WO2004017115A1

WO2004017115A1 - Fiberoptical ferrule

Info

Publication number: WO2004017115A1
Application number: PCT/SE2002/001355
Authority: WO
Inventors: Anders Sjöberg
Original assignee: Sjoeberg Anders
Priority date: 2002-07-04
Filing date: 2002-07-04
Publication date: 2004-02-26
Also published as: AU2002324389A1

Abstract

In a fiberoptical ferrule (3) an axial channel (11) is provided for the optical fiber (1), the channel having a diameter that is larger than that of the optical fiber. A hole (17) extends in the ferrule in to the channel. By an arm (26) the optical fiber is positioned in a desired lateral position in the channel and is secured in this position using for example an adhesive (35). The ferrule can also be used in the same way for receiving a hole fiber acting as an accurate guide as well as an inner guiding sleeve for an optical fiber. The axial channel in the ferrule has not to be manufactured with a high precision, this reducing the cost of fiber and mounted ferrule. Furthermore, the end surface (8) of the fiber can in rather simple way be placed well positioned at the plane of the front surface (6) of the ferrule, this reducing the time required for the necessary polishing of the end surface of the fiber and the front surface of the ferrule.

Description

FIBEROPTICAL FERRULE TECHNICAL FIELD

The invention relates to a fiberoptical ferrule and a method and a device for mounting an optical fiber or a fiber insert in such a ferrule. BACKGROUND

When using optical fibers for transferring signals, i.e. in fiberoptical communication systems, alignment of the fibers is required. It means that the ends of the optical fibers have to be aligned with each other or with input/output surfaces of optical components. Without such an accurate alignment a too large attenuation can be obtained in splices between optical fibers or in the transition region between a fiber and an optical component. When connecting optical fibers to each other or to components, connectors permanently attached to the ends of the fibers can be used. Such connectors can in the typical case contain a guide sleeve, also called a ferrule, in particular a fiberoptical ferrule, to provide the accurate positioning, in particular in the transverse direction of the fibers. Then, the end of an optical fiber has to be very carefully positioned in the center of the ferrule, this implying that the guiding of the fiber in the ferrule must be made with a high mechanical precision. The ferrule also has outer guiding surfaces for guiding the ferrule in relation to connectors that include the same type of ferrule, and also these guiding surfaces must be made with a high mechanical precision. The precision of the ferrule and the fiber end retained therein must be so good that an attenuation of 0 - 0.5 dB in the splice or the transition region is obtained. The high mechanical precision results in that the cost of a ferrule including an optical fiber mounted therein is relatively high.

Thus, it is desired to find alternative solutions to achieve mounted ferrules having the same high optical performance but at a lower cost, i.e. more rational methods of manufacturing ferrules and simplified mounting operations of the fibers in the ferrules. In U.S. Patent No. 3,938,895 a method for attaching a fiber guide 30 in an axial hole in a sleeve 40 is disclosed. The hole has a diameter that is considerably larger than the outer diameter of the fiber guide. First the fiber guide is displaced radially to a central position in the sleeve in an adaptive or feed back manner, by conducting light through the optical fiber in the fiber guide to its end surface thereof and detecting the light output at the end surface. In the displacement the fiber guide is retained in a V-groove in a mechanical translator. Finally the fiber guide is permanently attached in the sleeve.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a fiber optical ferrule in which an optical fiber or fiber guide, such as a fiber capillary, can be mounted with a high precision. It is another object of the invention to provide a method and a device for mounting, in a fiber optical ferrule, an optical fiber or a fiber guide, such as a fiber capillary, with a high accuracy as to the centering thereof.

In a fiberoptical ferrule an axial channel or axial hole or bore is provided for the optical fi- ber, the channel or bore having a diameter larger than that of the optical fiber or than the outer diameter of a fiber guide in which the fiber is mounted or is to be mounted and thus allows that the fiber can move somewhat laterally in the channel. At least one recess or at least one hole extends through the material surrounding the channel in to the channel in a substantially radial direction. Using active manipulation through this recess or hole the optical fiber or fiber guide is moved in a controlled way to a desired lateral position in the channel while an adhesive or some similar curable mass has been introduced or is being introduced in the space between the fiber or the fiber guide and the walls of the channel. The fiber or fiber guide respectively is maintained in the desired position until the adhesive or the curable mass has been transferred into a solid state, i.e. has been cured. The manipulation through the hole can be accomplished using an arm of a tool, the arm introduced into the hole and capable of gripping the fiber or fiber guide, for example using vacuum such as in a vacuum chuck. Alternatively, a plurality of recesses or holes can be used extending radially in to the channel and placed in the same plane perpendicular to the channel and the fiber or fiber guide. In these recesses or holes supporting arms belonging to the tool and cooperating with each other can be introduced to displace the fiber laterally to a desired position.

