Fiber Optic T-Coupler

This invention relates to an optical-fibre T-coupler comprising solid-glass cores and reflective systems. Fibre-optics T-couplers are already known in embodiments including guide members and also provided with reflective arrangements. For example, in U.S. Pat. No. 3,883,217 there are described T-couplers in which two solid-glass cores arranged in series are interposed between the terminal surfaces of an interrupted primary fibre bundle and have the light guides of the coupled branches (receiver/transmitter) and also a direct connection applied to their inner surfaces. The incoming light signal is thus divided into a component led to the receiver and a component passing directly through the coupler. The light passing directly through the coupler and the transmitter signal injected in the same direction combine into a new transmitted signal at the coupler output. The two series-arranged solid-glass cores serve the function of a diffusor (randomiser, scrambler). Beyond a certain length of core, light entering at any point is distributed over the entire cross-sectional area of the core and thus passes uniformly into all the fibers of the outgoing fibre bundle. The relatively complex construction is a disadvantage. Six individual components are present in the device, of which the end faces must be exactly aligned and cemented. Even the construction of the fibre bundle providing the direct connection is not without problems.

In accordance with U.S. Pat. No. 3,870,396 right-angled isosceles glass prisms are interposed in the primary fibre bundle and the hypotenuse surfaces, positioned so as to be inclined relative to the direction of the transmitted light, are bridged by a reflector. This reflector is only partly reflective at the position at which the receiver signal is coupled out and is fully reflective at the position for the introduction of the transmitter signal. The arrangement is directional, so that two such couplers are necessary for duplex operation. The large coupling cross-sections which must subsequently be reduced, by way of guide transition sections (naturally accompanied by radiation losses or reflections) to the cross-section of the outgoing fibre bundle are a disadvantage. Exact adjustment of the individual coupling conditions at the partly transmissive reflector can also be far from simple.

A principal disadvantate of the T-coupler is known to be the relatively high transmission attenuation of the order of 3-4 dB resulting from the losses through the spacer and sheathing cross-section of the outgoing fibre bundle. Applications may be envisaged, however, in which these characteristics are of less importance.

It is an object of the invention to provide an optical T-coupler which includes few individual components, does not require special alignment and may be cheaply manufactured.

According to the present invention there is provided a fibre-optics T-coupler wherein a solid transparent primary core is interposed in the primary light path and reflective surfaces are provided at the coupling positions which deflect the incident light through at least approximately a right angle and are of such sizes in comparison to the total cross-section of the solid core that the deflected receiver signal corresponds to a predetermined ratio with respect to the light traversing the coupler directly.

The invention will be further explained with reference to the accompanying drawings comprising FIGS. 1 through 6, in which housings and any necessary mountings have been omitted for the sake of simplicity in illustration. In the drawings:

FIGS. 1a and 1b show two schematic views of a coupler operating on the principle of coupling light in and out by means of a reflector system positioned in the side of the coupler opposite the branch connection;

FIG. 2 shows how the principle of FIGS. 1a and 1b may be modified for directional coupling;

FIGS. 3a and 3b show two schematic views of a coupler operating on the principle of coupling light in and out by means of a reflector system positioned on the side of the coupler adjacent the branch connection;

FIG. 4 shows how the principle of FIGS. 3a and 3b may be modified for directional coupling;

FIGS. 5a and 5b show two schematic views of a coupler operating on the principle of coupling light in and out by means of a reflector system arranged within the solid core;

FIG. 6 shows how the principle of FIGS. 5a and 5b may be modified for directional coupling; and

FIG. 7 is an enlarged view of the reflective surface for FIG. 1.

In the embodiment shown in FIGS. 1a and 1b, reference 1 denotes a solid glass core included in the primary transmission path and 2 denotes a branch leading, for example, to a receiver, the branch likewise being in the form of a solid glass core. Opposite the position of the branch 2 there is formed in the core 1 a triangular notch 3, of which the surfaces 4 are mirror coated and are inclined to the axis of the light guide at an angle which is at least approximately 45.degree.. Thus, as is indicated by arrowed lines, a part of the light entering the core 1 is deflected into the core 2. The solid cores are arranged as diffuser rods, that is, their length, taken from the coupling point to the coupled fibre bundles (not shown in FIGS. 1a and 1b) should be at least 10 times their respective diameters. Core 2 is advantageously optically coupled to core 1 by cement indicated at 11.

