Abstract:
Dual concentric core fiber is used for optical communication. Inbound messages lying in a first wavelength channel are received from a terminal portion of the fiber, and outbound messages lying in a second such channel are injected into the terminal portion. The optical fiber has at least one annular portion surrounding a central core portion. The inbound messages are received from the annular portion, and the outbound messages are injected into the central core portion. Alternatively, the inbound messages are received from the central core portion, and the outbound messages are injected into the annular portion.

Description:
FIELD OF THE INVENTION 
     This invention relates to optical communication systems in which electro-optical emitters and detectors are coupled to the ends of optical fibers for sending and receiving signals along the fibers. More specifically, this invention relates to those communication systems in which individual fibers carry signals bidirectionally. 
     BACKGROUND OF THE INVENTION 
     Because optical fibers can transmit information at much greater rates than copper wire, there is much interest in schemes for delivering telecommunication services to customer premises, such as residential homes, over optical fiber. These schemes fall into two general classes, depending on whether each terminal location receives transmissions from the central office over its own dedicated line, or whether multiplexing is used to reduce the number of fibers that fan out from the central office. The first scheme is said to have Point-to-Point (PTP) Architecture. When passive optical components are used at intermediate locations to demultiplex downstream signals (and, in some cases, to multiplex upstream signals), the second scheme is said to have Passive Optical Network (PON) architecture. 
     One problem encountered by designers of PTP networks is fiber congestion at the central office or active remote node (ARN), where downstream signals are placed on optical fibers. There is a need for line cards with high-density electro-optical interfaces, to alleviate this congestion. 
     One problem encountered by designers of both PON and PTP networks is the need for expensive equipment at the fiber terminations. That is, bidirectional coupling of signals into and out of the end of an optical fiber typically calls for optical splitters and couplers that are bulky and expensive to manufacture. In PON networks in particular, this is a problem for fiber installations within the customer premises. Typically, a fiber extends from the on-premises optical network unit (ONU) to a network PON fiber termination defining the physical interface between the network and the customer premises. There is a strong economic incentive to reduce the cost of the interfaces at the ends of this fiber. 
     SUMMARY OF THE INVENTION 
     We have discovered that these, and other, problems of bidirectional communication in optical networks can be alleviated by using dual concentric core fiber (DCCF) to carry optical signals bidirectionally. In one broad aspect, our invention involves a method for communicating messages, in the form of modulated optical signals, over an optical fiber. Inbound messages lying in a first wavelength channel are received from a terminal portion of the fiber, and outbound messages lying in a second such channel are injected into the terminal portion. The optical fiber has at least one annular portion surrounding a central core portion. The inbound messages are received from the annular portion, and the outbound messages are injected into the central core portion. In alternate embodiments of the invention, the inbound messages are received from the central core portion, and the outbound messages are injected into the annular portion. 
     As used herein, the directions indicated by the terms “inbound” and “outbound” are relative to the terminal device under discussion, irrespective of whether the signals so described are directed toward or away from the central office. Those directions are indicated, respectively, by the terms “upstream” and “downstream”. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an illustrative dual concentric core fiber (DCCF). 
     FIG. 2 is a schematic illustration of a DCCF in use for bidirectional communication in accordance with the invention. 
     FIG. 3 is a schematic illustration of a DCCF in use within a residence or other customer premises for coupling an ONU to an outside plant (OSP) fiber. 
     FIG. 4A is a schematic drawing of an exemplary DCCF having a polymeric outer cladding. 
     FIG. 4B is a schematic drawing of an exemplary DCCF having an outer cladding of depressed-index glass. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a typical DCCF useful for the practice of the invention. As shown in the figure, DCCF  10  includes central, single-mode core  15 , annular, single-mode cladding  20 , and annular, multimode cladding  25 . Although the refractive index may be constant within each of these three portions, the DCCF is more generally described in terms of respective, radially dependent refractive index profiles n 1 (r), n 2 (r),n 3 (r) for core  15 , cladding  20 , and cladding  25 , respectively. The effective refractive index is greatest for core  15 , and least for cladding  25 . The diameter of core  15  and the effective refractive index of cladding  20  relative to that of core  15  are advantageously chosen in such a way that only the fundamental mode TEM 00  is guided within core  15 . On the other hand, core  15  and cladding  20 , in combination, behave as a core for guiding multiple modes. The number of modes guided within this combination, serving as a multimode core, is determined by the outer diameter of cladding  20  and the effective refractive index of this combination, relative to that of cladding  25 . 
