1.  Field of the Invention
The present invention generally relates to optical fibers.  In particular, the invention concerns methods for evaluating optical fibers characteristic parameters, and, more specifically, methods for evaluating the Polarization Mode Dispersion (hereinafter, PMD) of optical fibers. 
2.  Description of the Related Art
In the field of optical fibers manufacturing, the PMD is an important parameter to be evaluated, because it is related to the communication rate that an optical fiber can sustain. 
The PMD relates to the polarization-pendent group velocities of optical signals propagating through the fiber.  This phenomenon, which is a consequence of the optical fiber birefringence, causes optical pulses to spread in the time domain as they propagate along the fiber, because different polarization components of the optical pulse have different arrival times. 
The spread of optical pulses poses limit to the communication bit rate, and a proper evaluation of the PMD of optical fibers is thus important in order to determine the maximum allowable data rate in an optical communication system in which the optical fibers are exploited.  In a world in which there is an increasing demand for bandwidth, this is of paramount importance. 
It is known in the art that the PMD in an optical fiber is a statistical parameter.  The statistical nature of the PMD in optical fibers is for example recognized in the IEC Standard 86A/658/NP (based on the Proposal IEC 60793-1-48 entitled “Polarisation mode dispersion measurement methods”).  Such document sets a standard and provides uniform methods of measuring the PMD of optical fibers and optical fiber cables; for this reason, the document will be hereinafter concisely referred to as the IEC standard, and the content thereof is to be considered as incorporated herein by reference. 
In the IEC standard there is explained that (for a sufficiently narrow band optical source) the effect that gives rise to the PMD can be related to a Differential Group Delay (shortly, DGD, a quantity usually measured in ps) between pairs of orthogonally polarized principal states of polarization at a given wavelength; in long fiber spans, the DGD is random in both time and wavelength, since it depends on the details of the birefringence along the entire fiber length, and it is also sensitive to temperature and mechanical perturbations on the fiber.  A useful way to characterize the PMD in long fibers is thus in terms of the expected value (or the mean value) of the DGD over wavelength. 
The IEC standard provides three basic methods for measuring the PMD, namely: the fixed analyzer method (Method A), the Stokes parameter evaluation method (Method B) and the interferometry method (Method C).  Methods A and B both call for evaluating the PMD by measuring a response to a change of narrowband light across a wavelength range; Method C, intended for evaluating the PMD of installed optical fiber cables (that may be moving or vibrating) is based on a broadband light source that is linearly polarized. 
The IEC standard prescribes that, in case of disputes, Method B is to be taken as the reference PMD evaluation method.  Summarizing, in Method B a light source is coupled to the fiber under test and the fiber output is coupled to a polarimeter, used for measuring the output Stokes vectors for each selected input polarization and wavelength.  The wavelengths are scanned across a range appropriate for the operative wavelength region and the desired precision, and with a suitably small wavelength increment.  The measurement data are gathered for each wavelength. 
Different approaches for performing calculations on the gathered measurement data are possible (Jones Matrix Eigenanalysis—JME—, Poincaré Sphere Analysis —PSA—or State Of Polarization—SOP), all approaches resulting in a distribution of DGD values across the wavelength range. 
A fiber is said to be in random mode coupling regime when it is longer than few hundreds meters, perturbed by external sources randomly distributed along its length, each perturbation site changing locally the state of polarization without altering significantly the intrinsic fiber birefringence; by mode coupling there is intended the energy transfer between the different polarization modes within the fiber.  In the ideal case of random mode coupling within the fiber under evaluation, the distribution of the DGD values obtained by varying the wavelength is a Maxwell curve.  The mean value of the DGD distribution is by definition the fiber PMD (which is therefore usually measured in ps).  Since the PMD increases as the square root of the fiber length in the random mode coupling regime, it is common practice to derive from the fiber PMD a coefficient PMDc defined as the fiber PMD divided by the square root of the length of the fiber under test.  The coefficient PMDc is measured in ps/sqrt(km). 
In order to achieve a Maxwell distribution, a wide wavelength range should be used (typically hundreds of nanometers), so as to measure many DGD values.  This wide range is often impractical for the limited width of spectral sources.  For this reason, the same curve is obtained varying randomly the spatial distribution of the perturbation on the fiber under test.  For example, in the IEC standard, Annex E, strategies for improving precision are set forth, suitable to achieve a distribution of measured DGD values better matching the ideal Maxwell distribution, and thus a better evaluation of the fiber PMD.  One of these strategies calls for merging data obtained by performing repeated DGD measurements on the optical fiber, changing the mode coupling of the fiber between the different measurements. 
One proposed way for changing the fiber mode coupling calls for varying the temperature of the fiber through the different DGD measurements.  In the case of already-installed optical fiber cables, the daily ambient temperature change can be expediently exploited, by properly timing the different measurements, as demonstrated by C. T. Allen et al.  in the technical paper “Measured Temporal and Spectral PMD Characteristics and Their Implications for Network-Level Mitigation Approaches”, Journal of Lightwave Technology, Vol.  21, No.  1, p. 79-86 (January 2003). 
Another way to change the fiber mode coupling in case the fiber to be tested is loosely spooled on a spool calls for rearranging the fiber on the spool by making the spool vibrate, turning the spool upside down or massaging the fiber by hand. 
The Applicant has observed that the perturbation of the fiber is critical: it is necessary to find a good method of modifying the fiber configuration in order the alter the evolution of polarization states, without modifying the original fiber birefringence. 
