BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method of measurement of birefringence of an optical fiber and a measurement device, and to an optical fiber polarization mode dispersion measurement method and an optical fiber, and relates to techniques for the precise and simple measurement of the birefringence and polarization mode dispersion of an optical fiber along the length direction. This application relates to and claims priority from Japanese Patent Application No. 2005-117030, filed on April 14, 2005, and from Japanese Patent Application No. 2005- 229263, filed on August 8, 2005, the entire disclosures of which are incorporated herein by reference. • 2. Description of the Related Art In recent years, the faster transmission rates and longer transmission distances of optical communications have been accompanied by a need to reduce polarization mode dispersion (hereafter "PMD") along transmission paths. PMD is dispersion which occurs due to group velocity differences among two eigenpolarization components which are orthogonal, propagating in an optical fiber (see Patent References 1 and 2 and Non-patent References 1 to 5). There are two parameters determining the PMD. One is the magnitude of birefringence in the optical fiber; the other is the magnitude of the polarization mode coupling, which indicates changes in the optical fiber length direction of the birefringent axis in the optical fiber. Specific factors determining PMD in an optical cable transmission path include ellipticity of the fiber core, asymmetry or the like of stresses occurring in the core, and other factors arising within the optical fiber, as well as asymmetry of stresses due to optical fiber bending in optical cable manufacturing processes and other factors arising from processes to produce optical cable. Hence in order to prevent worsening of PMD in optical cables due to factors within optical fibers, it is desirable that PMD arising from factors within the optical fiber be measured before processes to manufacture optical cable, and that optical fibers with poor PMD characteristics be excluded. Optical fiber is normally wound on a bobbin for transport to the site of the process for optical cable manufacture. But an optical fiber wound on a bobbin is subject to bending and to birefringence caused by lateral pressure while wound on the bobbin; in addition, optical fibers will come into contact with each other or will be subjected to considerable torsion while being taken up on the bobbin, so that polarization mode coupling is induced. Consequently the PMD of an optical fiber wound on a bobbin does not coincide with the PMD arising from factors within the optical fiber. Hence in order to measure PMD arising from factors within the optical fiber, a method is employed in which the optical fiber is removed from the bobbin and is wound with a diameter of from 20 cm to 100 cm, and by immersing the fiber in a liquid having a specific gravity close to that of the optical fiber, birefringence arising from lateral pressure and small-radius bending, as well as polarization mode coupling arising from contact between optical fibers, are eliminated, and the PMD is measured. This PMD measurement is for example described in Non-patent Reference 5. As is stated in Non-patent Reference 4, PMD has statistical properties, and so measurements are attended by uncertainty. In order to reduce this uncertainty, methods may be used to increase the total PMD of the optical fiber for measurement, or expand the wavelengths for measurement, or apply perturbations to the optical fiber being measured and perform measurements a plurality of times. Patent Reference 1: International Patent Publication No. WO 2004/010098 Patent Reference 2: International Patent Publication No. WO 2004/045113 Non-patent Reference 1: E. Chausse, N. Gisin, Ch. Zimmer, "POTDR, depolarization and detection of sections with large PMD", OFMC '95. Non-patent Reference 2: Tadao Tsuruta, Ouyou Kougaku 2, pp. 197-200, BAIFUKAN CO., LTD. Non-patent Reference 3: R.C. Jones, "A new calculus for the treatment of optical systems VI. Experimental determination of the matrix", JOSA, Vol. 37, pp. 110-112, 1947. Non-patent Reference 4: N. Gisin, "How accurately can one measure a statistical quantity like polarization-mode dispersion", PTL, Vol. 8, No. 12, pp. 1671-1673, Dec. 1996. Non-patent Reference 5: B.L. Heffner, "Automated measurement of polarization mode dispersion using Jones matrix eigenanalysis", IEEE Photonics Tech. Lett. Vol. 4, No. 9, Sept. 1992. However, there are the following problems with the PMD measurement methods of the prior art. In order to increase the total PMD of the optical fiber for measurement, the total length of the optical fiber for measurement must be made long if the optical fiber for measurement has a small PMD; but because an optical fiber used in PMD measurements in the free state cannot again be used as a product, this method requires a long optical fiber each time a measurement is performed, so that waste is substantial. Further, methods entailing expansion of the wavelengths for measurement are subject to constraints imposed by the operating wavelengths of