Coupling Element for Coupling Light Into and Out of an Optical Fiber

The present invention is directed to a coupling element for coupling light into and out of an optical fiber which has an end formed by a pair of end surfaces that extend at an angle to one another with one of the pair of end surfaces being essentially perpendicular to the axis of the optical fiber and the other end surface of the pair which end surface is inclined to the axis being provided with a reflective layer.

In the system fibers for an optical data bus which bus may be assumed to be realized as a star bus with a reflective mixer, light is to be coupled into the system fiber from the transmitter of the subscribers with as little loss as possible. In addition, light is to be coupled out of the system fiber to the receivers of the subscriber with a high degree of efficiency.

It is an object of the present invention to provide a coupling element of a specific type in which light can be coupled into an optical fiber with as little light loss as possible and light can also be coupled out of the optical fiber with a high degree of efficiency. To accomplish these objects, the present invention is directed to a coupling element for coupling light power into and out of a first optical fiber, said coupling element comprising a first optical fiber with an end having a first end surface and a second end surface inclined at an angle relative to one another, the first end surface extends essentially perpendicular to the axis of the first optical fiber and the second end surface is inclined relative to the axis and has a reflective layer, a second optical fiber is coupled to the first end surface of the first optical fiber, and a third optical fiber branches laterally from the first optical fiber adjacent the end, the diameter of the light conducting area of the second optical fiber is smaller than the diameter of the light conducting area of each of the first fiber and third fiber.

Preferably, the axis of the third fiber forms an angle 2.theta. with the axis of the first fiber which is twice as great as the angle .theta. which is formed by a surface normal of the second end surface with the axis of the first fiber. The second end surface forms an angle with the axis of the first fiber which angle is in a range of between 45.degree. and the arc sine (A.sub.N /n.sub.0), wherein A.sub.N denotes the numerical aperture of the first fiber and n.sub.0 is the index of refraction of the light conducting area of the first fiber. The numerical aperture of the third fiber is equal to or greater than the numerical aperture of the first fiber and the numerical aperture of the second fiber is equal to or less than the numerical aperture of the first fiber. The first, second and third fibers may all be fibers with a stepped index of refraction. Also, while the first and third fibers are fibers having a stepped index of refraction, the second fiber may be a fiber with a gradient index of refraction. Preferably, the first and third fibers are fibers which have a thick core, which has a diameter for the light conducting portion of approximately 200 .mu.m while the core diameter of the light conducting portion of the second fiber is approximately 50 .mu.m and preferably all of the fibers have a numerical aperture of 0.3

The FIGURE shows an embodiment of the coupling element in accordance with the present invention.

The principles of the present invention are particularly useful in a coupling element illustrated in the FIGURE. The coupling element comprises a first optical fiber f.sub.s, a second optical fiber f.sub.e, a third optical fiber f.sub.a and means for supporting the fibers in the desired relationship.

The first optical fiber f.sub.s is a system fiber that has an end formed by a first end surface A.sub.e and a second end surface A.sub.a which surfaces are illustrated as extending at an angle relative to each other. The first end surface A.sub.e is perpendicular to the axis A.sub.1 of the first fiber f.sub.s, however, the second end surface A.sub.a is inclined at an angle of 90.degree.-.theta. relative to the axis A.sub.1 which angle lies in a range between 45.degree. and the arc sine (A.sub.N /n.sub.0), wherein A.sub.N is the numerical aperture of the first fiber f.sub.s. As illustrated, the second end surface A.sub.a is covered with a reflective layer R.

The second optical fiber f.sub.e, which serves as an incoupling fiber for coupling light into the system fiber f.sub.s, is coupled with a butt-joint to the first end surface A.sub.e of the first optical fiber, which end surface, as pointed out hereinabove, extends perpendicular to the axis A.sub.1. As illustrated, the second fiber f.sub.e has a second axis which extends parallel to the axis A.sub.1 but is offset therefrom. The light conducting area or core of the second fiber f.sub.e has a diameter d.sub.e, which is smaller than a diameter d.sub.a of the light conducting area or core of the third optical fiber f.sub.a, which extends or branches laterally off from the first optical fiber adjacent the end. The third optical fiber f.sub.a serves as an out-coupling fiber for coupling light out of the first optical fiber f.sub.s. A reception diode (not illustrated) may be coupled to the far or opposite end of the output coupling fiber formed by the third fiber f.sub.a.

The fibers f.sub.s and f.sub.a should be fibers with a stepped index of refraction. However, the second fiber f.sub.e may either be of the same type or can also be a fiber that has a gradient index of refraction. The numerical aperture of the first fiber f.sub.s and the third fiber f.sub.a should be identical or the numerical aperture of the third fiber f.sub.a should be greater than the numerical aperture of the first fiber f.sub.s. The numerical aperture of the second fiber f.sub.e and of the first fiber f.sub.s should be equal or the numerical aperture of the second fiber f.sub.e should be smaller than the numerical aperture of the first fiber f.sub.s.

In a practical case, for example, the numerical aperture for each of the three fibers f.sub.s, f.sub.e, f.sub.a is 0.3. The diameter d.sub.s of the core of the first fiber f.sub.s and the diameter d.sub.a of the core of the third fiber f.sub.a can be 200 .mu.m and the diameter d.sub.e of the second light conducting area or core of the second fiber f.sub.e can be selected at 50 .mu.m.

As illustrated, the support means for the various fibers of the coupling element is formed by substrates having covers or by a plurality of interconnected block members. The coupling element can be manufactured in accordance with known methods or can be manufactured according to the teachings of copending U.S. application Ser. No. 246,100, filed Mar. 20, 1981, which was based on German OS No. 3,012,184 and whose disclosure is incorporated herein by reference thereto. The asymmetrical coupling element is particularly suited for data buses with reflex mixers.

The axis A.sub.3 of the third fiber f.sub.a is inclined relative to the axis A.sub.1 of the first fiber f.sub.s at an angle 2.theta. which is twice as great as the angle .theta. which is formed by a surface normal N of the second surface A.sub.a to the optical axis A.sub.1 of the first fiber f.sub.s. The third fiber f.sub.a abuts laterally obtuse to the first fiber f.sub.s. The mirror or reflective layer R on the second end surface A.sub.a is inclined relative to the axis A.sub.1 and should be highly reflective.

Although various minor modifications may be suggested by those versed in the art, it should be understood that I wish to embody within the scope of the patent granted hereon, all such modifications as reasonably and properly come within the scope of my contribution to the art.