Abstract:
An apparatus inserts a reference guide pin having a reference end surface into a guide pin receiving hole in an endface of a fiberoptic connector to prepare the fiberoptic connector for interferometric profiling of the endface. The reference guide pin has an end portion on which the reference end surface is located. A connector support mechanism supports the fiberoptic connector. A reference guide pin support mechanism receives the end portion to hold the reference guide pin in axial alignment with the guide pin receiving hole as one of the connector support mechanism and the reference guide pin support mechanism is moved toward the other to insert the reference guide pin into the guide pin receiving hole.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention relates to fiberoptic connectors, and more particularly to pin insertion tools for use in conjunction with testing multi-fiber fiberoptic connectors to ensure that all fiber ends are properly aligned with, and in physical end-to-end contact with, corresponding fiber ends in a mating fiberoptic connector.  
           [0002]    The MT (Mechanically Transferable) connector shown in FIG. 1 and MPO (Multi-path Push On) connector shown in FIGS.  2 A, originally developed by NTT in Japan, have been deployed primarily in Japan for several years. The advantageous technical features and price/performance ratios of MT and MPO connectors have meant that this non-traditional style of fiberoptic connector is gradually becoming more widely accepted, and is becoming quite widely used in both the U.S. and other worldwide markets. The main advantages of MT and MPO connectors are high optical fiber density (typically 2-12 fibers), small physical size, and low cost. A variant of the MT connector is the MT-RJ connector, which has a smaller design that fits within the footprint of a standard 8 pin modular telephone jack, and is being considered as one of the main contenders by the standards organizations for fiberoptic premise network wiring.  
           [0003]    This type of fiberoptic connector has extremely critical dimensional tolerances that must be maintained to ensure acceptable performance and “intermatability” of connectors. As these connectors (the MT, MPO, and MT-RJ) become more widely used in single-mode applications, their geometric tolerances are expected to become even tighter. As fiberoptic cable bandwidth requirements increase, the fiberoptic connectors can become one of the most critical components affecting the overall system performance of a fiberoptic transmission system.  
           [0004]    An optical fiber  1  shown in FIG. 4 typically is constructed in three distinct concentric layers, including a 250 micron diameter acrylic jacket  2  which coats the outside of the glass optical fiber. The jacket&#39;s main function is to provide basic environmental protection to the glass optical fiber. Without this jacket, just brushing the fiber over another surface could score the glass, leading to a crack which eventually would propagate through the glass, fracturing the fiber and rendering it inoperable. Since the acrylic coating can be colored, it also provides a useful method of fiber identification. The second layer is a 125 micron diameter cladding  3 . This has become the standard outer diameter for all but the most unusual and application specific fiber designs. The purpose of the cladding is to contain the light within the 8 micron fiber core  4 , using the principal known as “total internal reflection”. The secondary purpose of the cladding  3  is to increase the fiber diameter to a level that provides it sufficient mechanical strength, can be fairly easily seen and can be manipulated by human hands. The core is the part of the fiber that carries the light. The core  4  and the cladding  3  constitute one contiguous piece of glass; however, they have different refractive indexes to keep the light within the core. Multi-mode fibers have a typical core diameter of 62.5 microns, as opposed to the 8 micron core diameter typically used in single-mode fibers.  
           [0005]    Although the core of a single-mode fiber is much smaller than that of a multi-mode fiber, allowing only a single “mode” to propagate from the input to the output of the fiber dramatically increases the amount of data or “bandwidth” offered by single-mode fibers, when compared to multi-mode fibers. With the rapidly increasing demand for voice, video, and Internet communications, bandwidth can be a scarce and valuable resource. As such, most new long distance fiber deployment is single-mode. Even when using sophisticated multiplexing techniques, the maximum bandwidth capacity of a single fiber may be used up, and there is no other option than to add additional fibers to increase communication capacity. As the number of single fibers being added to a bundle increases, so does the diameter of the cable necessary to contain and protect them. Not only is this expensive, but it can also create problems in already crowded ducts and passages used to route cables. Therefore, manufacturers are looking for ways to achieve smaller physical size, higher performance, more manageable, and less expensive systems, and have begun to manufacture “ribbon fibers”. A ribbon fiber as shown in FIG. 5 includes a number of optical fibers (typically 2-12) laid side by side and sleeved with an additional outer coating. This technique provides very high fiber densities, while having the added advantage that installation and maintenance workers are able to handle up to 12 or more fibers at one time.  
           [0006]    Fiber preparation can be a very labor intensive and expensive part of terminating or joining fibers together. Having the ability to work on multiple fibers at one time using specialized tools has led to dramatic time and cost savings in optical fiber installation and maintenance for ribbon fiberoptic cable users.  
           [0007]    Optical fiber multiplexing and transmitter and receiver technology have made such great technological advances that data transfer rates of the order of Terabytes per second over a “perfect” optical fiber link have been demonstrated, using a combination of various multiplexing and data compression technologies. As a result, engineers and scientists now face the difficult task of simplifying system implementation (without losing performance) to a level such that workers with little experience, crawling down through manhole covers in harsh environments, can be reasonably expected to install and maintain such ribbon fiber links with a high degree of success and reliability.  
           [0008]    One of the most important and frequently overlooked factors involved when installing a fiberoptic transmission system is the proper installation and use of fiberoptic connectors. When it is necessary to join or patch two ribbon cables together, there are two primary choices: fusion of optical fibers and use of optical fiber connectors. Fusion involves accurately cleaving all of the fibers to the same length across the ribbon on the two cables to be joined, and then using a specialized machine known as a ribbon fusion splicer, which brings all of the fiber pairs together very accurately along the X, Y and Z axes. An electrical arc applied with a small compressive force pressing the cleaved surfaces together then is used to physically fuse the individual fiber pairs together as one contiguous fiber. This process, when performed properly, and after the application of additional splice protection, can join two multi-fiber ribbon cables together almost as effectively as if they were manufactured as a contiguous piece of fiber. For permanent joints, fusion splicing provides the most economical and robust solution to joining optical fibers.  
           [0009]    On the other hand, there are many situations where a permanent joint is not desired, not required, or not feasible, in which case connectors become the only viable alternative technique for joining the fibers. Examples of such applications would include (1) patch panels where reconfiguring of fiber routes may be necessary, and (2) attachment to system or test equipment and applications such as high speed optical back-planes which require automatic connection and disconnection of the optical path as circuit boards are inserted and removed. At the user level “consumers” expect multi-fiber fiberoptic connectors to work in the same way as electrical connectors, that is, the fiberoptic connectors are simply “plugged in” for a pair of MPO or a pair of MT connectors, and everything works. In reality, a great deal of sophisticated technology and precision engineering has to occur to make this happen.  
