Abstract:
An interferometric measurement method aims at calculating end face surface angle of multi-fiber connector or ferrule taking into account parallelism deviation angles of the connector or ferrule guide holes. The parallelism deviation angles are measured by scanning the side surfaces of guide pins inserted into the guide holes, which is done simultaneously with the end face scanning Interferometric data from connector or ferrule end face and side surfaces of the guide pins is gathered during one scanning session—either one simultaneous scan or several scans without moving or re-inserting the connector or ferrule.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to the field of measuring the geometry of multi-fiber optic ferrules and connectors by means of interferometric microscopes. More specifically, the invention relates to interferometric measurement of end face surface angle. 
         [0004]    2. Description of the Related Art 
         [0005]    Manufacturers of fiber optic connectors seek ways to produce the connectors with low transmission loss and low back reflection. End faces of fiber optic connectors must satisfy certain criteria for effective fiber mating as required by the industry standards. They must be clean and their surface geometry must provide for good physical contact and low signal loss. 
         [0006]    Connector manufacturing procedure includes end face polishing for achieving the surface parameters that ensure good mating of the connectors in plugs. End face flatness needs to be verified that can be done by measuring the end face surface angle. 
         [0007]    International Electrotechnical Commission standard IEC 61-300-3-30 defines surface angle as the angle between a reference plane which is perpendicular to the average guide pin axis and the connector end face surface. 
         [0008]    The end face parameters, including the surface angle, are measured in an interferometric system. Special fixtures for positioning the connectors in the system are used. In case of multi-fiber connectors, the fixtures enable precise connector alignment in the interferometric system with the use of two guide pins. In the same way, in multi-fiber connector plugs two guide pins enable precise alignment between mating male and female connectors to minimize the optical insertion loss, 
         [0009]    The fixtures are accurately adjusted and calibrated before measurement so that calibration angles of the pins relative to the optical axis of the interferometric microscope are known. The surface angle is calculated as the angle between the surface end face plane and a reference plane perpendicular to the optical axis of the interferometric microscope with known calibration angle correction. 
         [0010]    Guide holes of multi-fiber connectors or ferrules are not strictly parallel. There is always some parallelism deviation as demonstrated on  FIG. 1 . On this figure the reference number  1  represents a multi-fiber connector or ferrule, the top image being the top view of the connector or ferrule and the bottom image being its side view. 
         [0011]    Reference plane  5  is perpendicular to the optical axis  2  of the interferometric system. When guide pins  3  and  3 ′ of a fixture are inserted into the connector guide holes which have some deviation from being parallel, the guide pins become deviated too. The alignment angle of pins is changed. The reference number  6  demonstrates deviation angle of one of the guide pins. In the same way, the other guide pin has a deviation angle as well. 
         [0012]    Since during the surface angle calculation the deviation angle is not taken into account, the measured angle value has some calculation error. 
         [0013]    The present invention suggests measuring precise alignment angle of the guide pins by scanning them from side together with inspecting the connector end face in one measurement or in several subsequent measurement without re-inserting the connector or ferrule. The measured values of the angles between the guide pin axes are taken into account when calculating the surface angles. Such surface measurement method allows to increase measurement accuracy by considering the deviation angle of the guide pins in calculations. 
         [0014]    There is a known method for determining precise orientation of the axis of guide pin holes of a multi-fiber ferrule and precise angle of the ferrule (see Dean, D., (2004) U.S. Pat. No. 006,786,65). 
         [0015]    The present invention employs the same technique of measuring fiber optic connectors as described in a related patent application by the same inventor (see Towfiq, F., (2015) U.S. patent application Ser. No. 14/697,784). The interferometric data is obtained from a side face of the guide pins which allows to measure the alignment angles of the guide pins of the connector or ferrule with two degrees of freedom. 
       SUMMARY OF THE INVENTION 
       [0016]    The present method aims at interferometric measurement of surface angle of multi-fiber connectors and ferrules that takes into account precise alignment angle of the guide holes. The main concept of the present method is obtaining the interferometric data simultaneously from multi-fiber connector or ferrule guide pins by scanning them from side, and from the connector end face. The guide pins can be present in a male connector or can be inserted into a female connector or ferrule from a special fixture used for connector positioning. 
         [0017]    The measurement is performed in an interferometric system with the help of special fixtures for simultaneous scanning of side face and end face. The special fixtures position the connector or ferrule so that its end face is approximately perpendicular to the optical axis of the interferometric microscope and the axes of its guide holes are parallel to it. 
         [0018]    The special fixtures also contain a mirror element that turns the image obtained from the guide pins and directs it into the interferometer objective. Interferometric fringe patterns are created on the guide pins and on the end face. 
         [0019]    The method works well for both multi-fiber female connectors or ferrules and male connectors. The alignment angle is measured for either the guide pins of the male connector or guide pins of the fixture inserted into the guide holes of the female connector or ferrule. 
         [0020]    The required surface angles X and Y are calculated from the data obtained during the interferometric scanning First angles X and Y between each guide pin axis and system Z axis are calculated. These angles are then taken into account when calculating end face surface angles. Thus, the resulting angle values consider precise alignment angle of the guide holes. 
         [0021]    The provided method can be applied to multi-fiber MTP/MPO connectors and MT ferrules with 2, 4, 8, 12, 16, 24, 48, 72 and other numbers of fibers, of both PC and APC types. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The presented method can be better understood with reference to the following drawings. Like reference numerals designate corresponding parts throughout several drawings. 
           [0023]      FIG. 1  demonstrates related art problem with surface angle measurement. 
           [0024]      FIG. 2  shows a measurement system being prepared for testing. 
           [0025]      FIG. 3  demonstrates the use of the special fixture in interferometric system. 
           [0026]      FIG. 4  is a flowchart of the measurement method in accordance with the present invention. 
           [0027]      FIG. 5  illustrates the coordinates system. 
           [0028]      FIG. 6  is a top view of the object being measured with the illustrated guide pin angles. 
           [0029]      FIG. 7  is a side view of the object being measured with the illustrated guide pin angles. 
           [0030]      FIG. 8  is a top view of the object being measured with the illustrated surface angle. 
           [0031]      FIG. 9  is a side view of the object being measured with the illustrated surface angle. 
           [0032]      FIG. 10  is a flowchart of calibration of the mirror surface angle of the fixture. 
           [0033]      FIG. 11  is a side view of the object being calibrated with illustrated angles which are measured during the calibration procedure. 
           [0000]    
         
