Optical inspection of optical specimens supported by a work holder

An optical inspection system includes an optical inspection device and an interface. The optical inspection device houses optical imaging components that acquire microscope visual images and acquire interference fringe images of a plurality of optical specimens along an optical path. The optical path is located along an optical axis of the optical inspection device. The interface is coupled to the optical inspection device and is configured to removably engage a polishing work holder that supports the plurality of optical specimens. The interface allows an optical specimen axis of each of the plurality of optical specimens and the optical axis of the optical inspection device to be aligned.

BACKGROUND

An optical fiber connector includes a ferrule, which is a cylindrical ceramic element, having a centrally mounted optical fiber. An optical fiber connector also includes a connector housing which is physically mounted to a fiber optic cable and the ferrule. Optical fibers and ferrules in fiber optic connectors require a high quality, optical grade endface surface to maximize coupling efficiency and ensure proper operation. Generally a fiber endface has a desirable geometry or topography, such as a desirable radius of curvature, apex offset and fiber height. The fiber endface also has an acceptable surface quality free of any undesirable surface defects.

The proper surface quality and topography of a fiber endface in a fiber connector is achieved through an optical polishing process. During this process a plurality of fiber optic connectors or fiber optic connector ferrules are mounted in a polishing plate or work holder. The work holder determines the position and polish angle of the ferrules during the polishing process. The connectors are mounted in a way such that the endface of the ferrule protrudes slightly past the bottom surface of the polishing plate. The polishing plate is repetitively moved over an abrasive surface, or pad, in a systematic manner. The abrasive pad removes material from the protruding portion or endface of the ferrule. The desired ferrule endface geometry and surface quality is achieved by changing the polishing variables such as the type of abrasive material used, pressure and speed applied and the time of polishing.

There are a large number of factors that influence the end result of the polishing process. Examples of these factors are tool wear, temperature variations, material variations, external physical contaminants and skill of the operator. Depending on the level of control exercised over these variables, the result of the polishing process can vary significantly between iterations. Therefore, to ensure desired polishing results, it is necessary to inspect the endface quality and test the topography of the fiber connector during and after the polishing process.

The surface quality of the fiber endface can be evaluated using an optical microscope to magnify any surface defects that might be present on the fiber endface surface. The fiber endface geometry can be measured using several different contact, or non-contact, three-dimensional surface mapping methods. For example, optical interferometry is a very practical and accurate measurement technique and is commonly used for inspecting endfaces.

In general, testing and inspecting the connector endfaces using conventional means to requires that each ferrule, or connector, be physically removed from the work holder and placed into a separate testing instrument for evaluation. If the ferrule endface does not have the required parameters it must be re-inserted back into the polishing plate for further processing or considered rejected. It may be necessary to repeat this testing procedure several times with each ferrule during the fabrication process.

Such testing procedures require excessive material handling of the sensitive ferrules and can be very time consuming. Furthermore, the removal and subsequent re-insertion of the ferrule from the polishing plate may introduce minute positional changes and other variables that further impede the predictability of the polishing process. It is generally undesirable to remove the ferrule from the polishing tool until the polishing process is complete.

SUMMARY

An optical inspection system includes an optical inspection device and an interface. The optical inspection device houses optical imaging components that acquire microscope visual images and acquire interference fringe images of a plurality of optical specimens along an optical path. The optical path is located along an optical axis of the optical inspection device. The interface is coupled to the optical inspection device and is configured to removably engage a polishing work holder that supports the plurality of optical specimens. The interface allows an optical specimen axis of each of the plurality of optical specimens and the optical axis of the optical inspection device to be aligned. In one embodiment, the polishing work holder is rotated to align the optical specimens with the optical axis. In one embodiment, the optical inspection device is rotated to align the optical axis with the optical specimens.

DETAILED DESCRIPTION

Embodiments of the disclosure pertain to inspection of endfaces of optical fiber connectors that are mounted to a polishing work holder. Embodiments of the disclosure improve the speed, accuracy and dependability of fiber connector endface polishing processes by allowing the visual inspection and three-dimensional topographical measurements of fiber connector endfaces without the need to remove the connectors from the polishing plate or polishing work holder that supports the connectors during polishing.

A polishing work holder is configured to support a plurality of optical fiber connectors or fiber optic connector ferrules for polishing. Such optical fiber connector or fiber optic connector ferrules are example optical specimens. Optical fibers in fiber optic connectors require a high quality, optical grade endface surfaces to maximize coupling efficiency and to ensure proper operation with other fiber optic connectors. Generally a fiber endface has a desirable geometry or topography, as well as an acceptable surface quality free of any undesirable surface defects.

