, by having a structure capable of temporarily reducing the tension, it could be confirmed whether the PMD standard was satisfied after optical cabling, but when this structure was not used, confirmation was not possible. After the measurement, since the tension was returned to that before the measurement, there was no problem in feeding the optical fiber in the subsequent manufacturing process. Moreover, by using the P-OTDR, the PMD in the longitudinal direction of the optical fiber could be measured. Example 7 The present inventor investigated the beat length required for achieving a PMD equal to or less than 0.1 ps/√km as the whole transmission path, and the beat length required for achieving a PMD equal to or less than 0.1 ps/√km in an individual optical fiber cable, by using combinations of optical fibers and optical fiber cables having various structures. As a result, when optical fibers having a beat length of 15 m or longer after optical cabling were coupled, a PMD equal to or less than 0.1 ps/√km could be achieved as the whole transmission path. Moreover, with regard to an optical fiber having a beat length of 30 m or longer when used to form an optical fiber cable, the PMD was equal to or less than 0.1 ps/√km in all individual optical fibers. In other words, it is preferred that the beat length of the optical fiber in the state wound around the bobbin be 15 m or longer. Moreover if the beat length of the optical fiber in the state wound around the bobbin is 30 m or longer, the PMD after optical cabling can be made equal to or less than 0.1 ps/√km, regardless of the type of the optical fiber cable. Therefore, optical fibers having various birefringence were produced, and wound around a bobbin having a diameter of 300 mm at a tension of 40 gf in a length of 3000 m. Then the relationship between the beat length in the state wound around the bobbin, and the PMD after the optical fiber was used to form an optical fiber cable was measured. The results are shown in FIG. 4. From FIG. 4, a clear relationship can be understood between the beat length in the state wound around the bobbin, and the PMD after the optical fiber was used to form an optical fiber cable. The mean value of the PMD after optical cabling of the optical fiber having the beat length of 15 m or longer in the state wound around the bobbin was 0.05 ps/√km. All the optical fibers having a beat length of 15 m or longer in the state wound around the bobbin were connected after optical cabling to form one transmission path, and the PMD thereof was measured. The PMD of the transmission path was 0.06 ps/√km, which was smaller than 0.1 ps/√km. Example 8 Of the optical fibers used in Example 7, the mean value of the PMD of the optical fibers having a beat length of 30 m or longer in the state wound around the bobbin after optical cabling, was 0.015 ps/√km, and there was no optical fiber cable having a PMD exceeding 0.1 ps/√km. All the optical fibers having a beat length of 30 m or longer in the state wound around the bobbin were connected after optical cabling to form one transmission path, and the PMD thereof was measured. The PMD of the transmission path was 0.016 ps/√km, which was sufficiently smaller than 0.1 ps/√km. Comparative Example The PMD of the optical fiber used in Example 7 was measured in the state wound around the bobbin, and the relationship between the PMD in the state wound around the bobbin, and the PMD after the optical fiber was used to form an optical fiber cable was measured. The results are shown in FIG. 5. From FIG. 5, a clear relationship cannot be found between the PMD in the state wound around the bobbin, and the PMD after the optical fiber was used to form an optical fiber cable. The mean value of the PMD after optical cabling of the optical fibers having a PMD equal to or less than 0.1 ps/√km in the state wound around the bobbin was 0.09 ps/√km, which was worse than in Example 7, and was not sufficiently smaller than 0.1 ps/√km. All the optical fibers having a PMD equal to or less than 0.1 ps/√km in the state wound around the bobbin were connected after optical cabling to form one transmission path, and the PMD thereof was measured. The PMD of the transmission path was 0.11 ps/√km, which was larger than 0.1 ps/√km FIG. 6 is a cross-sectional view showing an example of the structure of the optical fiber according to present invention. In FIG. 6, reference symbol 1 denotes an optical fiber, reference symbol 2 denotes a core, and reference symbol 3 denotes a cladding. FIG. 7 is a cross-sectional view showing an example of the structure of a coated optical fiber. In FIG. 7, reference symbol 1 denotes an optical fiber, reference symbol 4 denotes a primary coating, and reference symbol 5 denotes a secondary coating. The primary coating 4 and the secondary coating 5 respectively constitute a protective layer for protecting the optical fiber 1. FIG. 8 is a cross-sectional view showing an example of the structure of the optical fiber cable according to the present invention. In FIG. 8, reference symbol 20 denotes an optical fiber cable (loose tube type), reference symbol 10 denotes a coated optical fiber, reference symbol 11 denotes a tension member, reference symbol 12 denotes a loose tube, 13 and 14 denote a jelly, reference symbol 15 denotes a pressed winding, and reference symbol 17 denotes a sheath. Preferable examples of the present invention has been described above, but the present invention is by no means limited to these examples. Addition, omission, replacement, and other modifications of the configuration are possible without departing from the scope of the present invention. The present invention is not limited by the explanation above, but limited only by the scope of the appended claims. INDUSTRIAL APPLICABILITY According to the method of measuring polarization mode dispersion of an optical fiber of the present invention, the PMD of the optical fiber after optical cabling can be estimated in a state where the optical fiber is wound around a transport bobbin. We Claim : 1. A method of measuring polarization mode dispersion of an optical fiber, comprising estimating polarization mode dispersion when an optical fiber is formed as an optical cable, from a beat length when the optical fiber is wound around a bobbin, and an average coupling length when the optical fiber is formed as the optical cable. 2. A method of measuring polarization mode dispersion of an optical fiber, comprising estimating polarization mode dispersion when an optical fiber is put in a free state, from a beat length when the optical fiber is wound around a bobbin, and an average coupling length when the optical fiber is when the optical fiber put in the free state. 3. A method of measuring polarization mode dispersion of an optical fiber as claimed in claim 1 or 2, comprising measuring Rayleigh scattered light intensity of the optical fiber by using a P-OTDR. 4. A method of measuring polarization mode dispersion of an optical fiber as claimed in claim 3, comprising setting a resolution of the P-OTDR shorter than a shortest beat length estimated for the optical fiber wound around the bobbin. 5. A method of measuring polarization mode dispersion of an optical fiber as claimed in claim 1 or 2, comprising setting a radius R of the bobbin, and a tension at the time of winding an optical fiber around the bobbin, so that a magnitude of birefringence of the optical fiber induced due to winding of the optical fiber around the bobbin becomes smaller than a magnitude of internal birefringence inherently held by the optical fiber, and then winding the optical fiber around the bobbin. 6. A method of measuring polarization mode dispersion of an optical fiber as claimed in claim 1 or 2, comprising setting a radius R of the bobbin, and a tension at the time of winding an optical fiber around the bobbin, so that a magnitude of birefringence of the optical fiber induced due to winding of the optical fiber around the bobbin becomes smaller than a magnitude of internal birefringence allowed in a standard for the optical fiber, and then winding the optical fiber around the bobbin. 7. A method of measuring polarization mode dispersion of an optical fiber as claimed in claim 1 or 2, comprising setting a radius R of the bobbin, so that the radius R of the bobbin, and a maximum value B' of a magnitude of internal birefringence allowed in a standard for the optical fiber satisfy the following Equation: where "n" denotes a refractive index of the optical fiber, p11 and p12 denote Pockels coefficients of the optical fiber, v denotes a Poisson's ratio of the optical fiber, and "r" denotes a radius of the optical fiber. 8. A method of measuring polarization mode dispersion of an optical fiber as claimed in claim 1 or 2, comprising measuring the beat length in a state where a tension applied to the optical fiber is temporarily reduced. 9. A method of measuring polarization mode dispersion of an optical fiber as claimed in claim 1 or 2, wherein the bobbin has a structure capable of temporarily reducing a tension on the optical fiber. 10. A method of measuring polarization mode dispersion of an optical fiber as claimed in claim 1 or 2, comprising measuring a distribution of polarization mode dispersion in a longitudinal direction. 11. An optical fiber comprising a core and a cladding arranged around the core, wherein the PMD measured by using the method as claimed in claim 1 or 2 is equal to or less than 0.1 ps/√km. 12. An optical fiber as claimed in claim 11, wherein a beat length in a state wound around the bobbin is equal to or longer than 15 m. 13. An optical fiber as claimed in claim 11, wherein a beat length in a state wound around the bobbin is equal to or longer than 30 m. 14. An optical fiber as claimed in claim 11, wherein a beat length in a state in which a tension due to the bobbin is reduced after wound around the bobbin, is equal to or longer than 15 m. 15. An optical fiber as claimed in claim 11, wherein a beat length in a state in which a tension due to the bobbin is reduced after wound around the bobbin, is equal to or longer than 30 m. 16. An optical fiber cable in which a plurality of coated optical fibers having a protective layer around an optical fiber as claimed in claims 11-15, are arranged side by side, and the plurality of coated optical fibers are housed in a sheath. A

