Abstract:
A passive optical network system and method in which at least part of the data is optically transmitted through a single optical fiber using a wavelength division multiplexing technique, with a plurality of signals being carried through the fiber in each direction, a different wavelength being used for each of the multiplexed upstream and downstream signals. The system may be retrofitted into existing telecommunications system to provide a multi-fold increase in the available bandwidth of long-distance optical fiber transmission.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical fiber communications network; and more particularly, to an optical fiber network system and method providing improved fiber utilization in the link between end-users and central stations. 
     2. Description of the Prior Art 
     Present telecommunications and computer systems require the high data-rate transmission of digital information between different circuits. These circuits may be in close proximity, such as within a single equipment cabinet, or they may be separated by very long distances. In the earliest stages, telecommunications involved transmission of electrical impulses carried using a wired connection, such as ordinary copper wires, a coaxial cable, or a conductive trace on a circuit board. Later, transmission was also carried wirelessly using microwave or satellite connections. 
     More recently, the alternative of transmitting data in the form of light pulses propagating through optical fibers has become increasingly prevalent, because of the vast increase in capacity afforded over what is possible either with either wired electrical connections or wireless satellite or microwave links. A single optical fiber, which may be thinner than a human hair, can carry far more data than a copper wire pair. In many circumstances, telecommunications providers already have physical right-of-way in the form of existing utility poles or underground conduit ducts. By replacing existing copper wires or cables in this right-of-way with optical fiber, demand for more bandwidth can be satisfied far more efficiently, and with less societal and environmental impact, than if new construction were required. 
     Optical fiber communication relies on the representation of binary digital data by a series of on/off light pulses. These pulses typically are generated by laser diodes (LDs) or light emitting diodes (LEDs) and injected into long fibers of glass or polymeric materials. The fibers are capable of propagating the light over extended distances with extremely low attenuation and dispersion, whereby information embodied in an on/off modulation pattern may be conveyed. The light pulses that emerge at the other end of the fiber can be detected and reconverted into electronic signals that reproduce the original electrical signal. Commonly, a single fiber is used for bidirectional communication, with the data transmitted in one direction represented by light pulses of one wavelength (color) and the data transmitted in the opposite direction represented by light pulses of a second wavelength. As used herein and in the subjoined claims, and in accordance with conventional parlance in the fiber optics art, the term “light” is employed for electromagnetic radiation that extends from the infrared to the ultraviolet, thus including both wavelengths perceptible to humans (about 380-750 nm) and wavelengths above and below the visible spectrum. For example, silica-fiber based systems frequently use wavelengths in the range of about 1.2-1.6 μm (1200-1600 nm), which are classified as near infra-red and are not visible to humans. Nevertheless, in the fiber-optic art, radiation at these wavelengths is still termed “light” and particular wavelengths are called “colors” by analogy. 
     Present-day optical fiber communication is often implemented in a telecommunications system in which a part of the network is generically called a passive optical network (PON) system. In the nomenclature of a typical telephony system, a central office or station may house some number of optical line termination stations (OLT) that provide an interface between electrical and optical signals. The OLTs are in communication on one side with data sources and on the other side are connected to an optical fiber that provides a bidirectional data path to end users. Each central office providing optical service must have at least one OLT, but usually there are a large number of OLTs. 
     At the other end, devices called optical network units (ONU) connect on one side to optical fiber from a central office OLT and on the other side through conductive wire to one or more customer devices, which can include telephones, computers, televisions, or the like. Normally, fiber from the central office is connected through a multiplexer/splitter to multiple ONUs. Each ONU is associated with a particular OLT, and at least one ONU is associated with each customer&#39;s location. The number of ONUs serviced by a given OLT depends on the amount of bandwidth each needs. The number of ONUs might typically be 32, but can also range up to about 256 or more in some circumstances. 
     The two directions of transmission are generally called “downstream” and “upstream,” and refer, respectively, to data going in the direction from the OLT to the ONU, and the reverse direction from the ONU to the OLT. The ONU includes the electronic devices needed to convert incoming optical signals to the electrical signals needed by the various customer devices. Likewise, the ONU receives electrical impulses from these devices and converts them to optical pulses for upstream transmission. Corresponding conversions between optical and electrical signals are performed by the OLT at the central office. 
     Various forms of PONs have evolved to meet the ever-increasing demand for higher bandwidth, i.e., the amount of information that can be communicated per unit time. In order to ensure continuing compatibility between systems, standardized protocols for PONs have been promulgated by governmental regulatory authorities and standards-setting bodies. Prominent standard-setting bodies include the Institute of Electrical and Electronics Engineers (IEEE) and the International Telecommunications Union (ITU). 
