Abstract:
A method for testing optical fibers includes using an optical testing instrument to measure a characteristic, such as clad non-circularity, of an optical fiber at a multiple angles of rotation of an optical fiber around its optical axis. From the measurements data points indicative of measured values of the characteristic at the respective angles of rotation are generated. A model is created of the optical fiber having the characteristic as a variable parameter, and from the model a functional relationship between an expected measured value of the characteristic and the angle of rotation and the variable parameter is generated. By varying the parameter a fit of the functional relationship to the data points is made according to one or more predetermined criteria, such as least-squares fit. The value of the characteristic can be found based on the fit. Instrumental parameters, such as fiber misalignment and cleave angle, can also be ascertained by the method.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Patent Application Ser. No. 62/077,036, filed on Nov. 7, 2014, the disclosure of which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    Telecommunication links contain increasing numbers of detachable multi-mode optical fiber interconnects. The maximum allowable attenuation per connection has to decrease, which is a motivation for conducting accurate fiber geometry measurements. Using a series of measurements with a transmitted near-field (TNF) method on a commercially available instrument, it has been recognized that the recorded multi-mode fiber geometries vary upon re-cleaving a fiber sample. 
         [0003]    Aspects of the present disclosure relate to devices, systems and methods to determine the fiber cleave angle on a general purpose confocal microscope. Other aspects relate to devices, systems and methods along with a model to estimate the true cladding non-circularity and the apparent fiber misalignments in a consistent manner. 
         [0004]    There are multiple methods to measure the fiber geometry, comprising the fiber core diameter and non-circularity, the fiber cladding diameter and non-circularity, and core-cladding concentricity. A known method employs the transmitted near-field method (TNF). After a fiber sample of at least 2 meters is cleaved on both ends, one end is illuminated with an LED, and on the other end is measured with a CCD camera detector. A problem with this approach is the poor reproducibility of the measurement after re-cleaving. 
       SUMMARY 
       [0005]    In the present disclosure, a new tool is used which allows rotation of a fiber around its optical axis in front of the camera detector of a commercially available TNF apparatus, to study the behavior of the fiber geometry parameters. The fiber cladding non-circularity reported by the instrument showed a periodic pattern with higher rotation harmonics as the fiber was rotated 360 degrees around its optical axis. A geometric model was created of a fiber and the projection of its perimeter onto a detector plane, taking into account the non-circularity (and orientation) of the fiber, the cleave angle (and orientation), and the angular misalignment (and orientation) of the fiber with respect to the detector. The output of the model is the non-circularity of the perimeter projected on the detector plane. A least-square fit of the model (taking the aforementioned six parameters as input) to at least six measurements of the fiber non-circularity at fixed (known) orientations, allows a user to output the true fiber non-circularity. This method allows improvements in the accuracy and repeatability of the measurement of other fiber geometry parameters. The model-based estimation of fiber alignment and fiber cleave imperfections with the fiber rotation method, allows for more accurate measurements of the fiber cladding diameter and non-circularity. This may be extended to the core diameter and non-circularity as well. This improvement may be of interest to the manufacturers of such equipment. Furthermore, it may be used to measure the fiber misalignment with respect to the instrument, and adjust the instrument accordingly. This improvement is also of interest to fiber manufacturers, and fiber component manufacturers and users. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  illustrates a relationship between the available link power budget and data rates; the available link power budget decreases with increasing data rates. 
           [0007]      FIG. 2  shows recorded variation in core diameter over several samples. 
           [0008]      FIG. 3  shows an example of a confocal measurement of a cleaved fiber end-face viewed from the side. 
           [0009]      FIG. 4  shows cleave angle measurements over several samples. 
           [0010]      FIG. 5  show measurements of cladding non-circularity as function of the orientation; the measurements show repeatability in orientational dependency. 
           [0011]      FIG. 6  illustrates (a) a perspective (side) view of a cleaved fiber, (b) view of the cleaved fiber from the TNF detector, and (c) a least-squares fit between a model (line) and the measured data (crosses) for the cleaved fiber according to an aspect of the present disclosure. 
           [0012]      FIG. 7  is a plot showing impact of fiber bend on core diameter as measured by the TNF method. 
