Abstract:
In the determination of the angular rotational position of an axial asymmetry, such as of optically inhomogeneous regions, in an optically transparent body, e.g. stress concentration zones of optical PM-fibers, where the body is located in arbitrary angular start positions, the body is illuminated during rotations thereof to different angular positions around its longitudinal axis. For different angular positions the difference is then determined between light, which has passed through the fiber end and in its position corresponds to the central part of the fiber, and light, which has passed through the fiber end and in its position corresponds to the region of the fiber located immediately outside the central part. These differences, considered as a function of the rotation angle, constitute a curve having a shape typical of the considered body. This curve is compared to a reference curve, for different translational positions of the reference curve, and that translation position is found, where a maximum agreement is obtained of the curves. The translation value of this translational position gives the angular offset of the body from a reference position. By this method a correct alignment can be made of for example two optical PM-fibers of different types and also the basic type of an unknown PM-fiber can be determined.

Description:
BACKGROUND 
     The present invention relates to methods and devices for determination of the angular position about a longitudinal axis of an optical body or fiber which is asymmetric in an azimuthal or circumferential direction, i.e. is asymmetric as seen in a cross-section, about the same longitudinal axis in regard of its optical properties, for positioning such a body or fiber in a chosen angular position of the axial asymmetry and for aligning two such bodies or fibers so that the positions of the axial asymmetries coincide, and for two optical fibers, splicing ends of the fibers to each other maintaining the alignment of asymmetries. 
     In a previously developed method for an angular alignment of e.g. PM-fibers in order to splice the fibers correctly, see the published International Patent Application WO-A1 95/14945, which is incorporated herein by reference, a correlation method was applied directly to intensity profiles (POL profiles) obtained from the two fibers to be spliced. The prior method can be called the direct correlation method. The angular offset between the two fibers was then found from the location of the maximum correlation point. One of the two fibers was finally rotated to eliminate the angle offset in order to get the maximum extinction ratio in a splice. After the rotation alignment, the angular offset between the two fibers is almost zero. However, both the initial rotational, relative position of the two fibers is random and, even worse, the final angular position after splicing is random which may cause some problems. 
     The previous method, the direct correlation method, works very well in obtaining a high extinction ratio of the splice between two similar PM-fibers, i.e. two PM-fibers of basically the same type. However, there are also some disadvantages of the method. 
     First of all, since the rotational position is random after the alignment, the core of the spliced fiber is often shaded by the stress applying parts (in e.g. fibers of the bow-tie and Panda types). Thus the core is in this case invisible in a hot or warm fiber image. Then it is often impossible to make a splice loss estimation using the direct correlation method. 
     Secondly, in splicing different types of PM-fibers, the direct correlation method works only well for the stress-induced birefringence fiber types, including bow-tie, Panda, and elliptical cladding PM-fiber types. The correlation profiles between those fiber types show a single maximum at 0 angle offset, as is obvious from the diagrams of FIGS. 1b, 2b, 3b. However, in the case where a stress-induced birefringence PM-fiber is to be spliced to a geometrical birefringence PM-fiber, which is usually an elliptical core PM-fiber, the prior alignment procedure does not give the maximum extinction ratio since at least two local maximum points appear at non-zero angle offset. This can be seen from the diagram of FIG. 4b showing the correlation for different angular shifts for splices of elliptical-core type to Panda type PM-fibers. The correlation curve has two distinct maxima and the desired alignment angle is found in one of the double peaks of one of the maxima. 
     Also, in the published International patent application WO-A1 96/12980 it is disclosed how a determination is made of the angular position of optical axial asymmetries, in this case the two cores of a twin-core fiber. 
     SUMMARY 
     It is an object of the invention to provide a method and a device for determining the angular position of e.g. PM-fibers and generally optical fibers and similar cylindrical bodies which are optically asymmetric, as seen in the longitudinal direction of such a fiber or body, for example as seen in a cross-sectional view. 
     It is another object of the invention to provide a method and a device for aligning axially optically asymmetric bodies such as optical PM-fibers to provide a predetermined relative angular position between the axial asymmetries, which are suitable for all types of bodies and PM-fibers and in particular are suitable for two bodies or fibers having different kinds of asymmetries. 
     It is another object of the invention to provide a method and a device for determining the type of an optical fiber, such as the type of a PM-fiber, which is optically asymmetric, or for distinguishing between different basic kinds of optical fibers having axial asymmetries. 
     Thus, determinations of indirect correlation are made wherein a profile height curve is compared to a simulated one, in particular by comparing the shape of the height curve to the shape of one or more simulated, predetermined typical height curves. A simulated curve is displaced translationally by adding displacements to its arguments for finding the best possible fit. 
     More specifically, the angular position of at least one axial optical asymmetry of a cylindrical body is determined where the asymmetry typically is an optically inhomogeneous region in an optical fiber. The asymmetry is thus located in parallel to the longitudinal axis of the body which is located in an arbitrary angular start position about its longitudinal axis. A height or profile curve is determined by illuminating the body with a light beam, which preferably is essentially perpendicularly to the longitudinal direction of the body. The light beam should naturally comprise light for which the body is transparent so that the body can work as a cylindrical lens for the light. The body is rotated through a predetermined angular interval such as half a full turn for a body having a two-fold axial symmetry and preferably a full turn, from its start angular position about its longitudinal axis. During the rotation the light profile is determined and therefrom the height of the central peak for a number of different angular positions, where thus the height is taken as the difference which is determined between the intensity of light, that has passed through the body and in its position corresponds to the central portion of the body as seen in the longitudinal direction, and of light which has passed through the body and in its position corresponds to regions located most close to and outside the body. The determined differences as a function of the rotation angle from the start angular position are then compared to values of a predetermined function for the same angular positions, where this predetermined function is selected to have essentially the same basic shape as the function formed of the determined differences depending on the rotation angle for the considered type of cylindrical body. The comparison is made for several translational positions of the predetermined function, where the transitional positions are formed by adding different angular values to the argument of the predetermined function. From this comparison that angular translational value for the predetermined function is determined, which gives the best agreement between the determined differences and the translated function values and this angular translational value is taken to be a measure of the angular rotational position of the body from a fixed or reference angular position. 
