MXPA01009925A - System and method for measuring polarization mode dispersion suitable for a production environment - Google Patents

Abstract

A system for measuring polarization mode dispersion (PMD) in a fiber using a polarizer controlling the polarization state of light input to the fiber and a polarization analyzer measuring the polarization state of light output from the fiber. Jones matrix analysis is applied to data derived from three input polarization states and two wavelenghts of probing radiation. Performance is improved by using incoherent light sources such as light emitting diodes in conjunction with two bandpass filters. However, a laser source and optical detector are used to align the fiber. The system is particularly useful in measuring PMD values in short lengths of fiber and mapping those values with a long fiber from which the test fiber was cut. Preferably, the PMD is measured for various values of twist experimentally induced in the test fiber, and the short-length PMD value is that associated with zero-internal twist in the fiber as calculated according to a model. The fiber may also be loaded during measurement.

Description

SYSTEM AND METHOD FOR MEASURING DISPERSION OF ADEQUATE POLARIZATION MODE FOR A PRODUCTION ENVIRONMENT This application claims the benefit of the provisional patent application of E.U.A No. 60 / 127,107, filed on March 31, 1999.

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates generally to optical measuring methods and equipment, and in particular to said equipment and methods for measuring birefringence in such measurements as differential group delay.

TECHNICAL BACKGROUND Optical fiber is the preferred transmission medium for long distance telecommunication systems due to its very large bandwidth (ie, data transport capacity) noise immunity, and relatively low cost. Silicon fiber optic attenuation has been reduced to such low levels that it is possible to transmit data over hundreds of kilometers without the need for amplifiers or repeaters. The data transport capacity of a fiber communication system in relatively short distance is largely governed by the speed of the electronic and optoelectronic elements used in the transmitter and receiver. Currently, the most advanced commercially available optical receivers and transmitters are limited to approximately 10 gigabits / sec (Gb / s), although systems of 40Gb / s are being considered. However, at typical longer distances for telecommunication, dispersion of various types may limit the useful bandwidth. A cylindrical optical fiber of considerably large cross section can transmit a number of waveguide modes having different spatial force distributions. The propagation speed differs between fundamental mode and high order modes in an effect called modal dispersion. An optical signal applied by a transmitter in the fiber will normally contain a distribution of all the modes that can be supported by the fiber. Due to the modal dispersion, the different modes after traversing a large fiber section will reach the receiver at slightly different times. The speed of transmissions is limited by the dispersion integrated along the length of the transmission. In order to avoid modal dispersion, most modern fiber communication systems designed for long distance transmission are based on a fiber in a simple way. In the case of a simple fiber with a core and a coating, the core of a fiber in a simple way is so small, taken together with the difference in refractive indexes between the core and the coating, that the fiber will only support the fundamental mode . All high-order modes are rapidly attenuated at distances associated with long-distance telecommunication. The description is more complicated for a profiled fiber or for a fiber having multiple layers of coating, but it is well known to manufacture and test a fiber for it to be simple. In fact, a single circularly symmetric fiber supports two fundamental transverse modes corresponding to the polarization states of the lower order modes. For a fair approximation, these two lower order modes are degenerate in the circular geometry of a fiber and have the same velocity of propagation, so that there is no polarization dependent dispersion. However, as will be explained below, the polarization dependent dispersion may arise in a realistic fiber. In the past, high-speed bit transmission over long fiber distances in a simple way has been limited by chromatic dispersion, also characterized as group velocity dispersion. A data signal applied on an optical carrier signal causes the optical signal to have a finite bandwidth, whether it is considered to be produced through the spectral decomposition of a pulse signal or through the data bandwidth of a signal. analog signal. Usually, the propagation velocity or propagation constant of an optical signal depends basically on the refractive index of the core, and varies with optical frequency. As a result, the different frequency components of the optical signal will reach the receiver at different times. Chromatic dispersion can be minimized by operating at wavelengths close to zero dispersion, approximately 1300 nm for silica, or by other methods to compensate dispersion. Despite its circularly symmetrical design, the actual optical fiber is typically birefringent. This means that the two axial modes of lower order do not degenerate, and the fiber at any point can be characterized as having a fast axis and a slow axis. The two modes that travel the fiber with their electric field vectors respectively aligned with the fast and slow fiber axes will propagate relatively faster or slower. As a result, the group speed of a signal traveling the fiber is a function of the polarization state of the optical signal. Birefringence can arise from internal or external sources. The fiber may have been stretched with a slight non-physical circularity. The fiber can be installed so that a fold, side load, anisotropic tension, or twist is applied to it. The birefringent interaction is complicated by coupling the two modes that also occur in fiber twists, bends or other causes. The coupling causes the energy to transfer between the orthogonal modes. But even with the mode coupling, the group delay continues to disperse, resulting in a significant polarization mode (PMD) dispersion or delay. The cause of mode coupling is not fully understood, but it is molded through a statistical model of coupling sites so that they occur randomly with an average distance between the sites (mode coupling length), which typically assumes a value between approximately 5m and 100m. The exact length of coupling mode depends on the unfolding of the fiber and is usually not characteristic of the intrinsic birefringence of the fiber. It is estimated that above about 10 Gb / s, the polarization mode dispersion limits the bit rates of the fiber more than other types of dispersion. Polarization mode dispersion also degrades cable television (CATV) systems by introducing composite second order distortion and signal fading. Some fiber manufacturers stretch their fiber with a small continuous twist applied to the fiber, so that manufacturing anisotropies do not allow fast and slow modes to always be aligned in a propagation mode. For this reason, the difference in propagation delay between the two modes is reduced, resulting in reduced PMD. An additional technique to reduce the net PMD over a long distance is to periodically reverse the direction of the manufacturing torque. In the past, polarization mode dispersion has been treated as an amount that depends on the time a statistical description requires. PMD has usually been measured in long lengths (one kilometer or more) of fiber wound under a lower tension around a large diameter spool. The fold and tension induced by high voltage winding on a smaller boarding spool affects the birefringence and coupling mode and therefore, PMD is experienced. However, installing such a test requires time and resources. In addition, sections of a fiber cut kilometer from the boarding reel or production line can not be used in the opposite way, and the test represents a loss of one kilometer of fiber, which for a standard 25 km spool It is a loss of 4%. Accordingly, it is desired to measure the dispersion effects of polarization mode which are expected to be experienced in a realistic environment without the need to test extensive fiber lengths. It is also desired, measure the dispersion effects of polarization mode in a precise and determinant way.

