Optical fiber tap utilizing reflector

Abstract: A tap for withdrawing light from an intermediate portion of an optical fiber core by passing light through a side of the optical fiber comprises an op­ tical coupler in contact with an outside surface of an optical fiber which is bent and disposed in a plane. A light reflector (41) extending transverse to the plane deflects the withdrawn light (3) towards the end surface (5) of a light element (6) disposed completely outside the plane. A similar arrangement may be used to inject light to an intermediate portion of an optical fiber. The tap may be used as a read tap to withdraw light, or as a write tap to inject light in optical fiber networks.

Description: The present invention relates to a optical fiber taps and to a method for making same. Numerous methods have been proposed in the prior art for distributing information using an optical fiber, preferred methods including star, ring, and bus architectural networks. Generally speaking, star and ring networks utilize point-to-point connections, whereas bus networks are capable of utilizing non-point-to-point connections whereby an optical signal is only partially interrupted by any one connection. For example Polczynski, U.S. patent 4,089,584 discloses a bus network which utilizes an optical fiber having a rectangular core and cladding, and connection or tapping of the fiber is accomplished by removing the cladding and disposing a prism or grading against an exposed rectangular core. Such networks are disadvantageous since the fiber and taps usable therewith are relatively complex in design and hence unduly expensive, and optical network performance is rather poor in view of relatively low tapping efficiencies that result using such methods. Miller, GB 2,126,749B and Dakin et al. "Experimental Studies into the Non-Invasive Collection and Distribution of Data on a Fiber-Optic Monomode Bus" propose designing a read optical fiber bus using taps whereby light is withdrawn through a side of the optical fiber by passing the light through a coating of the fiber. Miller collects the light from the bus fiber by disposing a photodetector at an end of a curved and grooved light pipe disposed around the bus fiber, and Dakin et al. collects the light by tightly pressing a polymeric fiber with part of its cladding removed against a curved portion of the bus fiber. Such techniques are also disadvantageous in that again the taps are complicated in design, are craft sensitive to install in the field, and are not sufficiently efficient when tight flux budgets are mandated by network design. Goell et al., U.S. patent 3,982,123 at Figures 5A and 5B discloses an optical fiber read tap whereby an exposed glass cladding of a bent optical fiber portion is glued to a top of a photodetector. Such taps are disadvantageous since a strength of the fiber is disadvantageously affected by removal of its outer protective buffer, and rather small coupling light efficiencies are obtained by simply using epoxy to secure a bent optical fiber onto a top surface of a photodetector. In addition, the optical fibre is not releaseable from the tap. Cross, U.S. patent 4,270,839 discloses a tap for an optical fibre whereby the fibre is bent in air, and downstream from the bent portion of the optical fibre a straight section of the optical fibre is glued within a straight light pipe which thereafter is curved and has a light detector at a remote end thereof. Again, such taps are disadvantageous since it has been found to yield relatively low light coupling efficiencies, and the optical fibre is not releaseable from the light pipe once glued thereonto. Campbell et al., U.S. serial no. 602,242, filed April 19, 1984; Campbell et al., U.S. serial no. 754,035, filed July 11, 1985; and Campbell et al., U.S. serial no. 614,884, filed May 25, 1984, all assigned to the assignee of the present invention, the disclosures of which are incorporated herein by reference, disclose several advantageous kinds of taps for either injecting light into or withdrawing light from optical fibres. However, there continues to be a need for yet more efficient taps which are also mechanically simple in structure. GB-A-2162657 discloses apparatus for injecting light into the core of a buffer coated optical fibre, comprising a transparent body having at least three faces: an arcuate contact face adapted to bear against a smoothly curved length of fibre, an entry face through which light emitted by a light source is injected into the body in a direction towards the contact face, and a reflecting face which reflects light in a direction towards the contact face. Light injected through the entry face is directed and reflected towards the smoothly curved contact face from where it will pass into the fibre against which the body bears. A similar apparatus for extracting ligt from an optical fiber is also disclosed. GB-A-2158607 discloses a fibre optic coupler having a transparent body through which a passage extends. The passage accommodates a waveguide and has an angled section with the apex of the angle sufficiently sharp that light propagating along the waveguide is emitted as a beam at the