Generally, the desired position is determined by outer mechanical guide surfaces of the ferrule. Therefore, the ferrule can be manufactured having a not too high precision, in particular as to the diameter and the centering of the axial channel. The only requirement is that the axial channel or bore allows a displacement of an introduced fiber of fiber guide to for example a well centered position. Since thereby the manufacturing cost of the ferrule can be significantly reduced, also the total cost of the combined component fiber and ferrule mounted at an end thereof, i.e. generally a fiber including a mounted connector, is reduced.

Furthermore, the optical fiber can in some simple way be positioned having its end well located in the same plane as the front surface of the ferrule. It reduces the time required for polishing the end surface and the front surface. Also, due to this feature, the cost of the combined component fiber and ferrule is reduced.

A ferrule of the same type but having an adapted diameter of the channel can be used for receiving a fiber guide having the shape of for example a hole fiber acting as an accurate guide, as well as generally an inner guide sleeve for an optical fiber, which then can be detachably mounted in the hole in the hole fiber or fiber guide.

The ferrule can be made from transparent plastics material, for example curable plastics material, or also from for example metal. The adhesive or curable mass surrounding the fiber in the channel can be the UV-curable type, i.e. the type that is cured when exposed to ultraviolet light.

Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the ap- pended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

While the novel features of the invention are set forth with particularly in the appended claims, a complete understanding of the invention, both as to organization and content, and of the above and other features thereof may be gained from and the invention will be better appreciated from a consideration of the following detailed description of non-limiting embodiments presented hereinbelow with reference to the accompanying drawings, in which:

- Fig. 1 is a schematic side view partly in a cross sectional view of device for mounting an optical fiber in an accurately determined position in a ferrule,

- Fig. 2 is a perspective view of the end of a positioning arm holding an optical fiber, - Fig. 3 is a side view, partly in a cross sectional view, showing parts corresponding to those illustrated in Fig. 2,

- Figs. 4a and 4b are schematic views of the front side of a ferrule having an optical fiber located therein,

- Fig. 5 is a schematic cross sectional view of a portion at the front surface of a ferrule having an optical fiber mounted therein,

- Fig. 6a is a cross sectional view of a ferrule having a hole for positioning arms,

- Fig. 6b is a perspective view of the front portion of the ferrule of Fig. 6a, and

- Fig. 7 is a view similar to Fig. 1 but including a ferrule in which a hole fiber acting as a fiber guide is to be mounted in an accurately determined position. DESCRIPTION OF PREFERRED EMBODIMENTS

In Fig. 1 a device for mounting an optical fiber 1 in a desired position in a ferrule 3 is shown. The ferrule 3 is part of or forms an optical connector and includes at its front end an outer mechanical guide surface 5 having a cylindrical shape that at its rear end can continue into a cylindrical collar 7 having a larger diameter or generally into one or more cylindrical, concentric portions having larger or smaller diameters. The guide surface 5 continues at the front end of the ferrule, for example through a conical surface or bevelling, into a front flat surface 6 that is perpendicular to the axis of the cylindrical surfaces. The guide surface is in the use of the ferrule, when an optical fiber is attached therein, intended to cooperate with another connector having an identical cylindrical guide surface, not shown, and to be centered or aligned in relation thereto by for example mounting an elastic guide sleeve, not shown, on the guide surfaces and then the end surface 8 of the fiber 1 is placed at a corresponding surface in the other connector.

Advantageously, the ferrule can be made from for example some curable plastics material and it can, at least in the portion located inside the guide surface 5, be transparent to ultraviolet light.