     The determination of the differences is suitably made along a line which is located essentially perpendicularly to the longitudinal axis of the body or forms a large angle therewith such as in the range of 80-100° and passes closely to or essentially through a focal line for the body considered as an optical lens. 
     In the determination of the differences for different angular positions for the body, for each angular position, a light intensity curve can be determined along a straight line which extends essentially perpendicularly to the longitudinal axis of the body or forms a large angle thereto as above, whereupon this curve is evaluated for determining the difference between the central portion of the curve and the portions of the curve located adjacent to the central portion. 
     This determination can be used advantageously in splicing two optical fibers such as PM-fibers, which each one comprises at least one axial asymmetry and thus can have at least one optically inhomogeneous region that extends in the longitudinal direction fiber and is eccentrically located in relation to a longitudinal axis of the considered fiber. The splice is to be made with a predetermined angle between the angular positions of the axial asymmetries in the two fibers, where this angle can be selected to give e.g. the least possible attenuation in the splice or a desired attenuation chosen for some special purpose. Then end surfaces of the optical fibers are placed close to or opposite each other so that the longitudinal axes of the fibers are essentially aligned with each other or so that the fiber axes are at least essentially in parallel to each other. Thereupon, the ends of the fibers are rotated about their longitudinal axes in order to produce an angular position in relation to each other, so that the axial asymmetries will have the predetermined position in relation to each other, in particular so that an alignment is obtained between the axial asymmetries, in particular between optically inhomogeneous regions in each fiber end. The fiber ends are fixed and/or clamped in this position in relation to each other, and they can be welded to each other by heating and fusioning or melting regions to each other that are located at the end surfaces of the fibers. In rotating the ends of fibers in relation to other the angular position of each fiber end in relation to a reference angular position is first determined as has been described above. A rotation angle for each fiber end is then determined from this determined angular position. 
     In the determination of the angular position of the fiber ends in relation to a reference position the ends can be placed close to each other so that the fiber ends can be illuminated from their sides simultaneously by the same light beam. 
     The determination as has been described above can be used for also determining the type of an optical fiber having an axial optical asymmetry about an longitudinal axis, such as typically a PM-fiber of unknown type. In this case predetermined functions for the same angular positions are provided, where each one of these predetermined functions has essentially the same basic shape as different types of optical fibers having axial asymmetries. The shape of the function formed of the determined differences for the unknown fiber having as an argument the rotation angle from the start angular position is compared to the shape of each one of these predetermined functions. That predetermined function is then determined, which gives the best agreement, and thus the type of optical fiber associated with this predetermined function can be taken as the type of the considered optical fiber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described by way of a non-limiting embodiment with reference to accompanying drawings in which: 
     FIGS. 1a and 1b are diagrams illustrating alignment using the direct correlation method for splices of bow-tie type to elliptical-cladding type PM-fibers, where in FIG. 1a POL profiles and in FIG. 1b the correlation profile of these two POL profiles in relation to each other are shown, 
     FIGS. 2a and 2b are diagrams similar to those of FIGS. 1a and 1b respectively for splices of bow-tie type to Panda type PM-fibers, 
     FIGS. 3a and 3b are diagrams similar to those of FIGS. 1a and 1b respectively for splices of elliptical cladding type to Panda type PM-fibers, 
     FIGS. 4a and 4b are diagrams similar to those of FIGS. 1a and 1b respectively for splices of elliptical-core type to Panda type PM-fibers, 
     FIGS. 5a and 5b are diagrams illustrating the correlation between a 3M elliptical cladding fiber (diameter 125 μm) and simulated profiles according to a first template function for two parameter values γ=1 and γ=29, where in FIG. 5a the POL profile and simulated profiles are shown and in FIG. 5b the correlation profiles as calculated for the POL profile in relation to the two simulated profiles for the same parameters, 
     FIGS. 6a and 6b are diagrams similar to those of FIGS. 5a and 5b respectively for a Panda fiber (125 μm) and simulated profiles for the same parameter values, 
     FIGS. 7a and 7b are diagrams similar to those of FIGS. 5a and 5b respectively for a Bow-tie fiber (125 ∞m) and simulated profiles the same parameter values, 
     FIGS. 8a and 8b are diagrams similar to those of FIGS. 5a and 5b respectively for an elliptical core fiber (125 μm) and simulated profiles for the same parameter values, 
     FIG. 9a is a diagram showing the POL profile of an elliptical cladding fiber (diameter 80 μm) and a simulated POL profile calculated from a second template function using suitable parameters, 
     FIG. 9b is a diagram illustrating the correlation of the measured profile and shifted simulated POL profiles according to FIG. 9a as a function of the shift angle, 
     FIGS. 10a and 10b are diagrams similar to those of FIGS. 9a and 9b for an elliptical core fiber (diameter 125 μm) and a simulated POL profile as calculated from the second template function, 
     FIG. 11 is a schematic picture, partly in block diagram form, showing a device for splicing optical fibers, 
     FIG. 12 is a flow diagram illustrating a rotational alignment procedure using the indirect correlation method for any PM-fiber types. 
    