angled section. JP 60-244908 discloses a cutless signal detector to detect a leaking optical signal. It has a tip of a first member formed of a transparent hard material, and a corrugated bent shape part is formed at the tip. A photodetector is arranged near the bent shape part in the first member. The bent shape part is shaped such that it meshes with a second member, and a reflection mirror is arranged nearby. When the first and second parts are brought together with a fibre between them any leaking optical signal is detected by the photodetector. Optical fibre Comm. Conf. and Sixth Int. Conf. on Integrated Optics and Optical Fibre Comm. Tech. Digest 19-22 January 1987 page 107 discloses that small scale fibre-axis distortions of the correct periodically create resonant power coupling between cladding and core that is more than 100 times greater than macrobend techniques. Accordingly, it is an object of the present invention to eliminate the above-noted drawbacks and to provide an optical read tap and/or optical write tap and network usable therewith which operates more efficiently than heretofore described. These and other objects are achieved by utilizing a tap which bends an optical fibre in a plane, preferably the plane being flat, and utilizes reflecting means for directing light withdrawn from the core of the optical fibre towards a light collection end surface for directing light from a light emitting end surface into the core of the optical fibre. Thus, the present invention provides a tap for extracting light from an intermediate portion of an optical fibre core by passing the light through a side of the optical fibre, comprising: a light receiving element having a receiving end surface; an optical coupler consisting of a transparent member provided with a concave outer surface portion for receiving the intermediate portion of the optical fibre bent thereagainst and disposed in a plane, so as to extract light at said optical fibre bent portion; means for maintaining the intermediate portion of the optical fibre bent in said plane against said concave outer portion of said transparent member; and a light reflector formed on another outer surface portion of said transparent member, said reflector being disposed in the path of the extracted light downstream of the bend and extending traversely to the plane in which the optical fibre bent portion is intended to be maintained so as to deflect the extracted light toward the light receiving element end surface, characterized in that the receiving end surface of the light receiving element is disposed completely outside said plane in which the optical fibre bent portion is intended to be maintained, and in that the transparent member constituting the optical coupler has an elongate and curved shape, said elongate and curved transparent member being provided in its concave side surface with a groove for receiving said optical fibre bent portion, and said light reflector being formed on another outer surface portion of said curved elongate member which is inclined in such a manner with respect to the optical fibre bent portion maintaining plane as to deflect said extracted light toward said receiving end surface of said light receiving element which is completely outside said optical fibre bent portion maintaining plane. The present invention also provides an analogous tap for injecting light into an intermediate portion of an optical fibre. The invention further provides a method of making a tap for coupling light between a light element and an intermediate portion of an optical fibre core by passing the light through a side of the optical fibre, which method comprises: providing a light element having an end surface; providing an optical coupler consisting of a molded transparent member, provided with a concave outer surface portion for receiving the intermediate portion of the optical fibre bent thereagainst and disposed in a plane, so as to couple light at said optical fibre bent portion; providing means for maintaining the intermediate portion of the optical fibre bent in said plane against said concave outer portion of said transparent member; and forming a light reflector on another outer surface portion of said transparent member, said reflector being disposed in the path of the coupled light between the bend and the light element end surface and extending traversely to the plane in which the optical fibre bent portion is intended to be maintained so as to form a continuous light path between the bent optical fibre portion and the light element end surface, characterised in that the end surface of the light element is disposed completely outside said plane in which the optical fibre bent portion is intended to be maintained, and in that the transparent member constituting the optical coupler has an elongate and curved shape, said elongate and curved transparent member being provided in its concave side surface with a groove for receiving said optical fibre bent portion, and said light reflector being formed on another outer surface portion of said curved elongate member which is inclined in such a manner with respect to the optical fibre bent portion