the light source, and so there are limits to the use of such methods. And, methods requiring a plurality of measurements require time to perform measurements and are inefficient. Next, another technology of the prior art, and problems with this technology, are described. Because there are large fluctuations in PMD depending on the preform and drawing conditions of the optical fiber, normally optical fibers manufactured under identical conditions exhibit substantially the same PMD value, but due to unanticipated causes, there are cases of partial worsening of the PMD, and so it is desirable that length-direction measurements be performed. Methods of the prior art for longitudinal measurement of the birefringence and PMD include the methods described in Patent References 1 and 2. These methods involve measurement of the birefringence and PMD based on the amount of scattering in the OTDR waveform observed when a polarizer is placed between the OTDR and the optical fiber for measurement. However, these measurement methods are accompanied by a number of problems. First, in methods of the prior art, the waveform amplitude differs depending on the incident polarization state and on the birefringence axis angle of the optical fiber, and so there is the problem that measurements cannot be performed accurately. For example, when the incident polarization is linear polarization, the amplitude is maximum when the angle between the direction of linear polarization and the birefringence axis is 45°, but when the two directions coincide, the amplitude is zero. This problem has a serious impact on the results of measurement of polarization mode dispersion using conventional methods. Further, in methods of the prior art, scattering from the least-square approximating line is used as an index of the scattering in the OTDR waveform; to this end, averaging must be performed over a certain interval, and so it is inherently not possible to obtain a high resolution. Moreover, a feature of methods of the prior art is the simple configuration obtained by using a general-purpose OTDR; but because the light source of a general-purpose OTDR has a broad spectral width from 5 nm to 20 nm, once a point with large PMD is traversed, a phenomenon occurs in which the polarization state of the pulse differs with the wavelength, and so the amplitude is averaged and becomes smaller; hence there is the problem that subsequent PMD measurements cannot be performed (see Non-patent Reference 1). SUMMARY OF THE INVENTION This invention was devised in light of the above circumstances, and has as an object the provision of a method of measurement, with accuracy and in a short length of time, of the birefringence and PMD of a short optical fiber having a comparatively small PMD in the free state and a device therefore. A further object of this invention is the provision of a method and device for the accurate longitudinal measurement of birefringence and PMD of an optical fiber in the free state with arbitrary resolution, and such that even when a point with large PMD exists midway in the fiber, there is no influence on subsequent measurement results. In order to attain the above objects, this invention provides a method of measurement of the birefringence of an optical fiber, in which the round-trip Jones matrix R(z) for a first interval (0, z) from a measurement starting point 0 in the optical fiber for measurement to a prescribed position z, and the round-trip Jones matrix R(z+Δz) for a second interval (0, z+Δz) from the measurement starting point 0 to a position z+Δz differing from the position z, are acquired, the eigenvalues p1, p2 of the matrix R(z+Δz)R(z)-1 are determined, and by computing the following equations (1) and (2), (where ϕ represents the phase difference between linear polarization components due to birefringence, An represents birefringence, and λ represents wavelength), the birefringence in the infinitesimal interval Δz from the position z to the position z+Δz is obtained. In an optical fiber birefringence measurement method of this invention, it is preferable that a polarization OTDR be used to acquire the round-trip Jones matrices of the optical fiber for measurement. Further, this invention provides an optical fiber birefringence measurement device, having timing control means; pulse light generation means, controlled by the timing control means; polarization conversion means which converts pulse light from the pulse light generation means into a polarized state; optical recirculation means which inputs pulse light from the polarization conversion means to one end of the optical fiber for measurement, and which outputs backscattered light returning to one end of the optical fiber for measurement; polarization detection means, controlled by the timing control means, which detects the polarization state of light output from the optical recirculation means as a time series; and analysis means, which, based on the output of the polarization detection means, uses the birefringence measurement method to measure the birefringence of the optical fiber for measurement. Further, this invention provides an optical fiber polarization mode