Tilt adjustable optical fibre connectors

The embodiments of the invention in which an exclusive property of privilege is claimed are defined as follows:

1. A coupling device for optically coupling a pair of optical fibres in end-to-end relation, each of said fibres terminating at a beam expanding or imaging lens, comprising base means for each of said fibres, each such base means including an axial bore and means for securing one of said lenses therein; a resilient member positionable between confronting end faces of said base means, said resilient member permitting passage of light energy thereacross; and axially extending first and second securing and adjusting means interconnecting said base means with said resilient member trapped therebetween; whereby said first securing and adjusting means are individually axially displaceable to effect a coarse angular adjustment of one base means relative to the other, and the second securing and adjusting means are individually axially displaceable to effect a fine angular adjustment of said one base means relative to the other so as to optimize the light energy transmissible from one fibre and its lens to the other lens and its fibre.

2. The coupling device of claim 1 wherein said first and second securing and adjusting means includes a plurality of alternating first and second threaded screws circumferentially spaced apart adjacent the outer periphery of said base means, each screw passing through a through bore in one base means and being receivable in a mating threaded bore in the other base means, each of said first screws having a coarser thread than each of said second screws.

3. A method of optimizing the light energy transmissible from a source of light energy to an optical fibre within a coupling device holding said fibre generally in position for energy transmission from said source to said fibre, said coupling device including first and second base means connectable to said source and said fibre respectively, beam expanding or imaging lens means in at least said second base means in association with said fibre, a resilient member, and first and second securing and adjusting means interconnecting said base means together with said resilient member trapped between confronting faces thereof, comprising the steps of: transmitting a test optical signal from said source; monitoring said fibre to determine the strength of the test signal received thereby; adjusting said first securing and adjusting means to coarsely alter the relative angular position of said base means until the received signal is close to its desired strength, and adjusting said second securing and adjusting means to finely alter the relative angular position of said base means until the received signal is at its desired strength, thereby indicating that no further adjustment is required.

4. A coupling device for optically coupling a pair of optical fibres in end-to-end relation, each of said fibres terminating at a beam expanding or imaging lens, at least one of said lenses being spherical, comprising: elongated cylindrical housing means; first ferrule means mounting one of said fibres and an associated lens and having means engageable with said housing means to secure said first ferrule means within said housing means at a predetermined position therein; second ferrule means containing said spherical lens therein and carrying a fibre holder surrounding the other of said fibres, said second ferrule means being positionable within said housing means with the spherical lens thereof adjacent the lens of said first ferrule means; and radially extending securing and adjusting means extending through said second ferrule means for engagement with said fibre holder away from said spherical lens; whereby said securing and adjusting means are individually displaceable to effect angular adjustment of said fibre holder relative to said housing means and said first ferrule means so as to optimize the light energy transmissible from one fibre and its lens to the other lens and its fibre.

5. The device of claim 4 wherein said second ferrule means includes a plurality of circumferentially spaced apart threaded bores therethrough and each of said securing and adjusting means is a threaded screw receivable in a corresponding one of said bores for bearing engagement with said fibre holder.

6. The device of claim 5 wherein said housing means has an externally threaded portion at each end and each of said ferrule means carries a nut for threaded engagement with a corresponding one of the threaded portions on the housing means.

7. The device of claim 4 wherein said second ferrule means includes a plurality of alternating circumferentially spaced apart first and second threaded bores therethrough and said securing and adjusting means includes a plurality of first and second screws receivable in said first and second bores respectively for bearing engagement with said fibre holder, said first bores and screws being coarser than said second bores and screws.