BRIEF DESCRIPTION OF THE INVENTION The invention includes a method and apparatus for measuring polarization mode dispersion in an optical fiber, preferably quantized as a differential group delay between two fundamental polarization modes. In one aspect of the invention, one or more incoherent light sources are used in conjunction with optical bandpass filters to provide light to a polarimeter arranged to measure birefringence in an optical fiber. The polarimeter measures how the fiber affects the polarization state of light that passes through it, preferably through a delay or dispersion measurement of polarization mode. The visible laser light can be changed on the fiber for visual alignment. The laser light of wavelength comparable with those of the inconsistent sources can also be changed in the fiber and electronically detected to complete the alignment. An optical switch can be placed on the output of the fiber under test to change the light alternatively to the polarimeter and the alignment detector, without affecting the polarization mode dispersion measurement. The fiber can be subjected to a selected amount of twist along its length. The measured torque-dependent polarization mode dispersion can be used to determine different optical properties of the fiber. The fiber can also be subjected to a selected amount of filler or otherwise strained during its test. The polarization mode dispersion value measured for a short fiber length can be empirically mapped to values for larger fiber, with the polarization mode coupling length being intermediate with the two fiber lengths. The mapping can be used to measure the coupling length of mode.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic illustration of the system for measuring polarization mode dispersion over a short length of optical fiber. Figure 2 is a graph of the torsion dependence of the differential group delay of a fiber. Figure 3 is a graph of the mapping between short-length and long-length values of the polarization mode dispersion. Figure 4 is an axial cross-sectional view of a load cell.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES The invention allows the measurement of polarization mode dispersion performed by an improved measurement system over a short fiber length. The fiber length is generally maintained at about 1 m, which is usually less than the value at which the modes are mixed randomly by environmental influences, i.e. the mode coupling length. The short length value can be mapped to much longer fiber lengths in order to predict their behavior in the field. In the basic Jones matrix measurement technique, the differential group delay? Tn between two orthogonal polarization modes is measured on a frequency scale between coi and? N. Under the normal circumstances described here, only the two end frequencies? N- need to be measured? and? n that enclose a wide region of interest, for example, wavelengths of 1300 nm and 1550 nm. The differential group delay? Tn is derived from Jones' T matrices measured for each of the two frequencies. A Jones matrix T is a 2x2 matrix with possibly complex elements that relate the polarization states of two orthogonal input signals, expressed as two-component vectors, to the corresponding polarization states of the output signals after traversing some component optical that is measured. An example of the optical measurement circuit used to measure the Jones matrices is illustrated in the schematic diagram of FIG. 1. A fiber under test (FUT) 10 having a length of about 1 m extends over a table in a line straight. Two light sources of narrow band 1214 are selectively changed by a 4 x 1 16 optical switch to a single mode input fiber 18. A first lens 20 collimates with the light of the output fiber through a controllable polarizer 22. A second lens 24 directs the polarized light from the input fiber 18 to the input end of the FUT 10. One of the lenses 20, 24 can be removed with a lens focused on both fibers 10, 18. The light output by the FUT 10 is changed through a 1 x 2 optical switch 26 to a single mode output fiber 28 that enters a polarization analyzer or polarimeter 30, such as an HP8509B available from Hewlett-Packard of Palo Alto, California. The fiber on both sides of the 1 x 2 switch 26 will be referred to as the output fiber 28. A polarimeter measures the polarization state of a detected signal, which can be characterized as a point on the Poincare sphere. The equator of the Poincare sphere represents linear polarizations, the poles represent the two circular polarizations, and the intermediate surface represents elliptical polarizations. For each optical frequency, the polarizer 22 is set to three different angular positions or the three polarizers