maintaining plane as to deflect the light between said end surface and the bent optical fibre portion. The light reflector preferably has a reflectance greater than 0.5, preferably has a reflectance greater than 0.5, preferably greater than 0.7 or 0.8, more preferably greater than 0.85 or 0.90, most preferably greater than 0.95, a polished light reflector being most preferred. Preferably, the light is coupled utilizing an optical coupler which has an index of refraction which optimally matches an outer surface of the optical fibre. The reflecting surface is formed in a vicinity of the optical coupler and in close proximity therewith and deflects light out of the plane of the bent optical fiber portion and towards the end surface of a light element, the light element either constituting ultimately a photodetector for light detection or a light emitter for light injection. In any case, the light emitter or the light detector can be substantially displaced from the tap of the invention by utilizing a pigtail optical fiber light element which facilitates testing of the apparatus prior to permanent installation and after disposing a bus optical fiber within the tap. The tap further includes means for bending the optical fiber which is releaseable therefrom which also facilitates testing and repairs. Since the light element is disposed outside the plane of the optical fiber bend, the bend profile of the optical fiber can be optimized for optimum optical efficiency and is not required to be unnecessarily further modified so as to accommodate mechanical size constraints imposed by a size of the light element, which is generally much larger than the fiber core. The invention is most suitable for use in a serial manner on an optical fiber for creating either a read or a write bus therewith. In addition, the invention allows the tap to be manufactured in a much more cost efficient way since simple molding techniques can be utilized to form a single component which incorporates both the bend profile features necessary to control and locate the fibre and the optical coupling and focusing elements required to optimally transfer the light between the light element and the fibre core in the bend. Also, a thickness of the component can be made sufficiently uniform to allow it to cool and harden in a stable manner when molding. Figure 1 illustrates preferred embodiment of the invention which includes a member for releasably pressing an optical fibre so as to maintain a constant bend attitude therein; Figure 2 illustrates an alternative embodiment of the invention which includes a waveguide for transmitting light to and from an optical fibre core; Figure 3 illustrates one preferred embodiment of a network utilizing taps of the invention. Figure 1 illustrates a geometry of a preferred tap embodiment whereby it is evident that in all cases a light element 68 has a reflective end surface 13 which is disposed in a plane which is parallel with plane 24 and whose optical axis is not parallel to plane 24 (but is preferably normal or perpendicular to that plane), the plane including the bent optical fibre portion which is to lie in groove 17 in the concave side surface 22 of the curved transparent member of optical coupler 14. In Figure 1, a substrate 16 has formed thereon the groove 17 sized to accept an optical fibre 1 (shown in Figure 2) to be tapped, the groove 17 including the bent portion 22. The substrate 16 includes first and second flanges 18 which define first and second grooves 19 along which a member 20 can slide along or parallel to the plane 24 which includes the bent portion 22 and the bent optical fibre portion. An end face 25 of the member 20 has a curved profile 26 complementary to the curved profile of the groove 17 such that the optical fibre can be securely maintained in a constant bent attitude within the groove 17 by urging the member end face 25 against the optical fibre and the groove 17 by utilizing a force which urges the member 20 along a direction of force arrow 28. Preferably the force is resilient, and can be applied by a spring (not shown) so as to maintain a continuous load on the fibre regardless of dynamic changes which may occur over time, such as temperature induced differential material expansions and contractions, material creep due to stress, etc. When the optical fibre is disposed in the tap of Figure 1 and is transmitting light in a direction from right to left in the drawing, light radiates outward from the fibre core, through its cladding, through its outer coating(s), and into the coupler 14 defining the groove bent portion 22 which bent portion acts to facilitate light transfer between the outer coating of the fibre and the coupler 14. If desired, a wetting agent, preferably stable, such as a partially cross-linked gel having finite elongation properties, such as an ultimate elongation in excess of 200 or 500% can be used to facilitate optical coupling. The light entering the optical coupler is deflected by the reflecting surface 4 of the coupler out of the plane 24 of the bent portion 2 of the fibre and groove 17 and towards the light element 68. Element 68 has an end surface 13 disposed entirely