Full Text

The magnitude of local birefringence of the optical fiber can be quantified by
using a beat length (hereinafter, abbreviated as "LB")- LB indicates a propagation
distance over which optional polarization entering into the optical fiber returns to the
incident polarization.
Another parameter expressing the local birefringence of the optical fiber is a
mode birefringence rate B. The relationship expressed by the following Equation (1) is
established between the mode birefringence rate B and LB.

In the above Equation (1), X indicates the wavelength of light.
When the length of the optical fiber is short, it can be considered that there is no
polarization mode coupling; therefore, the PMD is expressed by the following Equation
(2) as a function between light velocity C and the length L of the optical fiber.

From the above Equation (2), it is understood that the PMD increases in
proportion to the length L of the optical fiber.
On the other hand, when the length of the optical fiber is long, the PMD is
expressed by the following Equation (3).

From the above Equation (3), it is understood that the PMD increases in
proportion to the square root of the length L of the optical fiber.
In the above Equation (3), Lc is referred to as an average coupling length, which
is a parameter expressing the magnitude of the polarization mode coupling, and as the
polarization mode coupling increases, the magnitude of the average coupling length
decreases. The magnitude of the polarization mode coupling is mainly determined by the
distortion of the optical fiber, a force applied from the outside, and the like.
When the length L of the optical fiber is shorter than Lc, the PMD can be
expressed by using Equation (2). On the other hand, when the length L of the optical
fiber is longer than Lc, the PMD can be expressed by using Equation (3).
It is understood from Equations (2) and (3) that the PMD increases as LB becomes
shorter, and Lc becomes longer.
Generally, the optical fiber is transferred to an optical cabling process in a state
wound around a bobbin, or is shipped and transported just as an optical fiber unit.
Therefore, it is preferable that the PMD can be measured in the state with the optical
fiber wound around the bobbin.
However, by winding the optical fiber around the bobbin, disturbance such as
bending, lateral pressure, distortion and the like occurs in the optical fiber, thereby
changing LB and Lc, and hence the PMD varies. Therefore, in the same optical fiber, the
PMD of the optical fiber wound around the bobbin for transport, and the PMD of the
optical fiber after optical cabling, show quite different values (for example, see Non-
patent Document 1). As a result, the PMD of the optical fiber after optical cabling
increases, and in some cases, exceeds an upper limit of the PMD specified by the
standard, thereby causing a problem.

Moreover, the optical fiber is shipped to the optical cabling process in a length of
about 20 to 100 km, and at the time of optical cabling, the length of the optical fiber is
cut to 1 to 10 km. Therefore, if there is a place having locally large PMD, even if the
PMD of the whole length of the optical fiber before optical cabling is small, when the
optical fiber is cut and formed as a cable, there may be a place where there is large PMD,
which has been considered to be a problem.
Non-patent Document 1: Scott Grindstaff, Joseph Hill, Omid Daneshvar, "Extrinsic
Stress Effects on Polarization Mode Dispersion in Optical Fiber Cables", International
Wire & Cable Symposium Proceedings, 1993, pp. 647 to 654.
DISCLOSURE OF INVENTION
The present invention takes into consideration the above situation, and an object
thereof is to provide a method of measuring polarization mode dispersion of an optical
fiber, which can estimate the PMD of the optical fiber after optical cabling, in a state
with the optical fiber wound around the bobbin for transport, to provide an optical fiber,
and to provide an optical fiber cable.
In order to achieve the above object, the present invention provides a method of
measuring polarization mode dispersion of an optical fiber, including estimating
polarization mode dispersion when an optical fiber is formed as an optical cable, from a
beat length when the optical fiber is wound around a bobbin, and an average coupling
length when the optical fiber is formed as the optical cable.
In the abovementioned method of measuring polarization mode dispersion of an
optical fiber, preferably the Rayleigh scattered light intensity of the optical fiber is
measured by using a P-OTDR.

TECHNICAL FIELD
The present invention relates to an optical fiber and a method of measuring
polarization mode dispersion of an optical fiber.
Priority is claimed on Japanese Patent Application No. 2003-361812, filed
October 22, 2003, the content of which is incorporated herein by reference.
BACKGROUND ART
Recently, with progress in speeding up of transmission speed, and lengthening of
transmission distance in optical communication, reduction of polarization mode
dispersion (hereinafter, abbreviated as "PMD") is required for the optical fiber, being the
transmission path.
The PMD of the optical fiber is mode dispersion caused by an occurrence of a
group velocity difference in two orthogonal intrinsic polarization components, which
propagate in the optical fiber, due to non-circularity of the core shape of the optical fiber,
asymmetry of stress occurring in the core, etc.
There are two parameters for determining the PMD. One parameter is the
magnitude of local birefringence of the optical fiber, and the other is polarization mode
coupling expressing how the direction of a birefringent axis of the optical fiber changes
in the longitudinal direction of the optical fiber.