8. A method of optimizing the light energy transmissible to an optical fibre within a coupling device holding said fibre generally in position for energy transmission thereto, said coupling device including cylindrical housing means, ferrule means mounting a beam expanding or imaging lens within said housing means and carrying a fibre holding means surrounding said fibre, and radially extending first and second securing and adjusting means extending through said ferrule means for engagement with said fibre holding means, comprising the steps of transmitting a test optical signal for reception by said optical fibre; monitoring said fibre to determine the strength of the transmitted energy received thereby; adjusting said first securing and adjusting means to coarsely alter the angular position of said fibre holding means relative to said ferrule means until the received energy is close to the desired strength; adjusting said second securing and adjusting means into engagement with said fibre holding means; backing off said first securing and adjusting means slightly; and adjusting said second securing and adjusting means to finely alter the angular position of said fibre holding means relative to said ferrule means until the received energy is at the desired strength, thereby indicating that no further adjustment is necessary.

9. A method of optimizing the light energy transmissible to an optical fibre within a coupling device holding said fibre generally in position for energy transmission thereto, said coupling device including cylindrical housing means, ferrule means mounting a beam expanding or imaging lens within said housing means and carrying a fibre holding means surrounding said fibre, comprising the steps of: positoning a jig on said housing means such that an axial extension of said jig is radially spaced from a cylindrical extension of said fibre holding means; directing radially extending adjusting means through said jig extension for engagement with said fibre holding means extension; transmitting a test signal for reception by said optical fibre; monitoring said fibre to determine the strength of the transmitted energy received thereby; adjusting said adjusting means to alter the angular position of said fibre holding means relative to said housing means until the received energy is at the desired strength, thereby indicating that no further adjustment is necessary; fixing said fibre holding means in its adjusted position relative to said housing means; releasing said adjusting means; and replacing said jig with a ferrule retaining means.

10. The method of claim 9 wherein said fixing step includes introducing a settable bonding material between said fibre holding means and said ferrule means and allowing said material to set.

11. A coupling device for optically coupling a pair of optical fibres in end-to-end relation, each of said fibres terminating at a beam expanding or imaging lens, comprising: elongated cylindrical housing means; first and second ferrule means each carrying one of said fibres and an associated lens and having means engageable with said housing means to secure the respective ferrule means within said housing means at a predetermined position therein with the lens of said first ferrule means being adjacent the lens of said second ferrule means; at least one of said ferrule means carrying a fibre holder mounting the fibre associated therewith and connected to the lens associated therewith; a cylindrical resilient member surrounding a portion of said fibre holder and the lens associated therewith; and radially extending securing and adjusting means extending through said one ferrule means for engagement with said fibre holder away from the lens associated therewith; whereby said securing and adjusting means are individually displaceable to effect angular adjustment of said fibre holder relative to said housing means and the other ferrule means so as to optimize the light energy transmissible from one fibre and its lens to the other lens and its fibre.

12. A system for transmitting light energy from one optical fibre to a second optical fibre through an optical coupling device comprising: a housing containing said device and having light energy inlet and outlet means; a first optical coupler connected to said housing at said inlet means; a second optical coupler connected to said housing at said outlet means; first ferrule means receivable in said first optical coupler and containing a transmitter optical fibre and a beam expanding or imaging lens; second ferrule means receivable in said second optical coupler and containing a receiver optical fibre and beam expanding or imaging lens; and adjustment means for tiltably adjusting each of said first and second ferrule means relative to said housing so as to optimize the light energy transmissible from said transmitter fibre and lens through said optical coupling device to said receiver fibre and lens.

13. The system of claim 12 wherein each of said optical couplers comprises: a generally circular base plate having a boss extending therefrom and an axial bore therethrough for receiving the associated ferrule means therein; a resilient member positioned between opposing faces of said base plate and said housing; and a plurality of fine-pitch screws extending through said base plate and said resilient member and threadedly engageable with said housing, said screws being evenly circumferentially spaced about the circumference of said base plate; whereby said screws may be adjusted to alter the orientation of the axis of said base plate relative to said housing to thereby adjust the position of the optical axis of the lens contained in said base plate relative to said housing.

14. The system of claim 12 wherein each of said couplers includes: a cylindrical housing extending outwardly from said coupler housing and having a bore therethrough for receiving the associated ferrule means therein; a resilient member positionable between the ferrule means and the cylindrical housing adjacent said coupler housing; and a plurality of fine-pitch screws extending radially through the cylindrical housing at the end thereof away from said coupler housing, said screws being evenly circumferentially spaced around the cylindrical housing and passing therethrough to bear against the ferrule means; whereby said screws may be adjusted to alter the orientation of the lens optical axis relative to said coupler housing.