out of the plane 24, even through the end surface 13 can be in very close proximity to the bent portion 22. The reflection surface can simply be a smooth surface exposed to air, and shaped such that the withdrawn light hits the smooth surface at angles such that total internal reflection occurs from this surface with little or no light being refracted therethrough into the air. Preferably the smooth surface has a reflective coating thereon as well. It is preferred to form the surface so as to have a reflectance greater than 0.5, preferably greater than 0.6 or 0.7, most preferably greater than 0.8 or 0.85, optimally greater than 0.9 or 0.95. Preferably, though not necessarily, the reflection surface 4 is curved in one or preferably two directions so as to optimise focusing between the fibre core and the light element. A parabolic or an elliptical reflector are two preferred embodiments. According to particularly preferred embodiments, the actual shape of the reflecting surface is optimized so that optimum focusing into the optical fibre core or onto a photodetector or the light collecting surface is achieved. To this end, the surface 4 is shaped and positioned such that preferably more than 30% of the light withdrawn from the fibre core is reflected, more preferably more than 40%, 50% or 60%, most preferably more than 70% or 80%. For light injection, the surface 14 is shaped and positioned such that as much light as possible emitted by the light source is injected into the fibre core as a guided mode, e.g. preferably more than 0.05%, more preferably more than 0.1%, 0.5% or 1%, most preferably more than 10%, 30% or 40%. Differences in phase-space area between the end surface of the light emitting source and the core of the fibre necessarily result in lower light injection efficiencies than are possible with a similar geometry for withdrawing light. The invention produces several new and unexpected advantages. First by disposing the light element 68 and its end surface 13 completely out of the plane 24 of the bend, the bend profile of the optical fibre portion can be optimized as desired to achieve optimum optical detection or injection efficiency without being unduly further modified due to a physical size of the light element or its end surface. In the absence of this feature of the invention the difference between the physical size of the light element and the optical fibre (including its bent profile) requires that the end surface of the light element be disposed an undesirably large distance away from the beginning of the fibre bend which tends to decrease coupling efficiency. In addition, to allow the fibre to clear a lowermost edge of the end surface the fibre bend portion must necessarily extend a distance so as to include an arc of fibre, which is particularly disadvantageous since very little of the light escaping the fibre within that arc will be collected by the end surface. Also, if the light element were disposed so that its end surface were in the plane of the bend, the substrate housing the light element would need to be unduly thick in that region which contains the groove for defining the bend profile for the fibre. This would complicate manufacture of the substrate as molding tolerances become difficult to control. In the invention, however, the reflection surface 4 can be disposed as close as desired to the bent optical fibre portion 2 and can be made as large and shaped as desired so as to deflect as much light as is required escaping from the fibre or the light source to achieve highest coupling efficiency. As mentioned, for light withdrawal, since the end surface of the light collecting element is out of the plane 24 of the bend, preferably the bent portion 22 of the optical fibre is disposed entirely upstream of the reflection surface 4, and for light injection, is disposed entirely downstream of the reflection surface 14 to achieve highest efficiencies. Figure 2 illustrates another preferred embodiment of the invention. In this embodiment, the light 3 withdrawn from the optical fibre 1 at the bent portion 2 is confined within a waveguide 30 which forms part of the substrate 16 and forms the bend profile 22 for the bent optical fibre portion 2, with the reflection surface 4 being disposed at an end of the waveguide 30. Figure 3 illustrates one practical embodiment for using taps having reflecting surfaces as described, Figure 3 illustrating an optical fibre network 90 having first and second bus fibres 91, 92 interconnecting a plurality of terminals 93 in a bus architecture. The optical fibre 91 constitutes a read optical fibre, which the optical fibre 92 constituting a write optical fibre, the network being controlled by a CPU or central processing unit 94. According to the invention, the light is withdrawn from the read optical fibre 91 in a serial manner using a plurality of read taps 96 disposed in series constructed according to any combination of the embodiments described above, with the terminals 93 writing onto the write bus 92 via a plurality of taps 95 disposed in series and constructed according to any combination of the taps described above. Preferably signals going to any one