In the abovementioned method of measuring polarization mode dispersion of an
optical fiber, preferably the resolution of the P-OTDR is made shorter than the shortest
beat length estimated for the optical fiber wound around the bobbin.
In the abovementioned method of measuring polarization mode dispersion of an
optical fiber, preferably the optical fiber is wound around the bobbin by setting the radius
R of the bobbin and a tension at the time of winding the optical fiber around the bobbin,
so that the magnitude of birefringence of the optical fiber induced due to winding of the
optical fiber around the bobbin becomes smaller than the magnitude of internal
birefringence inherently held by the optical fiber.
In the abovementioned method of measuring polarization mode dispersion of an
optical fiber, preferably the optical fiber is wound around the bobbin by setting the radius
R of the bobbin and a tension at the time of winding the optical fiber around the bobbin,
so that the magnitude of birefringence of the optical fiber induced due to winding of the
optical fiber around the bobbin becomes smaller than the magnitude of internal
birefringence allowed in the standard for the optical fiber.
In the abovementioned method of measuring polarization mode dispersion of an
optical fiber, preferably the optical fiber is wound around the bobbin by setting the radius
R of the bobbin, so that the radius R of the bobbin and the magnitude B of internal
birefringence allowed in the standard for the optical fiber satisfy the following Equation
(4).

(where "n" denotes a refractive index of a glass material (normally, silica glass)
constituting the optical fiber, p11 and p12 denote Pockels coefficients of the glass material

constituting the optical fiber, v denotes a Poisson's ratio of the glass material constituting
the optical fiber, and "r" denotes a radius of a glass portion of the optical fiber.)
In the abovementioned method of measuring polarization mode dispersion of an
optical fiber, at the time of measurement, preferably the measurement of the beat length
is performed in a state where the tension applied to the optical fiber is temporarily
reduced, because the influence of birefringence due to the lateral pressure resulting from
the winding tension, and the influence of the PMD can be removed at the time of
measurement. Moreover, after measurement, by returning the tension to the original
tension before the measurement, a problem such as that the feed of the optical fiber
becomes difficult, can be prevented in the subsequent manufacturing process, which is
preferable.
In the abovementioned method of measuring polarization mode dispersion of an
optical fiber, preferably the bobbin has a structure capable of temporarily reducing the
winding tension on the optical fiber.
In the abovementioned method of measuring polarization mode dispersion of an
optical fiber, it is desirable to measure the PMD in the longitudinal direction of the
optical fiber.
The present invention further provides an optical fiber including a core and a
cladding arranged around the core, wherein the PMD measured by using the above
method is equal to or less than 0.1 ps/√km.
In the abovementioned optical fiber, preferably a beat length in a state wound
around the bobbin is equal to or longer than 15 m, and more preferably, is equal to or
longer than 30 m.
In the abovementioned optical fiber, preferably a beat length in a state in which a
tension due to the bobbin is reduced after wound around the bobbin, is equal to or longer
than 15 m, and more preferably, is equal to or longer than 30 m.

The present invention further provides an optical fiber cable in which a plurality
of coated optical fibers having a protective layer around the optical fiber are arranged
side by side, and the plurality of coated optical fibers are housed in a sheath.
ADVANTAGEOUS EFFECTS OF THE INVENTION
According to the method of measuring polarization mode dispersion of an optical
fiber of the present invention, the PMD of the optical fiber after optical cabling can be
estimated in a state with the optical fiber wound around the bobbin.
According to the method of measuring polarization mode dispersion of an optical
fiber of the present invention, it can be determined whether the PMD of the optical fiber
after optical cabling is within the standard, in a state with the optical fiber wound around
the bobbin.
According to the method of measuring polarization mode dispersion of an optical
fiber of the present invention, since it is not necessary to prepare the optical fiber for
measurement in a free state, the optical fiber can be used effectively.
According to the method of measuring polarization mode dispersion of an optical
fiber of the present invention, since the polarization mode dispersion of the shipped
optical fiber itself can be measured, without using the measurement result of the
polarization mode dispersion of the optical fiber in the vicinity thereof, an optical fiber
having higher quality can be provided.
According to the method of measuring polarization mode dispersion of an optical
fiber of the present invention, since the PMD value of the optical fiber in the longitudinal
direction can be measured, a place having large PMD locally can be found and removed.
As a result, an optical fiber having higher quality can be provided.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIG. 1 is a graph showing the results of calculating the magnitude of average
birefringence, at the time of applying a force from outside to two optical fibers having
different internal birefringence, to induce birefringence of a different magnitude.
FIG. 2 is a graph showing calculation results of PMD of an optical fiber.
FIG. 3 is a graph showing an example of a waveform of Rayleigh scattered light
intensity actually measured by using a P-OTDR.
FIG. 4 is a graph showing the relationship between the beat length of an optical
fiber in a state with the optical fiber wound around a bobbin, and the PMD of the optical
fiber after an optical fiber cable is formed by the optical fiber.
FIG. 5 is a graph showing the relationship between the PMD of the optical fiber
in a state with the optical fiber wound around a bobbin, and the PMD of the optical fiber
after the optical fiber cable is formed by the optical fiber.
FIG. 6 is a cross-sectional view showing an example of the structure of an optical
fiber.
FIG. 7 is a cross-sectional view showing an example of the structure of a coated
optical fiber.
FIG. 8 is a cross-sectional view showing an example of the structure of an optical
fiber cable.
DESCRIPTION OF THE REFERENCE SYMBOLS
1 ... optical fiber, 2 ... core, 3 ... cladding, 4 ... primary coating (protective layer), 5 ...
secondary coating (protective layer), 10 ...coated optical fiber, 11 ... tension member, 12
... loose tube, 13 ...jelly, 15 ... pressed winding, 16 ... tearing string, 17 ... sheath, 20
... optical fiber cable