aligned differently 22 are inserted in the beam path to produce a known consecutive set of linearly polarized state entering the FUT 10. The analyzer of polarization 30 measures the resultant complex output polarization state vector, which can be represented as h, v, and q. A set of angles commonly used are 0o, 6o and 120o although 0o, 45o and 90o can easily be replaced. From these six polarization states, the Jones matrix can be calculated within a multiplicative constant through a method such as the one described above. A set of complex relationships of the three measured states are calculated from the values of x and y of the measured state vectors: ki = hx / hy; k2 = vx / v and k3 = qx / qy; and k4 = (k3 - k2) / (k? - k3). Within a complex scalar multiplier ß, the Jones transmission matrix T is given by Because this is an analysis of eigenvalue, scalar constants such as ß are not important. A linear input polarizer 22 is preferred, but some type of polarizer is needed to adjust the input bias state while the polarimeter 30 measures the output bias state. The illustrated polarization analyzer 30 includes an optical output that can be selected between two Fabry-Perot lasers emitting about 1310 and 1550nm. This laser source can be switched to the FUT 10 through the 4x1 16 switch. However, as described below, other light sources are desired for the main measurement. Both optical switches 16, 26 can be implemented with commercially available switches, for example, those based on mechanically mobile optical fibers that selectively couple a port with any other different ports. The Jones matrices measured in the two frequencies are used to calculate a matrix product T (? N) 1 (? N-?), A matrix of 2x2 itself, where T1 denotes the inverse matrix, TT "1 = 1 , where 1 is the diagonalized unit matrix, then the differential group delay is calculated as = \ Arg. { p \ lp?) \ where pi and p2 are the complex eigenvalues of the matrix product T (? n) T "1 (? n-?) and Arg denotes the argument function Arg (A e1?) =? The eigenvalues are the two elements diagonals of a diagonalized version of the matrix product T (? n) T "1 (? n-?), where the diagonalization is performed with techniques of own analysis well known in quantum mechanics and optical system. The DGD (differential group delay) μm is a measure of the birefringence of the fiber or polarization mode dispersion for wavelengths within the wavelength scale of the measurement and normalized for the measured fiber length. In practice, to eliminate the effect of the output fiber 28 and associated components in the optical output path, the path between the polarizer 22 and the polarization analyzer 30 is divided into two parts, the path through the FUT 10. which has a Jones F fiber matrix and the output path having a residual Jones matrix R. A measurement of the Jones matrix M is made for the entire trajectory including both the FUT 10 and the output fiber 28. Then the FUT 10 is removed and the polarizer 22 and the associated optical system 20, 24 are brought to the corresponding point to the output end of the FUT 10. The Jones residual R matrix is measured for the output fiber 28 and other parts within the output path. The eigenvalues pi, p2 are then calculated for FUT 10 based on the product of matrix F? F12 = R-1 1M1M'12R2 This technique is applied to measure differential group delays of less than 12 femtoseconds (12X10"15 s) with a resolution of at least 50 attoseconds (50x10"18 s). The measurement circuit of Figure 1 is improved in different ways. Instead of the conventional lasers included in the polarization analyzer 30, light emitting diodes (LEDs) are used as the light sources 12, 14. Commercial LEDs are available which emit two wavelengths, for example, 1310nm and 1550nm. The outputs of the LEDs 12, 14, which have relatively wide spectra because they do not have laser action, are filtered by respective optical bandpass filters 40, 42, for example, thin film dielectric interference filters with a width of spectral bands of 3db of approximately 10nm centered near the optical output peaks of their respective LEDs 12, 14. Other non-coherent light sources can be used. A single light source could be used for the two wavelengths if it emits sufficient light at the two wavelengths. The combination of LED 12, 14 and bandpass filters 40, 42 reduces the problem of coherence noise. The coherence noise arises in the butt-coupled joint between the FUT 10 and the output fiber 28 in which the two fibers have two facets separated by a small space to reduce reflection. A laser has a coherence length of approximately 30 cm. As a result, multiple reflections of a coherent signal in space can interfere constructively or destructively, creating noise.