The proper surface quality and topography of each fiber endface for each of the plurality of optical fiber connectors supported in a polishing work holder is achieved through an optical polishing process of the fiber and ferrule assembly or of the connector. Many different types of connector styles can be supported by a polishing work holder for polishing. For example, ST, SC, FC, LC and other industry standard connectors can be supported. The polishing work holder also determines the position and polish angle of the ferrules during the polishing process, such as straight polish (PC) and angle polish (APC) versions.

The connectors are mounted and supported in a polishing work holder in such way that the endfaces of the ferrules protrude slightly past a bottom surface of the polishing work holder. During polishing, the polishing work holder is repetitively moved over an abrasive surface, or pad, in a systematic manner. The abrasive pad removes material from the optical specimen supported by the polishing work holder. There are a large number of factors that influence the end result of the polishing process. Examples of these factors are tool wear, temperature variations, material variations, external physical contaminants and skill of the operator. Depending on the level of control exercised over these variables, the result of the polishing process can vary significantly between iterations. Therefore, to ensure desired polishing results, it is necessary to inspect the endface quality and test the topography of the fiber connector throughout the polishing process.

FIG. 1is a schematic block diagram of an optical inspection system110coupled to a polishing work holder112under one embodiment. Optical inspection system110includes an optical inspection device114and an interface116. Optical inspection device114includes an optical axis118that is positioned along its optical path. Polishing work holder112includes a work holder central axis120and a plurality of optical specimens121(of whichFIG. 1illustrates two such specimens) each having a specimen axis123that are radially spaced about work holder central axis120. Interface116is coupled to optical inspection device114and is capable of engaging with polishing work holder112such that the plurality of optical specimens221supported by polishing work holder112can be inspected by optical inspection device114.

Optical inspection system110is placed in close proximity to a polishing machine. At any time during the polishing process, an operator can remove polishing work holder112containing in-process optical specimens121from the polishing machine and engage it with interface116. Optical inspection device114is used to complete a visual evaluation of surface quality of the optical specimens contained by the polishing work holder112as well as performing a three-dimensional topographical measurement of the optical specimens. Optical specimens that meet specified surface requirements can be removed from polishing work holder112. Polishing work holder112, which contains the remaining optical specimens that require further processing, is placed back in the polishing machine.

InFIG. 1, polishing work holder112is fixedly attached to interface116, while optical inspection device114is rotatable relative to polishing work holder112and interface116. Optical inspection device114is rotatable about work holder central axis120as indicated by arrow119such that optical axis118can come into alignment with each specimen axis123of the plurality of optical specimens121supported on polishing work holder112. In theFIG. 1configuration, optical inspection device114can be moved about work holder axis120to all of the optical specimen positions on polishing work holder112while polishing work holder112is fixed.

FIG. 2is a schematic block diagram of an optical inspection system210coupled to a polishing work holder212under one embodiment. Optical inspection system210includes an optical inspection device214and an interface216. Optical inspection device214includes an optical axis218that is positioned along its optical path. Polishing work holder212includes a work holder central axis220and a plurality of optical specimens221(of whichFIG. 2illustrates two such specimens) each having a specimen axis223that are radially spaced about work holder central axis220. Interface216is coupled to optical inspection device214and is capable of engaging with polishing work holder212such that the plurality of optical specimens221supported by polishing work holder212can be inspected by optical inspection device214. Optical specimens supported in polishing work holder212will be in the process of polishing or have already undergone one or more polishing processes.

Like optical inspection system110, optical inspection system210is placed in close proximity to a polishing machine. At any time during the polishing process, an operator can remove polishing work holder212containing in-process optical specimens221from the polishing machine and engage it with interface216. Optical inspection device214is used to complete a visual evaluation of surface quality of the optical specimens contained by the polishing work holder212as well as performing a three-dimensional topographical measurement of the optical specimens. Optical specimens that meet specified surface requirements can be removed from polishing work holder212. Polishing work holder212, which contains the remaining optical specimens that require further processing, is placed back in the polishing machine.

InFIG. 2, polishing work holder212is rotatably attached to interface216, while interface216is fixedly attached to optical inspection device214. Polishing work holder212is rotatable relative to both optical inspection device214and interface216. In particular, polishing work holder212is rotatable about work holder central axis220as indicated by arrow219such that each optical specimen221having specimen axis223is radially positioned about work holder central axis220can come into alignment with the fixed optical axis218of optical inspection device214. In theFIG. 2configuration, polishing work holder212can be moved to discrete positions to locate each specimen axis223of each optical specimen221in line with optical axis218.