BEST MODE FOR CARRYING OUT THE INVENTION
The method of measuring polarization mode dispersion of an optical fiber
according to the present invention will be described hereunder in detail.
At first, an individual study is carried out as to how the beat length LB and the
average coupling length Lc change, by winding the optical fiber around a bobbin.
LB changes when birefringence is induced from outside, substantially in the radial
direction of the bobbin, due to the influence of the bending radius, tension, or lateral
pressure at the time of winding the optical fiber around the bobbin.
Lc changes due to torsion of the optical fiber, or contact of optical fibers with
each other, at the time of winding the optical fiber around the bobbin.
Here, the change of LB after the optical fiber is wound around the bobbin is
studied from the relationship between the internal birefringence of the optical fiber, and
the birefringence of the optical fiber induced from outside.
The direction of birefringence induced from outside by winding the optical fiber
around the bobbin, is substantially in the radial direction of the bobbin, while the angle of
the birefringent axis inside the optical fiber can take any angle.
Therefore, the average birefringence of the optical fiber when a lateral pressure is
applied, can be considered to be an average when the lateral pressure is applied from
various angles.
FIG. 1 is a graph showing the results of calculating the magnitude of the average
birefringence, at the time of applying a force from outside to two optical fibers having
different internal birefringence, to induce birefringence of a different magnitude.
From the results shown in FIG. 1, it is understood that when the magnitude of
birefringence induced from outside is smaller than the magnitude of internal

birefringence, the magnitude of the average birefringence hardly changes. On the other
hand, when the magnitude of birefringence induced from outside is larger than that of the
internal birefringence, the magnitude of the average birefringence becomes substantially
equal to that of the magnitude of the birefringence induced from outside, and finally, the
magnitude of the average birefringence becomes larger than that of the magnitude of the
birefringence induced from outside. However, the magnitude of the average
birefringence after birefringence is induced from outside due to lateral pressure does not
become smaller than that of the internal birefringence. Therefore, by winding the optical
fiber around the bobbin, the average LB becomes shorter inevitably, which causes an
increase in the PMD.
On the other hand, Lc after the optical fiber is wound around the bobbin becomes
very short, due to torsion in the optical fiber or due to contact of the optical fibers with
each other, when the optical fiber is wound around the bobbin. Therefore, a change in Lc
due to winding of the optical fiber around the bobbin causes a decrease in the PMD.
Accordingly, when the influence of the change in Lc is larger than the influence
of the change in LB, the PMD decreases when the optical fiber is wound around the
bobbin, and when the optical fiber is formed as an optical fiber cable and put in a free
state, the PMD increases, thereby causing a problem.
A calculation example of the PMD is shown below.
For example, when the length of the optical fiber is 1000 m, and this optical fiber
is divided into fine sections of 0.1 mm, it is assumed that in the fine sections, the optical
fiber has only linear birefringence. A change in the PMD of the optical fiber is
calculated by simulation, using the Jones matrix method (refer to IEEE Photonics
Technology Letters, Sept. 1992, vol. 4, No. 9, pp. 1066 to 1069).

In a free state, when optical fibers having LB of 10 m and 20 m and Lc of 30 m
are wound around the bobbin, Lc decreases, and birefringence is induced in the radial
direction of the bobbin, and from this, how the PMD of the optical fiber changes is
calculated.
Both optical fibers are set so that when the optical fibers are wound around the
bobbin, Lc becomes 3 m. The average coupling length used for the calculation is a
typical value for when the optical fiber is put in a free state, and for when the optical
fiber is wound around the bobbin. The calculation results of the PMD of the optical fiber
are shown in FIG. 2.
From the results shown in FIG. 2, it is understood that when the optical fiber is
wound around the bobbin, and when the magnitude of birefringence induced from
outside is small, the PMD of the optical fiber wound around the bobbin becomes smaller
than that of the optical fiber in the free state.
It is also understood that when LB of the optical fibers in the free state is different,
the magnitude of the change in the PMD with respect to the magnitude of birefringence
induced from outside becomes different. Therefore, when the optical fiber is put in the
free state, though the PMD is different, the PMD of both optical fibers may take the same
value, according to the magnitude of birefringence induced from outside. If
birefringence larger than that is induced, the magnitude correlation of the PMD of the
two optical fibers is reversed to that of optical fibers in the free state. That is, the PMD
of the optical fiber in the free state cannot be estimated only by measuring the PMD of
the optical fiber wound around the bobbin.
Here, what the inventors want to know is the magnitude of the PMD of the optical
fiber after optical cabling. Since the optical fiber cable has such a structure that an
external force applied to the optical fiber is made as small as possible, the PMD of the

optical fiber after optical cabling is substantially the same as that of the optical fiber in
the free state.
Therefore, the PMD of the optical fiber after optical cabling cannot be estimated
only by measuring the PMD of the optical fiber wound around the bobbin.
However, it has been found that if LB of the optical fiber wound around the
bobbin is obtained, LB of the optical fiber after optical cabling does not become shorter
than that. Therefore, by obtaining Lc of the optical fiber after optical cabling, the PMD
of the optical fiber after optical cabling can be estimated by using the following Equation
(2).

Lc of the optical fiber cable is determined mainly by external factors such as the
structure of the optical fiber cable, and the material and the surface properties of the resin
used for the optical fiber and the optical fiber cable. Therefore, by measuring Lc
beforehand by using the same type of optical fiber as in other optical fiber cables formed
from a resin of the same material, having the same structure, and the same surface
properties, Lc of the optical fiber after optical cabling can be obtained, without using the
optical fiber for measurement of Lc separately.
Next a specific example of the measurement method of Lc of the optical fiber
after optical cabling will be described below.
At first, the PMD of the optical fiber for the whole length of the optical fiber
cable is measured, and then, a section of the optical fiber of about 5 m is taken out and
the PMD is measured.

In a short optical fiber cable of about 5 m, it can be considered that there is no
polarization mode coupling, and hence LB can be obtained by using Equation (2).
Lc can be determined by using the following Equation (3), from LB obtained here,
and the PMD of the optical fiber for the whole length of the optical fiber cable.

Accordingly, from the above Equations (2) and (3), the PMD after optical cabling
can be estimated from LB of the optical fiber wound around the bobbin, and the
previously calculated Lc.
Next, a method of estimating the PMD by using a P-OTDR (Polarization Optical
Time Domain Reflectometry) after an optical fiber cable is formed by using an optical
fiber to be measured, will be explained.
In this method, by using the P-OTDR, LB and Lc can be independently measured
with the optical fiber to be measured wound around the bobbin. Specifically, methods
described in Reference document A (F. Corsi, A Galtarossa, and L. Palmieri,
"Polarization Mode Dispersion Characterization of Single-Mode Optical Fiber Using
Backscattering Technique", Journal of Lightwave Technology, Vol. 16, No. 10, Oct.
1998, pp. 1832 to 1843), and Reference document B (M. Wuilpart, G. Ravet, P. Megret,
and M. Blondel, "PMD measurement with a polarization-OTDR", ECOC 2002) can be
used.
FIG. 3 is a graph showing an example of a waveform of Rayleigh scattered light
intensity actually measured by using the P-OTDR.
According to the Reference documents A and B, as one calculation method of LB,
when there are N extreme values of the waveform in a section length L, from which the

waveform of the Rayleigh scattered light intensity measured by using the P-OTDR is
obtained, there is a method of calculating LB by using the following Equation (5).