The optical fiber 1 is over a portion 9 at its end rid of its protective polymer coating 10. Also, through all of the ferrule a through-hole or bore is provided extending along the axis of the outer cylindrical surfaces. The through-hole has a front portion 13 having a diameter that somewhat exceeds the diameter of the naked optical fiber 9, e.g. by 10 - 50 %. The through-hole also has a rear portion 15 having a diameter that is larger than that of the front portion 13 and that is also larger than the diameter of the coating 10 of the optical fiber.

In the front portion of the ferrule 3 also at least one transverse hole 17 is provided that extends from the guide surface 5 in to the front portion 13 of the axial hole 11. In a first embodiment only one such transverse hole is provided that can optionally extend rectilinearly either only in to the axial hole 11, i.e. it can be a blind hole, or through all of the front portion of the ferrule, i.e. it can be a through-hole.

In order to achieve a good optical coupling to active surfaces of other connectors the end surface 8 of the optical fiber 1 must be mounted accurately centered in relation to the guide surface 5, i.e. the end surface 8 must have its center point exactly placed at the geometric axis of the guide surface. For this purpose, the device includes a mechanic support part 17 having an inner cylindrical surface that is accurately adapted to the guide surface 5 so that the ferrule 3 can be mounted in the support part in an exact position. Furthermore, a mechanical linear movement device 19 for axial movement is provided to grip, using a gripping surface 21, the optical fiber 1 at the protective coating 10 thereof at a distance of the fiber end 8 and to displace the fiber in a movement in directions parallel to the axis of the ferrule 3. The gripping surface 21 is so located that a fiber 1 held by the gripping surface with its end portion can be moved into the ferrule 3 placed in the support part 17. Alternatively, a detachable fiber holder, not shown, can be used in which the fiber is securely held and which also can be securely placed and held by the axial movement device 19. Furthermore, a movement device 23 for radial displacement is provided, including a motor 24 and a gripping surface or contact surface 25 for gripping the fiber at the naked portion 9 thereof through the transverse hole 17 or holes for moving the fiber in directions perpendicular to the axis of the ferrule 3 or in a plane perpendicular to the axis. The motor 24 can be connected to at least one arm 26 which passes through the transverse hole 17 in to the naked optical fiber and which there at its end has the gripping surface or contact surface. The gripping surface 25 is shown in greater detail in Figs. 2 and 3 and it has in the design illustrated there, a concave shape including part of a cylindrical surface, the diameter of which agrees with or is somewhat larger than the diameter of the naked fiber. It can be designed as a subatmospheric or vacuum chuck and then, a channel is provided, not shown, in the arm 26 which mouths in the gripping surface and is connected to a subatmospheric pressure source, not shown.

A light injection device 27 can inject light into the optical fiber at a portion extending from the end thereof to propagate along the fiber in the direction towards the end surface 8 and to pass out of the end surface. The light transmitted from the end surface is by a lens system 29 imaged on the active surface of a camera 31 of CCD type. The axial movement device 19, the transverse radial movement device 23 and in particular the motor 23 thereof and the CCD camera 31 are all electrically connected to a control unit 33, designed for example as a microprocessor or computer. By the movement devices 19, 23 and detecting the light emitted from the fiber end the optical fiber can be moved to a position having its end surface 8 located substantially in the same plane as or insignificantly projecting from the front surface of the ferrule and centered in relation to the guide surface 5, as will now be briefly described.

A ferrule 3 is mounted in the mechanical support part 17 having its guide surface in contact with and enclosed by the corresponding inner surface of the support part. From an end portion of an optical fiber 1 the protective coating thereof is removed so that a naked portion 9 is formed at the end of the fiber. The fiber 1 is at its remaining coating 10 gripped in a region located not too far away from the naked portion of the gripping surface 21 of the axial movement device 19. It is activated by a signal from the control unit 13 and displaces the end portion of the fiber into the bore 11 of the ferrule 3. Preferably the fiber is moved so that its end surface is located somewhat outside the front surface 6 of the ferrule 3. The end surface of the fiber can be observed in that image or those images that is/are generated on the active surface of the camera 31 by the lens system 29. In the image, for a suitable setting of a lens system, not shown, the lens system for example being the microscope type, of the camera, interference fringes can be observed which can advantageously be utilized. The electrical signals that correspond to the images are transferred to and analyzed by the control unit 33, in particular those portions of the signals which indicate the interference fringes. The control unit provides a signal to the axial movement unit 19 to control the axial displacement and stop it when the end surface 8 of the fiber is located in an intended position. In this position the naked end portion 9 of the fiber extends into the front narrow portion 13 of the bore 11 and is located with some play in relation to the wall of the front portion of the bore. The naked fiber portion is gripped by the gripping surface 25 of the arm 26 of the radial movement device 23 and the motor 24 of this movement device is activated by signals from the control unit for moving the arm and thereby the fiber end laterally. The fiber end is displaced to a position that is centered in relation to the guide surface 5 of the ferrule, in a feedback manner. The signals from the control unit are generated by automatic image processing of the video sig- nals from the camera 31 so that a feedback control is obtained.