     One common high-speed PON data transfer protocol which can be implemented using the present system is the GPON protocol, which specifies bidirectional operation with a data rate of about 2.5 gigabits per second (Gbps) in continuous-mode (CNT) transmission in the downstream direction and 1.25 Gbps in burst-mode (BM) transmission in the upstream direction. Other PON protocols are also compatible with practice of the present invention. 
     A typical, generic fiber optic system of the prior art for telephony is shown in  FIG. 1 . A PON system, depicted generally at  10 , includes a plurality of OLTs  12   a - 12   d , each being associated with a group  14   a - 14   d  of one or more ONUs. For example, OLT  12   a  serves ONU group  14   a . Each ONU, in turn, is associated with one or more user devices (not shown) for which it handles upstream and downstream data transmission. It will be understood that while  FIG. 1  illustratively shows a PON with four OLTs, the actual number may vary, typically from 1 to 8 or more. Likewise, each OLT may serve a number of ONUs ranging typically from 8 to 128 or more. Optical fibers  16 , typically made of silica, connect each pair of nodes M and N, which may be separated by distances of up to about 20 km or more. Most commonly, the OLTs  12  and nodes M are all located in a single central office  8  of the telecommunications provider. At the user end, such as in an office building housing one or more office tenants, nodes N might all be located in one or more interior equipment cabinets or closets, with branches running to the various users. Individual nodes N might also be located in an exterior cabinet, which might be mounted on a utility pole, a ground-level pad, or in an underground vault, to serve end users in one or more buildings. 
     Signals from the OLT to the ONU in system  10  are carried through a fiber  16  as light pulses of a downstream wavelength, e.g. 1490 nm, while signals from the ONU to the OLT (upload or upstream direction), also carried through fiber  16 , are assigned a different upstream wavelength, e.g. 1310 nm. The OLTs and ONUs provide an interface between optical fiber and electrical signals. That is to say, they convert between the upstream and downstream optical signals and corresponding electrical signals needed to connect with devices such as computers, telephone instruments, televisions, and other such implements. Each node N includes a splitter, which connects the incoming fiber to a plurality of fibers extending to the ONUs of that node&#39;s group. The splitter divides the optical intensity in the downstream data among the various ONUs, so each receives all the data. Conversely, the splitter aggregates (or multiplexes) upstream traffic from the various ONUs and injects the aggregated optical signal into the fiber serving the node for upload to the specified OLT. A suitable networking protocol implemented using a media access control (MAC) system is employed to identify and maintain the integrity of both the upstream and downstream data associated with each OLT, ONU, and end-user devices, and to govern the requisite routing and processing of the data particular to each end user. Frequently, the identity and integrity of the data in such a system is established by including in the data being exchanged suitable headers, addressing information, and delimiters and providing control signals that govern the timing of data transmission by the various devices and sources. MAC systems having the requisite capability for carrying out these functions are conventionally used in the telecommunications art. 
     The typical distance of up to about 20-30 km between nodes M and N in  FIG. 1  arises from an interplay between the available optical power of feasible light sources, the attenuation of optical signals propagating through typical fibers, the amount of optical power available in each channel after division by the splitter, and the electronic sensitivity of typical optical receivers. 
       FIG. 2  depicts a graph showing the loss characteristic (dB/km loss) of conventional, single-mode silica fiber versus wavelength in the range of interest. It can be seen that some wavelengths are substantially more strongly attenuated than others. Various phenomena are believed to contribute to the losses, including Rayleigh scattering, which dominates at low wavelengths, and infrared absorption  22  by the fiber itself, which dominates at high wavelengths. Both these loss mechanisms vary relatively smoothly with wavelength. In addition, localized absorption in the fiber, caused by various impurities, including metals (peaks  24 ) and hydroxyl ions (peaks  26 ), produces absorption peaks centered over certain characteristic and relatively narrow wavelength ranges. These contributions together result in wavelength-dependent loss characteristic 28. Although fiber manufacturers have worked assiduously to make purer, more uniform fibers that somewhat reduce the absorption both in the impurity bands and overall, they cost much more. Moreover, it would be expensive and difficult to replace the vast amounts of older-generation fiber already in service, so that systems compatible with the installed base are particularly sought. 
     The widely used 1490 and 1310 nm base wavelengths are chosen because they are at approximate local minima in the absorption characteristic curve, while the 1550 nm wavelength is kept for optical video service. Light of the 1490, 1310, and 1550 nm wavelengths can transit 20-30 km of fiber without excessive attenuation or dispersion. 