or more of the terminals 93 are multiplexed in time rather than controlled by a token passing algorithm. According to a particularly preferred embodiment, any one or more of the terminals 93 is connected to one or more telephones, personal computers, mainframe computers, or similar data assembling and generating equipment. The invention is useful for tapping both single mode and multimode fibre, including both step index and graded index, and is usable with both glass-on-glass and plastic clad silica fibre. According to a preferred embodiment, the invention includes the use of glass-on-glass fibre including a polymeric coating (e.g. buffer), the coating preferably having an index of refraction higher than the cladding, examples being an acrylate or silicone buffer, and/or any thin additional layers (e.g. jackets) surrounding the buffer. Typical preferred glass-on-glass fibres include single mode fibres having a core diameter of about 10 µm, and a cladding diameter of about 125 µm, and a buffer diameter in a range between about 250-500 µm, with multimode glass-on-glass fibre including a core/cladding diameter of roughly 50/125 µms, 100/140 µms, and 85/125 µm, for example. Preferred fibres include those having cylindrical cores, cylindrical claddings and cylindrical coatings (e.g. buffers and/or jackets) and preferably at least the buffer being maintained intact so as not to detrimentally degrade the strength of the fibre by exposing a glass surface thereof to moisture.

Claims: A tap for extracting light from an intermediate portion (2) of an optical fibre (1) core by passing the light through a side of the optical fibre, comprising: a light receiving element (68) having a receiving end surface (13); an optical coupler (14, 30) consisting of a transparent member provided with a concave outer surface portion (22) for receiving the intermediate portion of the optical fibre bent thereagainst and disposed in a plane (24), so as to extract light at said optical fibre bent portion; means (20) for maintaining the intermediate portion of the optical fibre bent in said plane (24) against said concave outer portion of said transparent member; and a light reflector (4) formed on another outer surface portion of said transparent member, said reflector being disposed in the path of the extracted light downstream of the bend and extending traversely to the plane in which the optical fibre bent portion is intended to be maintained so as to deflect the extracted light toward the light receiving element end surface, characterized in that the receiving end surface (13) of the light receiving element (68) is disposed completely outside said plane (24) in which the optical fibre bent portion is intended to be maintained, and in that the transparent member constituting the optical coupler (14, 30) has an elongate and curved shape, said elongate and curved transparent member being provided in its concave side surface (22) with a groove (17) for receiving said optical fibre bent portion, and said light reflector (4) being formed on another outer surface portion of said curved elongate member which is inclined in such a manner with respect to the optical fibre bent portion maintaining plane (24) as to deflect said extracted light toward said receiving end surface of said light receiving element which is completely outside said optical fibre bent portion maintaining plane. A tap for injecting light into an intermediate portion (2) of an optical fibre (1) core by passing the light through a side of the optical fibre, comprising: a light source having an end surface; an optical coupler consisting of a transparent member provided with a concave outer surface portion (22) for receiving the intermediate portion of the optical fibre bent thereagainst and disposed in a plane (24), so as to allow light from the source to enter the core at said optical fibre bent portion; means (20) for maintaining the intermediate portion of the optical fibre bent in said plane (24) against said concave outer portion of said transparent member; and a light reflector (4) formed on another outer surface portion of said transparent member, said reflector being disposed in the path of the light from the source and upstream of the bend and extending traversely to the plane in which the optical fibre bent portion is intended to be maintained so as to deflect the light from the source towards the optical fibre bent portion, characterised in that the end surface (13) of the light source (68) is disposed completely outside said plane (24) in which the optical fibre bent portion is intended to be maintained, and in that the transparent member constituting the optical coupler (14, 30) has an elongate and curved shape, said elongate and curved transparent member being provided in its concave side surface (22) with a groove (17) for receiving said optical fibre bent portion, and said light reflector (4) being formed on another outer surface portion of said curved elongate member which is inclined in such a manner with respect to the optical fibre bent portion maintaining plane (24) as to deflect said light from the light source towards the optical fiber bent portion. A tap