    
     DETAILED DESCRIPTION 
     By rotating end portions of two PM fibers 360°, and taking POL samples for equally spaced or distributed angular positions, i.e. determining the value h as described in the cited International Patent Application WO-A1 95/14945, see in particular the description of FIGS. 1a-1d, a POL profile is obtained comprising these h-values as a function of the angular, rotated position of the fiber end. For the two fiber ends their POL profiles are denoted by two vectors, L and R, for left and right sides of fibers respectively: 
     
         L={l.sub.1, l.sub.2, l.sub.3, . . . , l.sub.n }            (1) 
    
     
         R={r.sub.1, r.sub.2, r.sub.3, . . . , r.sub.n }            (2) 
    
     where n is the number of total samples taken during the rotation. 
     Such POL profiles have different basic shapes depending on the type of fiber, as is illustrated by the diagrams of FIGS. 1a, 2a, 3a and 4a. Then such a profile can be compared to some predetermined function describing the general basic shape of the curve. For example, one can generate a simulated POL profile by an analytic function s x  (d) where x is the argument and d is a translational value of the function from a zero position. Since the POL profile comprises a full turn of 360° and the real POL profiles generally have a periodicity of 180° with peaks thus located 180° apart, some sum of sine and/or cosine functions can be a suitable choice. The profile can then be described as the vector 
     