FIG. 3is top perspective view of an optical inspection system310engaged with a polishing work holder312under one embodiment. Optical inspection system310is similar to optical inspection system210ofFIG. 2and polishing work holder312is similar to polishing work holder212ofFIG. 2in that polishing work holder312is rotatable about a central work holder axis320, while an interface316is fixedly coupled to a fixed optical inspection device314. Interface316is configured to removably engage with polishing work holder312. Polishing work holder312includes a plurality of optical specimens322. Each of the plurality of optical specimens322are radially arranged about central work holder axis320and polishing work holder312is rotatably coupled to interface316about central work holder axis320.

FIG. 4illustrates a top exploded perspective view of optical inspection system310ofFIG. 3and polishing work holder312ofFIG. 3.FIG. 5illustrates a bottom exploded view of interface316ofFIG. 3and polishing work holder312ofFIG. 3.FIG. 6illustrates a sectional view of optical inspection system310and polishing work holder312ofFIG. 3.

Optical inspection device314(FIGS. 4 and 6) is a self contained, non-contact measurement instrument used for the visual inspection and optical interferometric measurement of fiber optic end faces. The highly compact design of the instrument facilitates easy integration into other types of systems, such as polishing systems. Optical inspection device314includes a main body324(FIGS. 4 and 6). Main body324has a slender cylindrical body such that an operator can easily grip it for portable use. Main body324includes a removable base325(FIGS. 4 and 6). If base325is attached to main body324, then optical inspection device314can be supported and placed on a level surface. Optical inspection device314, as illustrated inFIGS. 3,4and6, has a rather small dimension. In particular, but not by limitation, device314includes a height that is approximately 150 mm and a diameter that is less than 200 mm. For example, the diameter of device314can be approximately 70 mm. In addition, device314is rather light-weight. In particular, but not by limitation, device102has a weight that is less than 2.25 kg. For example, device102can be approximately 630 g.

Main body324is coupled to a plurality of interconnected electrical components (not shown). For example, optical inspection device314includes a microprocessor used in conjunction with a connector and a host computer to control the function of and report data from onboard electronics. For example, the connector can be a USB II interface. The host computer includes software configured to display measurement results and to control various electrical functions of the optical inspection device314. No external power supply is necessary for the optical inspection device314since all power can be supplied through the connector. It should be noted, however, that the optical inspection device314is not limited to any particular type of connector or any particular type of software.

Main body324houses optical imaging components and other electrical components327(FIG. 6) that can acquire visual images for surface inspection and acquire interference fringe images for three-dimensional interferometric metrology when evaluating optical specimens322. The visual images and the interference fringe images are acquired along an optical path located at an optical axis318(FIGS. 4,5and6) of the optical inspection device314. The optical imaging components are based on a Michelson Interferometer arrangement combined with a direct visual imaging capability.

As previously discussed, optical inspection device314is capable of operating in two distinct modes. These modes include an interferometric measurement mode and a visual scope mode. Optical inspection device314performs separate functions in fiber endface inspection in each mode. The interferometric measurement mode and the visual scope mode are selectable by opening and closing a mechanical shutter. The mechanical shutter, placed in front of the reference mirror, is used to block the optical path to the reference mirror. When the shutter is in an open position, the optical inspection device314functions as a Michelson Interferometer. In particular, interference fringes between the reference path and test paths of the interferometer are imaged onto the area array detector. When the shutter is in a closed position and the reference path is blocked, the system functions as a finite conjugate imaging system and the surface under test is directly imaged onto the area array detector.

Main body324of optical inspection device314also includes an interface receiving surface328(FIG. 4). Interface receiving surface328includes a plurality of kinematic seats330(FIG. 4). In particular, main body324of optical inspection device314includes three kinematic seats330. Each kinematic seat330includes a magnet. Kinematic seats330are used in aligning interface316to optical axis318of optical inspection device314and coupling optical inspection device314with interface316.