dispersion measurement method, in which the birefringence of the optical fiber for measurement in the free state, measured using the optical fiber birefringence method, and the relation to the polarization mode dispersion of the optical fiber for measurement in the free state, are used to measure the polarization mode dispersion of the optical fiber for measurement in the free state. Further, this invention provides an optical fiber polarization mode dispersion measurement method, in which a portion of an optical fiber wound onto a bobbin is removed, and after using the optical fiber polarization mode dispersion measurement method to measure the polarization mode dispersion, the measured value of the polarization mode dispersion is taken to be the polarization mode dispersion when the entire optical fiber wound around the bobbin is placed into the free state. Further, this invention provides an optical fiber polarization mode dispersion measurement method, in which the birefringence of the optical fiber for measurement in the free state, measured using the optical fiber birefringence measurement method, and the relation to the polarization mode dispersion of the optical fiber for measurement in the free state, are used to measure the polarization mode dispersion of the optical fiber for measurement in the free state, while in the state of being wound around a bobbin. Further, this invention provides an optical fiber polarization mode dispersion measurement method, in which the birefringence of the optical fiber for measurement in the state of being wound around a bobbin, measured using the optical fiber birefringence measurement method, and the relation to the polarization mode dispersion of the optical fiber for measurement in the free state, are used to measure the polarization mode dispersion of the optical fiber for measurement in the free state, while in the state of being wound around the bobbin. In the polarization mode dispersion measurement method, the amount of twist applied to the optical fiber for measurement in the state of being wound around the bobbin may be 1 rad/m or less. In the polarization mode dispersion measurement method, the birefringence may be measured for a portion in which the effects of the tension of takeup on the bobbin and of lateral pressure due to the taken-up optical fiber itself are small, and this birefringence of the optical fiber for measurement may be used as a representative value, and used as the polarization mode dispersion of the optical fiber when the entire optical fiber wound around the bobbin is placed in the free state. In the polarization mode dispersion measurement method, cushion material may be positioned at a place at which the optical fiber for measurement is in contact with the bobbin around which the optical fiber is wound, to reduce lateral pressure on the optical fiber, and in addition the effect of polarization state fluctuations during measurements arising from expansion and shrinkage of the bobbin due to temperature changes in the measurement environment may be eliminated. In the polarization mode dispersion measurement method, upon temporarily relaxed the tension of the optical fiber, the birefringence of the optical fiber for measurement may be measured while in the state of being wound around the bobbin, and the polarization mode dispersion of the optical fiber in the free state may be measured. Further, this invention provides an optical fiber the polarization mode dispersion of which, as measured by the above optical fiber polarization mode dispersion measurement method, is 0.1ps/km or less. In this optical fiber, the amount of twist applied, in the state of being wound around the bobbin, may be 1 rad/m or less. In this optical fiber, the value of, or the upper limit of, the measured polarization mode dispersion may be displayed. According to this invention, the round-trip Jones matrix R(z) for a first interval (0,z) from a measurement starting point 0 in the optical fiber for measurement to a prescribed position z and the round-trip Jones matrix R(z+Δz) for a second interval (0, z+Δz) from the measurement starting point 0 to a position z+Az differing from the position z in the optical fiber for measurement are acquired, the eigenvalues p1, p2 of the matrix R(z+Δz)R(z)_1 are determined, and the birefringence of the infinitesimal interval Δz is obtained by calculation, and, the PMD of the optical fiber is obtained from the optical fiber birefringence thus obtained, so that a method and device for the accurate measurement, in a short length of time, of the birefringence and PMD of a short optical fiber, in the free state and having comparatively small PMD, can be provided. Further, this invention can provide a method and device to measure, accurately and with arbitrary resolution, the length-direction birefringence and PMD of an optical fiber in the free state, and moreover even when a point with large PMD exists midway in the fiber, there is no influence on subsequent measurement results. Further, by means of this invention the PMD of an optical fiber in the free state can be estimated for the optical fiber in the state of being wound around