For light from an inconsistent source that produces light with a coherence length of less than approximately 200μm (twice the smallest air space in the system), light can not significantly interfere with multiple reflections. It is preferred to use the LEDs 12, 14 instead of the originating light of the polarization analyzer 30. The LEDs have no laser action and therefore have a very short coherence length. They do not have a relatively broad emission wavelength, but the bandpass filters 40, 42 reduce the bandwidth to an acceptable value to allow accurate polarization measurements, but the bandpass of the filters 40, 42 should not be so narrow to lead to coherence noise. Another means for reducing coherence noise is to segment the output end of the FUT 10 at an angle that differs from the facet angle of the output fiber input 28 by at least about 11. It is unlikely that the light will resonate in said variable space. Preferably, the FUT 10 is segmented perpendicularly, and the input end of the output fiber 28 is segmented by approximately 11 as indicated by the slant line in Figure 1. The alignment of different fibers and the alignment required to measure the Jones' R matrix for the output path, is performed through the non-polished translational stages at the fiber output end, at either end of the FUT 10, and at the input end of the output fiber 28. Output fiber 28 must be rigidly supported so that it does not introduce variable bias mode dispersion between measurements. The approximate alignments, normally made after bank maintenance, are facilitated by changing the output of a visible laser through the optical switch from 4x1 16 to the FUT 10 or, during the residual measurement, towards the output fiber 28. The light Visible propagates with relatively high loss in the fibers in a simple way in infrared 10, 18, 28 and causes the fibers to shine, and either the brightness or the light output, can be observed visually for initial alignment. The optical intensities of the light output of the LEDs 12, 14 are relatively low in comparison with the laser light of the laser sources in the polarization analyzer 30. For precise alignment, the 4x1 16 switch and the 1x2 26 change light from the laser source of the polarization analyzer, which is wavelength in a simple manner in the fibers, to an optical power detector 46, and the steps are adjusted to maximize the signal from the detector 46. has observed that the contribution of the 1x2 optical switch for polarization mode dispersion remains relatively stable, so that once it is considered in the residual matrix R, it does not interfere with the measurement of the Jones F-matrices of the FUT10. Of course, it is possible to incorporate the detector 46 in the polarization analyzer 30, which already includes at least one detector. The laser sources in the polarization analyzer 30 can also be used to detect phase identification. This effect arises from the fact that the measured values are essentially phase angles mapped to the Poincare sphere, and these phase angles are ambiguous within 180 ° factors. To detect possible identification, either the analyzer laser source or another laser having a wavelength a little different from that of the two LEDs 12, 14, is used to measure another Jones matrix. If the three DGD values associated with wavelength are almost constant, then the measurement is probably valid. If the values for the average wavelength are different, there is a good possibility that the measured polarization mode spread is artificially low due to identification. An alternative device to that in Figure 1 includes, instead of the HP polarization analyzer, a polarimeter using a revolving half-wave plate, such as the PA430 model commercially available from Thor Laboratories of Newton, New Jersey. The input end of the fiber 28 and the polarimeter are placed in a transversely movable stage. The fiber 28 is directly connected to the optical ergometer 46 without intervention switch 26. With the step placing the fiber 28 at the outlet of the FUT 10, the stages at the two ends of the FUT 10 are adjusted to align the FUT 10 with the assistance of the ergometer 46. The transverse stage then moves the polarimeter to face the exit of the FUT 10 with intermediate free space. The DGD measurement is then performed as described above. The apparatus offers more stability and eliminates the need to consider the residual matrix R. Even other types of polarimeters are available, for example, those using optical temporal domain reflectometry. The fiber torsion effect can be investigated by adding one end of the FUT 10 to a torsion unit 50 which can rotate about the longitudinal axis of the FUT 10. The other end of the FUT 10 is immobilized for torsion by a fastener not illustrated Because the length of the FUT 10 is short and selected to be less than the blend length of mode, the torque effect induced in polarization mode dispersion is deterministic and can be predicted through the photoelastic effect with minimal effect from the mode mix. The torsion unit 50 should be designed to minimize the anisotropic forces in the fiber since it would contribute to its own birefringence. A prototype design includes two cylindrical clamps that hold the fiber. A jig supports the brackets spaced about 2 cm in a sufficiently firm manner to support the fiber circumferentially as it is rotated, but soft enough not to induce additional birefringence in the fiber. A template attached to one end of the fiber is fixed, while the other template attached to the other end of the fiber is mounted on a rotating stage that can rotate, for example, five turns in each direction. The torque unit 50 can be used for a number of different purposes. It can measure the torsion effect stored in a fiber and incurred during winding. Previous attempts to do this have used fiber lengths of 100 m. It can be used as an alignment tool if the installation of the FUT 10 inadvertently induces a torsion, as often happens in a production environment. As will be evident in this discussion, it can be used to separate torsion-induced birefringence from the intrinsic birefringence of the fiber, sometimes reported as interference length. It is considered that a PMD value associated with zero torsion and measured in a short fiber is the best predictor of PMD for a long fiber. The net value of zero torsion in a low torsion region can be derived in spite of the experimentally induced torque and fabrication by using a model for polarization mode dispersion induced or present torsion in a short length of non-spun fiber (fiber without significant twist in the stretching procedure) as a function of the torsion angle? ? torsion where? ttorsion is the net value of DGD of zero torsion,? or is an offset angle of torsion, and? ß is the inherent birefringence of the fiber, which is inversely proportional to the interference length Lß. An example of the polarization mode dispersion measured as an applied torsion function is shown in the graph of Figure 