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FIELD OF THE INVENTION

The present invention relates to connectors used to effect connections between separate lengths of optical fibres or between a light source and a receiver fibre. In particular the invention relates to connectors which are adjustable to optimize the transmission of light energy through the joint and which do not lose their adjustment if the joint is broken and then remade.

BACKGROUND OF THE INVENTION

The use of optical fibres for the transmission of data or optical information has increased dramatically in recent years. The heart of such transmission systems is an optical fibre of silica glass or other suitable material which has been clad with an appropriate material to achieve a "light tube" or waveguide along which light energy can travel in a controlled manner. Optical fibres are extremely small (maybe 100 microns in diameter) and when they are incorporated into a data transmission system it is necessary to effect interconnections between separate lengths of such fibres. The primary function of an optical connector is to provide a low-loss coupling of light energy from one fibre to the next and it is necessary to align, in an extremely precise manner, the cores of the coupled fibres so as to keep the losses at the joint to an absolute minimum.

The best coupling possible between two fibres is achieved by polishing the ends of the fibres to a smooth finish and then directly butting the ends together. Disregarding any Fresnel losses at the glass-air interface such a connection should have losses in the order of 0.2 dB. This type of connection requires high precision equipment and is best suited for permanent splices. For repeated connections a more rugged connector is required, but such can lead to increased losses.

There are six main sources of losses in any fibre to fibre coupling system. The greatest losses are due to lateral misalignment, when the mating fibres are not aligned along their central axes. Also, although manufacturers place tight tolerances on the position of the core within the cladding, any eccentricity of the central core is treated as a lateral misalignment condition. Angular losses occur when the central axes of the two fibres are tilted with respect to each other. End separation losses occur when the ends of the mating fibre are separated. Greater separations result in greater losses since light emanating from the end of an optical fibre is projected in the form of a cone. Dirt, surface irregularities and non-perpendicular ends conspire to keep the ends apart and generate losses. Extrinsic connector (intrinsic fibre) losses are caused by variations in the optical parameters of the fibre, including its "numerical aperture" (NA), concentricity of the core, core ellipticity and diameter variations. Finally, Fresnel losses occur whenever light passes from one transparent medium into another medium of a different index of refraction, since part of the transmitted light will be lost to a reflected beam. For transmission from glass to air the Fresnel losses can be 0.2 dB for each surface. This loss can be eliminated by using index-matching fluids, or reduced by using anti-reflection coatings.

In order to minimize losses such as described above the tolerances of butt-joint connections must be extremely tight. However, any small piece of dirt which enters the joint can drastically increase the losses of the connection and accordingly the ends of the fibre must always be protected from ambient conditions.

The problems associated with connections as described above can be reduced by the use of "expanded beam" technology through which the optical beam diameter is increased from the core diameter of 100 microns up to a more manageable size of a few millimeters. Since the resulting beam is considerably larger than a speck of dirt the losses associated therewith are reduced. Furthermore since one is dealing, relatively speaking, with a macro rather than a micro situation all aspects of the connection become simpler, from manufacture, to maintenance.

If a fibre is placed at the focal point of a lens then the beam emerging from the lens is collimated with diameter much larger than that of the fibre core and if each fibre has an appropriate lens the spot image from one will be formed on the other at the focal point of its lens. Expanded beam connectors obviously reduce losses due to lateral misalignment and end separation. However, due to the autocollimation such connectors increase the losses due to angular misalignment.

In principle, if the fibres are positioned at the focal point of the lenses with the same accuracy as with end-to-end butt joint connections the losses should be the same with an expanded beam coupler. Several couplers using expanded beam technology are presently available commercially. One of the easiest lenses to use in fibre connectors is the graded index (GRIN) lens.