           [0013]      FIG. 8  illustrates (a) a perspective (side) view of a cleaved fiber, (b) view of the cleaved fiber from the TNF detector, and (c) values, as predicted by the model, of clad non-circularity as a function of fiber orientation for a combinations of zero actual clad non-circularity, zero cleave angle and zero fiber angular misalignment. 
           [0014]      FIG. 9  illustrates (a) a perspective (side) view of a cleaved fiber, (b) view of the cleaved fiber from the TNF detector, and (c) values, as predicted by the model, of clad non-circularity as a function of fiber orientation for a combinations of zero actual clad non-circularity, non-zero cleave angle and zero fiber angular misalignment. 
           [0015]      FIG. 10  illustrates (a) a perspective (side) view of a cleaved fiber, (b) view of the cleaved fiber from the TNF detector, and (c) values, as predicted by the model, of clad non-circularity as a function of fiber orientation for a combinations of zero actual clad non-circularity, non-zero cleave angle and non-zero fiber angular misalignment. 
           [0016]      FIG. 11  illustrates (a) a perspective (side) view of a cleaved fiber, (b) view of the cleaved fiber from the TNF detector, and (c) values, as predicted by the model, of clad non-circularity as a function of fiber orientation for a combinations of non-zero actual clad non-circularity, zero cleave angle and non-zero fiber angular misalignment. 
           [0017]      FIG. 12  illustrates (a) a perspective (side) view of a cleaved fiber, (b) view of the cleaved fiber from the TNF detector, and (c) values, as predicted by the model, of clad non-circularity as a function of fiber orientation for a combinations of non-zero actual clad non-circularity, non-zero cleave angle and non-zero fiber angular misalignment. 
           [0018]      FIG. 13  shows an example of a clamp for holding an optical fiber in TNF measurements, and a fiber rotation tool. 
           [0019]      FIG. 14  shows another view of the clamp shown in  FIG. 13 . 
           [0020]      FIG. 15  show another view of the fiber rotation tool shown in  FIG. 13 . 
           [0021]      FIG. 16  schematically illustrates a TNF imaging setup. 
           [0022]      FIG. 17  schematically illustrates orienting an optical fiber at different rotational angles by rotating the fiber about its optical axis. 
       
    
    
     DETAILED DESCRIPTION 
     1. Introduction 
       [0023]    The optical networks that are deployed in datacenters typically employ multi-mode fibers for a number of reasons. The bandwidth-length product of these fibers have improved significantly over the years and have kept up with the demand for high-speed data transmission over workable distances. The technology to manufacture the active components for these multi-mode fibers, such as the transmitters and receivers, are typically cheaper compared to their single-mode fiber counterparts. Furthermore, the fiber alignment in a connection is typically less demanding in terms of attenuation, dust or defects compared to single-mode fiber connections. 
         [0024]    In order to meet the increasing demand for bandwidth, increasing numbers of multi-mode fiber links may be deployed in parallel. In order to manage the backbone of such networks in a structured manner, trunk cables contain large numbers of fibers and are terminated with connectors, for example with the Multi-fiber Push-On (MPO) connector. With the aid of cassettes, the individual fibers in the trunk cable may be rerouted to other connector types, which facilitates the connection to the individual transceivers. With these components, an entire network topology may be configured in a flexible and structured manner, albeit that there are multiple interconnects in each link. 
         [0025]    As data rates increase, the available link power budget decreases rapidly, as shown in  FIG. 1 . The challenge is to meet the power budget, without sacrificing the transmission distance or the ability to provide multiple interconnects in each link. Therefore, the maximum allowed attenuation per connector should be decreased. To reduce the fiber intrinsic contributors to attenuation, the tolerances on fiber geometry parameters, as regards the core diameter and the numerical aperture, can be tightened. 
         [0026]    Methods to measure the core diameter include the transmitted near-field (TNF) method, the refractive near-field (RNF) method and the transverse interferometric (TI) method. The latter two methods aim to measure the shape of the refractive-index profile directly, whereas the first method measures the near-field at the end of the fiber when excited using an overfilled launch (OFL). 