In the case of an optical fiber having a waveform as shown in FIG. 3, since there
are 19 extreme values in a 100 m section, it is understood that LB is 21 m. Thus, by
measuring LB of the optical fiber wound around the bobbin by using the P-OTDR, and
separately measuring Lc of the optical fiber cable in the free state, the PMD of the optical
fiber formed as an optical fiber cable can be estimated.
Here, as the method for calculating LB, the above Equation (5) is used. However,
the calculation method of LB is not limited thereto, and other methods may be used.
In the Reference documents A and B, LB and Lc of the optical fiber to be
measured in the free state are measured by using the P-OTDR. On the other hand, in the
present invention, the calculation method is different from the conventional method in
that LB is measured by winding the optical fiber around the bobbin, to estimate the PMD
of the optical fiber after optical cabling.
Next the relationship between LB of the optical fiber to be measured, and the
resolution of the OTDR used for the measurement of the PMD will be described.
In a single-mode fiber excluding a polarization maintaining fiber, since LB is
normally 10 cm or longer, if the resolution of the OTDR is shorter than 10 cm, LB and Lc
can be measured independently in any single-mode fiber.
In such an application, resolution equal to or shorter than 1 cm can be obtained,
for example, by using a technique referred to as photon counting OTDR. Therefore, it
can be applied to any single-mode fiber other than the polarization maintaining fiber.

The measurement method of the PMD using the P-OTDR can also be applied to a
multi-mode fiber, by using a device which selectively excites and receives only a mode
to be measured.
Next an optical fiber in which the PMD after the optical fiber is formed as an
optical fiber cable, can be estimated in a state with the optical fiber wound around the
bobbin, will be described.
When the magnitude of birefringence of the optical fiber induced due to winding
of the optical fiber around the bobbin is smaller than that of internal birefringence, then
as described above, the magnitude of the birefringence of the optical fiber wound around
the bobbin is substantially the same as the magnitude of the birefringence of the optical
fiber after optical cabling.
Therefore, LB of the optical fiber measured in the state wound around the bobbin
is substantially equal to LB of the optical fiber after optical cabling. Accordingly, an
optical fiber can be provided, in which the PMD of the optical fiber after optical cabling
can be estimated by measuring Lc of the optical fiber cable beforehand using the same
type of optical fiber.
Lc after cabling is mainly determined by the internal birefringence of the optical
fiber, and the external force induced due to the structure of the optical fiber cable. In
many cases, there is no problem in considering that Lc is constant regardless of LB.
However, when LB is short, that is, when the internal birefringence of the optical fiber is
large, then even if an external force is applied to the optical fiber, polarization mode
coupling is hardly induced. Hence there is a case in which Lc cannot be regarded as
constant. Even in this case, by obtaining Lc as a function of LB, the present method can
be applied.

Japanese Unexamined Patent Application, First Publication No. H11-208998
discloses a method of winding an optical fiber so that the PMD of the optical fiber wound
around the bobbin matches with the PMD of the optical fiber in the free state. As
described above, however, the way of change of the PMD of the optical fiber wound
around the bobbin changes due to the internal birefringence of the optical fiber in the free
state. Therefore, in this method, the winding method cannot be determined unless the
internal birefringence of the optical fiber in the free state is known beforehand.
Therefore, when LB is different for each optical fiber, that is, in the actual
manufacturing process, the effect thereof is smaller than that obtained by the present
invention.
Next an optical fiber in which it can be determined whether the PMD standard of
the optical fiber is satisfied after optical cabling, in a state wound around the bobbin, will
be described.
If it is found that the magnitude of the birefringence induced by winding the
optical fiber around the bobbin is smaller than the internal birefringence allowed in the
standard for the optical fiber, it is understood that the birefringence after optical cabling
is smaller than the internal birefringence allowed in the standard for the optical fiber.
Therefore, by measuring the magnitude of the induced birefringence, and the
magnitude of the internal birefringence allowed in the standard for the optical fiber, in a
state with the optical fiber wound around the bobbin, it can be determined whether the
optical fiber satisfies the PMD standard after optical cabling.
The maximum value of the internal birefringence allowed in the standard for the
optical fiber can be calculated from a predetermined upper limit of the PMD, and the
average coupling length of the optical fiber cable, by using the above Equations (1) and
(3).

Next the radius of a bobbin, which can determine whether the optical fiber
satisfies the PMD standard of the optical fiber in an optical fiber cable after optical
cabling, in a state with the optical fiber wound around the bobbin will be described here.
According to Reference document C (R. Ulrich, S. C. Rashleigh, and W. Eickhoff,
"Bending-induced birefringence in single-mode fibers", Optics Letters, Vol. 5, No. 6,
June 1980, pp. 273 to 275), birefringence B induced by bending at the time of winding
the optical fiber around the bobbin is expressed by the following Equation (6), from a
refractive index "n" of the optical fiber, Pockels coefficients p11 and p12, Poisson's ratio v,
a radius "r" of the optical fiber, and a radius R of the bobbin.

From the above Equation (6), if the magnitude B of the birefringence induced by
bending of the optical fiber is smaller than the magnitude of the internal birefringence
allowed in the standard for the optical fiber, it can be determined whether the optical
fiber satisfies the PMD standard after optical cabling, in the measurement with the
optical fiber wound around the bobbin.
Accordingly, it is preferable that the radius R of the bobbin satisfies the
relationship in the following Equation (4).