Cylindrical GRIN lenses are functionally identical to conventional spherical lenses except that they have flat end surfaces. The change in the index of refraction along its axis generates the unique properties of the GRIN lens and lenses can be tailored by the manufacturer to generate a wide range of optical parameters. The length of a lens defines its pitch, or the fraction of a complete wavelength, that is contained within the lens at a particular wavelength. For the production of a collimated beam from a point source it is necessary to use a quarter-pitch lens.

If one quarter-pitch GRIN lens in a joint is tilted by an angle .theta. relative to the other lens then the transmitted image will be displaced relative to the receiving lens axis by an amount given by the equation z=tan .theta./NoA where .theta. is the tilt angle; No and A are GRIN lens parameters which determine the focal length of the lens, since f=1/NoA. For different types of specific GRIN lenses the losses due to a tilt angle of 1 degree can range from about 6 dB to well over 10 dB. Furthermore, as the fibre core size decreases the tilt losses will become more severe. In a GRIN lens connector if there is any tilt variation in the lenses or even in the placement of the fibres then the transmitted image will not be focused on the receiving fibre. It therefore is very desirable to achieve a connector in which the tilt losses are minimized without demanding extremely high (costly) manufacturing tolerances.

The principles stated above apply to other imaging lenses, not just to GRIN lenses. If the image is formed at the focal point of the lens then a tilt through the angle .theta. will produce a translation of ##EQU1## at the fibre end face. For small angles .theta..perspectiveto. tan .theta..

The aforementioned U.S. patent application described and claimed several embodiments of tilt-adjustable fibre optic connectors which meet all of the requirements indicated above. Connectors for fibre-to-fibre joints and for source-to-fibre joints were disclosed therein, with the connectors using either axial tilt or radial tilt principles to achieve the desired ends.

SUMMARY OF THE INVENTION

The present invention is intended to overcome specifically the tilt problems associated with beam expanding or imaging lens type connectors or couplers and the extremely high tolerance requirements of placing the fibre end at the focal point of the lens. The present invention is embodied in a new connector or coupling device which is economical to manufacture, may be easily hermetically sealed in use, is effortlessly manipulated during disconnection and reconnection, and is adjustable to optimize the transmission of light energy therethrough. The coupling device of the present invention uses the properties of lenses in combination with novel tilting techniques to achieve a compact structure capable of submicron resolution. Furthermore, with very little, or even no, adjustment devices embodying the principles of the present invention could be used as source couplers, attenuators or connectors to couple light into any size or number of receiving fibre(s).

Throughout the disclosure and claims it should be understood that the word "optimum" and its variants is intended to have a broad meaning, such as "most favourable under defined conditions". The "optimum" signal strength for a coupler might be the maximum obtainable, whereas for an attenuator it would be a desired signal strength, less than maximum.

In one form the present invention utilizes a pair of base plates each having a threaded boss thereon and an axial bore therethrough. Each bore is adapted to receive in a predetermined position therein a holder which carries a beam expanding or imaging lens and an optical fibre associated therewith. The lens holder has a nut thereon for threaded connection to the boss of the base plate such that the holder can be disconnected from the base plate and reconnected thereto. A resilient member is sandwiched between confronting faces of the base plate and two sets of threaded screws interconnect the base plates by passing from one plate through the resilient member to the other plate. The central void area of the resilient member contains the opposing faces of the lenses and may be hermetically sealed from the surrounding atmosphere by sealing contact with the base plates. Once the connector has been assembled a test light can be transmitted from one fibre through the connection to the other fibre and then to a suitable receiver. The first set of threaded screws can then be adjusted to initially or coarsely alter the angular orientation of one base plate relative to the other so as to alter the angular orientation of one lens and its fibre relative to the other. During adjustment the receiver is monitored and the first screws are adjusted in a pattern until the detected output is approximately at the desired level at which point the second set of screws is adjusted to fine tune the adjustment until the detected output is optimized. The screws of the second set have a finer thread than those of the first set. Even if one or both of the fibres is disconnected from the joint as described above the base plates will hold their adjusted condition and the fibres can be reconnected to the joint without fear of any increase in losses after reconnection.