In this respect, the IEC standard also sets general prescriptions for carrying out the measurements directed to evaluating the PMD, valid independently of the method adopted.  In particular, it is recognized that the deployment of the optical fiber can influence the result; thus, the optical fiber deployment should be selected so as to minimize any externally-induced mode coupling (i.e., any externally-induced birefringence).  The IEC standard points out that sources of such externally-induced mode coupling can be an excessive fiber tension, an excessive fiber bending, induced from fiber cross-overs on a shipping reel and/or crimping of fiber within a cable on a spool that is too small and/or too small a bend radius (concerning the effects of fiber bending on the fiber birefringence, see for example R. Ulrich et al., “ Bending-induced birefringence in single-mode fibers”, Optics Letters, Vol.  5, No.  6, p. 273-275, (June 1980)), an excessive twist of the fiber. 
For these reasons, the IEC standard prescribes that the fiber under evaluation is to be kept at a minimal tension either by deploying the fiber in loops on a flat smooth surface (e.g., the floor of a test room), or by loosely wrapping the fiber onto a spool having a smooth surface; the IEC standard also sets a prescribed minimum smoothness, corresponding to that of paper of grade 88 gsm.  In either case, according to the IEC standard, the fiber is to be deployed without crossing over itself, and with a bend radii in excess of 15 cm. 
The former type of fiber deployment, which in jargon is referred to as the “floor test” deployment, allows closely matching the conditions that the optical fiber under testing will encounter in the normal operating environment (i.e., in an optical cable).  For example in K. Walker, “Fibers and cables for Ultralow PMD”, WJ4, Proceedings of OFC 2003, it is shown that there is a good correspondence between PMDc when measured in cables and “in large coils on a floor”. 
The second fiber deployment, also referred to as the “low-tension bobbin” deployment, is more suitable than the floor test deployment in view of an industrial, mass-scale fiber production. 
The Applicant observes that the known methods of changing the fiber mode coupling are time consuming and error-prone, so they are hardly compatible with the necessity of performing, in an industrial environment, repeated DGD measurements in different fiber configurations for achieving a precise evaluation of the PMD. 
In particular, in the floor test deployment, after each DGD measurement, one or more human operators are required for slightly modifying manually the fiber deployment. 
In the low-tension bobbin deployment, when one tries to rearrange the fiber on the spool by vibrating the spool or repeatedly turning the spool upside-down, or by manually massaging the fiber, so as to change the fiber configuration, it turns out that the fiber is normally not loose enough to obtain the desired Maxwell distribution of DGD measured values. 
Besides, the Applicant observes that the operation of wrapping an optical fiber to be tested around a spool (i.e., spooling the optical fiber to be tested) with a minimal tension not to affect the measures to be conducted may pose some problems.  In particular, the fiber spooling process is rather time consuming, because the fiber spooling speed needs to be kept low in order to keep the fiber tension under tight control.  So, this process cannot be repeated many times on the same fiber to obtain a good Maxwell distribution. 
A different way to evaluate the PMD statistics calls for performing a limited number of DGD values measurements on a limited wavelength range for a given fiber, evaluating the coefficient PMDc thereof, then repeating the operation for different fibers of a same production lot, i.e., fibers produced by the same manufacturing process.  If all the PMDc coefficients derived for the different fibers are represented in the same distribution, a characterization of the overall fiber manufacturing process from the PMD viewpoint is obtained. 
The Applicant observes that, even if this procedure to obtain a PMD statistical description does not require perturbing the fiber configuration, it is in any case mandatory to choose a deployment that avoids any alteration of the fiber intrinsic birefringence. 
In the U.S. Pat.  No.  6,020,584 a method for measuring the PMD of a chromatic dispersion compensating optical fiber is described, according to which a plurality of PMD values is obtained.  A length of the fiber is wound under a tension around a rigid center hub of a spool, in radially outward overlapping layers; the fiber tension is in the range of 40 to 80 grams, preferably at least 50 grams, in order to provide a packing fraction greater than about 0.7, more preferably greater than about 0.8, and most preferably at least 0.85.  Then, a force is applied to the flanges of the spool proximate to an outermost layer of wound optical fiber, to urge the two flanges together, so as to squeeze the optical fiber between the flanges and radially outward from the center hub.  The force is then relieved, and the fiber PMD is measured and recorded.  The steps of applying the force, relieving the force and measuring the fiber PMD are repeated so as to obtain a plurality of post mechanically perturbed PMD measurement values. 
The Applicant observes that the method disclosed in U.S. Pat.  No.  6,020,584 does not comply with the IEC standard prescriptions concerning the minimisation of externally-induced mode coupling, because the fiber is subjected to a non-negligible tension, due to the fact that the fiber is not loosely wrapped on the spool, rather it is wound around the spool under a tension in the range of 40 to 80 grams.  Also, it may be expected that the fiber tension increases as a result of the repeated squeezes to which it is subjected, with the result that the measured PMD values hardly reproduce the Maxwell distribution corresponding to the substantially zero tension measurement, contrarily to what desired. 
In view of the state of the art outlined in the foregoing, it has been an object of the present invention to improve the known methods for evaluating the PMD in optical fibers. 
In particular, the Applicant has observed that most of the PMD evaluation methods and apparatuses known in the art do not allow performing an evaluation of the PMD in an optical fiber that takes into due account the statistical nature of the PMD.  The Applicant has also observed that the lack of a real statistical description reduces the measurement accuracy, making it impossible to properly point out the effects of the different possible fiber deployments on the intrinsic birefringence.  The method disclosed in U.S. Pat.  No.  6,020,584 fails to provide the desired statistical description, because the PMD measurements are biased by externally-induced mode coupling caused by the fiber tension.