         [0027]    The TNF method is one preferred method for fiber geometry measurements, because it is typically less labor intensive and less time consuming compared to an RNF measurement. Fiber geometry parameters, including the fiber outer (cladding) diameter and non-circularity, the fiber core diameter and non-circularity, and core-cladding concentricity can be obtained from a single TNF measurement. After stripping away the coating on both ends of a sample of typically 2 meters, the bare fiber is cleaved on both ends and inserted in the measurement apparatus. An 850 nm light emitting diode (LED) couples light into the fiber core, and the other end of the fiber is projected onto a CCD camera. Under ideal circumstances, a single image of the illuminated fiber end-face is sufficient to determine the fiber geometry parameters. A TNF imaging setup is schematically illustrated in  FIG. 16 . 
         [0028]    However, upon re-cleaving the fiber, the outcome of a TNF geometry measurement may vary. This variation on the core diameter measurement may indeed easily attain several tenths of a micrometer. For every new measurement, the fiber is repositioned in the fixture after the coating around the protruding fiber was stripped, and the bare fiber is re-cleaved. A mechanical tool that replaces the standard tool of the fiber geometry apparatus, allows a user to rotate the fiber around its optical axis. A measurement method determines the end-face angle relative to the optical axis of the fiber, as well as the angular misalignment of the fiber with respect to the tool. 
         [0029]    The recorded cladding non-circularity depends on the orientation of the fiber with respect to the optical axis of the instrument. With the aid of a model, several factors play a role in the observed variation in cladding non-circularity. Even for circular fibers, the combination of a finite cleave angle, and either an angular misalignment of the tool with respect to the optical axis, or an improper calibration of the two Cartesian camera axes of the instrument, or both, will result in an apparent orientation-dependent cladding non-circularity. On top of that, the fiber may actually be non-circular, causing a higher rotation angle harmonic to become apparent. 
         [0030]    With measurements for various cleaves, consistent results are obtained for the true cladding non-circularity, and apparent tool misalignment on the instrument (or instrument miscalibration). For all these measurements, the launch into the fiber was left unchanged. Besides the effect of rotating the fiber at the camera-side, there may also be an effect of changing the fiber bend radius on the recorded core diameter. 
       2. Fiber Geometry Measurements 
       [0031]    A fiber sample is aligned in the TNF measurement apparatus. While keeping the tool and cleave on the LED-side untouched over all subsequent measurements, measurements were made on the receiving side in nine orientations, from zero degrees with steps of 45 degrees to 360 degrees. The resulting recorded core diameters are shown in  FIG. 2 , for twelve cleaves on the receive side. 
         [0032]    For each cleave and each orientation, the fiber geometry parameters were measured. The TNF core diameter is defined as the diameter obtained after fitting an ellipse to the measured contour at a 2.5% intensity level. The non-circularity is then defined by, 
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         [0033]    where nc denotes the non-circularity, D max , and D min  the major and minor axes of the fitted ellipse, and D mean  is the TNF core diameter. After a sample was measured, the fiber was directly cut behind the tool, so that it can be moved to a general purpose confocal microscope, while maintaining the reference to the orientation. An example of a scan is shown in  FIG. 3 . It clearly shows a spiky shaped defect at the front-edge where the cleave is initiated. A plane was fitted through every measured end-face surface. Although the shape may not seem to be represented well by a plane, it must be noted that the image is not displayed to scale. The plotted height difference (measured with respect to the optical axis of the microscope) is less than 2.5 μm for this example, whereas the diameter is 125 μm. With the aid of at least two scans with known rotation (about the fiber axis), one can discriminate between the cleave angle and orientation with respect to the cross-sectional plane, and the angle (and orientation) of the fiber axis with respect of the optical axis of the microscope. Scans of four orientations were collected, which gives six permutations resulting in a cleave angle (and orientation) and fiber-tool misalignment (and orientation). The average of these outcomes was used for the estimate of the cleave angle (and orientation). The standard deviation of the cleave angle is smaller than 0.05 degrees, but usually better. The misalignment of the fiber axis with respect to the microscope was about 0.1 degrees. 
         [0034]    In this example, the cleave angles are often as large as 1 degree. The resulting cleave angles for twelve samples are shown below in  FIG. 4 . 