The foregoing represents an embodiment wherein axial adjustment of the screws achieves the desired end. The same end can be achieved using screws oriented radially with respect to a fibre and lens holder mountable in a housing. Also, the end can be achieved in radial-tilt connectors by utilizing reusable adjustment jigs which fit over the fibre and lens holder and over the housing so that the fibre and lens holder can be adjusted relative to the housing. Then the fibre and lens holder can be fixed relative to the housing, as by potting, and the jig removed. In such a structure the fibre and lens holder could not be removed from the housing without destroying the adjustment previously made.

Other aspects of the invention will become apparent from the description and claims which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of an optical fibre coupling device of the present invention.

FIG. 2 is a composite view of the major components of the coupling device of FIG. 1.

FIG. 3 is a cross-section of an optical fibre and lens holder receivable in a base plate of the FIG. 1 embodiment.

FIG. 4 shows an embodiment of the invention, in cross-section, utilizing radial adjustment techniques and a spherical lens instead of a cylindrical lens.

FIG. 5 shows another embodiment of the invention wherein a temporary jig is used to achieve adjustment.

FIG. 6 shows a further development of the invention as depicted in FIG. 5.

FIGS. 7 and 8 show schematically the use of tilt adjustable connectors used on the transmitter and receiver sides of optical coupling devices through which light energy is transmissible.

DESCRIPTION OF THE PREFERRED EMBODIMENT

One embodiment of an optical fibre coupling device according to the present invention is illustrated in FIG. 1 under reference number 10. The coupling device 10 is used to join a pair of optical fibre assemblies 12, 14 in end-to-end relation so that an optical signal in the form of light energy can be transmitted from one assembly to the other with minimum losses at the joint. The optical fibre assemblies 12, 14 typically include the clad optical fibre core 12a, 14a, plastic coating 12b, 14b surrounding the core and a protective cable or sheath 12c, 14c surrounding the coating. With reference to FIG. 3 it is seen that each clad fibre core 12a, 14a terminates at a graded index lens (GRIN) 72, which with the fibre core end being positioned at the focal point of the lens, expands and collimates the optical signal for improved transmission to the receiving GRIN lens. Suitable GRIN lenses for the present invention are available under the SELFOC (Trademark) designation from the Nippon Sheet Glass Company.

With reference to FIGS. 1 and 2 it will be seen that each optical fibre assembly 12, 14 is received in a corresponding base plate 16, 18 via an appropriate connection mechanism to be described herein. Each base plate, 16, 18 is provided with a central boss 20, 22 projecting outwardly from one face thereof. Each boss carries external threads 36 and a central bore 38 extends axially through the boss and its base plate to exit at the flat obverse face thereof. A keyway 40 is machined in the sidewall of the bore 38 for a purpose to be described.

Each fibre assembly 12, 14 carries at its end a lens holder 24, 28 and each lens holder in turn carries a nut 26, 30 which is free to rotate thereon. Furthermore each lens holder carries a key 42, which is matable with the keyway 40 provided in the axial bore 38 of each base plate. When it is desired to assemble a fibre assembly 12, 14 to its base plate 16, 18 it is only necessary to slide the lens holder 24, 28 into the appropriate axial bore 38 with the key 42 engaging the keyway 40 and to then rotate the nut 26, 30 to engage the internal threads thereof with the external threads 36 on the boss 20, 22. When the mating threads are fully engaged the lens holder will be held in a predetermined position within its base plate.