a bobbin, or in a state of being wound around a bobbin with the tension temporarily relaxed. BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS FIG. 1 is a summary diagram to explain the measurement interval in an optical fiber birefringence measurement method of this invention; FIG. 2 is a configuration diagram of an aspect of an optical fiber birefringence measurement device of this invention; FIG. 3 is a configuration diagram showing an example of pulse light generation means of an optical fiber birefringence measurement device of this invention; FIG. 4 is a configuration diagram showing another example of pulse light generation means of an optical fiber birefringence measurement device of this invention; FIG. 5 is a configuration diagram showing an example of polarization conversion means of an optical fiber birefringence measurement device of this invention; FIG. 6 is a configuration diagram showing another example of polarization conversion means of an optical fiber birefringence measurement device of this invention; FIG. 7 is a configuration diagram showing another example of polarization conversion means of an optical fiber birefringence measurement device of this invention; FIG. 8 is a configuration diagram showing another example of polarization conversion means of an optical fiber birefringence measurement device of this invention; FIG. 9 is a configuration diagram showing another aspect of an optical fiber birefringence measurement device of this invention; FIG. 10 shows an example of birefringence measured by a measurement method of this invention, when twist in one direction is applied after optical fiber solidification; FIG. 11 shows an example of actual birefringence and of birefringence measured by a measurement method of this invention, when twist in one direction is applied before optical fiber solidification; FIG. 12 shows an example of actual birefringence and of birefringence measured by a measurement method of this invention, when sinusoidal spin is applied before optical fiber solidification; FIG. 13 compares the birefringence measured by a method of this invention with the measurement results for ten measurements of PMD by a conventional method; FIG. 14 compared the measurement results for a single measurement of PMD by a conventional method with the measurement results for ten measurements of PMD by the conventional method; FIG. 15 shows an example of longitudinal measurement of the birefringence of an optical fiber wound around a bobbin; FIG. 16 shows an example of longitudinal measurement of the birefringence of an optical fiber wound around a bobbin; FIG. 17 shows the relation between the longitudinal birefringence distribution, measured with the optical fiber in the state of being wound around a bobbin, and the PMD when the optical fiber is divided in two at the center and placed in the free state; FIG. 18 shows the relation between the longitudinal birefringence distribution, measured with the optical fiber wound around a bobbin configured for temporary relaxation of tension, in a state of relaxed tension, and the PMD when the optical fiber is divided in two at the center and placed in the free state; FIG. 19 is a graph showing the result of comparison of the birefringence in the state of being wound around a bobbin, and the PMD of the optical fiber placed in the free state; FIG. 20 is a graph showing the result of comparison of the birefringence of an optical fiber placed in the free state, and the PMD of the optical fiber placed in the free state; and, FIG. 21 is a graph showing the result of comparison of the birefringence in the state of being wound around a bobbin, and the birefringence of the optical fiber placed in the free state. BEST MODE FOR CARRYING OUT THE INVENTION Below, preferred embodiments of the invention are explained, referring to the drawings. However, the invention is not limited to the following embodiments, and for example constituent elements of these embodiments may be appropriately combined. First, a method of measurement of birefringence of an optical fiber according to this invention is explained. FIG. 1 is a summary diagram to explain the measurement interval in an optical fiber birefringence measurement method of this invention. In the birefringence measurement method of the invention, a first interval (0, z) is set from a measurement starting point 0 to a prescribed position z in the optical fiber for measurement, and a second interval (0, z+Δz) from the measurement starting point 0 to a position z+Az different from the position z is set; the interval from the position z to the position z+Δz (the interval which is the difference between the first interval and the second interval) is the infinitesimal interval Az. Further, if the one-way Jones matrix for the first interval (0, z) is J1, the one-way Jones matrix for the infinitesimal interval Δz is J2, and the round-trip Jones matrix for the first interval (0, z) is R (z), then the relation of the following equation (3) obtains. Here, upon considering the matrix R(z+Δz)R(z)"', the following equation (4) is obtained. In optical fiber in the free state and in optical fiber within optical