     As a result of the continually increasing demand for high bandwidth digital data transmission, existing fiber installations are beginning to lack sufficient capacity to carry the desired amount of information. The problem is particularly acute in metropolitan areas, where installing new lines is especially difficult and expensive. Techniques that would increase the available bandwidth of existing fiber links are highly sought, in order to forestall or eliminate the need to install and maintain additional fiber connections. Especially desired are systems in which a single fiber could be used to connect multiple OLTs with multiple ONUs. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention provides a system and method in which at least part of the data is optically transmitted through a single optical fiber using a wavelength division multiplexing technique, with a plurality of signals being carried through the fiber in each direction, a different wavelength being used for each of the multiplexed upstream and downstream signals. The system in this aspect may comprise a plurality of “N” optical line termination stations (OLTs) having an OLT optical connection. (For convenient reference herein and in the subjoined claims, the OLTs, and other components and features of the system associated with a particular one of the OLTs are designatable by sequential numbers 1 to N.) The system also includes a plurality of ONUs, each ONU having an ONU optical connection and being appointed to be in communication with a predetermined one of the OLTs. Each user device is appointed to be in communication with a predetermined one of the ONUs. A media access control system, including a media access controller (MAC) for each OLT, is configured to identify and maintain the integrity of both the upstream and downstream data associated with each OLT, ONU, and end user device. N OLT-side wavelength converters have an OLT port and a multiplexer port; each is associated with a particular one of the OLTs. An OLT-side optical multiplexer provides a common port and at least N branching ports, with the multiplexer ports of the OLT-side wavelength converters being connected to respective ones of these branching ports. 
     N ONU-side wavelength converters have an ONU port and a demultiplexer port, with each being associated with a corresponding one of the OLT-side wavelength converters. An ONU-side optical demultiplexer has at least N branching ports and one common port, with the demultiplexer port of each ONU-side wavelength converter being connected to one of the branching ports. The common ports of the OLT-side optical multiplexer and the ONU-side optical demultiplexer are connected by a multiplex optical fiber. 
     For all values of a descriptor “i” ranging from 1 to N, a unique intermediate downstream wavelength λ iD  and a unique intermediate upstream wavelength λ iU  are assigned to the “i-th” of the OLT-side wavelength converters and the “i-th” of the ONU-side wavelength converters; and the “i-th” OLT is connected for data communications via its OLT optical connection to the OLT port of the “i-th” OLT-side wavelength converter. The “i-th” OLT is configured: (i) to be communicatively coupled to one of the data sources, from which downstream data for the end user devices is to be received and to which upstream data from the end user devices is to be transmitted; (ii) to transmit the downstream data to the ONU predetermined for the OLT; (iii) to receive the upstream stream data from the predetermined ONU, (iv) transmit the downstream data at a downstream base wavelength “λ BD ” and receive the upstream data at an upstream base wavelength “λ BU .” 
     Each of the ONUs is connected for data communications via its ONU optical connection to the ONU port of the ONU-side wavelength converter with which said ONU is associated. Each ONU is configured: (i) to be communicatively coupled to at least one of the end user devices, to which the downstream data is to be transmitted and from which the upstream data is to be received; (ii) to transmit said downstream data to the ONU predetermined for that OLT; (iii) to receive the upstream stream data from the predetermined ONU; and (iv) transmit the downstream data at a downstream base wavelength “λ BD ” and receive the upstream data at an upstream base wavelength “λ BU ” through an OLT optical connection; 
     Further provided is an improved method for bidirectional optical transmission of downstream and upstream digital data traffic between a plurality of “N” OLTs, designatable by sequential numbers from 1 to “N” and communicatively connected to a plurality of data sources, and an equal plurality of “N” ONU groups. Each ONU group is associated with a specific one of the OLTs and comprises at least one ONU, with each ONU being communicatively connected to at least one end user device. The data traffic comprises downstream data sets, each appointed to be transmitted from one of the data sources to a specific one of the end user devices and upstream data sets, each appointed to be transmitted from one of the end user devices to a specific one of the data sources. Each said downstream data set is optically transmitted from one of the OLTs and received by the ONU associated therewith as downstream light having a downstream base wavelength λ BD . Each upstream data set is optically transmitted from one of the ONUs and received by the OLT associated therewith as upstream light having an upstream base wavelength λ BU . 