Returning to FIG. 2 it will be seen that one base plate 16 is provided with first and second sets of circumferentially spaced and alternating through holes 44, 44a positioned adjacent the periphery of the base plate. The other base plate 18 is provided with first and second sets of circumferentially spaced and alternating threaded bores 46, 46a positioned adjacent the periphery of that base plate, the bores 46, 46a being alignable with the holes 44, 44a respectively. Threaded screws 34, 34a are provided for threaded engagement with the threaded bores 46, 46a respectively and for a close fit within the holes 44, 44a respectively. The screws 34 should have at least 56 threads per inch and the screws 34a should have at least 80 threads per inch. A greater number of threads per inch provides higher resolution in the adjustment step. Finally, a resilient washer member 32 is provided, the washer member having through holes 48, 48a alignable with the bores 46, 46a and the holes 44, 44a respectively and also having a central void area 50, the configuration of which is not critical to the invention.

The components of FIG. 2 are assembled together as shown in FIG. 1, with the fibre assemblies 12, 14 connected to the base plates 16, 18 and with the base plates 16, 18 connected together with the obverse faces thereof in confronting relation and with the resilient washer member 32 sandwiched between the obverse base plate faces. The screws 34 pass through aligned holes 44 and 48 and are threadedly received in threaded bores 46 such that when the screws are tightened they sealingly clamp the washer member 32 between the base plates 16, 18. The screws 34a pass through aligned holes 44a and 48a and are threadedly received in threaded bores 46a. Initially they are loosely attached to the base plates so as not to contribute to the clamping forces. When initially assembling the base plates and the washer member together it is advantageous to slide the base plates on to a centering rod which fits closely within the bores 38, to ensure that the axes of the base plates are initially aligned when the screws 34 are set at their initial positions.

The washer member 32 is shown in FIG. 2 as being continuous peripherally and as having flat surfaces which abut the confronting faces of the base plates. Such a member is particularly useful if it is desirable to hermetically seal the interior of the coupler, as in an underwater application. In such an application an O-ring (not shown) could be placed between the end face of the boss 20, 22 and the inner face of the nut 26. If hermetic sealing is not required the washer member 32 could be formed as an annular spring member, such as a Belleville washer, having appropriate holes through which the screws 34, 34a could pass. Instead of an annular spring, individual springs located at each screw 34, 34a could bias the base plates apart. Alternatively the annular spring member (or the washer member 32) could be located within the circumference defined by the screws so that it would then not be necessary to have the screws pass through the washer or spring member itself. Usually the washer member 32 would be formed from a rubber or soft plastic material, although it would be possible to use a soft metal (e.g. indium) if desired.

As an alternative to the washer member 32 described above it would be possible to hermetically seal the interior of the connector with a commercially available O-ring. One base plate could be provided with an annular groove in its confronting face, in which the O-ring is receivable, a portion of the O-ring projecting away from the face of the base plate. The other base plate need not have a mating groove as its face will be forced into sealing engagement as the screws 34 are drawn tight. In this embodiment the O-ring preferably lies within the circumference of the screws 34, 34a.

With reference now to FIG. 3, the internal structure of a typical fibre assembly 12 will be described, it being understood that FIG. 3 is drawn to a much larger scale than the components themselves.

The fibre assembly 12 is made up of several components, namely the clad core 12a which is typically a silica or a doped silica glass of extremely small diameter (e.g. 100 microns), the plastic coating 12b which surrounds the clad core, and the cable or sheath 12c which may be formed from a resilient flexible plastics material and serves to protect the clad core and the plastic coating. The outer diameter of the sheath will be in the order of 4 mm.

At the end of the fibre assembly the sheath is removed or stripped from the plastic coating over a short length of about 13 mm and an optional, yet desirable, inner crimp sleeve 52 is fitted over and crimped to the exposed coating 12b. An optional, yet desirable, outer crimp sleeve 54 is fitted over and crimped to the sheath 12c adjacent the inner end of the inner crimp sleeve 52. At the free end of the assembly a very short length of the clad core 12a is exposed.

The assembly 12 having the sleeves 52 and 54 crimped thereon is slid into a fibre ferrule 56. Ferrule 56 includes three distinct sections, namely an enlarged first section 58 having an axial bore 60 therein adapted to loosely receive the outer crimp sleeve 54, a reduced diameter second section 62 having a reduced diameter blind axial bore 64 therein adapted to receive the inner crimp sleeve 52, and an end section 66 having a s