Apparatus for extending bandwidth of large core fiber optic transmission links

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CROSS REFERENCE TO RELATED APPLICATION

U.S. application Ser. No. 08/328,177 to Cina et al. filed on the same day herewith is entitled and directed to a "Method for Extending Bandwidth of Large Core Fiber Optic Transmission Links", the teaching of which is incorporated herein by reference describes a method for making an optical fiber transmission apparatus for limiting the optical modes which were emitted from a source in such a way to impinge on an optical fiber to extract a high bandwidth from the fiber. The apparatus includes a lens or aperture to control the angle and distribution of light launched into the fiber. The apparatus achieves reproducibly high bandwidths in large core step-index optical fibers of short transmission length distances. The lens or aperture introduces light from the source into the fiber at an angle at which substantially no intermode delay occurs as the light propagates down the fiber. An integral fiber optic coupling assembly that includes an optical electronic component receptacle, the lens and/or aperture, and an optical fiber connector interface which provides low cost easy to manufacture assembly is also disclosed. A unitary plastic housing provides the function of a lens and mechanical reference or locating features for the light source and optical fiber.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to optical fiber launch controls for improving the bandwidth of large core fibers of relatively short communication links and more particularly to an optical coupling apparatus that limits the optical modes launched into such fibers. In addition the invention is directed to a low cost, easy to assemble fiber optic coupling sub-assembly.

2. Description of the Prior Art

Fiber-optical communication links, because of their ability to carry vastly more information than equivalent sizes of electrical wires, will be the "infrastructure" of the much talked about information highway. In optical communication, sources of light, such as lasers, are connected to optical fibers which carry the signal over both long and short distances to optical receivers. Optical fibers typically have a central region of a high index of refraction surrounded by a cladding region of a lower index of refraction. Optical signals launched into the fiber propagate along its length in a multiplicity of optical modes. A problem with long fibers such as 1 km or longer, is that the time taken for the transmission of a signal via the different optical modes through the fiber varies, so that a rapid sequence of individual pulses fed into the fiber at one end can be smeared out into a continuous stream at the other end and all information is lost. To prevent this, fibers are being made with a radial gradient of refractive index having a parabolic cross section of index. In this way, the velocity of the various modes is compensated to reduce transit time variations among the modes. The number of possible modes can also be reduced by making the fiber very thin, i.e., of dimensions comparable to the wavelength of the light.

For such long, thin fibers, it is assumed that all optical modes, described the by angle .theta. at which they propagate down the fiber, are equally coupled to near neighbor modes and so equally couple and mix as the modes propagate down the fiber. If one allows the light to propagate for a sufficiently long distance, such as a kilometer, down such a fiber, all of the modes should be mixed together and from then on the effective fiber bandwidth will not vary with launch conditions.

Most data communications applications, however, require links under 500 m, the majority of which are under 100 m in length. In this length regime, the standard theory predicts great variation in fiber bandwidth with launch angle, even for small angles.

Fibers used in these shorter links are typically of a large core design, such as a step-index optical fiber. These fibers, because of their large cross-sectional areas, have a smaller bandwidth than fibers with small cross-sectional areas. Large core step-index optical fibers are known to exhibit intermode dispersion due to various postulated optical mode propagation and diffusion mechanisms. There has been experimental work which shows a large variation in fiber bandwidth upon changing the source (LED or laser) or the launch optics. This variation in bandwidth with launch has relegated the use of large core fibers to rather low data rates because the details of the variation in fiber performance with launch are not understood. Thus, there is a need to understand the mode propagation in a large core fiber and from that understanding derive launch design rules and optical structures to achieve reproducibly higher bandwidth in large core, short link fibers.