In the arrangement shown in FIG. 2, two separate couplings 2a, 2b are provided on the primary core, for the purpose of directionally separate coupling in and coupling out of light, for example, in networks of ring form. For this purpose the solid core 1 has a trapezoidal shaped recess about its centre. The terminal faces 4a, 4b of the recess are inclined at 45.degree. opposite the positions of the T-couplers 2a, 2b, while the long face 4c bounds the direct connection 5 extending between the two T-couplers. At the reflective surface 4a a part of the incident light is, as illustrated, deflected at a right angle into the guide 2a leading to the receiver E. By the reflector 4b, the light signal coming from the transmitter S over the guide 2b is injected into the primary guide again in the original direction of transmission. The cross-section of the direct connection 5 is smaller than the full cross-section of the primary core 1 by an amount determined by the percentage of the light intensity received in the coupler that is diverted by means of the reflector 4a into the receiver branch. At the position at which the transmitter is coupled-in by means of the reflector 4b the smaller cross-section of the connection 5 is again increased to the full cross-section of the coupler output. Under these conditions the losses for light which traverses the coupler directly are kept to a minimum. By means of the relative reduction in cross-section of the connection 5, the power coupled-out to the receiver may be given a predetermined ratio to the light intensity incident on the coupler.

The coupling of the branch core 2 (FIG. 1) or of the cores 2a, 2b (FIG. 2) to the primary core 1 is not critical. In any case, however, the optical relations must be such that as full use as possible is made of the received light. There are in principle several possibilities, in accordance with the degree of coupling. With strong coupling (e.g. 3 dB corresponding to half the primary cross-section) the transition should be optically homologous, which can readily be achieved by cementing, as indicated at 11, in FIG. 1. For weaker coupling on the other hand, it is possible, as indicated in FIG. 2, for the branch cores 2a, 2b to merely abut the primary core 1, without cementing. The curvature of the surface of the primary core opposed to the reflective surfaces in fact effects a considerable focusing of the incident light, as in a cylindrical lens. In addition, it is possible to provide the reflector surfaces with a convex curve of appropriate curvature in the longitudinal direction of the primary guide 1, so that light transmission similar to a plano-convex lens results.

In the coupler systems in accordance with FIGS. 3a, 3b and 4, the recesses in the primary guide 1 are of rectangular form and are placed on the side of the guide which faces the coupling guides. Here the reflector surfaces for deflecting the light are, as shown, on the end faces of the coupled guides. FIGS. 3a and 3b show the construction for bi-directional transmission, while FIG. 4 shows the arrangement for directionally separated transmission. Preferably, a rectangular or square cross-section of the guides is used here, since optimum conditions for light transmission can then be easily achieved. The mode of operation is in general the same as for the arrangements of FIGS. 1a, 1b and 2.

Finally, FIGS. 5a, 5b and 6 show arrangements in which the 45.degree. diverting reflectors are arranged concentrically in a circular primary core. Both of the systems are arranged for directionally separate operation. In FIGS. 5a and 5b the primary core is provided with a cylindrical bore arranged at a right angle to the longitudinal axis of core 1 and adapted to receive the two coupling guides 6a, 6b. Between the end surfaces of the two guides, inclined at 45.degree., is a metallic foil 7 which is reflective on both sides to deflect the incident light appropriately for both opposing directions. A thin air-gap may be employed instead of the foil, reflection then resulting from the difference in the refractive indices of the material of the guides 6a, 6b and of air. The degree of coupling to the receiver is again determined by the ratio of the diverted cross-section to the full primary cross-section.