           [0010]    It is important to recognize that the performance of optical connectors can have a dramatic impact on the overall performance, integrity and reliability of an entire optical link. The main “enemies” of an optical signal at a connectorization point are “loss” and “back-reflection”. Since the core diameter of a single-mode fiber is only 8 microns, when connecting two fibers together using a connector, a lateral misalignment smaller than 1 micron can cause significant optical power loss at the connector interface. This uses some of the loss “budget”, and therefore reduces the distance through which the light can continue to propagate before regeneration or optical amplification is required to maintain the data integrity of the signal. One cause of back-reflection occurs in a connector when two mating fiber ends do not achieve physical contact with each other, creating a small air gap in the transmission path of the signal, and causing “back-reflections” of the laser light from the unmatched interface. Such spurious reflections can affect the stability of the transmission equipment and greatly degrade the useable bandwidth of the fiberoptic transmission system.  
           [0011]    Both high loss and high back-reflection are common problems associated with poorly terminated connectors. Loss is most often caused by either a defective ferrule not maintaining stringent lateral tolerances for fiber alignment, a lack of physical contact between two fiber endfaces (due to fiber recess or poor endface geometry), or surface imperfections/contamination on one or both of the fiber endfaces. Back reflection most often is caused by fiber recesses in the connector endface or poor endface geometry wherein the connector&#39;s ferrules physically come into contact with each other before the mating fiber ends can physically contact each other. This results in back-reflection caused as the laser wave-front hits the silica/air interface of the discontinuous fiber path. Additionally, high fiber endface surface roughness can also increase both back-reflection and loss.  
           [0012]    With conventional single fiber connectors, it is relatively easy using modem processes to assure the necessary physical contact between two inter-mating fibers, simply by spherically polishing the ends of each ferrule containing the fiber. By controlling factors such as the radius of curvature, fiber height and apex offset of the polish with respect to the center of the fiber, physical contact between the fibers can be assured, thereby minimizing loss and back reflection. For further explanation, see my U.S. Pat. No. 5,459,564 entitled “Apparatus and Method for Inspecting End Faces of Optical Fibers and Optical Fiber Connectors”, issued Oct. 17, 1995, incorporated herein by reference.  
           [0013]    For MT and MPO connectors as shown in FIGS. 1 and 2, the problem of maintaining physical contact simultaneously between all 2-12 fibers becomes significantly more difficult to achieve consistently than for a single fiber connector. Since a truly spherical polish can only have one apex, and can thus only resolve the physical contact problem for one fiber, manufactures have had to resort to alternative methods to solve the problem. A common method is to use a polishing process that leaves the fibers very slightly protruding above the ferrules “flat” or “angled flat” endface surface for MT and MPO connectors, respectively.  
           [0014]    It should be appreciated that there is a very small difference between leaving enough protruding fiber to ensure physical contact of all fibers, and the alternatives which would be either (1) to leave too much protruding fiber, thereby damaging the fragile fibers when two connectors are mated, or (2) to leave the fibers recessed below the ferrules surface, thereby eliminating any possibility of physical contact.  
           [0015]    Another popular polishing technique that has evolved involves polishing the rectangular upper endface  35 A of the MT connector  35  (or MPO connector  350 ) much like the bowed or elongated convex shape found on the top of a long loaf of bread, e.g. as indicated in FIG. 8. The idea is that the centerline  10  of the transverse axis of the connector which contains the row of fiber ends  1  would be slightly higher than the surrounding perimeter of endface  35 A, thereby encouraging physical contact. It should be noted that the difference between adequate physical contact of the fiber ends and none at all is only a few microns. Therefore, having an accurate measurement of the surface topography of the connector endface, including the fiber ends, is essential to predicting the performance of all types of multi-fiber connectors.  
           [0016]    MT and MPO fiberoptic connectors have a significantly larger endface area of interest than single-fiber connectors do, not only because they contain more fibers, but also because the polish of the connector endface more closely resembles a flat or angled flat surface than a sphere. Thus, the first point of contact between the endfaces of the two ferrules encapsulating the fibers could fall anywhere across a relatively large surface. This first point of contact (with respect to the fiber heights) determines the magnitude of any possible separation or gap between the ends of the corresponding fibers of the mated connectors, and ultimately determines the overall performance of the connector. Until the present invention, there has been no way to determine the location or height of the above-mentioned first point of contact.  
           [0017]    To further explain the foregoing difficulty in determining the first point of contact, FIGS. 10A and 10B show a precisely coupled pair of MT connectors  35  and  35 - 1 . All of the corresponding fiber ends extending from the endfaces of connectors  35  and  35 - 1  are precisely aligned and in perfect end-to-end physical contact so that there is minimal light loss or back-reflection. In contrast, FIGS.  11 A-D show some of the various forms of defective end-to-end connector couplings that can occur as a result of imperfect connector endface profiles. FIG. 11A shows a connection where the endface  35 A of lower connector  35  has excessive fiber “undercut”, preventing any of the ends of any of the corresponding fibers  1  and  1  -  1  from achieving physical contact. FIG. 11B shows a connection where the endface  35 A of lower connector  35  is at an excessive angle relative to a plane perpendicular to the longitudinal axis of guide pin holes  40 , whereby the first point of ferrule endface contact prevents the ends of some of the corresponding fibers  1  and  1 - 1  from achieving physical contact, resulting in both loss and back-reflection. FIG. 11C shows a connection where lower connector  35  has excessive protrusion of fibers  1  above endface  35 A, causing the fibers  1  to bend as shown as a result of end abutment forces. Such endfaces result in stress, bending, poor coupling and potential fiber damage. FIG. 11D shows a connection where the radius of curvature of endface  35 A of lower connector  35  is too small; the result is that the outermost pairs of corresponding fibers  1  and  1 - 1  fail to achieve physical contact, which causes harmful loss and back-reflections in those fibers.  
           [0018]    To determine the presence of the conditions shown in FIGS.  11 A-D, it is necessary to obtain the surface topography profiles of the connector endfaces  35 A and  35 A- 1  and the fiber ends  1  and  1 - 1  protruding therefrom, using interferometric measurements. Despite the many advantages of the MT and MPO connectors, their design inherently creates many difficulties for interferometric measurement. A first difficulty is that the region of interest on an MT or an MPO connector can be as large as 6.4×2.5 millimeters, in contrast to a diameter of only approximately 250 microns (i.e., 0.25 millimeters) being required for a standard single fiber connector. Simply reducing the magnification of the interferometer to view the entire surface is not acceptable, as too much resolution is lost by doing this. Consequently, most manufacturers have independently chosen a “trade-off” magnification, trying to optimize the trade-off between field of view and resolution. Consequently, using the prior art techniques, multiple interferometric measurements of the separate continuous areas of endface of an MT or MPO connector must be made and “pasted” together to build up the required view of the endface surface topography.  