         S(d){=s.sub.1 (d), s.sub.2 (d), s.sub.3 (d), . . . , s.sub.n (d)}(3) 
    
     For example, a sum of cosine functions can be used and a suitable one has proved to be 
     
         s.sub.x (d)=a{c.sub.1 cos .sup.γ  4π(d+x)/n!+c.sub.2 cos  8π(d+x)/n!}+b                                         (4) 
    
     The argument x is equal to the values of the index i which is a natural number in the range of  1, n! and d is the translational value, 0≦d ≦n. 
     In Eq. (4) a, c 1  and c 2  are amplitude constants, b and d are displacements in amplitude and phase, respectively. The power constant γ is used to simulate different birefringence types. For examples, stress-induced birefringence PM-fiber can be simulated by setting γ=1, and the geometrical birefringence PM-fiber can be simulated by setting γ=29, see FIGS. 5a, 6a, 7a, 8a. In the diagrams of FIGS. 5a-8b the parameters are set as c 1  =4, c 2  =1.5. Since a and b have no impact to the further linear correlation calculation, they can be set to any values which make the simulated POL curves more close to the measured POL profiles. 
     A standard correlation function is defined as follows: ##EQU1## where the vector X is defined as 
     
         X={x.sub.1 x.sub.2, x.sub.3, . . . , x.sub.n }(6) 
    
     which can be any of the left or right fiber POL profiles, L or R. Using the correlation function (5) it is possible to scan one of the POL profiles X with the simulated POL profile S for different phase displacements d to find the position where the maximum correlation between the two POL profiles, that is between the measured one and the simulated one, can be obtained, see FIGS. 5b, 6b, 7b, 8b. Since d is a continuous variable, the scan can be made in any desirable length of step. For instance, if D is the point where the maximum correlation is found: 
     
         Max{C X, S(d)!, 0≦d≦n}=C X, S(D)!            (7) 
    
     then the polarization position a of the fiber can be accurately calculated by 
     
         α=360·d/n (degrees)                         (8) 
    
     By executing this procedure for the two fiber ends, the angular polarization position of the left and right fiber ends can be determined. Then each fiber can be rotated in the opposite direction through an angle according to their current polarization position (i.e. both fiber ends are rotated to their 0° position) and the fibers will thus be rotationally aligned. 
     comparing this indirect correlation method to the former direct correlation method, we find that an interpolation of the POL data is no more necessary when using the procedure described above. This interpolation is the part which consumes most computing time and most memory in the automatic calculations performed in the direct correlation method. Secondly, with the indirect correlation method the PM-fiber type, i.e. whether the fiber is the stress-induced birefringence type or the geometrical birefringence type, can be automatically found out by comparing the maximum correlation value resulting from different values in the scan of the correlation profiles, see FIGS. 5a-8b. 
     Another template function that can be used instead of (4) is a sum of the first terms of a simple Fourier series for the expected symmetry of the fibers, as given by ##EQU2## 
     Here the four parameters of the set {a 2 , a 4 , a 6 , a 8  } give the essential shape of the function. a 0  is only a level constant, physically signifying the average intensity of light when making a fit of this function to a measured one. It must thus be chosen appropriately. In Eq. (9), d is as above the displacement in phase. The coefficient set can be computed from each measured set of POL-profile values. Since every PM fiber type has its own distinguished coefficient set {a j  }, j=2, 4, 6, 8, thus the computed set {a j  } can also be used to identify the PM fiber type. In FIGS. 9b and lob, two examples are given to show the correlation profiles between real and simulated POL arrays. The parameters of the simulated POL curves in FIGS. 9a and FIG. 10a are computed for {a 2  =6, a 4  =5, a 6  =-2, a 8  =0.2} and {a 2  =2, a 4  =6, a 6  =10, a 8  =5}, respectively. Since a 0  has no impact on the linear correlation calculation, it can be set to any value which makes the simulated POL curves close to the measured POL profiles for people to compare with. From FIGS. 9a and 10a, one also observes that the simulated curves are not necessary to fit the measured POL curves very well. The criterion for a valid simulation is that the maximum correlation should be large enough, for example larger than 0.5, to distinguish itself from other peaks in the correlation profile, and sharp enough to yield a high accuracy of the magnitude of order less than 0.1° for searching its location. The correlation curves in FIGS. 9b and 10b show that the simulation functions works very well giving easily distinguishable maxima. 
     In the table below these parameters are listed for eight fiber types. Also the value &#34;contrast&#34; is given signifying the range of the profile, that is the difference between the highest POL-profile value and the lowest POL-profile value. 
     
