Patent
1989-02-02
1991-05-14
Lee, John D.
65 42, 350320, G02B 626
Patent
active
050150588
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates generally to fiber optic couplers, and is concerned in particular with the fabrication of asymmetric fiber optic couplers
The simple symmetric fiber optic coupler, in which input light at any branch is split substantially equally between output branches, has little practical application to linear fiber optic buses and networks since only a trivial number of taps are possible before optical signal strengths become impracticably small. Larger linear fiber optic networks require asymmetric couplers in which tap-off coupling is substantially less than half, but tap-on coupling is much greater than the tap-off coupling. In the context of this specification, the terms "asymmetric coupler" and "asymmetric coupling" refer to coupling asymmetry of this kind. Such a requirement arises, for example, in networks where each component tied to a bus includes both an optical transmitter and an optical receiver, the receiver tapping the bus in front of the tap for the transmitter.
It has been proposed that asymmetric couplers having, e.g. a throughput power coupling factor, e.g. on a bus, of 95 and a tap-off coupling factor of 1.5%, might be fabricated by employing conventional techniques to merge two optical fibers of substantially different diameters or by simply reducing the nominal coupling factor of two similar fibers. The resultant couplers are satisfactory for single-mode applications, e.g. in basic telecommunication systems, but unacceptable for multi-mode operation because of significant power losses and the restricted proportion of modes which successfully couple to and from the tap fibers
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide asymmetric multi-mode fiber optic couplers which exhibit better coupling properties than those hitherto produced.
The invention accordingly provides a method of fabricating an asymmetric multi-mode fiber optic coupler comprising assembling the coupler from two or more multi-mode optical fibers of different core and cladding diameters but substantially equal cladding refractive indices, and wherein the fibers are selected so as to have substantially equal effective refractive indices for their respective highest order cladding modes, at least in the fabricated coupler.
The invention further provides an asymmetric multi-mode fiber optic coupler having at least four optical fiber branches of at least two different core and cladding diameters but substantially equal cladding refractive indices, wherein the fiber branches further have substantially equal effective refractive indices for their respective highest order cladding modes. In one embodiment, the invention affords a coupler fabricated by the aforedescribed method.
If the cladding refractive indices of the fibers are not similar, high order core modes of the fiber having the lower cladding refractive index will be unable to couple to corresponding core modes of the other fiber and will be lost. By matching the cladding refractive indices and the effective refractive index of the highest cladding modes, the effective refractive index of each core mode of each fiber will lie above the cladding refractive index but below the core refractive index of the other fiber. Equilibration of n.sub.e 's in both fibers is required to minimize the coupler excess loss and maximize the mode coupling between the fibers in the coupling region.
In order to fabricate an asymmetric multi-mode fiber optic coupler having a predetermined or predictable power coupling factor, the method of the invention preferably further includes selecting the optical fibers in regard to their numerical apertures NA, and their core-to-cladding radii ratios .rho..sub.co /.rho..sub.cl and subjecting the fibers to respective taper ratios T, so that, on assembly of the fibers to form the coupler, the products ##EQU1## for the respective fibers are at least momentarily equal, and wherein the assembly is thereupon completed.
In one embodiment, the values of NA and .rho..sub.co /.rho..sub.
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Love John D.
Lyons Peter R. A.
Maine Johnathan A.
Rashleigh Scott C.
Thorncraft David A.
Lee John D.
The Commonwealth of Australia
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