In Figs. 4a and 4b schematic pictures of the front surface of the ferrule when placing the optical fiber in its position are shown. The ferrule 3 shown in Figs. 4a and 4b has an axial bore 11, 13 that is not concentric with the guide surface 5. When the fiber is introduced into the ferrule by the axial movement device 19 it obtains the position illustrated in Fig. 4a and is neither centered in the hole 11, 13 or in relation to the guide surface 5, the center of which is shown at 39. By a lateral displacement obtained by a suitable activation of the motor 24 the fiber can obtain the well centered position as shown in Fig. 4b. Images similar to those of Figs. 4a and 4b are captured by the camera 13 and are evaluated by the control unit 33.

When the end surface 8 of the fiber is located in the intended position, into the front portion of the narrow portion 13 of the bore, in the region between the transverse hole 17 and the front surface 6 of the ferrule an adhesive or curable mass 35 is introduced. The adhesive or the mass can preferably also be introduced before the final positioning of the fiber and surface 8, possibly also before the fiber is introduced in the axial bore 11. The adhesive or mass is cured in a suitable way such as by exposing it to ultraviolet light which is conducted therein through the fiber, from for example the front end surface thereof. The ferrule 3 can in its front portion be made from a material that is transparent to ultraviolet light, this facilitating the introduction of the ultraviolet light to the curable adhesive or mass. After curing the adhesive or the mass 35 another curable mass is introduced into the rear wider portion 15 of the axial bore 11 of the ferrule and is cured, for example by heat. The ferrule including the fiber accurately positioned therein can then be re- moved from the device.

After the mounting, the front portion of the ferrule can have a configuration as illustrated by the cross sectional view of Fig. 5. The naked end of the fiber is here illustrated as somewhat projecting beyond the front surface 6 of the ferrule and the adhesive or mass 35 secures the end in a desired centered position. However, the end surface 8 of the fiber can generally be located in the same plane as the front surface as has been mentioned above, even somewhat retracted in relation to or inside the end surface. By the fact that images of the end surface of the fiber can be captured during the introductory axial displacement and that these pictures can be automatically evaluated, the end surface 8 of the fiber can be made to be located in a desired position axially in 5 relation to the front surface 6 of the ferrule, such as substantially in the same plan as it. Such a position in the plane of the front surface allows that polishing the end surface in order to give it an intended shape and to make it be located accurately in the plane of the front surface can be made within a time period that is short compared to the time period which is required when using a conventional, direct mechanical guiding of the optical fiber by the axial bore to a centered posi-

10 tion.

An alternative design of the radial movement device 23 is illustrated by the schematic views of Figs. 7a and 7b. The radial movement device here includes a plurality, such as three, arms 26' that are arranged in holes 17 in the same plane perpendicular to the axis of the ferrule and that are evenly distributed over the full turn, i.e. in the design shown forming an angle of

15 120° to each other, and that extend in to those places where the cylindrical surface of an introduced optical fiber is located. The arms 26' are here not required to have some gripping surfaces but can have smooth end surfaces 25' for engaging with and displacing an introduced optical fiber. The arms 26' are connected to motors, not shown, for moving them and thereby an introduced optical fiber to a desired transverse position.