           [0019]    A second difficulty is that the material chosen by manufacturers for MT and MPO connectors primarily has been a black epoxy, filled with silica particles. When polished, the resulting endface often looks like “silica islands in a sea of epoxy”, as indicated by numeral  9  in FIG. 12, which shows the endface of a typical MT or MPO connector. Good interferometric data representing the surface profile of the endface of an MT or MPO connector often is available only from the highly reflective silica islands. Therefore, it is quite a complex operation to paste all of the captured regions together in three dimensions. Since the good data is situated on such islands  9  as shown in FIG. 12, it becomes necessary to perform broad-band interferometry on MT and MPO connectors, because otherwise it is impossible to know the height relationship of the good data regions with respect to each other. Unfortunately, broad-band interferometry generally is much slower than narrow-band interferometry.  
           [0020]    A third difficulty can be understood by comparison to the calibration of conventional single fiber connectors, wherein the precision cylindrical ferrule can be rotated in the interferometer fixture. By watching the interferogram, or alternatively by measuring the apex offset at different rotational orientations, it is possible to verify and compensate for any misalignment between the connector fixture and the interferometers optical axis. Since single fiber connectors use a precision split sleeve to couple two connectors, the endface geometry measurement can be considered to be calibrated because the same surfaces are referenced to measure the endface geometry in the interferometer as are used to locate the ferrule in an actual mating of a pair of joined connectors. In contrast, this is not the case for MT and MPO connectors wherein two precision “guide pins” slide into precision aligned mating holes  40  in a pair of MT or MPO connectors being joined instead of using locating features on the outsides of the ferrules to join a pair of the connectors together. Because the mating reference surface of an MT or MPO connector is not on the outside of the ferrule, and more importantly, because the ferrule cannot be rotated (since it is rectangular), it has not been possible to achieve an accurate calibration of the surface to be measured by the interferometer to obtain a calibrated profile of the connector endface and fiber ends which need to be aligned with and brought into physical contact with ends of corresponding fibers of a mating MT or MPO connector.  
           [0021]    As stated earlier, the first point of contact determines the overall performance of a fiberoptic connector. Without an accurate calibration of the measured connector endface data this first point of contact cannot be determined, making any surface topography test data far less useful in judging whether the multi-fiber fiberoptic connector should be accepted, re-worked, or discarded.  
           [0022]    Thus, there is an unmet need for a technique and apparatus for providing a profile of a multi-fiber fiberoptic connector endface which is precisely calibrated with respect to a feature of the connector that aligns it to mate with a like multi-fiber fiberoptic connector, and more specifically, there is an unmet need for a pin insertion tool for inserting precision guide pins into pin-receiving holes in a multi-fiberoptical connector to allow accurate, referenced interferometric measurements to be made of the end-faces of multi-fiber optical connectors.  
         SUMMARY OF THE INVENTION  
         [0023]    Accordingly, it is an object of the invention to provide a pin insertion tool which aids in providing a calibrated profile of a multi-fiber fiberoptic connector such as an MT connector or an MPO connector, which profile identifies the point of initial contact between the endface of the multi-fiber fiberoptic connector with the endface of a “like”, but theoretically perfect, mating fiberoptic connector.  
           [0024]    It is another object of the invention to provide a method and apparatus to aid in precisely evaluating the intermatability of multi-fiber fiberoptic connectors.  
           [0025]    It is another object of the invention to provide a pin insertion tool and technique to aid in profiling endfaces of multi-fiber fiberoptic connectors so as to predict which connectors may cause loss and back reflection.  
           [0026]    Briefly described, and in accordance with one embodiment thereof, the invention provides an apparatus for inserting a reference guide pin ( 12 - 1 ) having a reference end surface ( 13 A) into a guide pin receiving hole ( 40 ) in an endface ( 35 A) of a fiberoptic connector ( 35 ) to prepare the fiberoptic connector ( 35 ) for profiling of the endface ( 35 A). The guide pin ( 12 - 1 ) has an end portion ( 13 ) on which the reference end surface ( 13 A) is located. A connector support mechanism ( 34 ) supports the fiberoptic connector ( 35 ). A reference guide pin support mechanism has an end portion for receiving and retaining the end portion ( 13 ) of the first reference guide pin ( 12 - 1 ). The reference guide pin support mechanism ( 26 ) supports the end portion ( 13 ) of the reference guide pin to hold the reference guide pin in axial alignment with the guide pin receiving hole ( 40 ) as one of the connector support mechanism ( 34 ) and the reference guide pin support mechanism ( 26 ) is moved toward the other to insert the reference guide pin into the guide pin receiving hole ( 40 ). The reference guide pin support mechanism releases the reference guide pin before one of the reference guide pin support mechanism and the connector support mechanism is moved away from the other.  
           [0027]    In one embodiment, the apparatus inserts a first reference guide pin ( 12 - 1 , 2 ) having a reference end surface ( 13 A) into a first guide pin receiving hole ( 40 ) in an endface ( 35 A) of a multi-fiber connector ( 35 ) to prepare the multi-fiber connector ( 35 ) for interferometric profiling of the endface ( 35 A). The first guide pin ( 12 - 1 ) has a head ( 13 ) on which the reference end surface ( 13 A) is located. A connector support mechanism ( 34 ) supports the multi-fiber connector ( 35 ). A reference guide pin support mechanism has an end portion with a first recess ( 49 ) for receiving and retaining the head ( 13 ) of the first reference guide pin ( 12 - 1 ). The reference guide pin support mechanism ( 26 ) is closed on the head ( 13 ) of the first reference guide pin to securely hold the first reference guide pin in axial alignment with the first guide pin receiving hole ( 40 ) as one of the connector support mechanism ( 34 ) and the reference guide pin support mechanism ( 26 ) is moved toward the other to insert the first reference guide pin a predetermined depth into the first guide pin receiving hole ( 40 ). The reference guide pin support mechanism is opened to release the first reference guide pin before one of the reference guide pin support mechanism ( 26 ) and the connector support mechanism ( 34 ) is moved away from the other. The multi-fiber connector can be an MT connector or an MPO connector. In the described embodiment, the jaw mechanism includes a first jaw member ( 26 ) pivotally mounted in fixed relation to the base, the first jaw member ( 26 ) having an end portion with the first recess ( 49 ) therein for receiving and retaining the head ( 13 ) of the first reference guide pin. The jaw mechanism also includes a second jaw member ( 27 ) pivotally mounted in fixed relation to the base, the second jaw member ( 27 ) having an end portion with a second recess ( 49 ) therein for receiving and retaining the head of the first reference guide pin. In one described embodiment, the shape of the head ( 13 ) of the first reference guide pin is cylindrical, and in another described embodiment the shape of the head ( 13 ) of the first reference guide pin is non-cylindrical to ensure a predetermined axial rotational orientation of the first reference guide pin when its head is retained in the first and second recesses of the first and second jaw members, respectively. In the described embodiment, a pivotal stop mechanism ( 29 ) is connected in pivotal relation to the base ( 21 ) and operative to support the second jaw member ( 27 ) at a level such that the first reference guide pin ( 12 - 1 ) held by the jaw mechanism is axially aligned with the first guide pin receiving hole ( 40 ) when the jaw mechanism is closed on the head ( 13 ) of the first reference guide pin ( 12 - 1 ), the pivotal stop mechanism ( 29 ) pivoting clear of the second jaw member ( 27 ) to allow opening of the jaw mechanism before the moving of the jaw mechanism ( 26 , 27 ) away from the carriage mechanism ( 34 ). In the described embodiment, the jaw mechanism includes an additional first recess ( 49 ) for receiving and retaining a head of a second reference guide pin to securely hold the second reference guide pin in axial alignment with a second guide pin receiving hole ( 40 ) in the endface of the multi-fiber connector.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    [0028]FIG. 1 is a perspective view of an MT fiberoptic connector.  