     It should be noted in this regard that unlike single-mode propagation, multimode propagation is subject to modal dispersion. As a consequence, a multimode fiber generally has less transmission bandwidth than a comparable single-mode fiber. However, we believe that DCCFs are readily provided having a transmission bandwidth of 50 MHz-km or more. A transmission bandwidth of 50 MHz-km is enough to be useful for downstream transmissions over the distances encountered within residences and other customer premises, since these typically call for data rates of 155 Mb/s or less, over distances of 300 m or less. The same transmission bandwidth is also enough to be useful for upstream transmissions, not only within a residence, but also over the distances encountered within a distribution area of the access plant, which typically calls for data rates of 50 Mb/s or less over distances of a few kilometers or less. 
     One advantage of a DCCF is that it can be directly coupled to an electro-optical source and detector without the need for passive splitting and coupling elements. Advantageously, the source and detector are integrated in a single, compact device with a spacing close enough for both to be butted against the DCCF such that one is coupled to the single-mode core, and the other is coupled to the multimode core. 
     Even more advantageously, the source and detector are integrated in a concentric configuration in which the source occupies an annular region about the detector, or the detector occupies an annular region about the source. In such a device, it is desirable for the cross section of the central device to match that of the single-mode core, and for the cross section of the annular device to match that of the outer portion of the multimode core (i.e., of the single-mode cladding). 
     Such an arrangement is shown in FIG.  2 . As shown in the figure, electro-optical device  30  at user terminal  35  includes annular, face-emitting source  40  and central detector  45 . As shown in expanded view  50  of the near end of DCCF  55 , detector  45  butts against and aligns with core  15 , and source  40  butts against and aligns with cladding  20 . (In all of the figures, like reference numerals denote like elements.) At upstream terminal  60 , as shown in expanded view  65  of the near fiber end, central source  70  butts against and aligns with core  15 , and annular detector  75  butts against and aligns with cladding  20 . 
     For even greater compactness, it is advantageous to fabricate the concentric source-detector pairs in arrays, and to couple such arrays, in unitary fashion, to multifiber cables such as ribbon cables. 
     We believe that concentric source-detector pairs useful for operation as described above are readily made using conventional fabrication techniques for light-emitting diodes (LEDs) and surface detectors. An example of fabrication technology of this kind is provided by U.S. Pat. No. 5,283,447, issued on Feb. 1, 1994 to G. R. Olbright et al. under the title “Integration of Transistors with Vertical Cavity Surface Emitting Lasers”. Described there are monolithically integrated optoelectronic circuits including a vertical-cavity surface-emitting laser and a transistor, such as a phototransistor. 
     It should be noted in this regard that capacitance in monolithically integrated devices of this kind may in some cases be high enough to preclude applications for high-speed data transmission. Alternate devices are readily provided for use in such applications. For example, U.S. Pat. No. 4,314,740, issued on Feb. 9, 1982 to G. W. Bickel under the title “Optical Fiber Beam Splitter Coupler” describes a passive beam splitting device that is readily interposed between the FTTH and one member of the source/detector pair, while juxtaposed to the other member of the source-detector pair. In particular, an array of such devices will be useful for providing, in a very compact fashion, passive coupling between an array of sources and detectors, and an FTTH fiber array. 
     Another passive coupling device, useful in this regard, is described in U.S. patent application Ser. No. 08/897,195, filed on Jul. 21, 1997 by D. J. DiGiovanni et al., and commonly assigned herewith. As described there, a coupler is made from a bundle of multimode fibers packed around a central fiber having a single-mode core. Coupling is effectuated by heating and tapering the bundle, and fusion splicing it to a single-mode main fiber (which, in the example given in the cited patent application, is a cladding-pumped laser). Light in one wavelength region can be coupled between the respective single-mode cores of the bundle and the main fiber. Optoelectronic sources or emitters (according to the example described in the cited patent application, they are semiconductor broad stripe emitters) are coupled to the distal ends of the individual multimode fibers, and through those fibers, to the cladding of the main fiber. 
     An exemplary application of the DCCF for a PTP network is conveniently discussed with further reference to FIG.  2 . In this application, terminal  60  is the central office or an active remote node (ARN). From there, the downstream transmission signal is launched into core  15  and propagated as a single-mode transmission to an ONU at the customer premises, which includes source  40  and detector  45 . At the ONU, source  40  launches the upstream signal into the multimode core (i.e., core  15  and cladding  20  in combination) for multimode transmission to terminal  60 . 
     FIG. 3 shows an exemplary application of DCCF  55  within customer premises. Downstream end  80  of the DCCF (shown in expanded view) is coupled to ONU  85 , which includes a concentric source-detector pair or the like. Upstream end  90  (also shown in expanded view) is coupled to network fiber termination  95 . Fiber termination  95  is the connector to network outside plant (OSP) fiber  100 . Fiber termination  95  is typically situated in a Network Interface Device (NID) at the side of the residence or other customer premises. OSP fiber  100  is a single-mode fiber such as standard 5D fiber. By way of example, fiber  100  supports 1.55 μm downstream transmission and 1.31 μm upstream transmission. 