Here, B' denotes the maximum value of the magnitude of the internal
birefringence allowed in the standard for the optical fiber, and can be calculated from the

predetermined upper limit of the PMD, and the average coupling length of the optical
fiber cable, by using the above Equations (1) and (3).
Here, if the bobbin has a structure capable of temporarily reducing the tension
applied to the optical fiber, then this is preferable because the influence of the
birefringence generated inside of the optical fiber due to the tension can be removed.
Here it is preferable to estimate the distribution of PMD in the longitudinal
direction of the optical fiber.
Prior to the present invention, there was no measurement method for performing
measurement of the beat length, without cutting the optical fiber. Accordingly, a fiber in
which the PMD after optical cabling can be known before the optical cabling process,
can be provided only by using the method of the present invention.
Moreover, in the present invention, since the beat length can be measured in the
longitudinal direction of the optical fiber, then even if there is a place where the beat
length is short partially, the place can be determined. Then, by using the method of the
present invention, and first determining the place where the beat length of the optical
fiber is short partially, the cause of shortage can be specified and the manufacturing
process can be improved. As a result, even after the optical fiber is divided in the optical
cabling process, an optical fiber which can obtain a favorable PMD in the whole optical
fiber cable can be provided.
In 40 Gb/s transmission, preferably the PMD of the optical fiber is equal to or less
than 0.1 ps/√km. Moreover, since the transmission path is formed of a plurality of
optical fibers, it is required at least as the whole transmission path, that the PMD be equal
to or less than 0.1 ps/√km. The present inventor has executed investigations by using
combinations of optical fibers and optical fiber cables having various structures, to find
that when optical fibers having a beat length of 15 m or longer after optical cabling are

connected to form one transmission path, the PMD can be set to 0.1 ps/√km or less as the
whole transmission path. For optical fibers having a beat length of 30 m or longer after
optical cabling, it has been found that the PMD is 0.1 ps/√km or less in all individual
optical fibers. Accordingly, it is preferred that the beat length measured by using the
method of the present invention is equal to or longer than 15 m, and more preferably,
equal to or longer than 30 m.
Hereunder the present invention will be described more specifically by way of
examples, but the present invention is by no means limited to the examples below.
Example 1
An optical fiber to be measured before cabling, was wound in a length of 3000 m
around a bobbin having a diameter of 300 mm at a tension of 20 gf. When the magnitude
of the internal birefringence of the optical fiber to be measured was measured, LB was 30
m.
On the other hand, when an optical fiber cable having a length of 3000 m was
formed by using the same type of optical fiber, separately from the optical fiber to be
measured, the PMD of the optical fiber constituting the optical fiber cable was 0.05
ps/√km, and LB was 30 m. Consequently, it could be estimated that Lc after forming an
optical fiber cable by using the optical fiber to be measured was about 85 m, from the
above Equation (3).
From the above result, it could be estimated that the PMD after forming a cable
by the optical fiber to be measured was 0.05 ps/√km. When the optical fiber to be
measured was actually used to form an optical fiber cable, the PMD was 0.05 ps/√km.
Example 2

An optical fiber to be measured before cabling was wound in a length of 3000 m
around a bobbin having a diameter of 300 mm at a tension of 20 gf. When LB was
measured by a measurement using the P-OTDR, LB was 25 m.
On the other hand, when an optical fiber cable having a length of 3000 m was
formed by using the same type of optical fiber, separately from the optical fiber to be
measured, the PMD of the optical fiber constituting the optical fiber cable was 0.07
ps/√km, and LB was 20 m. Consequently, it could be found that Lc after forming an
optical fiber cable by using the optical fiber to be measured was about 73 m, from the
above Equation (3).
From the above result, it could be estimated that the PMD after forming a cable
by the optical fiber to be measured was 0.06 ps/√km. When the optical fiber to be
measured was actually used to form an optical fiber cable, the PMD was 0.06 ps/√km.
Example 3
An optical fiber to be measured was wound in a length of 3000 m around a
bobbin having a diameter of 300 mm at a tension of 20 gf. When the P-OTDR
measurement was carried out by using an OTDR having a resolution of 2 m, the
waveform as shown in FIG. 3 required for measuring LB could not be observed.
On the other hand, when the P-OTDR measurement was carried out by using an
OTDR having a resolution of 1 cm, the waveform as shown in FIG. 3 could be observed,
and it was found that LB was 1.2 m.
On the other hand, when an optical fiber cable having a length of 3000 m was
formed by using the same type of optical fiber, separately from the optical fiber to be
measured, the PMD was 1.0 ps/√km, and LB was 2.0 m. Consequently, it could be found
that Lc after forming an optical fiber cable by using the optical fiber to be measured was
about 150 m, from the above Equation (3).

From the above result, it could be estimated that the PMD after forming a cable
by the optical fiber to be measured was 1.67 ps/√km. When the optical fiber to be
measured was actually used to form an optical fiber cable, it was found that the PMD
was 1.60 ps/√km, and that the PMD could be estimated accurately.
Example 4
3000 m of an optical fiber was prepared, which was the same type as that used in
Example 1, and had LB of 30 m in a state where birefringence was not induced by an
external force, and this optical fiber was wound around a bobbin having a diameter of
300 mm at a tension of 20 gf.
After winding the optical fiber around the bobbin, when LB was measured by the
P-OTDR, LB was 30 m. When the optical fiber was wound around the same bobbin at a
tension of 70 gf, LB was 16 m, which was shorter than that for in a state where
birefringence was not induced by an external force.
When the same optical fiber cable as in Example 1 was formed by this optical
fiber, Lc was 85 m. Therefore, it could be estimated that the PMD of the optical fiber
after optical cabling was 0.05 ps/√km, when the optical fiber was wound around the
bobbin at a tension of 20 gf. Moreover, it could be estimated that the PMD of the optical
fiber after optical cabling was 0.1 ps/√km, when the optical fiber was wound around the
bobbin at a tension of 70 gf.
The PMD after forming cables, respectively, by these two optical fibers were 0.05
ps/√km in both cases, and the PMD could be accurately estimated from the beat length
when the optical fiber was wound around the bobbin at a tension of 20 gf. However, the
PMD could not be estimated from the beat length when the optical fiber was wound
around the bobbin at a tension of 70 gf.

Example 5
In 40 Gb/s transmission, it is preferable that the PMD of an optical fiber of an
optical fiber cable be equal to or less than 0.1 ps/√km. When the same type of optical
fiber cable as in Example 1 is formed by using the same type of optical fiber as in
Example 1, since Lc of the optical fiber cable is about 85 m, then according to Equation
(3), if LB at the time of optical cabling is longer than 15 m, a PMD equal to or less than
0.1 ps/√km can be achieved.
An optical fiber of the same type as that mentioned in Example 1 and having a
length of 3000 m was prepared. Then when this optical fiber was wound around a
bobbin having a diameter of 300 mm at a tension of 20 gf, and LB was measured by using
the P-OTDR, it was found that LB was 20 m. Next, when this optical fiber was wound
around a bobbin having a diameter of 150 mm at a tension of 70 gf, and LB was
measured by using the P-OTDR, it was found that LB was 10 m.
Moreover, when the optical fiber was used to form optical fiber cables, the PMD
was 0.06 ps/√km for both cases, which was less than 0.1 ps/√km.
In other words, when the optical fiber was wound around the bobbin having a
diameter of 300 mm at a tension of 20 gf, the magnitude of the birefringence induced by
winding the optical fiber around the bobbin was smaller than the magnitude of the
internal birefringence allowed in the standard for the optical fiber. Therefore it could be
confirmed that the PMD standard could be satisfied after optical cabling, in the state
where the optical fiber was wound around the bobbin. However, when the optical fiber
was wound around the bobbin having a diameter of 150 mm at a tension of 70 gf, it could
not be confirmed that the PMD standard could be satisfied after optical cabling, in the
state where the optical fiber was wound around the bobbin.
Example 6