         [0029]    [0029]FIG. 2A is a front elevational view of an MPO fiberoptic connector.  
         [0030]    [0030]FIG. 2B is a side elevational view of the MPO fiberoptic connector of FIG. 2A.  
         [0031]    [0031]FIG. 3 is a perspective view of a conventional “single fiber” fiberoptic connector.  
         [0032]    [0032]FIG. 4 is a perspective view of a single optical fiber.  
         [0033]    [0033]FIG. 5 is a partial plan view of a typical ribbon fiberoptic cable.  
         [0034]    [0034]FIG. 6 is a plan view of two MT connectors aligned to be joined or mated.  
         [0035]    [0035]FIG. 7 is a side view of two MPO connectors aligned to be mated.  
         [0036]    [0036]FIG. 8 is a perspective view of an MT connector with a slightly convex endface.  
         [0037]    [0037]FIG. 9 is a side view of an MPO connector with a slightly convex endface polish.  
         [0038]    [0038]FIG. 10A is a plan view of a pair of precisely mated MT connectors.  
         [0039]    [0039]FIG. 10B is an enlarged view of feature  15  of FIG. 10A.  
         [0040]    [0040]FIG. 11A shows feature  15  of FIG. 10B with an excessive undercut of the fiber ends of the lower connector so as to prevent ends of corresponding fibers from achieving physical contact.  
         [0041]    [0041]FIG. 11B shows feature  15  of FIG. 10B with an excessive endface polish angle of the lower connector preventing the ends of some of the corresponding fibers from achieving physical contact.  
         [0042]    [0042]FIG. 11C shows feature  15  of FIG. 10B with excessive protrusion of the fiber ends from the endface of the lower connector, resulting in fiber stress, poor fiber coupling and potential fiber damage.  
         [0043]    [0043]FIG. 11D shows feature  15  of FIG. 10B wherein the endface of the lower connector is too convex, preventing the ends of corresponding outermost fibers from achieving physical contact.  
         [0044]    [0044]FIG. 12 shows silica islands in a “sea of epoxy” on the endface of a typical MT or MPO connector.  
         [0045]    [0045]FIG. 13A is a plan view of a standard guide pin used to align a pair of MT connectors or a pair of MPO connectors as they are mated.  
         [0046]    [0046]FIG. 13B is a plan view of a reference guide pin for aiding interferometric profiling of the endface of an MT connector.  
         [0047]    [0047]FIG. 13C is a plan view of a reference guide pin for aiding in interferometric profiling of the endface of an MPO connector.  
         [0048]    [0048]FIG. 13D is a partial left end view of FIG. 13C illustrating a keying feature.  
         [0049]    [0049]FIG. 14A is a plan view of a tool for loading and unloading reference guide pins into and from guide pin holes of MT and MPO connectors.  
         [0050]    [0050]FIG. 14B is a side elevational view of the tool of FIG. 14A.  
         [0051]    FIGS.  15 A-C show a sequence of operations using the tool shown in FIG. 14B to load the reference guide pins of FIG. 14B into guide pin holes of an MPO connector.  
         [0052]    [0052]FIG. 16A is a diagram useful in explaining a first method of interferometric profiling of the endface of an MT connector.  
         [0053]    [0053]FIG. 16B is another diagram useful in explaining the first method of interferometric profiling of the endface of the MT connector of FIG. 16A.  
         [0054]    [0054]FIG. 16C is an elevation view diagram showing the clamping mechanism  42 A and adjustment mechanism  42  which positions an MT or MPO connector in the field of view of an interferometer and provides precise adjustment of the connector in the X, Y, or Z axes and about the tip and tilt axes.  
         [0055]    [0055]FIG. 16D is a plan view diagram of the arrangement shown in FIG. 16C.  
         [0056]    [0056]FIG. 17 is an elevation view illustrating using the tool of FIGS. 14A and 14B to load the reference guide pin of FIG. 13B or  13 C into guide pin holes of an MT or MPO connector.  
         [0057]    [0057]FIG. 18 is a diagram useful in explaining interferometric profiling of the endface of an MT connector.  
         [0058]    [0058]FIG. 19 is a diagram useful in explaining interferometric profiling of the endface of an MPO connector.  
         [0059]    [0059]FIG. 20 is a diagram useful in explaining interferometric profiling of the endface of an MPO connector.  
         [0060]    [0060]FIG. 21 is a section view diagram useful in describing an alternative embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0061]    To obtain an accurate rather than relative measurement of the endface of an MT or MPO connector, the invention provides several possible techniques for calibration of surface topography data interferometrically acquired from the endface of a multi-fiber fiberoptic connector such as an MT or MPO connector. Each method uses at least one reference guide pin to achieve accurate calibration of the interferometrically measured profile of the connector endface relative to the guide pin holes  40  (e.g., see FIG. 16A) by means of which mated connectors are aligned. It is important to note that the novel “reference guide pins” of the present invention having precisely flat polished right angled endfaces are used when working with MT connectors, and precisely polished “angled flat” endfaces are used for MPO connectors. Referring to FIG. 13A, the standard cylindrical guide pin  11  commonly used to perform the optical alignment during joining or mating of a pair of MT connectors or a pair of MPO connectors has an endface  11 A which is not polished or toleranced.  
         [0062]    A “reference” guide pin  12 - 1  according to the present invention is shown in FIG. 13B, and is used when interferometrically measuring endfaces of MT connectors, wherein the endface  13 A on head  13  of reference guide pin  12 - 1  is precisely flat, precisely polished, and oriented at a right angle to the longitudinal axis of reference guide pin  12 - 1 . What is meant by a “precisely polished” surface is simply a smooth, flat, or angled flat highly reflective surface. A reference guide pin  12 - 2  shown in FIG. 13C is used when interferometrically profiling the endface of an MPO connector. The endface  14 A of head  14  of reference guide pin  12 - 2  is precisely flat, precisely polished, and usually is oriented at an angle of 8 degrees from a plane perpendicular to the longitudinal axis of reference guide pin  12 - 2 .  