2, where a turn has 360 ° of torsion. The experimental data is marked with circles. The data have been adjusted to in curve 60 with the above equation according to the two parameters? To (the peak of curve 60) and the inherent birefringence? ß, which corresponds to an interference length of L = 9.75 cm. However, the effective induced torsion is assumed to equal 0.92 to that of the actual mechanical torsion, where the difference is due to a photoelastic effect in the opposite direction. The interpolation provided by the curve that conforms to the previous equation, provides a more accurate value of the net polarization mode dispersion of zero torque? To. In this curve, the torsion phase shift? Or experimentally induced or otherwise present has already been aligned. The internal torsion can be induced by the operator, and there are unusual values of 0.75 turns / m. The fiber winding operation can twist the fiber and the twist is not reversed by the operator. Typical values of 0.3 turns / m are typical. The manufacture of fiber can inadvertently introduce a net unidirectional spin. It is not time that a fiber that is manufactured with a spin oscillation (clockwise, then counterclockwise) with an amplitude of approximately 3 turns / m has a unidirectional spin of 0.1 spin / m. The spin differs from torsion, in that there is no photoelastic force of restoration for induced spin during the stretching procedure. To consider the torsion induced in the determination of an intrinsic birefringence in the fiber, the following procedure can be followed. After the aforementioned intensity alignment has been performed, the polarization mode dispersion must be mediated for a number of torsion values. Between each measurement, the input side of the FUT 10 is realigned to compensate for any rotation offset. The torsion angle? which presents the maximum value of polarization mode dispersion, as measured with the polarization analyzer, is considered as the net zero torsion position? o. It is not uncommon to require 90 ° of torsional compensation, and at least part of this is considered induced during fiber assembly. When using the initial measured value for polarization mode dispersion, a very low value would normally result according to the dependence shown in Figure 2. Of course, it is appreciated that the repetitive measurements required for polarization mode test and Torsion experiments can be easily automated. In addition, the torsion equation can be generalized to the unknown angular offset or to combine the torsional alignment and generation of the torsion data. It is also appreciated that the residual polarization mode dispersion, ie, the Jones residual R matrix, needs to be tested only infrequently, because it is assumed to be independent of the fiber used as the FUT 10. The torsion dependence predicted by the above equation and observed experimentally in figure 2, assumes that the photoelastic effect is relatively small, so that the torsion does not induce significant tension in the fiber. Expressed alternatively, the inherent birefringence is assumed to be large compared to the photoelastic effect. A more complete version of the equation that incorporates stress effects is given by TO :. torsion where g is the photoelastic constant and g 'is its derivative with respect to frequency, dg / d ?. Any negative value of? T must be changed to positive value. This equation also takes advantage of the relation? ß =? To. Typical values for silica are g = 0.14 and g '= 1.036x101"7 when the angles están are expressed in rad / m, t t in s / m, and en in rad / s, for very small values of inherent birefringence? , the observed torsion dependence? torsion begins with a very low value and increases in a monotonous way with the torsion difference angle (? -? o) for positive and negative values of the difference angle For fibers with said inherent birefringence low in the that your non-twisted DGD can not be measured, the DGD of zero torsion can be calculated from the decrease of the larger values of the sides.In intermediate scales of inherent birefringence and photoelastic effect, the peak of the figure is surrounded by clearly ascending vignettes The polarization mode dispersion measured for a short fiber length, i.e., significantly shorter than the mode coupling length, needs to be somewhat associated with a value for a long fiber for which the coupling of unmeasured mode has a significant effect. The association can be done with an empirically developed cartography. A fiber length of 1 km is tested for polarization mode dispersion, for example, according to the conventional procedure described above. The long-length measurement is made under a number of predetermined conditions of temperature, diameter of the fiber spool, fiber tension in the spool, and the type of cable in which the fiber is embedded. A fiber length of 1 m is cut from one end of the fiber of 1 km (or possibly both with repetition of the procedure) or from the same reel, and that short length is tested for polarization mode dispersion according to the method of the invention described above. Preferably, any rotational torque introduced during manufacture is removed by the torsional alignment, although the mapping can be performed without zeroing the torsion. The short-length polarization mode scattering coefficient measured? TcoRTA is then matched with the long-length scattering coefficient measured? TLA GA- In practice, the DGD of short length is normalized to the length of the fiber that is measured, while the long-length DGD is normalized to the square root of the length of the fiber because these are the observed dependencies of the differential time delay in the two regimes. A large number of samples is measured, perhaps 200 to 1000 samples for each cartography. Each sample is taken from a single fiber boarding reel. It is expected that the long and short normalized differential group delays are related by where LMCL is the average length of mode coupling. As a result, the mapping largely simply quantifies the average coupling length of mode for a particular type of deployment, as long as the deployment conditions do not additionally change the short-length DGD. It is expected that the relationship of the previous equation will support lengths greater than 1 km. A preliminary mapping for 15 fibers is presented in the data marked with circles in figure 3. The indicated linear adjustment of these data corresponds to an average coupling length of LMCL mode of 2.9 m. This value satisfies the conditions that the fiber LCORTA short length is less than the LMCL mode coupling length and the long length LLARGA is greater than the coupling length of mode. The cartography demonstrates the validity of the relationship of the previous equation. Reels of posterior fibers, at least made with the same general manufacturing techniques, are tested only for a short length value. The empirical cartography is used to predict the weathering behavior that depends on the longitude value.