     The improvement comprises: (i) converting the light at wavelength λ BD  transmitted by the “i-th” OLT to converted downstream light having a unique intermediate downstream wavelength λ iD ; (ii) multiplexing the converted downstream light into multiplexed downstream light comprising a plurality of downstream spectral components, each encompassing one of the intermediate downstream wavelengths λ iD ; (iii) transmitting the multiplexed downstream light through a multiplex optical fiber; (iv) demultiplexing the multiplexed downstream light transmitted through the multiplex optical fiber to separate the downstream spectral components; (v) reconverting light of each of the downstream spectral components back into reconverted light at the downstream base wavelength λ BD ; (vi) receiving light reconverted from the downstream spectral component encompassing wavelength λ iD  at the “i-th” ONU; (vii) converting the light at wavelength λ BU  transmitted by the “i-th” ONU to converted upstream light having a unique intermediate upstream wavelength λ iU ; (viii) multiplexing the converted upstream light into multiplexed upstream light comprising a plurality of upstream spectral components, each encompassing one of the intermediate upstream wavelengths λ iU ; (ix) transmitting the multiplexed upstream light through the multiplex optical fiber; (x) demultiplexing the multiplexed upstream light transmitted through the multiplex optical fiber to separate the upstream spectral components; (xi) reconverting light of each of the upstream spectral components back into reconverted light at the upstream base wavelength λ BU ; and (xii) receiving light reconverted from the upstream spectral component encompassing wavelength λ iU  at the “i-th” OLT. 
     Beneficially, the improved system can be retrofitted into existing telephony systems, still maintaining the existing OLTs and ONUs, and their respective connections to existing data sources and existing end user devices. The additional hardware and connection changes required can be located, respectively, at the central office near the existing OLTs and at the equipment cabinets or other like locations of the existing ONUs. 
     In some implementations of the present system and method, an auxiliary channel is also provided. This channel may be used for any requisite function, including data management and control or monitoring or surveilling the interconnecting optical fiber, which may be many km long. In many instances, the data rate needed for these functions is lower than that desired for the base data communications, so lower data rates may be used in both directions. As a result, the auxiliary wavelengths employed may include choices that would experience too much attenuation to sustain higher transfer rates. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be more fully understood and further advantages will become apparent when reference is had to the following detailed description of the various embodiments of the invention and the accompanying drawings, wherein like reference numerals denote similar elements throughout the several views, and in which: 
         FIG. 1  is a block schematic diagram showing a prior art PON system; 
         FIG. 2  is a graph showing the loss characteristic of conventional, single-mode silica optical fiber; 
         FIG. 3  is a block schematic diagram showing a PON system of the invention; 
         FIG. 4  is a block schematic diagram showing portions of a PON system of the invention; 
         FIGS. 5A and 5B  are block schematic diagrams of exemplary wavelength converter circuits used in OLT-side and ONU-side interfaces, respectively; 
         FIGS. 6A and 6B  are block schematic diagrams of exemplary wavelength converter circuits respectively used in the OLT-side and ONU-side interfaces of the present system as part of an optional auxiliary channel; and 
         FIG. 7  is a block schematic diagram showing a fuller, system-level depiction of an implementation of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One implementation of the present system is schematically depicted generally at  50  in  FIG. 3 . A single fiber  52  is used to connect an OLT-side interface  54  with an ONU-side interface  56 . These interfaces in turn connect to plural OLTs and ONUs. A conventional system ( FIG. 1 ) requires a separate fiber carrying both upstream and downstream base wavelengths (e.g. 1490 and 1310 nm) to connect each OLT to a specific ONU node that serves one or more particular ONUs. In contrast, the present system  50  preferably uses a single fiber  52  to convey data bidirectionally between multiple OLTs (e.g. OLTs  58   a - 58   d ) and a corresponding number of ONU nodes (e.g. the nodes serving ONU groups  60   a - 60   d ). Each ONU group may comprise one or more individual ONUs (e.g. ONUs  60   a   1  . . .  60   a   4  seen in  FIG. 3 ). The elimination of multiple fibers is made possible by multiplexed transmission of data at multiple upstream and downstream wavelengths, instead of just one pair of wavelengths. It will be understood that FIG.  3 &#39;s depiction of four OLTs and associated ONU groups is exemplary, and other numbers of OLTs and ONUs are also possible. The number of ONUs served from each ONU node may also vary. 