cable, changes in the birefringence axis of the optical fiber and torsion applied to the optical fiber are gradual, and so the infinitesimal interval Δz can be regarded as having only a linear birefringence, and the birefringence axis angle can also be regarded as constant. Then the one-way Jones matrix J2 for the infinitesimal intervalh Δz is given by the following equation (5), where the angle of the fast axis of birefringence is 0 and the phase difference between orthogonal polarization due to birefringence is occurring between two orthogonal intrinsic polarizations which are intrinsic to the infinitesimal interval, and after diagonalizing the Jones matrix for the infinitesimal interval to equation (14), An' can be computed from the following equations (15) and (16). In this case also, the birefringent axis direction in the infinitesimal interval Δz is not constant, and so may affect the value of birefringence measured using a method of this invention. This effect was computed using numerical calculations, to study the range of applicability of methods of this invention. The calculation conditions were the same. First, FIG. 11 shows the calculated differences occurring in the magnitude of the effective birefringence in the interval Δz and in the magnitude of birefringence measured by a method of this invention, when spin is applied in a constant direction before optical fiber solidification, and the amount of applied spin is varied. FIG. 12 shows the results of similar calculations for cases in which sinusoidal spin is applied before optical fiber solidification. Sinusoidal spin is a method of application of spin such that the following equation (17) obtains between the spin angle 9 at a point at a distance z, the spin amplitude A, and the spin period P. From FIG. 11 and FIG. 12, it is seen that even when spin is applied in a constant direction before optical fiber solidification, and even when sinusoidal spin is applied before optical fiber solidification, the magnitude of birefringence measured by a method of this invention agrees well with the effective birefringence magnitude. Hence when the effective birefringence is lowered by applying spin before optical fiber solidification, a method of this invention can be used for accurate measurement of birefringence. Next, another PMD measurement method of this invention is explained. If the effect of an external force applied to the optical fiber for measurement by the bobbin is small, a method of this invention can be used to measure the PMD of the optical fiber placed in the free state, but when the tension of takeup onto the bobbin is high, there is an effect of lateral pressure due to the tension, and there are cases in which it is difficult to reduce the effect on birefringence of the external force applied across the entire length of the optical fiber for measurement. FIG. 15 shows the results of longitudinal measurement, from the outermost periphery, of the beat length of optical fiber wound around a bobbin. From FIG. 15, it is seen that in the state of being wound around the bobbin, the further toward the inner periphery, the larger is the birefringence. On the other hand, the birefringence of an optical fiber is frequently due to the preform of the optical fiber; if the preform is the same, often the magnitude of the birefringence is substantially the same. In such cases, such locations that the effect on the birefringence due to the applied external force is small, the birefringence in the vicinity of the outermost periphery of the wound optical fiber can usually be measured and taken as a representative value of the birefringence for the optical fiber for measurement, and used to measure the PMD of the optical fiber placed in the free state. FIG. 16 compares the birefringence actually measured over an interval of 500 m from the outermost periphery in the state of being wound on a bobbin with the PMD measured when the entire optical fiber wound on the bobbin was placed in the free state. From FIG. 16 it is seen that by measuring the birefringence of the outer peripheral portion while in the state of being wound on the bobbin, the result can be used as a representative value of the PMD when the entire optical fiber wound on the bobbin is placed in the free state. Next, the form of bobbins for which use of the methods of this invention are suited is discussed. If the effect of external force applied by the bobbin to the optical fiber for measurement is small, then when the above method is used to measure the PMD of an optical fiber placed on the free state, measurements can be performed over a longer distance from the outer peripheral portion. To this end, it is preferable that a cushion material be placed at locations at which the bobbin and the optical fiber for measurement make contact, to reduce the effect of external force applied to the optical fiber for measurement. One perturbing factor acting on an optical fiber for measurement during measurements is the perturbations due to changes in lateral pressure applied to the fiber when the bobbin around which the optical fiber is wound expands or contracts due to changes in temperature; it is