     In operation, DCCF  55  carries 1.31 μm upstream transmissions from the ONU via core  15  to OSP fiber termination  95 . From there, the upstream transmissions couple into the core of fiber  100  and propagate toward the central office. 
     Downstream transmissions, at the longer wavelength of 1.55 μm, are initially coupled from fiber termination  95  into the core of DCCF  55 . However, in this application, a feature of the DCCF to be described below causes the energy at this longer wavelength to escape from the single-mode core and couple into cladding modes (with respect to core  15  and cladding  20 ). That is, the energy is guided in the multimode core described earlier. Thus, the injection of the downstream transmission at fiber end  90  has the effect of injecting the transmission into both core  15  and cladding  20 . As a consequence, when downstream signals in DCCF  55  reach ONU  85 , they can be detected there by, e.g., an annular detector. 
     Although the 1.55 μm downstream signals are to be coupled into cladding modes, a DCCF effective for this purpose is readily made that will remain an effective single-mode waveguide for the 1.31 μm upstream signals. Such a DCCF behaves as a directional coarse wavelength-division multiplexer (CWDM) when used in conjunction with a concentric source-detector pair. 
     One special feature that will effectuate the desired outcoupling of energy into cladding modes is a long-period grating written into single-mode core  15 . Techniques for writing these gratings using, e.g., actinic radiation are well known, and need not be described here in detail. One useful description of some such techniques may be found in U.S. Pat. No. 5,430,817, issued on Jul. 4, 1995 to A. M. Vengsarkar under the title “Optical Systems and Devices Using Long Period Spectral Shaping Devices.” A long-period grating is a refractive index Bragg grating. The repeat distance A of such a grating is selected such that forward-propagating fundamental mode light is coupled into forward propagating higher-order-mode light. Periods typically are several hundred micrometers, and typical grating lengths are 1-5 cm. Index change induced by actinic radiation is on the order of 10 −4 . 
     An alternative feature for effectuating the same purpose is provided by tailoring the refractive index profile of the DCCF. That is, the Δn, or relative refractive index difference, between core  15  and cladding  20  is selected such that 1.3 μm signals are confined by the single-mode core, but 1.55 μm signals are not. This could be achieved, for example, in a DCCF having core  15  and cladding  20  similar to the core and cladding of standard non-dispersion-shifted fiber, dispersion shifted fiber, or non-zero dispersion fiber, but in which one or both of the core diameter and Δn are adjusted to make the fiber only weakly guiding at 1.55 μm. For example, reducing the core index of Lucent 5D fiber from Δn=0.0045 to Δn=0.0035 and keeping the same core diameter of 8.2 μm leads to a fiber in which attenuation of the energy at 1.55 μm is estimated to be greater than 30 dB over 100 m due to bends induced during normal cabling or installation. 
     One exemplary design for a DCCF is shown in FIG.  4 A. As shown in the figure, DCCF  105  includes core  110  and cladding  115  of a standard Lucent 5D single-mode glass fiber. Polymer coating  120 , which is chosen to have a refractive index lower than that of cladding  115 , serves as the multimode cladding. This design is advantageous because the manufacturing process is the same as for a standard product, except for the addition of a simple polymer overcoating step to provide coating  120 . Thus, production costs are relatively small. Moreover, interconnection with other fibers, such as an OSP fiber, is facilitated because the single-mode core and cladding are a very close match to those of at least some standard fibers. 
     A second exemplary design for a DCCF is shown in FIG.  4 B. To make the preform from which this fiber is drawn, a standard core rod, such as a Lucent 5D core rod, is first fabricated using MCVD (Modified Chemical Vapor Deposition). The inner and outer portions of the standard core rod are the precursors for single-mode core  125  and pure silica single-mode cladding  130 , respectively. According to conventional techniques, the standard core rod would then be enclosed in an overclad tube of pure silica. In our design, however, the overclad tube is not made of pure silica. Instead, it is made from silica doped with fluorine to depress its refractive index. When the fiber is drawn from the resulting preform, the overclad tube provides multimode cladding  135 . If the core rod and overclad tube have the same dimensions as those for making 5D fiber, the resulting DCCF is readily made to match 5D fiber in core diameter and outer cladding diameter. 
     Alternatively, core and cladding portions of a preform are grown from silica soot by VAD (Vapor Axial Deposition). In a subsequent growth step, fluorine-doped silica is deposited to form the region that will ultimately give rise to the multimode cladding.