           
                 
               
                 
                 
               
             
                 
                     
                 
                 
                   REFERENCE NUMERALS IN THE DRAWINGS 
                 
                 
                     
                 
               
               
                 
                     
                 
               
            
             
                 
                   1 multi-fiber connector or ferrule 
                   2 optical axis of interferometric 
                 
                 
                     
                   system 
                 
                 
                   3, 3′ guide pins inserted into 
                   4, 4′ guide hole axes 
                 
                 
                   guide holes of the connector 
                 
                 
                   or ferrule 
                 
                 
                   5 reference plane 
                   6, 6′ deviation angles of the 
                 
                 
                     
                   guide holes 
                 
                 
                   7 interferometric microscope 
                   8 opening on the microscope front 
                 
                 
                     
                   panel 
                 
                 
                   9 special fixture 
                   10 mirror element of the fixture 
                 
                 
                   11 fiber holes 
                   12, 12′ guide holes 
                 
                 
                   13 system Z axis 
                   14, 14′ X angles between the 
                 
                 
                     
                   system Z axis and the axes of 
                 
                 
                     
                   the guide holes 
                 
                 
                   15, 15′ Y angles between the 
                   16 end surface plane 
                 
                 
                   system Z axis and the axes 
                 
                 
                   of the guide holes 
                 
                 
                   17 guide pins 
                   18 end surface angle of polish 
                 
                 
                     
                   along X axis 
                 
                 
                   19 end surface angle of polish 
                   20 pin axis in mirror 
                 
                 
                   along Y axis 
                 
                 
                   21 perpendicular to connector 
                   22 Y angle between optical axis 
                 
                 
                   end face 
                   and pin image in mirror 
                 
                 
                   23Y angle between optical axis 
                   24 Y angle between optical axis 
                 
                 
                   and perpendicular to connector 
                   and mirror 
                 
                 
                   end face 
                 
                 
                   25 mirror surface 
                 
                 
                     
                 
               
            
           
         
       