         [0063]    In accordance with the invention, the above mentioned standard guide pins  11  of FIG. 13A are used to align a pair of properly, precisely manufactured MT fiberoptic connectors or a pair of properly, precisely manufactured MPO fiberoptic connectors. The diameter, straightness and surface finish of both the guide pins and guide pin holes are held to extremely tight tolerances so as to achieve sub-micron lateral and angular repeatability in the alignment of mated MT connectors and mated MPO connectors.  
         [0064]    The close piston fit between the guide pins  11  and the guide pin holes  40  in MT connectors  35  and MPO connectors  350  makes standard guide pins  11  ideal for modification according to the present invention to provide polished endfaces  13 A and  14 A of reference guide pins  12 - 1  and  12 - 2 , respectively; the reference guide pin endfaces  13 A and  14 A are suitable for use as reference surfaces when measuring MT and MPO connectors, respectively. Such a close fit between reference guide pin  12 - 1  or  12 - 2  and hole  40  (FIG. 16A) results in very stable and repeatable fringe patterns on the endfaces  13 A or  14 A of the reference guide pins during interferometric measurement.  
         [0065]    The endface of a standard guide pin  11  can be modified to have either an extremely precise “flat” polish or an “angled flat” polish produced on its previously un-toleranced endface to obtain reference guide pins  12 - 1  and  12 - 2 , respectively. Referring to FIG. 13A, numeral  11 A designates the previously un-toleranced endface of a standard guide pin  11  before such modification. FIG. 13B shows the “reference guide pin”  12 - 1  having a head  13  with a precisely “flat” polished endface  13 A perpendicular to the longitudinal axis of reference guide pin  12 - 1 , which is used for profiling endfaces of MT connectors. FIG. 13C shows a reference guide pin  12 - 2  with a head  14  having a precisely “angled flat” polished endface  14 A, and is used for profiling endfaces of MPO connectors. FIG. 13C also shows one possible configuration of a keying feature  14 B of reference guide pin  12 - 2  to accurately control the rotational orientation (about the longitudinal axis of guide pin  12 - 2 ) of its angled endface  14 A when reference guide pin  12 - 2  is inserted into the guide pin hole  40  of an MPO connector. Such keying feature  14 B also can be used on a reference guide pin  12 - 1  to allow the rotational orientation of a guide pin suitable for profiling the endfaces of MT connectors to be controlled. Such rotational control can yield even more accurate calibration of MT style connectors.  
         [0066]    Note that it is possible, although not preferable, to use any known surface configuration on the endfaces of the reference guide pins including, but not limited to angled, spherical and parabolic shapes, as all such variants could perform essentially the same tip/tilt calibration function.  
         [0067]    A “first method” and associated apparatus according to the present invention are more suitable for manual interferometer systems, where the interferogram needs to truly represent the surface topography or profile of the fiberoptic connector under test such that an operator controlled interferometer system can quickly identify the first point of contact with a theoretically precisely flat endface of a mating fiberoptic connector, and also can interpret the overall shape or profile of the connector endface.  
         [0068]    Referring to FIG. 13B, one example of the first method involves providing reference guide pin  12 - 1  such that its endface  13 A is polished precisely flat at a right angle with respect to the longitudinal axis of reference guide pin  12 - 1 . Such reference guide pin  12 - 1  is inserted into one end of a guide pin hole  40  of an MT connector  35  as shown in FIG. 16A, using a special reference guide pin insertion/removal tool  20  shown in FIGS. 14A, 14B,  15 A, and  15 B to set the insertion depth of the reference guide pins  12 - 1 . (Optionally, insertion/removal tool  20  can be used to also precisely align the rotational orientation of the reference guide pin  12 - 1  about its longitudinal axis, as subsequently explained with reference to FIGS. 13C, 13D, and  17 .)  
         [0069]    Note that another example of the “first method” would be to perform the foregoing procedure using a reference guide pin  12 - 2  and an MPO connector  350 .  
         [0070]    Referring to FIGS. 14A and 14B, insertion/removal tool  20  includes a stationary base  21  including a pair of spaced uprights  22  supporting a pivot rod  25 . A pair of handle/jaw elements  23  and  24  are supported on pivot rod  25  as shown, with semi-cylindrical bearing surfaces that mate with the outer surface of pivot rod  25 . One or more tension springs  28  urge the upper handle/jaw element  24  and the lower handle/jaw element  23  together on the right-hand side of pivot rod  25 . Handle/jaw element  24  includes a jaw  26  having a semi-cylindrical recess  49  therein which matches the head  13  or  14  of a reference guide pin  12 - 1  or  12 - 2 , respectively, which is to be securely held therein. Similarly, lower handle/jaw member  23  includes a like recess  49 .  
         [0071]    A slidable carriage  34  moves on a stationary track  33  attached to base  21  in the directions of arrows  37 , and carries an MT or MPO connector  35  or  350 , as shown in FIGS. 14A and 14B, respectively, so that guide pin hole  40  is precisely aligned with recesses  49  of jaws  26  and  27 . As shown in FIG. 14A, jaws  26  and  27  include two recesses  49 , to allow one or two reference guide pins to be inserted into the two guide pin holes  40  of the MT connector  35  (or MPO connector  350 ). Pivotal stop  29  in FIG. 14B establishes the elevation of jaw  27  in its closed position. Element  31  is rigidly attached to carriage  34 , and displaces pivotal stop  29  as shown in FIG. 14B when carriage  34  is at its left-most location to allow jaws  26  and  27  to be opened as shown in FIG. 15C to release the “captive” reference guide pin.  
         [0072]    An example of the sequence of steps using insertion/removal tool  20  to “load” reference guide pin(s)  12 - 2  into an MPO connector  350  for interferometric profiling of connector endface  350 A is shown in FIGS.  15 A-C. When carriage  34  moves on carriage track  33  in the direction of arrow  37  in FIG. 15A so that one or two reference guide pins  12 - 2  held by insertion/removal tool  20  are inserted into guide pin hole(s)  40  of MPO connector  350 , pivotal stop  29  is moved from the position shown in FIG. 15A to the position shown in FIG. 15B to allow jaws  26  and  27  to be opened later as shown in FIG. 15C. In FIG. 15B the left-most point of connector  350  abuts the endface of upper jaw  26 , to limit the depth to which pins  12 - 2  are inserted into guide pin holes  40 . In FIG. 15C, arrow  37  illustrates movement of carriage  37  and MPO connector  350  with reference pin(s)  12 - 2  inserted after jaws  26  and  27  are opened to release the head  14  of reference guide pin  12 - 2 .  