The apparatus for measuring the polarization mode dispersion shown in Figure 1 can also be used to measure the effects of wiring in a weather environment. The FUT 10 is positioned between a loading jig, illustrated schematically in Figure 4, comprising a table 70 and a loading block 72. A variable load L is applied to the loading block 72 to impose a lateral load on the fiber 10. , and the apparatus of Figure 1 is used to measure the DGD, that is,? to. The experiment is repeated for a number of different values of the load to demonstrate the loading effect. This measurement was made for three fibers that have low, medium and high DGD without load. When the load was increased to 2400 g / m, the DGD low fiber showed a very large relative increase, the average DGD fiber presented only a modest increase, and the fiber with high DGD showed a decrease. The fiber lengths mentioned in the examples are only illustrative. Although a length of 1 m is preferred for the fiber under test, the experimental equipment can be extended to 5 m without undue inconvenience. Shorter lengths at 1 m are possible, but introduce difficulty in measuring small dispersion values in polarization mode. Attempts to use lengths of 30 cm have proven to be difficult due to the small measured values. Lengths less than 2 m are of convenient size, with 1 m being preferred. Although conventionally, fiber lengths of 1 km have been measured for polarization mode dispersion, in many circumstances the mixture of suitable polarization mode can be achieved in lengths greater than 100 m. These lengths will be compared with typical 25 km spool lengths, although reel lengths can be on the 4 km to 50 km scale. In this way, it is appreciated that the invention provides an effective and simple apparatus and method for measuring birefringent properties, such as differential group delay, in an optical fiber. The invention also provides a method for predicting the birefringent behavior of long lengths without having to measure the long fiber lengths. It will be apparent to those skilled in the art that various modifications and variations may be made to the present invention without departing from the spirit and scope thereof. Thus, it is intended that the present invention encompass the modifications and variations of this invention as long as they fall within the scope of the appended claims and their equivalents.