20 The device as described above can also be used for positioning a fiber guide or fiber insert in an accurately determined position in a ferrule, compare the published European Patent application 0 301 775. Such a fiber insert can according to this European patent application comprise a hole fiber or channel fiber, also called fiber capillary, of substantially the same material as that of the optical fiber. As is illustrated in Fig. 7, in this case the ferrule 3' is used having an axial hole

25 11 ' which has a diameter that is adapted to the diameter of the hole fiber 41 and that can comprise for example about the double fiber diameter. The hole fiber 41 is, when positioned in the ferrule, supported by the front narrow part of a support rod 43 that at its rear end is held by the gripping surface 21 of the axial movement device 19. The hole fiber 41 has such a length that it extends, after it has been placed in a desired axial position by the axial movement device with its

30 front surface substantially in the same plane as the front surface of the ferrule, only through the front portion of the ferrule. The support rod 43 is elastic and allows transverse displacement of the hole fiber using the radial movement device, not visible in Fig. 7.

In the design shown no light is introduced into the hole fiber 41 or support rod but the camera 31 captures an image using ambient or general illumination. However, light can easily be introduced if required, for example through the support rod if it is made from substantially the same material as the optical fiber and the insert.

The hole fiber 41 is secured in the desired position using a curable adhesive or a curable mass 35' in the same way as been described above. While specific embodiments of the invention have been illustrated and described herein, it is realized that numerous additional advantages, modifications and changes will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative devices and illustrated examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general in- ventive concept as defined by the appended claims and their equivalents. It is therefore to be understood that the appended claims are intended to cover all such modifications and changes as fall within a true spirit and scope of the invention.

Claims

1. A device for mounting an optical fiber or fiber guide in a ferrule, the ferrule including an elongated body having an axial bore extending therethrough in a direction parallel to the axis of the elongated body, the device including: - a support part for holding a ferrule, and

- a radial movement device for displacing an optical fiber or a fiber guide held thereby transversely, perpendicularly to the axis, in the axial bore in the ferrule held by the support part, characterized in that the radial movement device includes at least one arm that has a gripping or support surface at its end for engagement with an optical fiber or fiber guide held by the radial movement device and that passes through a radial hole in the ferrule.

2. A device according to claim 1, characterized in that the radial movement device includes an arm having gripping surface made as a vacuum chuck for holding a optical fiber or fiber guide introduced in a held ferrule.

3. A device according to claim 1, characterized in that the radial movement device in- eludes at least three arms arranged in a plane perpendicular to the axis of a ferrule held by the support part, the arms extending in radial directions in relation to the axis and evenly distributed over the full turn and at their ends having support surfaces for cooperating with the outer surface of an optical fiber or fiber guide introduced in a held ferrule.

4. A device according to claim 1, characterized by an axial movement device for displac- ing an optical fiber or fiber guide held by the radial movement device in an axial direction in the axial bore in a ferrule held by the support part.

5. A device according to claim 1, characterized by an image capturing device for capturing an image of a front surface of a ferrule held by the support part and an optical fiber or fiber guide introduced in the ferrule, and a unit for automatic image processing of the captured image.

6. A method of mounting an optical fiber or fiber guide in a ferrule, the ferrule including an elongated body having an axis and an axial bore extending through the body in a direction parallel to the axis of the elongated body, including the steps of:

- holding the ferrule in a predetermined or fixed position,

- displacing the optical fiber or fiber guide transversely, perpendicularly to the axis, in the axial bore in the ferrule to a desired position, and

- securing the optical fiber or fiber guide in a desired position, characterized in that in the step of displacing the optical fiber or fiber guide, the optical fiber or fiber guide is displaced by engagement with at least one arm passing through a radial hole of the ferrule.

7. A method according to claim 6, characterized by the further step of moving, before the step of displacing the optical fiber or fiber guide transversely, the optical fiber or fiber guide in an axial direction in the axial hole in the ferrule to a position in which the end surface of the optical fiber or fiber guide takes a predetermined or desired position in relation to a front surface of the ferrule, in particularly is located substantially in the same plane as this front surface or projecting with a relatively short distance from this front surface.

8. A method according to claim 7, characterized in that images of the end surface of the fiber or fiber guide are observed and interference fringes developed therein and that the axial displacement is controlled depending on the images and the pictures therein of the interference fringes to achieve the predetermined or desired position of the end surface of the fiber or fiber guide.