according to claim 1 or claim 2, characterized in that the light reflector (4) has a reflecting surface thereon with a reflectance greater than 0.50 and extending in a position so as to reflect at least 20% of the withdrawn or injected light. A tap according to any of claims 1-3, characterized in that the means (20) for maintaining is releasable so that the optical fibre (1) can be completely removed therefrom when so released. A tap according to claim 1, characterized in that the light reflector (4) is non-planar and curved in at least one dimension so as to focus the withdrawn light onto the end surface (13) of the light receiving element (68). A tap according to any one of claims 1-5, characterized in that the light reflector (4) has a reflecting surface thereon with a reflectance greater than 0.95. A tap according to any one of claims 1-6, for extracting light from or injecting light into an optical fibre comprising a core, a cladding, and a polymeric coating, such that the withdrawn or injected light (3) passes though the coating, the bent optical fibre intermediate portion (2) being disposed in a substantially flat plane (24), characterized in that the optical coupler (16) contracts the bent optical fibre portion. A tap according to any one of claims 1-7, for extracting light from or injecting light into an optical fibre comprising a core and cladding characterized in that the bent optical fibre portion (2) defines a macrobend having a bend amplitude greater than twice a diameter of the cladding. A tap according to any one of claims 1-8, for extracting light from or injecting light into an optical fibre comprising a core and a cladding characterized in that the bent optical fibre portion (2) defines a series of microbends having bend amplitudes less than twice a diameter of the cladding. A tap according to any one of claims 1-9, characterized in that the light reflector (4) is a deflection surface located a distance less than 4mm from a furthermost upstream, as viewed for extracted light, section of the bent optical fibre portion (2). A tap according to any one of claims 1-10, characterized in that the light reflector (4) has a reflective coating disposed thereon. A tap according to any one of claims 1-11, characterized in that the transparent member includes a light waveguide (30) extending from the bent optical fibre portion (2) to the light reflector (4). A tap according to any one of claims 1-12, characterized in that the means (20) for maintaining comprises a resilient force means for urging the optical fibre portion within the groove (17). A tap according to any one of claims 1-12, characterized in that the light receiving element (68) includes a pigtail waveguide having an end which forms the light element end surface. A tap according to any one of claims 1-14, characterized in that the light receiving element or light source end surface (13) is located beneath said plane (24) and under a deflection surface constituted by the light reflector (4) and within 4mm of a furthermost section of the bent optical fibre portion (2). A method of making a tap for coupling light between a light element and an intermediate portion (2) of an optical fibre (1) core by passing the light through a side of the optical fibre, which method comprises: providing a light element (68) having an end surface (13); providing an optical coupler (14, 30) consisting of a molded transparent member, provided with a concave outer surface portion (22) for receiving the intermediate portion of the optical fibre bent thereagainst and disposed in a plane (24), so as to couple light at said optical fibre bent portion; providing means (20) for maintaining the intermediate portion of the optical fibre bent in said plane (24) against said concave outer portion of said transparent member; and forming a light reflector (4) on another outer surface portion of said transparent member, said reflector being disposed in the plath of the coupled light between the bend and the light element end surface and extending traversely to the plane in which the optical fibre bent portion is intended to be maintained so as to form a continuous light path between the bent optical fibre portion (2) and the light element end surface, characterised in that the end surface (13) of the light element (68) is disposed completely outside said plane (24) in which the optical fibre bent portion is intended to be maintained, and in that the transparent member constituting the optical coupler (14, 30) has an elongate and curved shape, said elongate and curved transparent member being provided in its concave side surface (22) with a groove (17) for receiving said optical fibre bent portion, and said light reflector (4) being formed on another outer surface portion of said curved elongate member which is inclined in such a manner with respect to the optical fibre bent portion maintaining plane (24) as to deflect the light between said end surface and the bent optical fibre portion.

Inventors: UKEN WILLIAM DAVID, UKEN, WILLIAM DAVID
Applicants: RAYCHEM CORP, RAYCHEM CORPORATION (A DELAWARE CORPORATION)
Application year: 1988
Main classification: G02B6
IPC codes: H04B10, G02B6