preferable that cushion material be used to prevent the application of perturbations to the optical fiber for measurement even when expansion or contraction of the bobbin occurs. Further, it is preferable that the bobbin be configured so as to enable temporary removal of tension on the optical fiber for measurement, and that a measurement method be used in which, having removed tension temporarily from the optical fiber during measurements, after measuring the PMD using a method of this invention, the tension is returned to the original state. This method is particularly effective when the bobbin takeup tension is high, and the birefringence arising from winding onto the bobbin is high. Next, a method of longitudinal measurement of the PMD and birefringence of an optical fiber is described. Using a method of this invention, the birefringence can be determined longitudinally along an optical fiber, and so using the relation between birefringence and PMD described above, the PMD can be measured longitudinally. FIG. 17 compares the results when an optical fiber of total length 5000 m, drawn from molten glass such that circularity of the optical fiber was partially worsened, and in a state of being wound on a bobbin, was subjected to longitudinal birefringence measurements by a method of this invention, and was then divided in two at the 2500 m point and subjected to PMD measurements in the free state. From FIG. 17, it is seen that by using a method of this invention, the PMD in the free state in the length direction can be measured longitudinally even when the optical fiber is wound on a bobbin. Further, it is preferable that a configuration be adopted in which cushion material is placed in locations at which the bobbin and the optical fiber for measurement make contact or the tension on the optical fiber for measurement due to the bobbin be temporarily removed, and that, with the tension on the optical fiber temporarily removed during measurements, that the PMD be measured in the length direction by a method of this invention, enabling detection of longitudinal PMD fluctuations with extremely high precision. FIG. 18 compares the results when an optical fiber of total length 3000 m, drawn from molten glass such that circularity of the optical fiber was partially worsened, was wound onto a bobbin configured to enable temporary removal of tension on the optical fiber for measurement, after which tension was temporarily removed and the longitudinal birefringence was measured by a method of this invention, following which the fiber was divided in two at the 1500 m point, and PMD measurements were performed in the free state. From FIG. 18, it is seen that by using a method of this invention, even extremely small longitudinal PMD changes can be detected. This invention provides an optical fiber the polarization mode dispersion of which, as measured by the above-described optical fiber polarization mode dispersion measurement method, is 0.1 ps/^lkm or less. An optical fiber of this invention may be a quartz glass single-mode optical fiber (hereafter "SM fiber") or a polarization-maintaining optical fiber or similar, but is not limited to these. An optical fiber of this invention can be provided in a state of being wound on a bobbin; it is preferable that, in the state of being wound on a bobbin, the amount of torsion applied be 1 rad/m or less. If the torsion amount is 1 rad/m or less, then the birefringence measured in the state of being wound around the bobbin agrees with the magnitude of the birefringence when there is no twist with a difference of approximately 10%, and so the birefringence of the optical fiber can be measured in the state of being wound around a bobbin. On the other hand, in cases in which the twist amount exceeds 1 rad/m, and the twist amounts are different for each optical fiber, the relation between the measured birefringence and the PMD of the optical fiber placed in the free state is weakened, and the PMD cannot be accurately measured. It is preferable that the optical fiber of this invention display, either on the optical fiber itself or on the bobbin on which it is wound, the PMD value measured using the above-described PMD measurement method of this invention or the upper limit thereof. It is preferable that the displayed contents be, for example, "PMD 0.01 to 0.05 ps/ 4km ", "PMD 0.1 ps/Jkm or less", or similar. Display may be accomplished by affixing a label on which are printed the display contents, by attaching a tag with the display, or similar. The PMD value or upper limit may also be printed on an explanatory document listing the performance of the optical fiber, and this document may be packed or packaged with the optical fiber wound on the bobbin. Optical fibers of various lengths, wound around a bobbin of diameter 300 mm under a tension of 40 g, were in this state subjected to birefringence measurements over a 1300 m interval from the outermost periphery. Then, this 1300 m was placed in the free state, and both the birefringence and the PMD were measured ten times each. Vibrations were applied to the optical fibers upon each measurement (as described in IEC 60793-1-48, Annex E). The