Claims (10)

NOVELTY OF THE INVENTION CLAIMS

1. - A polarization mode dispersion measurement system for measuring a fiber under test comprising: at least one incoherent light source that emits light at a first wavelength and at a second wavelength; an optical polarizer adjustable to at least three polarization states; two band pass filters passing light respectively at said first and second wavelength and which can be inserted into a light path between at least one said light source and said optical polarizer, the light output of said polarizer being received for said fiber under test; and a polarimeter that receives an optical output of said fiber under test and that measures a state of polarization of light received from said fiber under test.

2. The system according to claim 1, further characterized in that at least one light source comprises a first and a second light emitting diode emitting respectively in said first and second wavelengths and the first and second wavelength filters. bandpass receive respective optical outputs from the first and second light emitting diodes; and further comprising a first light switch having two first switch inputs that receive respective outputs from said first and second bandpass filter and that can be selectively connected to a first switch output, the polarizer receiving the first switch output.

3. The system according to claim 2, further comprising: a second optical switch comprising a second switch input that receives the light output from the fiber under test, and at least two second switch outputs that are can selectively connect to the second switch input, an output of a first of the switch outputs being received by the polarimeter; and an optical detector that receives an output of a second of the second switch outputs.

4. The system according to claim 3, further comprising a laser that emits within a bandwidth associated with the first and second light emitting diodes and wherein the first switch a third input of the first switch that receives a laser output and which is selectively connected to the first switch output.

5. The system according to claim 3, further comprising a visible laser that emits at a visible wavelength and wherein the first switch includes a fourth switch input that receives a visible laser output and is connected from selective way to the first switch output.

6. The system according to claim 1, further comprising: an optical switch comprising a switch input that receives the light output from the fiber under test, and at least two switch outputs that can be connected from selectively to the switch input, an output of a first of the switch outputs being received by the polarimeter; and an optical detector that receives an output of a second one of the switch outputs.

7. The system according to claim 1, further comprising: a laser emitting within a bandwidth associated with the first and second wavelengths; and a switch that includes at least two inputs that receive respective outputs from the laser and the two bandpass filters and that is selectively connected to an output that provides light to the fiber under test.