         [0035]    The cleave angle of sample four is small, and this sample will prove very useful for the analysis. Furthermore, comparing  FIGS. 2 and 4 , there seems to be no straightforward connection between the cleave angle and the recorded variation in the core diameter. For instance, sample four does not show a smaller variation in recorded core diameter than sample seven in  FIG. 2 . 
         [0036]    In a fashion similar to the cleave angle measurement, the fiber inside the tool was rotated, but now on the commercially available TNF measurement apparatus. The motivation for that is that if standard fixtures are used, the fiber may be misaligned with the instrument, which would be difficult to determine. 
         [0037]    The recorded cladding non-circularity was periodic with the orientation of the fiber in the fixture. Although the non-circularity is small, the measurement was quite repeatable, as shown in  FIG. 5  below, for which the fiber was rotated three times. 
         [0038]    Apart from the clear outlier at 135 degrees, measurement method is repeatable. 
       3. Model-Based Data Interpretation 
       [0039]    To make a general description of the perimeter of the fiber cladding, a parametric description of an ellipse with a non-circularity and orientation was used. The cleave angle and cleave orientation is accounted for by taking an appropriate cross-section. The angularly misaligned fiber with respect to the camera is modeled by projecting the cross-section onto the detector plane. A misconfiguration of the two magnification factors of the Cartesian axes of the detector may also lead to an apparent non-circularity, and this may be accounted for by an appropriate scaling along the detector axes. However, in this example, there is no intention to discriminate between tool misalignment and the detector miscalibration, the example uses unit scaling factors and assumes the system is calibrated. 
         [0040]    In case the fiber is perfectly circular, the rotation of the fiber only leads to a change in the recorded cladding non-circularity when there is a non-zero cleave angle and a tool misalignment. This would lead to one period in 360 degrees as shown by the dashed curve in  FIG. 10( c ) . In case the fiber also has a finite cladding non-circularity, a second rotation angle harmonic becomes apparent as shown by the solid curve, as shown by the example in  FIG. 6( c ) . ( FIG. 6  also illustrates (a) a perspective (side) view of a cleaved fiber, (b) view of the cleaved fiber from the TNF detector.) 
         [0041]    In this case, the true cladding non-circularity is found to be 0.16%. The peak-to-through amplitude is due to a tool misalignment of about two degrees and a finite cleave angle. Given that a 0.16% non-circularity in equation (1) amounts to an orientation dependent diameter of 125+0.10 μm, the true cladding non-circularity is consistently measureable by this approach. Even though this cladding non-circularity is very small, from a connector manufacturing standpoint it is the maximum dimension that may pose difficulties inserting the fiber into a connector ferrule. 
         [0042]    In order to align the two meter fiber sample with the receptacles on the instrument, the fiber may have to be bent. The bend radius may have an impact on the recorded core diameter of a multi-mode fiber. In one experiment a bend diameter of 280 mm in diameter, which coincides with a marking on the instrument, indicated with a dashed line in  FIG. 7 . We subsequently reduced the bend diameter by a factor two, while keeping the arc length the same. The TNF core diameter reduces rapidly when the bend diameter reduces. In the first step, the diameter reduced by 0.3 μm. In the second step, this is already more than a micrometer, although the diameter of that bend is still larger than the inner-marking on the instrument. There is no mentioning of the bend diameter in the fiber geometry standard, but possibly this should be accounted for, in order to perform reliable measurements. 
         [0043]    When a multi-mode fiber is bent, the high-order modes tend to leak into the cladding, which affects the measured near-field pattern. 
         [0044]    Additional examples of modelling the apparent (measured) fiber characteristics are given in  FIGS. 8-12 . Each of  FIGS. 8-12  illustrates (a) a perspective (side) view of a cleaved fiber and (b) view of the cleaved fiber from the TNF detector. In  FIG. 8( c ) , values of apparent clad non-circularity, as predicted by the model, as a function of fiber orientation are shown for a combinations of zero actual clad non-circularity, zero cleave angle and zero fiber angular misalignment. As expected, the values are zero regardless of fiber orientation. 
         [0045]      FIG. 9( c )  illustrates values, as predicted by the model, of apparent clad non-circularity as a function of fiber orientation for a combinations of zero actual clad non-circularity, non-zero cleave angle and zero fiber angular misalignment. Again, the values are zero regardless of fiber orientation. 