         [0073]    MPO connector  350  or MT connector  35  with reference guide pin  12 - 2  inserted is then removed from carriage  34  and secured on an XYZ tip and tilt stage  42  of interferometer  45  as shown in FIGS. 16C and 16D so the procedure for profiling the reference pin endface and connector endface can be carried out, as subsequently described. After connector  350  or  35  has been “calibrated” and profiled according to the present invention, connector  350  is again affixed to carriage  34 , and the foregoing procedure is reversed to “unload” reference guide pins  12 - 2  back into jaws  26  and  27  as shown FIG. 15A, where the reference guide pins are held to be loaded into the next connector.  
         [0074]    After a reference guide pin  12 - 1  is loaded into the guide pin hole  40  of an MT connector  35 , or a reference guide pin  12 - 2  is loaded into the guide pin hole  40  of an MPO connector  350 , the connector is mounted in fixed relationship to interferometer  45  by means of XYZ tip and tilt stage  42  in such a way that the endface, e.g.,  13 A of reference guide pin  12 - 1 , lies approximately on the optical axis  19  of interferometer  45  and can be viewed by interferometer  45  generally as indicated in FIG. 16A. Tilt stage  42  in FIGS. 16C and 16D is somewhat similar to the fiberoptic connector holding fixture shown in FIG. 3 of my above mentioned U.S. Pat. No. 5,459,564, and is suitable for holding an MT connector  35  or an MPO connector  350  securely during measurement.  
         [0075]    The structure of interferometer  45  can be understood by reference to FIG. 16D, which shows a CCD camera  66  controlled by a known control circuit. CCD camera  66  receives an interferogram along the “folded” interferometer optical axis through a power zoom lens  67 . The interferogram is produced by interference between an image beam from a light emitting diode  65  impinging on the endface to be profiled and a corresponding reference beam reflected from a reference mirror  69 . The video output signal produced by CCD camera  66  then is digitized and processed in accordance with conventional phase shifting interferometry techniques, to compute the three-dimensional profile of the endface, to indicate if the endface has been sufficiently precisely polished to ensure low loss and low back reflection mating of the MT or MPO connector with a “like” connector. (The term “like” connector as used herein refers to a fiberoptic connector which has an ideal or perfect endface and is mateable with the fiberoptic connector being profiled.)  
         [0076]    As shown in FIG. 16C, XYZ tip and tilt stage  42  holds MPO connector  350  (or an MT connector  35 ) securely during the interferometric measurement. The position and orientation of the connector, MPO connector  350  in this example, is adjusted, either manually or automatically, in the X, Y and Z directions as necessary to ensure that interferometer  45  is focused on the interferometric image of the precisely polished right angled endface  14 A of a reference guide pin  12 - 2  in MPO connector  350 .  
         [0077]    Referring to FIGS. 16C and 16D, XYZ tip and tilt stage  42  is supported on a base  54  rigidly attached to the base of interferometer  45 . Tilt stage  42  includes a vertical support member  53  rigidly attached to base  54 . In XYZ tip and tilt stage  42 , either an MT connector or an MPO connector is fastened by a clamp  42 A in alignment with interferometer optical axis  19 . In FIGS. 16C and 16D, MPO connector  350  is shown, attached by clamp  42 A to tip stage  60 . Tip stage  60  is attached to and supported by tilt stage  59 . Tip stage  60  can be adjusted to provide precisely controlled amounts of “tip” about the Y axis. Tilt stage  59  provides precisely controlled amounts of tilt about the X axis. Tilt stage  59  is attached to and supported by X translation stage  58 , which in turn is attached to and supported by Z translation stage  56 . Z translation stage  56  is attached to Y translation stage  61  using an L-bracket  62 . Y translation stage  61  is attached to and supported by vertical support  53 .  
         [0078]    Referring to FIGS. 16C and 16D, according to the “first method” of the present invention, XYZ tip and tilt stage  42  with an MT connector  35  with a reference guide pin  12 - 1  loaded therein is “tipped” slightly about the X coordinate axis using tip stage  60  and/or “tilted” slightly about the Y coordinate axis using tilt stage  59  as necessary to cause only a single large fringe to be produced across the entire right angled endface  13 A of reference guide pin  12 - 1 , to thereby “null” endface  13 A so it is perpendicular to interferometer optical axis  19 . (Note that tilting the reference mirror of interferometer  45  instead of the connector endface also can “null” the connecting endface  35 A.)  
         [0079]    Then, the X,Y position of XYZ tip and tilt stage  42  is adjusted so interferometer  45  views a fringe pattern across MT connector endface  35 A and acquires the interferogram data necessary to compute the profile thereof.  
         [0080]    The interferometric profile of endface  35 A thus attained is inherently “calibrated” with respect to guide pin hole  40  of MT connector  35 , and hence also to the guide pin hole  40  of a like theoretically perfect MT connector  35 - 1 . Therefore, the calibrated profile of endface  35 A provides much more accurate information about the overall topography and especially the “highest” point of endface  35 A than would be available without the use of the foregoing reference guide pin  12 - 1  and the foregoing procedure of the present invention, and hence helps predict the performance (such as susceptibility to excessive loss and back reflection) of MT connector  35 .  
         [0081]    After the interferometric profile has been obtained, reference guide pin  12 - 1  is removed from MT connector  35  using insertion/removal tool  20  and is held therein for insertion into the next MT connector to be profiled. The present MT connector  35  then is advanced in its normal production path. The sequence of steps used to unload the one or two reference guide pins  12 - 1  is the reverse of the loading sequence shown in FIGS.  15 A-C.  
         [0082]    It should be understood that the above mentioned inherent calibration of the profile of connector endface  35 A occurs because during the nulling of reference guide pin endface  13 A it undergoes exactly the same “tip” and “tilt” angles required to precisely align the reference pin  12 - 1 , and hence guide pin hole  40 , with interferometer optical axis  19 .  
         [0083]    As indicated above, the foregoing method can include use of two rather than one of the precisely polished reference guide pins  12 - 1  inserted by insertion/removal tool  20  into one or two guide pin holes  40  as shown in FIG. 16B. One would normally expect that after “nulling out” the interferogram fringes on the endface  13 A of one reference guide pin  12 - 1 , the endface of the other reference guide pin would also have only one broad fringe across its endface  13 A. In the event that it does not, one would have learned and been able to quantify the amount by which the two guide pin holes are not parallel with respect to each other.  
         [0084]    [0084]FIG. 19 indicates how the “first method” of the present invention also can be performed to obtain a “calibrated” profile of the angled endface  350 A of an MPO connector  350 . First, interferometer  45  is focused onto the flat, polished angled endface  14 A of a reference guide pin  12 - 2  as shown in FIG. 13C. As previously explained, the keying feature  14 B mates with a complementary keying feature in the recesses  49  of jaws  26  and  27  of insertion/removal tool  20  so that reference pin endface  14 A is substantially parallel to MPO connector endface  350 A. The MPO connector  350  then is tipped and tilted to “null” reference guide pin endface  14 A so it is perpendicular to optical axis  19  of interferometer  45 . Then interferometer  45  is focused on MPO connector endface  350 A, and the resulting interferogram data is acquired by interferometer  45 . Interferometer  45  then computes the profile of connector endface  350 A, which profile is precisely calibrated relative to reference pin endface  14 A, hence to guide pin hole  40  (and hence the precisely angled endface of a theoretically perfect mating MPO connector  350 - 1  (FIG. 7)).  
