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AssigneeAmerican Telephone and Telegraph Company, AT&T Bell Laboratories
InventorsJames A. Aberson, Jr., Ian A. White
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Optical radiation can be efficiently removed from, or injected into, single mode optical fiber at an intermediate point along the fiber, by causing quasi-resonant coupling of the guided mode LP.sub.01 to an appropriate tunneling leaky (TL) mode, e.g., LP.sub.11. Such coupling is caused by means of a "grating" in the fiber, with the grating being formed by impressing a periodic (or pseudo-periodic) modulation on the fiber, or by causing a periodic (or pseudo-periodic) variation of the refractive index of the fiber by means of the photoelastic or the photorefractive effect. The nominal grating spacing .LAMBDA.(z) is chosen such that .OMEGA..sub.o =(2.pi./.LAMBDA.)>.beta..sub.01 -2.pi.n.sub.cl /.lambda..sub.o, where .LAMBDA. is the average grating spacing, .beta..sub.01 is the propagation constant of the LP.sub.01 mode, n.sub.cl is the refractive index of the fiber cladding, and .lambda..sub.o is the wavelength of the radiation to be coupled from or into the fiber. Furthermore, .LAMBDA.(z) is to be chosen such that .OMEGA..sub.o .about..beta..sub.01 -.beta..sub.rs, where .beta. .sub.rs is the propagation constant of the selected TL mode. In order for the coupling to be quasi-resonant, it is necessary that .alpha..sub.rs, the attenuation constant of the selected TL mode, be relatively small, typically <1 dB/cm. By appropriately choosing .LAMBDA.(z) and/or the amplitude function of the grating, it is possible to increase the coupling efficiency above what is possible with a constant spacing, constant amplitude grating. Devices according to the invention can be advantageously used not only as radiation couplers but also as filters and, if they are of the photorefractive type, as amplitude modulators.
Means for coupling electromagnetic radiation of wavelength .LAMBDA..sub.o into or from an intermediate portion of an optical fiber, the fiber comprising a core and a cladding surrounding the core and having optical characteristics including an attenuation constant .alpha..sub.ij and a propagation constant .beta..sub.ij for each mode LP.sub.ij of the radiation that can be present in the fiber, where i is a non-negative integer and j is a positive integer,
An optical communications system comprising a source of first electromagnetic radiation of wavelength .lambda..sub.o, an optical fiber adapted for guiding the first radiation, first means for coupling the first radiation into the fiber at a first fiber location, and means for detecting, at a second fiber location spaced apart from the first fiber location, the first radiation that is transmitted from the first to the second fiber location through the fiber, the fiber comprising a core and a cladding surrounding the core, the fiber having optical characteristics including an attenuation constant .alpha..sub.ij and a propagation constant .beta..sub.ij for each mode LP.sub.ij of the first radiation that can be present in the fiber, where i is a non-negative integer and j is a positive integer, the optical communications system further comprising second means for coupling first radiation into or from the fiber at a third fiber location intermediate the first and the second fiber locations without breaking or terminating the fiber,
Dependencies are inferred from the claim text.