8. The system according to claim 1, further comprising: a visible laser that emits light at a visible wavelength; and a switch that includes at least two inputs that receive respective outputs from the laser and two bandpass filters and that is selectively connected to an output that provides light to the fiber under test.

9. The system according to claim 1, further comprising a torsion unit for inducing a selected amount of twist along the fiber under test.

10. The system according to claim 1, further comprising a load cell capable of inducing a selected load on the fiber under test. 1 - . 1 - A method for measuring a birefringent property in an optical fiber comprising the steps of: (a) passing incoherent light through a bandpass filter to form probing light; (b) adjusting a polarization state for the polling light; (c) passing the probing light with the adjusted state of polarization through a fiber under test; (d) detecting a polarization state for the polling light with the adjusted state of polarization after the fiber is removed; and (e) from repeated sequences of steps (a) to (d), determine the birefringent property of the fiber under test. 12. The method according to claim 11, further comprising aligning the fiber under test with coherent light. 13. The method according to claim 11, further characterized in that the determination step uses polarization states measured for two optical wavelengths passed through two bandpass filters and three sets of polarization states. 14. The method according to claim 13, further characterized in that the determination step uses Jones matrix analysis. 15. The method according to claim 11, further characterized in that the birefringence property is a differential group delay. 16. The method according to claim 11, further comprising: (f) twisting the fiber under test to a twist value, wherein the determining step defines the birefringence property value corresponding to the twist value. «17. The method according to claim 16, further comprising: (g) repeating steps (a) to (f) for a plurality of torsion values; and (h) selecting a representative birefringent property of the fiber under test from the plurality of birefringent property values. 18. The method according to claim 17, further characterized in that the selected birefringent property is a differential group delay. 19. The method according to claim 18, further characterized in that the differential group delay is a determined maximum value of the plurality of differential group delay values. 20. The method according to claim 11, further comprising applying a plurality of charges to the fiber under test and measuring the birefringent property for each of the charges. 21. The method according to claim 11, further characterized in that said birefringent property is a differential group delay. 22. A method for measuring polarization mode dispersion comprising the steps of: (a) passing light from an incoherent light source through a bandpass filter, having a transmission peak at a wavelength of transmission, to a polarization system that passes a state of polarization of light that can be selected; (b) passing light of the polarization system through a fiber under test; (c) measuring a state of polarization of light output from the fiber under test; (d) performing steps (a) to (c) at least six times for all combinations of three light polarization states of said polarization system and for two transmission wavelengths; and (e) calculating a polarization mode dispersion from the six polarization states measured in step (). 23. The method according to claim 22, which includes aligning the fiber under test with a laser light source incident on said polarization system and an optical detector that receives an output from the fiber under test. 24.- A method to qualify a reel of at least 4 km of fiber comprising: cutting a length of no more than 5m of fiber from the reel, 5m of fiber forming a fiber under test; measure a polarization mode dispersion of the fiber under test; and associating the polarization mode dispersion measured for the fiber under test with the fiber remaining on the reel through an empirically derived cartography. 25. The method according to claim 24, further characterized in that the empirically derived cartography comprises: (a) a first fiber wrapped on a first reel comprising at least 4km of fiber, cutting a length of at least 100m of the first fiber; (b) measuring a first dispersion value of polarization mode in at least 100m fiber; (c) cutting a length of not more than 5m from the first fiber; (d) measuring a second polarization mode dispersion value of no more than 5m fiber; (e) associating the first dispersion value of polarization mode with the second dispersion value of polarization mode; and (f) repeating steps (a) to (e) on other fiber spools to thereby form a mapping between said first and second spreading values of polarization mode. 26.- The method according to claim 25, further characterized in that step (b) of measuring the first dispersion value of polarization mode includes: externally twisting the fiber under test to a twisting sequence and measuring the torsion values respective polarization mode dispersion; and selecting for the first polarization mode dispersion value a value derived from the respective torsion values associated with a predetermined amount of torsion internally experienced in the fiber. 27. The method according to claim 26, further characterized in that the selected value is a maximum value derived from the respective torsion values. 28. The method according to claim 27, further characterized in that the selection step includes adjusting the respective torsion values to a ratio that can be described with a number of parameters lower than the number of respective torsion values and calculating the first polarization dispersion value from the parameters.