In the construction shown in FIG. 6, the primary core is divided into two preferably similar portions 1a, 1b. From the meeting plane P.sub.M there are formed two inwardly directed blind bores which contain light-transmissive inserts 8a, 8b, as well as a preferably metallic peg-shaped spacer 9, of which the ends 7a, 7b, sloped at 45.degree., are reflective. Light falling on the reflective surfaces 7a, 7b is thus, as indicated, diverted into the guide 6a to the receiver E and from the transmitter S over the guide 6b into the primary core portion 1b. With accurate optical cementing of the components 8a,8b and 9 with guide portions 1a, 1b only the reflective surfaces 7a, 7b appear in the light transmission path. In principle the inserts 8a, 8b could even be omitted, but this would necessitate exact optical finishing of the end faces of the blind bores, or the supporting spacer 9 could be provided with perpendicular end surfaces and the 45.degree. end surfaces of the inserts 8a, 8b could be made reflective, and so on, though however, no substantial advantage would result. For light transfer between the coupling guides 6a, 6b and the primary core 1, the lens effect of the surface curvature of the primary core already explained with reference to FIG. 1, is advantageously made use of. With additional cylindrical curvature of the reflective surfaces 7a, 7b in the plane of the beam there may again be obtained a focusing action similar to that of a plano-convex lens. It is true that this assumes that the two inserts 8a, 8b are present so that the material in the vicinity of the reflective surface has a refractive index about equal to that of the primary core. In addition, the coupling guides 6a, 6b may also be mutually rotated through any angle, instead of lying in a common plane, as shown. The concentric introduction into the primary core 1b of the light transmitted directly into the coupler through the connection 10 surrounding the blind bores and of the transmitter signal coupled in by way of the reflector 7b effects advantageous homogenisation of the light distribution particularly suitably. The amount of receiver coupling is determined in principle by the width of the blind bore in comparison to the full cross-section of core 1a.

The possibilities are shown in principle in the first place by the arrangements represented in FIGS. 1-6. In addition, other analagous modifications are also possible, for example, to form on the primary core 1 two sawtooth-shaped recesses at diametrically opposed positions each having a 45.degree. inclination, or to form the trapezoidal or rectangular recesses in the primary core (in accordance with FIG. 2 or FIG. 4) merely as grooves. Any transparent material (silicate glass, transparent acrylic material) may be used as the solid-glass core. Instead of being circular in form the guide cross-sections may if necessary be square or rectangular and the ends of the fibre bundle may readily be adapted to either form. For the purpose of homogenising the mixing of the light the solid core may in addition contain suspended light-scattering centres or diffusing plates arranged at certain distances, forming a milky translucent material. In some cases optically true surfaces of the guide material may be used instead of mirrors for diversion of the light, so that the reflection takes place as a result of the difference in the refractive indices of the guide material and of the surrounding air.

While the degree of coupling of the receiver corresponds more or less to the ratio of the coupling surface to the primary transmission cross-section, at the transmitter it is practically equal to one. This difference is significant insofar as, in a linear fibre network, large section attenuations are thus acceptable, or more stations may be interposed for optimum transmission conditions. Maximum transmitter coupling requires concentration of all of the transmitted light upon the small cross-section of the respective coupling area. This is possible, for example, if a light-emissive diode or laser diode is used as the light source, the active radiating surface of which is smaller than or at most equal to the coupling-in cross-section of the reflector surface. The transmitting medium may then be merely an homogeneous light guide matched to the required transmission cross-section.

In the arrangements providing directional coupling, there is a marked decoupling between the two branches, which may be significant in certain applications of the coupler system. Solid material is advantageously employed for the receiver and transmitter coupling guides, because of the low intrinsic losses, especially for short lengths. However, the receiver and transmitter diodes may also be arranged on the primary core 1, directly opposite the respective reflector surfaces. In such a case the coupler may be advantageously placed in a common housing together with the receiver and transmitter.

Although there has been described a preferred embodiment of this novel invention, many variations and modifications will now be apparent to those skilled in the art. Therefore, this invention is to be limited, not by the specific disclosure herein, but only by the appending claims.