If the same type of optical fiber cable as in Example 1 is formed by using the
same type of optical fiber as in Example 1, since Lc of the optical fiber cable is about 85
m, then according to Equation (3), it is found that if LB at the time of winding the optical
fiber around the bobbin is longer than 15 m, a PMD equal to or less than 0.1 ps/√km can
be achieved, at the time of optical cabling. Here, according to Equations (1) and (6), the
radius ofthe bobbin making LB 15 mis about 0.07 m for a wavelength of 1.55 µm.
To calculate this value, as values for the wavelength of 1.55 µm, n = 1.444, p11 =
0.1486, p12 = 0.2959, and v = 0.186 were used. These values are those of synthetic
quartz for the wavelength of 1.55 |xm, and since members constituting the optical fiber
are mainly synthetic quartz, these values can be used. Moreover, it is assumed that r =
62.5 x 10-6 m, and the influence of a fiber coating is not taken into consideration.
Accordingly, when the optical fiber was wound around a bobbin having a radius
of 0.07 m or larger, it could be confirmed that the PMD of the optical fiber after optical
cabling became equal to or less than 0.1 ps/√km, if LB at that time was equal to or longer
than 15 m.
On the other hand, when the optical fiber was wound around a bobbin having a
radius smaller than 0.07 m, LB became shorter than 15 m under the influence of
birefringence induced by bending. Therefore, in the state where the optical fiber was
wound around the bobbin, it could not be confirmed whether the PMD after optical
cabling became 0.1 ps/√km or less.
An optical fiber of the same type as that mentioned in Example 1 and having a
length of 3000 m was prepared. Then when this optical fiber was wound around a
bobbin having a diameter of 300 mm at a tension of 20 gf, and LB was measured by using
the P-OTDR, it was found that LB was 20 m.

Next, when this optical fiber was wound around a bobbin having a diameter of
100 mm at a tension of 20 gf, and LB was measured by using the P-OTDR, it was found
that LB was 7 m.
Moreover, when the optical fiber was used to form optical fiber cables, the PMD
was 0.06 ps/√km in both cases, which was less than 0.1 ps/√km.
In other words, when the optical fiber was wound around the bobbin having a
diameter of 300 mm, it could be confirmed that the PMD standard could be satisfied after
optical cabling, in the state where the optical fiber was wound around the bobbin, but it
could not be confirmed, when the optical fiber was wound around the bobbin having a
diameter of 100 mm.
An optical fiber of the same type as that mentioned in Example 1 and having a
length of 10000 m was prepared. Then when this optical fiber was wound around a
bobbin having a diameter of 300 mm at a tension of 100 gf, and LB was measured by
using the P-OTDR, it was found that LB was 10 m.
The bobbin around which the optical fiber was wound had a structure capable of
reducing the tension. Therefore, when the tension was temporarily reduced, to perform
the same measurement, it was found that LB was 25 m in a section of from 0 m to 9000 m.
However LB was 10 m in a section of from 9000 m to 10000 m.
Moreover, when the optical fiber was cut into ten pieces of 1000 m, and bundled
to form an optical fiber cable of ten cores, the PMD of the coated optical fiber formed by
using the section of 0 m to 9000 m was within the range of from 0.03 to 0.06 ps/√km, but
in the core line using the section of 9000 to 10000 m, the PMD exceeded 0.1 ps/√km.
In other words, by having a structure capable of temporarily reducing the tension,
it could be confirmed whether the PMD standard was satisfied after optical cabling, but
when this structure was not used, confirmation was not possible. After the measurement,

since the tension was returned to that before the measurement, there was no problem in
feeding the optical fiber in the subsequent manufacturing process.
Moreover, by using the P-OTDR, the PMD in the longitudinal direction of the
optical fiber could be measured.
Example 7
The present inventor investigated the beat length required for achieving a PMD
equal to or less than 0.1 ps/√km as the whole transmission path, and the beat length
required for achieving a PMD equal to or less than 0.1 ps/√km in an individual optical
fiber cable, by using combinations of optical fibers and optical fiber cables having
various structures. As a result, when optical fibers having a beat length of 15 m or longer
after optical cabling were coupled, a PMD equal to or less than 0.1 ps/√km could be
achieved as the whole transmission path. Moreover, with regard to an optical fiber
having a beat length of 30 m or longer when used to form an optical fiber cable, the PMD
was equal to or less than 0.1 ps/√km in all individual optical fibers.
In other words, it is preferred that the beat length of the optical fiber in the state
wound around the bobbin be 15 m or longer. Moreover if the beat length of the optical
fiber in the state wound around the bobbin is 30 m or longer, the PMD after optical
cabling can be made equal to or less than 0.1 ps/√km, regardless of the type of the optical
fiber cable.
Therefore, optical fibers having various birefringence were produced, and wound
around a bobbin having a diameter of 300 mm at a tension of 40 gf in a length of 3000 m.
Then the relationship between the beat length in the state wound around the bobbin, and
the PMD after the optical fiber was used to form an optical fiber cable was measured.
The results are shown in FIG. 4. From FIG. 4, a clear relationship can be understood

between the beat length in the state wound around the bobbin, and the PMD after the
optical fiber was used to form an optical fiber cable.
The mean value of the PMD after optical cabling of the optical fiber having the
beat length of 15 m or longer in the state wound around the bobbin was 0.05 ps/√km.
All the optical fibers having a beat length of 15 m or longer in the state wound
around the bobbin were connected after optical cabling to form one transmission path,
and the PMD thereof was measured. The PMD of the transmission path was 0.06 ps/√km,
which was smaller than 0.1 ps/√km.
Example 8
Of the optical fibers used in Example 7, the mean value of the PMD of the optical
fibers having a beat length of 30 m or longer in the state wound around the bobbin after
optical cabling, was 0.015 ps/√km, and there was no optical fiber cable having a PMD
exceeding 0.1 ps/√km.
All the optical fibers having a beat length of 30 m or longer in the state wound
around the bobbin were connected after optical cabling to form one transmission path,
and the PMD thereof was measured. The PMD of the transmission path was 0.016
ps/√km, which was sufficiently smaller than 0.1 ps/√km.
Comparative Example
The PMD of the optical fiber used in Example 7 was measured in the state wound
around the bobbin, and the relationship between the PMD in the state wound around the
bobbin, and the PMD after the optical fiber was used to form an optical fiber cable was
measured. The results are shown in FIG. 5. From FIG. 5, a clear relationship cannot be
found between the PMD in the state wound around the bobbin, and the PMD after the
optical fiber was used to form an optical fiber cable.