         [0046]      FIG. 10( c )  illustrates values, as predicted by the model, of clad non-circularity as a function of fiber orientation for a combinations of zero actual clad non-circularity, but instrumental imperfections, with non-zero cleave angle (in this case 20°) and non-zero fiber angular misalignment (in this case 20°). The apparent clad non-circularity exhibits a single period of variation per revolution of the fiber. 
         [0047]      FIG. 11( c )  illustrates values, as predicted by the model, of apparent clad non-circularity as a function of fiber orientation for a combinations of non-zero actual clad non-circularity (in this case 10%), zero cleave angle and non-zero fiber angular misalignment (in this case 20°). The apparent clad non-circularity exhibits a higher harmonic variation, i.e., with a component of multiple cycles variation in apparent clad non-circularity per revolution of the fiber. 
         [0048]      FIG. 12( c )  illustrates values, as predicted by the model, of apparent clad non-circularity as a function of fiber orientation for a combinations of non-zero actual clad non-circularity (in this case 10%), non-zero cleave angle (in this case 20°) and non-zero fiber angular misalignment (in this case 20°). Similar to the case demonstrated in  FIG. 11 , the apparent clad non-circularity exhibits a higher harmonic variation. 
         [0049]    It is noted that the method described above not only yields the actual fiber characteristics (e.g., clad non-circularity) but also instrumental imperfections (e.g., cleave angle and fiber angle misalignment). In the example shown in  FIG. 6 , in addition to obtaining the true clad non-circularity of 0.16%, the method further found the cleave angle (0.28°) and fiber angle misalignment (4°). It is additionally noted that the fiber characteristics (e.g., clad non-circularity) obtained by this method is typically different from the mere average over all measurements. For example, the average value of clad non-circularity is approximately 0.26, but the true clad non-circularity is about 0.16. 
       4. Conclusions 
       [0050]    Through a series of experiments on a commercially available TNF measurement apparatus, it was revealed the various causes of variation in the recorded core diameter. This is most apparent when the fiber sample is subjected to re-cleaving. Moreover, it was observed that a periodic behavior in the recorded cladding non-circularity occurred when the fiber was rotated. With a model, a user is able to measure the true cladding non-circularity, as well as a fiber misalignment on the instrument in a consistent manner. Furthermore, it was found that a fiber bend does have an impact on the recorded TNF core diameter. 
         [0051]      FIGS. 13-15  show an example for a tool that allows fiber rotation during measurement of the fiber. A standard clamp A that comes with a known TNF apparatus. It has a V-groove block B to align the fiber, and two springs are used to push the V-groove block to the mechanical stop to keep the fiber locked in the V-groove. With the knob C, one can push the V-groove block away from the mechanical stop, and insert (or remove) the fiber from the clamp. This standard tool can be used on the LED-side of the apparatus, where the light is coupled into the fiber. 
         [0052]      FIGS. 13 and 14  show an example rotation tool D that can be used on the camera side of the apparatus. These Figures show an aluminum block E that contains a ferrule. The end-face of the circular ceramic ferrule is visible at the front-end of the block. The block also contains the mechanical part that holds on to the fiber and allows a user to rotate the fiber. Because the fiber fits inside in the narrow ferrule bore, the protruding part of the fiber can only rotate around its own axis. The fiber is thus rotated inside the ferrule bore. The rear-view shows the part F that facilitates the rotation. It shows the markings that may allow to keep track of the orientation of the fiber. The aluminum block itself is thus kept untouched in the apparatus throughout the measurements. Like the standard tool, it has a magnetic fixation to the instrument. Although the markings on the part that rotates the fiber allow tracking of the orientation, it is also possible to follow the orientation of some small but noticeable defect or contamination on the cladding contour from the detector camera image. Other devices can be used to permit fiber only rotation, including V-groove devices. 
         [0053]    The ferrule in the example tool D is similar to, or identical to, a ferrule used in a fiber optic connector, like an SC connector. 
         [0054]    Thus, a method of efficient and accurate measurement and quantification of imperfections in optical fiber as well as instrumental imperfections has been achieved according to the present disclosure. Because many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.