                       TABLE 1______________________________________Measured parameters and POL contrasts of eight differentPM (polarization-maintaining) and PZ (polarizing) fiber types.No.  Fiber type   contrast                     a.sub.2                          a.sub.4                                a.sub.6                                      a.sub.8______________________________________1    Bowtie       114.45  55.81                          -14.10                                -14.51                                      -1.772    Panda        130.25  25.56                          40.69 -12.48                                      -19.363    Panda coupler             11.08   1.36 4.05  1.21  -0.374    Elliptic jacket (PM)             12.60   4.15 3.16  -1.81 0.225    Elliptic jacket             27.62   10.61                          -0.78 -5.55 -2.90coupler6    Elliptic core by             53.95   1.83 12.38 10.21 7.62MCVD7    Elliptic core by             7.97    3.51 -1.15 0.32  -0.22OVD8    Elliptic jacket (PZ)             23.89   9.98 3.17  2.08  0.99______________________________________ 
    
     For a piece of an unknown fiber, one can then measure its POL profile and then find the function of the kind (9) which fits best to the measured profile by making some kind of regression analysis. Then its characteristic parameter-set denoted by {a u ,n }n=2, 4, 6, 8, is obtained. To identify the fiber type, one can calculate the linear correlation of {a u ,n } and all known characteristic parameter-sets denoted by {a t ,n }n=2, 4, 6, 8, in turn for example using a correlation function analogous to that of (5). Here t=1, 2, . . . , T indicates one of the T known fiber types. The maximum correlation should be larger than a suitably chosen threshold value cr, if the unknown fiber is to be determined to be one of the types in the known fiber set. The correlations of different fiber types are listed in table 2. 
     Table 2. Computed cross correlation between the characteristic parameter sets of the fiber types listed in Table 1. 
     
                                           TABLE 2__________________________________________________________________________Computed cross correlation between the characteristicparameter sets of the fiber types listed in Table 1.Fibertypes   1    2    3    4    5    6    7    8__________________________________________________________________________1  1.0000   0.2904        -0.2050             0.6438                  0.9514                       -0.9563                            0.945                                 10.92842  0.2904   1.0000        0.8606             0.8546                  0.5669                       -0.0004                            0.1271                                 0.56143  -0.2050   0.8606        1.0000             0.4815                  0.0877                       0.4819                            -0.2890                                 0.14184  0.6438   0.8546        0.4815             1.0000                  0.8405                       -0.4260                            0.4104                                 0.74385  0.9514   0.5669        0.0877             0.8405                  1.0000                       -0.8232                            0.8375                                 0.96536  -0.9563   -0.0004        0.4819             -0.4260                  -0.8232                       1.0000                            -0.9362                                 -0.79027  0.9451   0.1271        -0.2890             0.4104                  0.8375                       -0.9362                            1.0000                                 0.89058  0.9284   0.5614        0.1418             0.7438                  0.9653                       -0.7902                            0.8905                                 1.0000__________________________________________________________________________ 
    