The mean value of the PMD after optical cabling of the optical fibers having a
PMD equal to or less than 0.1 ps/√km in the state wound around the bobbin was 0.09
ps/√km, which was worse than in Example 7, and was not sufficiently smaller than 0.1
ps/√km.
All the optical fibers having a PMD equal to or less than 0.1 ps/√km in the state
wound around the bobbin were connected after optical cabling to form one transmission
path, and the PMD thereof was measured. The PMD of the transmission path was 0.11
ps/√km, which was larger than 0.1 ps/√km
FIG. 6 is a cross-sectional view showing an example of the structure of the
optical fiber according to present invention. In FIG. 6, reference symbol 1 denotes an
optical fiber, reference symbol 2 denotes a core, and reference symbol 3 denotes a
cladding.
FIG. 7 is a cross-sectional view showing an example of the structure of a coated
optical fiber. In FIG. 7, reference symbol 1 denotes an optical fiber, reference symbol 4
denotes a primary coating, and reference symbol 5 denotes a secondary coating. The
primary coating 4 and the secondary coating 5 respectively constitute a protective layer
for protecting the optical fiber 1.
FIG. 8 is a cross-sectional view showing an example of the structure of the
optical fiber cable according to the present invention. In FIG. 8, reference symbol 20
denotes an optical fiber cable (loose tube type), reference symbol 10 denotes a coated
optical fiber, reference symbol 11 denotes a tension member, reference symbol 12
denotes a loose tube, 13 and 14 denote a jelly, reference symbol 15 denotes a pressed
winding, and reference symbol 17 denotes a sheath.

Preferable examples of the present invention has been described above, but the
present invention is by no means limited to these examples. Addition, omission,
replacement, and other modifications of the configuration are possible without departing
from the scope of the present invention. The present invention is not limited by the
explanation above, but limited only by the scope of the appended claims.
INDUSTRIAL APPLICABILITY
According to the method of measuring polarization mode dispersion of an optical
fiber of the present invention, the PMD of the optical fiber after optical cabling can be
estimated in a state where the optical fiber is wound around a transport bobbin.

We Claim :
1. A method of measuring polarization mode dispersion of an optical fiber,
comprising estimating polarization mode dispersion when an optical fiber is formed as an
optical cable, from a beat length when the optical fiber is wound around a bobbin, and an
average coupling length when the optical fiber is formed as the optical cable.
2. A method of measuring polarization mode dispersion of an optical fiber,
comprising estimating polarization mode dispersion when an optical fiber is put in a free
state, from a beat length when the optical fiber is wound around a bobbin, and an average
coupling length when the optical fiber is when the optical fiber put in the free state.
3. A method of measuring polarization mode dispersion of an optical fiber as
claimed in claim 1 or 2, comprising measuring Rayleigh scattered light intensity of the
optical fiber by using a P-OTDR.
4. A method of measuring polarization mode dispersion of an optical fiber
as claimed in claim 3, comprising setting a resolution of the P-OTDR shorter than a
shortest beat length estimated for the optical fiber wound around the bobbin.
5. A method of measuring polarization mode dispersion of an optical fiber as
claimed in claim 1 or 2, comprising setting a radius R of the bobbin, and a tension at the
time of winding an optical fiber around the bobbin, so that a magnitude of
birefringence of the optical fiber induced due to winding of the optical fiber around the
bobbin becomes smaller than a magnitude of internal birefringence inherently held by the
optical fiber, and then winding the optical fiber around the bobbin.

6. A method of measuring polarization mode dispersion of an optical fiber as
claimed in claim 1 or 2, comprising setting a radius R of the bobbin, and a tension at the
time of winding an optical fiber around the bobbin, so that a magnitude of birefringence
of the optical fiber induced due to winding of the optical fiber around the bobbin
becomes smaller than a magnitude of internal birefringence allowed in a standard for the
optical fiber, and then winding the optical fiber around the bobbin.
7. A method of measuring polarization mode dispersion of an optical fiber as
claimed in claim 1 or 2, comprising setting a radius R of the bobbin, so that the radius R
of the bobbin, and a maximum value B' of a magnitude of internal birefringence allowed
in a standard for the optical fiber satisfy the following Equation:

where "n" denotes a refractive index of the optical fiber, p11 and p12 denote Pockels
coefficients of the optical fiber, v denotes a Poisson's ratio of the optical fiber, and "r"
denotes a radius of the optical fiber.
8. A method of measuring polarization mode dispersion of an optical fiber as
claimed in claim 1 or 2, comprising measuring the beat length in a state where a tension
applied to the optical fiber is temporarily reduced.
9. A method of measuring polarization mode dispersion of an optical fiber as
claimed in claim 1 or 2, wherein the bobbin has a structure capable of temporarily
reducing a tension on the optical fiber.

10. A method of measuring polarization mode dispersion of an optical fiber as
claimed in claim 1 or 2, comprising measuring a distribution of polarization mode
dispersion in a longitudinal direction.
11. An optical fiber comprising a core and a cladding arranged around the core,
wherein the PMD measured by using the method as claimed in claim 1 or 2 is equal to or
less than 0.1 ps/√km.
12. An optical fiber as claimed in claim 11, wherein a beat length in a state wound
around the bobbin is equal to or longer than 15 m.
13. An optical fiber as claimed in claim 11, wherein a beat length in a state wound
around the bobbin is equal to or longer than 30 m.
14. An optical fiber as claimed in claim 11, wherein a beat length in a state in which
a tension due to the bobbin is reduced after wound around the bobbin, is equal to or
longer than 15 m.
15. An optical fiber as claimed in claim 11, wherein a beat length in a state in which
a tension due to the bobbin is reduced after wound around the bobbin, is equal to or
longer than 30 m.

16. An optical fiber cable in which a plurality of coated optical fibers having a
protective layer around an optical fiber as claimed in claims 11-15, are arranged side by
side, and the plurality of coated optical fibers are housed in a sheath.
A

OPTICAL FIBER AND METHOD OF MEASURING POLARIZATION MODE DIFFUSION FOR OPTICAL FIBER

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