results of comparison of the birefringence in the state of being wrapped around a bobbin, and the PMD of the optical fiber placed in the free state, appear in FIG. 19. In FIG. 19, the PMD measurement results are averages often measurements. From FIG. 19 it is seen that by measuring the birefringence in the state of being wound around a bobbin, the PMD of the optical fiber placed in the free state can be measured. FIG. 20 shows the results of comparison of the birefringence of an optical fiber placed in the free state, and the PMD of the optical fiber placed in the free state. The points in FIG. 20 are averages often measurement results for each measurement; error bars indicate the standard deviation. From FIG. 20, by measuring the birefringence when placed in the free state, the PMD of the optical fiber placed in the free state can be measured. It is seen that the standard deviation of birefringence measurements is extremely small compared with the standard deviation of PMD measurements. Hence it is seen that the PMD measurement method of this invention has extremely good measurement reproducibility. FIG. 21 shows the results of comparison of birefringence when in the state of being wound around a bobbin, and the birefringence of the optical fiber placed in the free state. From FIG. 21 it is seen that even in the state of being wound around a bobbin, the state of birefringence is unchanged from the case of being placed in the free state, so that this method is suitable for measurement of optical fiber wound around a bobbin. The measured values of the birefringence are equal for the state of being wound around a bobbin and for the free state because no torsion occurs when winding around the bobbin, or, the effect on the birefringence due to the bobbin bending radius and lateral pressure is very small. WE CLAIM: 1. A method of measurement of the birefringence of an optical fiber, comprising the steps of: generating pulse light from a pulse light generating means; converting polarized state of the pulse light from the pulse light generation means; inputting the converted pulse light to one end of the optical fiber for measurement; and detecting the polarization state of backscattered light returning to the one end of the optical fiber for measurement as a time series by a polarization detection means; characterized in that the method comprises the steps of: acquiring the round-trip Jones matrix R(z) for a first interval (0,z) from a measurement starting point 0 in the optical fiber for measurement to a prescribed position z; acquiring the round-trip Jones matrix R(z+Δz) for a second interval (0,z+Δz) from said measurement starting point 0 to a position z+Az differing from said position z; determining the eigenvalues p1, p2 of the matrix R(z+Δz)R(z)-1; and obtaining the birefringence in the infinitesimal interval Δz from said position z to said position z+Δz by computing the following equations (1) and (2), where φ represents the phase difference between linear polarization components due to birefringence, Δn represents birefringence, and λ represents wavelength. 2. The optical fiber birefringence measurement method as claimed in claim 1, wherein a polarization OTDR is used to acquire the round-trip Jones matrices of the optical fiber for measurement. 3. An optical fiber birefringence measurement device, comprising: timing control means; pulse light generation means, controlled by the timing control means; polarization conversion means which converts polarized state of pulse light from the pulse light generation means; optical recirculation means which inputs pulse light from the polarization conversion means to one end of the optical fiber for measurement, and which outputs backscattered light returning to one end of the optical fiber for measurement; polarization detection means, controlled by the timing control means, which detects the polarization state of light output from the optical recirculation means as a time series; and analysis means, which, based on the output of the polarization detection means, uses the birefringence measurement method as claimed in claim 1 or claim 2 to measure the birefringence of the optical fiber for measurement. 4. An optical fiber polarization mode dispersion measurement method, comprising the steps of: determining in advance the relation between the birefringence of optical fibers in the free state, measured using the optical fiber birefringence measurement method as claimed in claim 1 or claim 2, and the polarization mode dispersion of the optical fibers measured in the free state, measuring the birefringence of an optical fiber for measurement in the free state by the optical fiber birefringence measurement method; and measuring the polarization mode dispersion of the optical fiber for measurement in the free state based on the measured birefringence of the optical fiber for measurement using the relation between the birefringence and the polarization mode dispersion. 