SUMMARY OF THE INVENTION

The present invention is directed to an optical fiber light transmission apparatus for improving the bandwidth of large core, step-index, short length optical fiber links. The bandwidth improvement is achieved by providing a launch condition control assembly that includes a launch means for introducing light from an optoelectronic component into the fiber at an angle less than or equal to a predetermined angle .theta..sub.M, where .theta..sub.M is the maximum angle at which substantially no intermode delay will occur as the light propagates down the fiber. An optical fiber launch condition control assembly incorporates three basic components, an optoelectronic component for generating light, an optical fiber and a fiber optic coupling assembly. The usefulness of the present invention is in relatively short length fiber optic links which typically are 500 meters or less as well as in large core fibers which have a diameter of typically 200 microns or greater. The inventors herein have determined that for such short, large core fibers, all modes within the fiber are not equally coupled as previously assumed. Rather, some low angle modes quickly scatter toward the zero angle mode which was quite unexpected from previous theory. Thus, the inventors determined that based on this new theory, a reproducibly high bandwidth can be achieved by eliminating the high angle modes. For each particular application, a predetermined angle .theta..sub.M is found by experimentation to be the maximum angle at which substantially no intermode delay will occur without resulting in an improved fiber optical bandwidth. Launch means are then designed to limit the angle of light introduced into the filter to .theta..sub.M or less.

One launch means for introducing light at less than the maximum angle is by the use of an optical lens. Another option is an aperture that blocks out the unwanted angles. In practice, it has been determined that a poor optical lens such as one having a substantial amount of spherical aberrations actually works well in limiting the launch angles introduced into the fiber. In a preferred embodiment, the launch means is integral with the fiber optic coupling assembly. The coupling assembly includes an optoelectronic component receptacle, the launch means and an optical fiber connection means. In a more particular preferred embodiment the integral fiber optic coupling assembly is composed of integrally molded plastic in which the launch means is a plastic lens formed in the molding process.

Thus, the prior art practice of maximizing the power launched into an optical fiber is the incorrect design criteria for large core, step-index, short distance fiber optic links. Higher data rates are achieved when the launch modes are limited to those which will mix together quickly in a fiber over a short length. The optical fiber light transmission apparatus of the present inventions launches only a fraction of the power so that the light will propagate in desired modes and provide an improved fiber optical bandwidth.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of one embodiment of the optical fiber light transmission apparatus of the present invention.

FIG. 2 is a schematic of a second embodiment of the optical fiber light transmission apparatus of the present invention.

FIG. 3 is a cross-sectional diagram of an integral fiber optic coupling assembly of the present invention.

FIG. 4 is a graph plotting the intermode delay as a function of launch angle showing in theory the .theta..sub.M for a 200 meter fiber.

FIG. 5 is a cross-sectional view showing the formation of a spherical lens integral with a coupling assembly.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to an optical fiber apparatus for providing reproducibly higher data rates by exploiting heretofore unknown transmission properties of large core short distance optical fiber links. In the prior art, it was known that launch conditions affected fiber bandwidth and, further, that large core optical fibers were avoided where high data rates were desired because a large variation in fiber bandwidth existed for large core fibers. Experimental work of the inventors herein have shown that previous theoretical descriptions of optical mode propagation within a large core fiber were derived from incorrect assumptions. The power flow equations of Gloge (1972) assumed all optical modes equally couple and mix as the modes propagate down the fiber. This theory, however, does not hold true for short distance cable links of 500 meters or less. The inventors study of pulse propagation in large core plastic clad silica fiber has shown that all modes within the fiber are not equally coupled. Rather, some low angle modes quickly scatter toward the zero angle mode. Practically, this means that for a 500 meter or less data transmission link, one can launch into these modes which are quickly homogenized and retrieve a reproducible and higher bandwidth. To achieve this reproducibly higher bandwidth, the launch control assembly must include means for limiting the angular spread of light launched into the fiber. The launch means utilized in the fiber optic transmission apparatus of the present invention introduces light from an optoelectronic source component to the fiber at an angle less than or equal to a predetermined maximum angle .theta..sub.M, where .theta..sub.M is the maximum angle at which substantially no intermode delay will occur as the light propagates down the fiber. By providing a launch angle sufficiently less than the angle light is diverging from the source component, eliminates the high angle modes and results in an improved optical fiber bandwidth.