         [0085]    According to either of the “first method” or a subsequently described “second method” of the present invention, the precisely polished endface of the reference guide pin is inclined or “angled” so as to have a known angle, for example 8 degrees from a plane perpendicular to the longitudinal axis of reference guide pin  12 - 2 , for use in conjunction with an MPO connector, or zero degrees from a plane perpendicular to the longitudinal axis of a reference guide pin  12 - 1  for use in conjunction with an MT connector. (Those skilled in the art know that 8 degrees is the accepted angular connector endface offset angle used in high performance fiberoptic connectors to minimize the effects of back-reflection. This 8 degree offset angle of the endface  350 A of an MPO fiberoptic connector  350  results in most of the back-reflection not being transmitted back down the fiber to the transmission equipment; the back reflection therefore causes very little adverse effect on system performance. Note that some manufacturers use endface offset angles different than 8 degrees, in which case reference guide pins could be manufactured with such different endface offset angle.)  
         [0086]    Reference guide pin  12 - 2  of FIG. 13C and insertion/removal tool  20  can include keying features to ensure that the rotational orientation of the 8 degree angled endface  14 A is established and maintained as reference guide pin  12 - 2  is being inserted into guide hole  40  of MPO connector  350  such that angled endface  14 A is approximately parallel to endface  350 A of MPO connector  350 , as shown in FIG. 17. The keying can be achieved by introducing a suitable keying feature  14 B onto head  14  of reference guide pin  12 - 2  and a complementary keying feature on the surfaces of recesses  49  in jaws  26  and  27  of insertion/removal tool  20 . Jaws  26  and  27  grip head  14  of reference guide pin  12 - 2 , and the keying features in recess  49  line up with matching keying features  14 B to orient head  14  so that reference guide pin endface  14 A is approximately parallel to MPO connector endface  350 A. (Note that even for MT connector  35 , it may be useful to maintain a known rotational orientation for the reference guide pins, to improve measurement repeatability, so it makes sense to use the foregoing rotational keying arrangement  14 B on the reference guide pins  12 - 1  used for MT connectors. The same reference guide pin insertion/removal tool  20  then can be used for insertion/removal of reference guide pins for both MT and MPO connectors.)  
         [0087]    It should be understood that the fringe density viewed by interferometer  45  on poorly polished endfaces during the above described “first method” may not be viable, i.e., may be too high for interferometer  45  to accurately interpret over the large endface area. This is the case because the MT and MPO connectors manufactured by some companies have a very poor polish on their endfaces, due to the unavailability of any practical way of accurately profiling the endfaces of such connectors.  
         [0088]    The foregoing problem of unacceptably high connector endface fringe density as viewed by interferometer  45  on a poorly polished connector endface using the “first method” can be solved according to the “second method” of the invention. This is accomplished by first focusing interferometer  45  on the poorly polished MT or MPO connector endface instead of on the precisely polished reference guide pin endface as in the “first method”, and by “nulling” the interferogram on the connector endface by adjusting the orientation of the connector endface (relative to the reference mirror of interferometer  45 ) to minimize the fringe density. At the same time, the interferogram data needed to compute the “uncalibrated” profile of the connector endface is acquired from the “nulled” connector endface. Then interferometer  45  is focused on the reference pin endface, and its profile is measured. Then the profile of the reference pin endface relative to interferometer optical axis  19  is computed. The profiles of the reference pin endface and the connector endface are combined or utilized to compute a “calibrated” connector endface profile that is calibrated and referenced to guide pin hole  40  of the MPO connector, and hence to the guide pin hole  40  of a theoretically perfect mating connector as shown in FIGS. 6 and 7.  
         [0089]    For example, in one implementation of the “second method” the connector is an MT connector  35  as shown in FIG. 18 (wherein the non-perpendicular “error” angle of endface  35 A is exaggerated for the purpose of illustration). Interferometer  45  is focused on connector endface  35 A, which is “nulled” by adjusting the orientation of the connector endface relative to the reference mirror of interferometer  45  to minimize the fringe density on connector endface  35 A. The interferogram data necessary to compute the profile of connector endface  35  then is acquired by interferometer  45 . Interferometer  45  then is focused on reference pin endface  13 A, which necessarily was tipped and tilted the same amount as connector endface  35 A to accomplish the above nulling. Although a large number of fringes may appear on reference pin endface  13 A, its small surface area presents a relatively small scan range for interferometer  45 . The interferometer  45  then is operated to accurately acquire the interferogram data representing all of the fringes on reference guide pin endface  13 A and then compute the profile of reference guide pin endface  13 A, which profile includes the tip and tilt angles (relative to guide pin hole  40 ) needed to null connector endface  35 A. Consequently, the calibrated profile computed for connector endface  35 A can be easily obtained by combining the two profiles. The calibrated profile of connector endface  35 A referenced to guide pin hole  40  then accurately indicates which point of connector endface  35 A will first contact the connector endface of a theoretically perfect mating MT connector.  
         [0090]    The “second method” of the present invention also can be utilized as indicated in FIG. 20, wherein interferometer  45  initially is focused onto an MPO connector endface  350 A. The XYZ tip and tilt stage  42  is then tipped or tilted about the X and/or Y axes, respectively, to “null” connector endface  350 A, i.e., to minimize the fringe density thereon. The interferogram data representing the profile of the “nulled” connector endface  350 A then is acquired by interferometer  45 , which uses that data to compute the “uncalibrated” profile of connector endface  350 A. Then, without changing the orientation of clamping fixture  42  and MPO connector  350 , interferometer  45  is focused on endface  14 A of reference pin  12 - 2 . Interferometer  45  then acquires data representing the interferogram of flat-angled, polished reference pin endface  14 A, which represents the amount of the foregoing tip and/or tilt relative to guide pin hole  40  which was required to null connector endface  350 A, and uses that data to compute the profile of reference guide pin endface  14 A. The profile of reference guide pin endface  14 A thus is effectively subtracted from or combined with the uncalibrated profile of connector endface  350 A to provide a calibrated profile of MPO connector endface  350 A that is precisely referenced to guide pin hole  40  of MPO connector  350  and hence to the reference guide pin hole of a mating theoretically perfect MPO connector (FIG. 7). The reducing of the scan range as explained above significantly reduces the amount of time for the MPO endface profile measurement.  