5. An optical fiber polarization mode dispersion measurement method, wherein a portion of an optical fiber wound around a bobbin is separated, and after measuring the polarization mode dispersion of the separated portion using the optical fiber polarization mode dispersion measurement method as claimed in Claim 4, the measured value is taken to be the polarization mode dispersion when the entire optical fiber wound around the bobbin is placed in the free state. 6. An optical fiber polarization mode dispersion measurement method, comprising the steps of: determining in advance the relation between the birefringence of optical fibers in the free state measured using the optical fiber birefringence measurement method as claimed in claim 1 or claim 2, and the polarization mode dispersion of the optical fibers measured in the free state, measuring the birefringence of an optical fiber for measurement in the state of being wound around a bobbin by the optical fiber birefringence measurement method; and measuring the polarization mode dispersion of the optical fiber for measurement in the free state based on the measured birefringence of the optical fiber for measurement using the relation between the birefringence and the polarization mode dispersion. 7. An optical fiber polarization mode dispersion measurement method, comprising the steps of: determining in advance the relation between the birefringence of optical fibers in the state of being wound on a bobbin, measured using the optical fiber birefringence measurement method as claimed in claim 1 or claim 2, and the polarization mode dispersion of the optical fibers measured in the free state, measuring the birefringence of an optical fiber for measurement in the state of being wound around a bobbin by the optical fiber birefringence measurement method; and measuring the polarization mode dispersion of the optical fiber for measurement in the free state based on the measured birefringence of the optical fiber for measurement using the relation between the birefringence and the polarization mode dispersion. 8. The optical fiber polarization mode dispersion measurement method as claimed in claim 6 or claim 7, wherein me amount of torsion applied to the optical fiber for measurement in the state of being wound around a bobbin is 1 rad/m or less. 9. The optical fiber polarization mode dispersion measurement method as claimed in any one of claims 6 to 8, wherein the birefringence of a portion for which the effects of the tension of winding onto the bobbin and of lateral pressure by the wound optical fiber itself are small is measured, and the result taken as a representative value of the birefringence of the optical fiber for measurement, and used as the polarization mode dispersion of the optical fiber when the entire optical fiber wound around the bobbin is placed in the free state. 10. The optical fiber polarization mode dispersion measurement method as claimed in any one of claims 6 to 9, wherein cushion material is placed at locations at which the bobbin which takes up the optical fiber for measurement makes contact with the optical fiber, reducing the lateral pressure on the optical fiber, and that the effects of polarization state fluctuations during measurements due to expansion and contraction of the bobbin due to temperature changes in the measurement environment, are eliminated. 11. The optical fiber polarization mode dispersion measurement method as claimed in any one of claims 6 to 9, wherein, with the tension on the optical fiber relaxed temporarily, the birefringence of the optical fiber for measurement in the state of being wound around the bobbin is measured, and the polarization mode dispersion of the optical fiber in the free state is measured. 12. An optical fiber, wherein the polarization mode dispersion, measured by the optical fiber polarization mode dispersion measurement method as claimed in any one of claims 4 to 11, is 0.1 ps/KM or less. 13. The optical fiber as claimed in claim 12, wherein, in the state of being wound around the bobbin, the amount of torsion applied is 1 rad/m or less. 14. The optical fiber as claimed in claim 12 or claim 13, wherein the measured polarization mode dispersion value, or the upper limit thereof, is displayed. ABSTRACT METHOD AND DEVICE FOR MEASURING DOUBLE REFRACTION OF OPTICAL FIBER, METHOD OF MEASURING POLARIZATION MODE DISPERSION OF OPTICAL FIBER AND OPTICAL FIBER It is an object of the invention to measure the birefringence and PMD of a short optical fiber (2) in the free state with accuracy. This method comprises the steps of generating pulse light from a pulse light generating means (12); converting polarized state of the pulse light by a polarization conversion means (13); inputting the converted pulse light to one end of the optical fiber (2); and detects the polarization state of backscattered light returning to the one end; acquiring the round-trip Jones matrix R(z) for a first interval (0,z); acquiring the round-trip Jones matrix R(z+Δz) for a second interval (0,z+Δz); determining the eigenvalues p1, p2 of the matrix R(z+Δz)R(z)-1; and obtaining the birefringence in the infinitesimal interval Δz by computing the following equations (1) and (2). φ represents the phase difference between linear polarization components due to birefringence, Δn represents birefringence, and λ represents wavelength.