Interface316includes an interface plate332(FIGS. 4 and 5) having a top surface334(FIG. 4) and a bottom surface336(FIG. 5). Interface plate332can be made of a variety of magnetic and non-magnetic materials. For example, interface plate332can be made of aluminum. Interface plate332aligns with, and magnetically couples to interface receiving surface328of optical inspection device314by a six-point kinematic contact arrangement. Interface plate332includes a plurality of kinematic tooling balls338(FIG. 5) attached to bottom surface336. The alignment of interface316with optical axis318includes a repeatable positional accuracy of less than two microns. Such precise alignment is accomplished by each ball contacting the two pins in each kinematic seat in exactly two points. The magnets ensure constant contact and load between the kinematic contacts (i.e., each seat330to each ball338). This arrangement facilitates a very simple method of quickly removing interface316while guaranteeing a high level of position repeatability. Complex alignment provisions are not needed. In addition, kinematic tooling balls338and kinematic seats330are radially arranged such that there is only one unique, mechanically stable position that can occur when interface316and optical inspection device314are assembled. A plane formed by kinematic tooling balls338seated in kinematic seats330is substantially precisely perpendicular to optical axis318of optical inspection device314.

Interface316also includes a moveable or fixed hub feature340(FIGS. 4 and 6) and a plurality of spherical surfaces342(FIGS. 4 and 6). Hub feature340protrudes from top surface334of interface plate332and is configured to removably engage polishing work holder312. Polishing work holder312is rotatable about hub feature340around its central work holder axis320(FIG. 4). The plurality of spherical surfaces342are aligned in such a manner such that a plane defined by a bottom surface350(FIG. 5) of polishing work holder312when engaged with the plurality of spherical surfaces342and hub feature340is parallel to the plane formed by kinematic tooling balls338seated in kinematic seats330. The plurality of spherical surfaces342define the position of polishing work holder312when seated on hub feature340. In addition, interface316includes a magnet343(FIGS. 4 and 6) for each spherical surface342. Magnets343impart a pulling force on polishing work holder312to ensure proper contact with spherical surface342. As illustrated, in one embodiment, interface316includes three spherical surface342and three magnets343. Therefore, polishing work holder312is magnetically engaged with interface316

Minor adjustments to the location of hub feature340and therefore central work holder axis320can be made in the plane perpendicular to optical axis318. Such minor adjustments can be made using a knob344(FIGS. 4,5and6). Upon rotation of knob344, hub feature340will translate in a direction perpendicular to optical axis318of optical inspection device314and thereby translate the location of the polishing work holder312with respect to the optical axis318. Such minor adjustments in the location of hub feature340allow optical specimens322to be centered into the field of view or optical path of optical inspection device314.

Polishing work holder312fits on hub feature340and rests on the plurality of spherical surfaces342, of which, in one embodiment, interface plate332includes three. After polishing work holder312is engaged with interface316and minor location adjustments are made of hub feature340, the plurality of optical specimens322supported by the polishing work holder312line up with optical axis318of optical inspection device314. Polishing work holder312is rotatable about axis320on hub feature340to bring each optical specimen322into a field of view or optical path of optical inspection device314. To bring each optical specimen322into the optical path of optical inspection device314, interface316includes an indexing mechanism346(FIG. 4). Index mechanism346clocks polishing work holder312about hub feature340. A screw348(FIG. 4) attached to indexing mechanism346can be used to fine-tune the exact position of each index stop.

Polishing work holder312can be made of a magnetic material. The magnetic material of the polishing work holder312can be different than the material of interface316or the same. Bottom surface350of polishing work holder312is made substantially perfectly flat to ensure that central work holder axis320remains substantially perfectly parallel with optical axis318of optical inspection device314for every position of which polishing work holder312rotates to. For example, bottom surface350can be flat within a range of 0-4 microns. To achieve a substantially perfectly flat bottom surface350, the bottom surface can be machined using precise methods, such as by grinding or lapping. It should be noted that other types of machining techniques can be utilized to achieve the substantially perfectly flat polishing contact surface.

Index stops352(FIGS. 4,5and6) are small, equally spaced features that protrude radially from an outer edge354(FIGS. 4,5and6) of polishing work holder312. Index stops352engage with indexing mechanism346of interface316and define discreet stopping positions as polishing work holder312is rotated to each position.

Although polishing work holder312is described as being particular to optical inspection system310, it should be recognized that optical inspection system310and can be coupled to any pre-existing conventional polishing work holder that is particular to any conventional polishing mechanism for testing fiber ferrules during the polishing process. This allows optical inspection device314and interface316to be versatile in scope and use. In addition, optical inspection system310is cost effective and highly versatile over other types of systems which inspect optical fiber connectors or fiber optic connector ferrules during a polishing process. For example, optical inspection system310is cost effective and time efficient over other systems that require manual release of each optical fiber connector from a polishing work holder. In another example, optical inspection system310is cost effective and highly versatile over large automated systems which require the use of specific polishing systems.