         [0091]    It should be understood that although it would be possible to “null” endface  350 A before acquiring the interferogram data without the use of reference guide pin  12 - 2 , it would not be possible to calculate the amount of tip and tilt about the Y axis and X axis, respectively, that was required in order to accomplish the nulling, because the number of fringes representing such tip and tilt amounts would not have been “transferred” to another surface. Therefore, even though the necessary interferogram data could be acquired without use of reference guide pin  12 - 2 , the resulting connector endface profile could not be referenced or calibrated with respect to a theoretically perfect mating MPO connector.  
         [0092]    The interferometric image of the precisely polished reference guide pin endface  14 A then includes tilt information representative of how much MPO connector endface  350 A had to be tipped and/or tilted about the X and Y axes in order to reduce the fringe density on MPO connector endface  350 A to a suitable, i.e., viable, level. Since the amount of tip and tilt can be exactly quantified and broken down into each of its two tip and tilt angle components about the X and Y axes, respectively, the interferometric image of MPO connector endface  350 A then can be corrected by the tip and tilt information transferred to the reference guide pin endface  14 A, allowing accurate computation of the actual profile of the endface  350 A that will be presented to a theoretically perfect mating MPO connector. The profile of MPO connector endface  350 A therefore provides much more accurate information about the susceptibility of the MPO connector to loss and back-reflection than otherwise would be available from use of prior techniques.  
         [0093]    After the profile of MPO connector endface  350 A has been completed, the reference guide pin(s)  12 - 2  is removed and held for the next connector to be profiled, using insertion/removal tool  20 .  
         [0094]    It should be understood that in accordance with the “second method” of the invention, the fringe density problem is eliminated by effectively “transferring” the same fringe density from a large, irregular, lower quality connector endface to the much smaller, highly polished, and reflective reference guide pin endface.  
         [0095]    Consequently, non-viable fringe densities that would have been difficult or impossible to interpret on a poorly polished MT connector endface  35 A or on a poorly polished MPO connector endface  350 A using the “first method” can be readily interpreted once transferred onto the reference guide pin endface  13 A or  14 A, respectively. When this “tip and tilt angle transferring” technique is used in conjunction with a Zoom Interferometer, such as the present assignee&#39;s Direct Optical Research ZX-1 Zoom Interferometer, which has the ability to change the field of view without interferometric re-calibration, the invention becomes even more useful. That interferometer is illustrated in FIG. 16D, and is further described in detail in my above referenced U.S. Pat. No. 5,459,564). In this case, even if the fringe density transferred to the reference guide pin endface is still too high to measure, this type of interferometer could zoom in as necessary when measuring the reference guide pin, effectively reducing the field of view until a viable fringe density is reached, and thereby still permit error correction of the connector endface profile, and thereby still provide a calibrated measurement.  
         [0096]    The foregoing technique and structure is more suited to, but not limited to, incorporation by automated measurement systems, due to the fact that the MT or MPO connector endface interferogram at the time of the measurement does not directly represent the calibrated surface topography of the connector, as it will be presented to a theoretically perfect mating connector. By the end of the measurement sequence the error, which is effectively transferred to the reference guide pin is compensated for, which results in an accurately calibrated measurement.  
         [0097]    It should be appreciated that the only way to meaningfully reduce the fringe density across the connector endface or guide pin endface is to precisely tip and tilt each individual connector to be measured with respect to the interferometer&#39;s optical axis until a viable fringe density is reached. Without using one of the reference guide pin methods described above, the connector endface profile cannot be meaningfully calibrated relative to the endface of a theoretically perfect mating connector, and therefore the connector performance is not accurately determinable.  
         [0098]    The above described embodiments of the invention provide the advantages of calibrated rather than relative measurement of the connector endface geometry, thus enabling the operator and/or interferometer to more accurately predict the expected performance of the connector. The “tip and tilt angle transfer” method of FIGS. 18 and 20 can additionally reduce the scan range required to acquire the surface data, while maintaining an accurate measurement. For example, even if a sample connector endface is flat but tilted at an angle with respect to the interferometer axis, a large scan range is required. By nulling out on the polished connector endface, the angle is effectively transferred to the polished endface of the reference guide pin  12 - 1  or  12 - 2 . Since the endface of the reference guide pin has much smaller lateral dimensions than the connector endface itself, the scan range can be reduced. Since the reference guide pin endface needs to be qualified only once the reference guide pin endfaces can even be measured using red rather than white light, which further reduces the overall required scan range, and significantly increases measurement speed.  
         [0099]    All of the methods described above provide the additional advantage of allowing accurate measurement of fiberoptic connectors, even with their housings installed; this has been impossible previously because insufficient amounts of the connector&#39;s endfaces protrude from the housing to allow for repeatable fixturing (which is essential for repeatable measurements even if the measurement is not calibrated by use of the present invention). In contrast, the invention eliminates the need for repeatable fixturing to obtain repeatable connector endface profiles; instead, the measurement fixture just has to be rigid and stable enough to hold the connector steady for the duration of the interferometric measurement.  
         [0100]    While the invention has been described with reference to several particular embodiments thereof, those skilled in the art will be able to make the various modifications to the described embodiments of the invention without departing from the true spirit and scope of the invention. It is intended that all elements or steps which are insubstantially different or perform substantially the same function in substantially the same way to achieve the same result as what is claimed are within the scope of the invention. For example, one connector could be provided with two rigidly attached reference guide pins  11 ′ which slide into corresponding edge grooves  40 ′ rather than the guide pin holes of a mating connector, as shown in FIG. 21.  
         [0101]    Although the foregoing methods have relied on the use of precisely flat polished “reference guide pins”, it would be possible, but not preferable, to achieve the objective of accurate calibration by using reference guide pins having a wide variety of endface profiles, providing that the exact endface angle, radius or shape is known. Examples of surfaces that can be fairly easily used to meet the objective include angled flats, radiuses or parabolic endfaces. Use of any reference guide pin endface profile other than a precisely flat endface profile would necessitate either rotation of the reference guide pin to at least two positions (preferably ninety degrees apart) or the use of both reference guide pins, each one having a known endface profile. Numerous other reference guide pin insertion/removal tools could be designed and used. For example, the reference guide pins could be of constant diameter, rather than having a head  13  connected to the main body  12 - 1  by a reduced neck section as illustrated in FIG. 13B. The insertion tool could have cylindrical holes into which end portions of the reference guide pins would be inserted, and then the insertion tool could be used to insert the opposite ends of the reference guide pins into the guide pin receiving holes of the connectors. As another alternative, end portions of the reference guide pins could be gripped by jaws that do not have recesses to accommodate end portions of the reference guide pins. Although the above examples illustrate multi-fiber connectors, the invention is applicable to a single-fiber connector using similar ferrule technology. Although the profiling is described as being accomplished by interferometry, it also could be accomplished in other ways, for example by a mechanical profiler or an atomic force microscope.