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0034]    Reference will now be made in detail to the presented measurement method illustrated in the accompanying drawings. 
         [0035]      FIG. 2  illustrates an exemplary embodiment of the present invention. The reference number  7  represents an interferometric microscope. A special fixture  9  for simultaneous measurements of the connector end face and the side face of the guide pins is mounted on the microscope which has an opening  8  to receive the fixture. A multi-fiber connector or ferrule  1  is inserted into the fixture for interferometric measurements. 
         [0036]      FIG. 3  provides a section view of the special fixture  9  with inserted connector or ferrule  1 . The fixture contains a mirror element  10  that turns the image from guide pins  17 . The light from the side surface of the guide pins is reflected from the mirror surface and directed into the interferometer. Thus simultaneous measurement of the connector end face and the side surfaces of the guide pins is possible. 
         [0037]      FIG. 4  represents a method flowchart of end face surface angle measurement of multi-fiber connectors or ferrules according to the present invention. The surface angle is measured by an interferometric microscope. Special fixture for simultaneous scanning of the end face and side surfaces of the guide pins is provided together with the microscope for positioning of the ferrule in the measurement system. The special fixture is mounted on the interferometric microscope and the connector or ferrule is inserted into the fixture. 
         [0038]    After the connector or ferrule is inserted into the special fixture and is ready for measurements, the interferometric microscope is focused and a fringe pattern is created on the guide pins and the end face of the connector or ferrule. Interferometric data is obtained from the side surfaces of the guide pins and from the end face of the connector or ferrule. 
         [0039]    The position of the system Z axis which is the averaged line of the two guide pin holes is determined. Then calculations of alignment angles of the guide holes along axis X and along axis Y are performed. 
         [0040]    The final step is calculations of end face surface angles X and Y in which alignment angles of the guide holes along axis X and along axis Y are taken into account. 
         [0041]      FIG. 5  explains a system of coordinates X and Y. An X-axis passes through centers of first and second guide holes  12  and  12 ′ on the ferrule end face. A perpendicular Y-axis passes through the midpoint of the line connecting the guide holes&#39; centers. 
         [0042]    The top and side views of the connector or ferrule  1  represented on  FIG. 6  and  FIG. 7  respectively demonstrate relative angles of the axes  4  and  4 ′ of the guide holes with inserted guide pins  3  and  3 ′. The relative angles of the guide holes represent their alignment angles. 
         [0043]    The reference numerals  14  and  14 ′ designate the angles along axis X between the averaged line  13  (the system Z axis) and the guide hole axes  4  and  4 ′ respectively. 
         [0044]    The reference numerals  15  and  15 ′ designate the angles along axis Y between the averaged line  13  (the system Z axis) and the guide hole axes  4  and  4 ′ respectively. 
         [0045]    The top and side views of the connector or ferrule  1  represented on  FIG. 8  and  FIG. 9  respectively demonstrate end face surface angles. The surface angles  18  and  19  along axes X and Y are calculated as the angles between the reference plane  5  and the end face surface  16 . In the resulting surface angles  18  and  19 , the alignment angles  6  and  6 ′ of the guide holes are taken into account. 
         [0046]      FIG. 10  illustrates the method of calibrating angle of the mirror surface of the fixture that turns the image from the side surface of the guide pins and directs it into the interferometric microscope. The fixture is mounted on the interferometric microscope and the connector or ferrule is inserted into the fixture in its first position. 
         [0047]    After the connector or ferrule is inserted into the special fixture and is ready for measurements, the interferometric microscope is focused and a fringe pattern is created on the guide pins and on the end face of the connector. Interferometric data is obtained from the side surfaces of the guide pins and from the end face of the connector. 
         [0048]    Angles between the optical axis and the pin image in mirror and between the optical axis and the line perpendicular to the connector end face are calculated for the first position of the connector. 
         [0049]    Then the connector is rotated by 180 degrees and inserted into the fixture in its second position. The interferometric microscope is focused again and a fringe pattern is created on the guide pins and on the end face of the connector or ferrule. Second set of the interferometric data is obtained from the side surfaces of the guide pins and from the end face of the connector. 
         [0050]    Angles between the optical axis and the pin image in mirror and between the optical axis and the line perpendicular to the connector end face are calculated for the second position of the connector. 
         [0051]    The final step is calculation of angles between the mirror surface of the fixture and the optical axis of the interferometric system. 
         [0052]      FIG. 11  illustrates the angles Y measured during the calibration of the mirror surface angle. The side view of the connector and fixture is provided. The angles along the axis X are defined similarly. 
         [0053]    Reference number  2  represents optical axis of interferometric microscope  7 . The multi-fiber connector or ferrule  1  is inserted into a special fixture with a mirror element. For simplicity, only the mirror element  10  of the fixture is shown and all other parts of the fixture are omitted. The mirror surface is designated by the reference number  25 . 
         [0054]    The angle between the optical axis  2  and the pin axis in mirror  20  is represented by the reference numeral  22 . The angle between the line  21  perpendicular to the connector end face is represented by the reference numeral  23 . The resulting angle between the mirror surface  25  and the optical axis is represented by the reference numeral  24 .