Two examples of means for limiting the optical modes of the light launched into the fiber are an optical lens and an optical aperture. FIG. 1 is a schematic of an optical fiber light transmission apparatus in which a lens is used as a launch means for limiting the optical modes launched into the fiber. As shown in FIG. 1, the optical fiber light transmission apparatus 10 includes an optoelectronic component 12 for generating light diverging at an angle .theta..sub.S. Optical fiber 14 is a large core fiber of, for example, 200 microns in diameter or greater, used in a short distance data links of, for example, 500 meters or less. Launch means 16 is an optical lens that introduces light from the optoelectronic component 12 into the fiber 14 at an angle .theta..sub.L less than or equal to an angle .theta..sub.M. .theta..sub.M is less than .theta..sub.S for typical sources, and is the maximum angle at which substantially no intermode delay will occur as a light propagates down the fiber 14. In the embodiment shown in FIG. 1, the size and positioning of the optical lens 16 results in limiting the launch modes introduced into the fiber 14. As can be seen, the high angle modes, transmitted at spaces 18, are not captured by lens 16 and therefore are not introduced into fiber 14. An alternative embodiment is to employ a poor quality lens that captures all of the light diverging from the source but having less coupling efficiency than normally achieved with good optical lens components. For example, a poor lens could have a substantial amount of spherical aberrations which result in the high angle modes not being launched into the fiber 14.

An example of an optical fiber light transmission apparatus of the present invention was constructed employing a Mitsubishi 40116 laser diode as the optoelectronic light source component 12 for introducing light into a plastic clad 200/230 .mu.m silica fiber 100 meters long. A poor quality lens, having a substantial amount of spherical aberrations, was used as lens 16. Operation of the light source resulted in the fiber having a bandwidth of 130 MHz. A very good Grin lens was then substituted for lens 16 so that the optical modes launched into the fiber were maximized and a bandwidth of under 90 MHz resulted. Thus, an increase of about 45% in the bandwidth was achieved. The present invention will result generally in an increased bandwidth in the range of 30% to 60%.

Another embodiment of the apparatus of the present invention is shown in FIG. 2. The optical fiber light transmission apparatus 20 includes an aperture 26 which limits the angle of light from light source 22 launched into the fiber 24 to the angle .theta..sub.L, which is less than .theta..sub.S and also less than or equal to .theta..sub.M.

The launch means of the optical fiber light transmission apparatus of the present invention is manufactured as a component of a fiber optic coupling assembly for coupling a light source to an optical fiber. Optical coupling subassemblies as known in the art consist of three basic components: an optoelectronic component receptacle, a lens retainer and a fiber connector. Each of these components must be actively aligned to each other and then fixed in place by soldering or other means. This assembly process is both labor and capital intensive resulting in expensive coupling assemblies. The high cost of these assemblies is one of the greatest impediments to the general proliferation of fiber optics and data communications. Alignment tolerances for large core fibers is significantly less than for thin fibers. Therefore, the launch control means of the present invention which results in significantly higher and reproducible bandwidths for large core fibers will now permit such fibers to be more widely used so that the cost savings resulting from the higher alignment tolerances of the large core fibers can be realized. In addition, by permitting the use of poor lenses which allows plastic molded lenses to be used in place of the standard high precision glass lenses results in an overall low cost, easy to assemble fiber optic coupling subassembly providing significantly higher fiber bandwidth.

In a preferred embodiment of the present invention the launch means for limiting the optical modes launched into the fiber is integral with the fiber optic coupling assembly. The integral fiber optic coupling assembly in this embodiment includes an optoelectronic component receptacle, the launch means and an optical fiber connector means. In a further preferred embodiment of the present invention, the launch means is a single plastic molded lens integral with an optical coupling assembly having mechanical locating and fixing features that eliminates the alignment procedures necessary with prior art coupling assemblies. The optical coupling assembly having the integral lens together with the locating and fixing features are all made in the same molding step. Thus, a light source such as a laser diode can be press fit into the optoelectronic device receptacle on one end of the coupling assembly while the other end will be adapted to conform to the desired optical fiber connector. The launch conditions may be controlled by changing the lens shape in the mold or by including an aperture in order to improve the bandwidth of low cost large core fibers. The optical assembly of the present invention is easy to assembly and has a reduced part count while eliminating the special assembly facilities and expensive capital tooling required by prior art precision optical coupling assemblies. In another