     From these cross correlation values the threshold value cr can be chosen as 0.98. This value can successfully distinguish between these different fiber types. If for a fiber its correlation values are all smaller than the threshold value cr, the fiber will be identified as belonging to an unknown fiber type. 
     A device for splicing two optical fibers is schematically shown in FIG. 11. This device is principally a conventional automatic splicing device for welding optical fibers to each other supplemented with devices for orienting the fibers angularly and provided with special routines for determining intensity curves and analysing them. 
     The two optical fibers 1, 1&#39; which are to be spliced to each other, are placed with their ends in special retainers 3, by means of which the fiber ends can be rotated about their longitudinal axes. These retainers 3 are, in addition, arranged on the usual alignment supports 5 for the fiber ends of the splicing device. The fiber supports 5 can further be displaced in relation to each other in the perpendicular directions which are indicated by the directions of light rays from two lamps 7, and also in the longitudinal direction of the fiber ends by means of drive motors 9, which are controlled by logical circuits and software in a processor logic module 11 through suitable driver circuits 13. The lamps 7 are activated through their own driver circuits 15 by the processor logic 11. Welding electrodes 17 are driven by corresponding driver circuits 19 controlled by the processor logic circuits 11. A video camera 21 makes a picture of the fiber ends and provides the corresponding video signals through a video interface 23 to an image processing and image analysis module 25. The result of the image processing and the image analysis in this module 25 is fed to the processor logic module 11 and the result can be shown on a monitor 27. Also the directly obtained picture of the end regions of the fibers as depicted by the video camera 21 can be shown on the monitor 27. 
     The procedure which is to executed by the splicing machine of FIG. 11 comprising a rotational alignment using an indirect correlation technique is also illustrated by the flow diagram of FIG. 12. This procedure will now be described with reference to FIGS. 12 and 11. 
     The ends of two optical fibers 1, 1&#39; are thus supposed to be placed in the rotational retainers 3, so that the fiber ends are aligned in parallel to and opposite each other. By means of the conventional control procedure performed by the processor logic module 11 the two fiber ends are aligned with each other in the transverse direction in relation to the longitudinal axes of the fiber ends and their end surfaces are also brought close to each other, as illustrated by the start block 1201 of the block diagram of FIG. 12. A picture of the end regions of the fibers can then be shown on the monitor 27. Then the optical system of the machine is re-focused to get a maximum lens effect for each fiber end, by adjusting the video camera 21. The rotational retainers 27 are rotated, e.g. manually by operating operational knobs 29, so that the rotational angles of the fiber ends are varied from a start position to angular values equally distributed over a full turn. For each value the light intensity profile is determined for at least one line adjacent the end surface of each fiber and extending essentially perpendicularly to the longitudinal direction of a considered fiber end. The height h of such a light intensity profile is determined by analysing the curves automatically and determining the heights of their central peaks, this being performed by the image module 25. When the h values have been determined for the two fiber ends, it is asked in a block 1203 whether the left fiber is a known type. 
     This question can e.g. be shown in a window of the monitor or indicated in some other way. If the fiber type is not known, a calculation is made in a block 1205 where a simulated POL-profile having a best agreement with the measured h-values is determined. In the calculation the simulated profile is shifted in regard of its argument and then parameter values characterizing the simulated profiled are determined and also the d-value D --  left corresponding to the shifted angle giving the best agreement. Then it is asked in a block 1207 whether the operator wishes to determine the fiber type. The operator can indicate his wishes by some manual input. In the case where an identification of fiber type is not desired, the evaluation procedure is finished for the left fiber, and otherwise a block 1209 is executed. There the correlation values co are calculated of the parameter set determined by the measured values to stored parameter sets typical of different fibers. That stored fiber type is then determined giving the maximum correlation value co max  . Then in block 1211 it is decided whether this determined maximum correlation is larger than the criterion value cr. If it is larger, then in a block 1213 the fiber type having the parameter values giving this maximum correlation is indicated to the operator in some manner. If instead the determined maximum correlation value is not larger than cr, another message is displayed, as is illustrated by the block 1215, indicating to the operator that the type of the left fiber cannot be determined. Then the evaluation procedure is finished for the left fiber. 
     If it was decided in the block 1203, that the type of the left fiber is known, a block 1217 is executed where the correlation profile is calculated for the measured h-profile and a simulated profile, that is defined by parameter values stored for this known fiber type. Then the shift value D --  left is calculated giving the largest correlation. Then the evaluation procedure is finished for the left fiber. 
     The same evaluation is then made for the right fiber end, in the blocks 1221-1237. Thus an angular translational value D --  right of the right fiber end is determined. 
     When a splicing of the fibers is to be performed, the fiber ends are rotated to optimal positions for taking warm fiber images, this being performed in blocks 1241 and 1243, the rotational values being -360°·D 13  left/n and -36020 ·D --  right/n, from their start positions respectively. Then, also of course the polarization planes or the planes through the stress zones of the two fiber ends will be located so that they are aligned with each other. Finally, in these positions the fiber ends are welded to each other, see block 1245.