9. A method according to claim 6, characterized in that in the step of displacing the optical fiber or fiber guide, it is displaced to a position that is centered in relation to an outer guide surface of the ferrule, this guide surface being intended for mechanical guiding and alignment of the ferrule.

10. A method according to claim 9, characterized in that images are captured of a front surface of the ferrule, the images including outlines of the outer guide surface and of the optical fiber or fiber guide, the displacement being made so that the outline of the optical fiber or fiber guide is located centered in relation to the outline of the outer guide surface.

11. A method of mounting an optical fiber or fiber guide in a ferrule, the ferrule including an elongated body having an axial hole extending therethrough in a direction parallel to the axis of the elongated body, including the steps of:

- holding the ferrule in a predetermined or fixed position, the optical fiber or fiber guide having been introduced or being introduced or thereafter being introduced into the axial hole, - displacing the optical fiber or fiber guide transversely, peφendicularly to the axis, in the axial hole in the ferrule to a desired position, and

- securing the optical fiber or fiber guide in the desired position, characterized in that in the step of displacing the optical fiber or fiber guide, images of a front surface of the ferrule are captured, the images including outlines of the outer guide surfaces and of the optical fiber or fiber guide, the displacement being made so that the outline of the optical fiber or fiber guide is located in a desired position, in particularly in a centered position, in relation to the outline of the outer guide surface.

12. A method of mounting an optical fiber or fiber guide in a ferrule, the ferrule including an elongated body having an axial bore extending therethrough in a direction parallel to the axis of the elongated body, including the steps of:

- holding the ferrule in a predetermined or fixed position, the optical fiber or fiber guide having been introduced or is being introduced or thereafter is to be introduced in the axial bore,

- displacing the optical fiber or fiber guide transversely, perpendicularly the axis, in the axial bore in the ferrule to a desired position, and

- securing the optical fiber or fiber guide in the desired position, characterized in that in introducing the optical fiber or fiber guide in the axial bore, the fiber or fiber guide is gripped by an axial movement device and displaced thereby, so that the end surface of the fiber or fiber guide is positioned in a predetermined or desired axial position, in particu- larly a position located substantially in the same plane as or at a relatively short distance outside a front surface of the ferrule.

13. A method according to claim 12, characterized in that images of the end surface of the fiber or fiber guide are observed and interference fringes developed therein and that the displacement made by the axial movement device is controlled depending on the images and the pictures located therein of the interference fringes to achieve a desired position of the end surface of the fiber or fiber guide.

14. A ferrule for mounting an optical fiber or fiber guide, including an elongated body having an axis and an axial bore extending through the body in a direction parallel to the axis of the elongated body for receiving the optical fiber or fiber guide, the axial hole having a diameter sub- stantially exceeding the diameter of the optical fiber or of the hole fiber respectively, characterized by at least one radial hole extending from the outside of the ferrule to the axial hole, said at least one radial hole allowing an active positioning of an optical fiber or fiber guide when mounting it the ferrule.

15. A ferrule according to claim 14, characterized by at least three radial holes extending from the outside of the ferrule to the axial hole and located in the same plane perpendicular to the axis of the ferrule.

16. A ferrule according to claim 15, characterized in that the radial holes are symmetrically or regularly distributed over the full turn.

17. A ferrule according to claim 14, characterized in that the axial bore has a diameter at a front surface of the ferrule that is smaller than the diameter of the axial bore at places located at a distance of the front surface.

18. A ferrule according to claim 14, characterized in that the axial bore has a diameter exceeding the diameter of the optical fiber or of the fiber guide respectively by 10 - 50 %.

19. A connector including a ferrule having an outer surface for mechanical guiding and an axial bore and an optical fiber mounted in the axial bore, the axial bore having a diameter significantly exceeding the diameter of the optical fiber and a cured material arranged in the axial bore around a front portion of the optical fiber to hold the optical fiber in a desired position in relation to the outer guide surface of the ferrule, characterized by at least one radial hole extending from the outer guide surface of the ferrule in to the axial bore.

20. A connector according to claim 19, characterized by at least three radial holes extending from the outer guide surface of the ferrule in to the axial bore and located in the same plane perpendicular to the axis of the axial bore.

21. A connector according to claim 20, characterized in that the at least three radial holes are symmetrically or regularly arranged around the axial bore.