     Use of an arrangement like that of  FIG. 3  is especially beneficial in high density metropolitan areas. For example, a single large office building might house many separate businesses with hundreds or even thousands of employees. The telecommunications needs for these entities might involve many OLTs at a central office and many more ONUs in the building. Typically, a given OLT might service up to 128 or more ONU groups, each typically comprising 1 to 128 ONUs. In the conventional implementation of  FIG. 1 , each OLT  12  would require a single fiber  16  for the connection with its group of ONUs  14 . Substituting the present system permits up to 8 or more OLTs to share a single fiber, providing an immediate eight-fold increase in bandwidth without installing any new fibers, and while retaining both the existing OLTs and ONUs and their respective connections to data sources and end user devices. The change only requires installing certain new devices at the central office location of the OLTs and at the location of the various ONUs. While the present system may be employed beneficially in new construction, it is especially useful in retrofitting existing PONs, wherein the OLTs and ONUs are already installed and interfaced with data sources and end-use devices. These connections need not be disrupted. Instead, new hardware is only disposed between the existing OLTs and ONU nodes. 
     The configuration of an implementation of the present PON system  50  of  FIG. 3  is further elucidated by  FIG. 4 , which provides additional detail of the configuration within OLT-side interface X ( 54 ) and ONU-side interface Y ( 56 ). This version of the system is configured to be connected to eight OLTs, designated as  58   a - 58   h  (not shown in  FIG. 4 ). Each of the OLT-side wavelength converters A 1 -A 8  is associated with one of the connected OLTs. OLT-side interface X also includes an optional auxiliary channel converter AS, whose structure and function are discussed in more detail later. 
     On the ONU side, interface Y includes eight counterpart ONU-side wavelength converters B 1 -B 8  and optional auxiliary channel converter BS. Each of the ONU-side wavelength converters is, in turn, associated with the node serving one particular ONU group, as is apparent from  FIG. 3 . 
     A pair of unique intermediate wavelength pairs, one each for upstream and downstream, is assigned to each of the complementary OLT-side and ONU-side wavelength converters. In one possible implementation, these intermediate wavelengths are chosen in accordance with a coarse wavelength division multiplexing (CWDM) arrangement. Each of interfaces X and Y further includes multiplexer/demultiplexer (MUX/DEMUX) circuitry that connects on one side through branching ports to a plurality of fibers, each carrying data at one of the intermediate wavelength pairs and on the other side through a common port to a single fiber that links the interfaces and carries multiplexed data between them. 
       FIGS. 5A and 5B  show exemplary OLT-side and ONU-side wavelength converter circuits used in the interfaces X and Y of  FIG. 4 , respectively. These circuits convert the base wavelength (e.g., 1310/1490 nm) optical data conventionally used in both the ONU and OLT to and from a pair of unique, predetermined intermediate wavelengths that are multiplexed with light of other intermediate wavelengths for bidirectional transmission over a single fiber  52  between interfaces X and Y. 
     The OLT-side wavelength converter circuit  64  shown in  FIG. 5A  and designated A 1  is exemplary. Optical fiber  72  operably connects converter circuit  64  at its OLT port to an OLT optical connection of an OLT (e.g., OLT  58   a ) configured to process 1490 nm downstream continuous-mode (CNT) data and 1310 nm upstream burst-mode (BM) data. Circuit  64  is operable to convert λ BU =1310 nm/λ BD =1490 nm optical data borne by fiber  72  to optical data at λ 1U =1270 nm/λ 1D =1450 nm borne on fiber  74 . 
     Within circuit  64 , downstream 1490 nm CNT optical signals are coupled to a receiver circuit within ONU TRX circuit  66  that converts incoming optical data pulses to corresponding electrical pulses on one wire of differential pair  68 . A clock-data recovery (CDR) circuit  224 , as shown in  FIG. 7 , is optionally included to reshape the electrical pulses. The downstream electrical pulses are then reconverted to light within CWDM transceiver A 1  circuit  70 . This light, at a new wavelength (e.g., 1450 nm for converter A 1 ), is injected into fiber  74 , which is connected at the wavelength converter&#39;s multiplex port. In the upstream direction, incoming BM light (at 1270 nm for converter A 1 ) is received by BM Digital RX circuitry within transceiver  70  and converted to an electrical signal on the other wire of differential pair  68 . This electrical signal is reconverted to light at 1310 nm using a first-bit valid (FBV) BM TX circuit in ONU TRX  66 , and thereafter injected back into the OLT connection via fiber  72 . The FBV feature is preferred so that the integrity of a burst-mode data stream is fully maintained. 
     Further within OLT-side interface X ( 54 ), wavelength converter A 1  circuit  64  communicates bidirectionally with one side of MUX circuit  76  through optical fiber  74 , which connects at one of the MUX&#39;s branching ports and carries data at the 1450 and 1270 nm intermediate wavelengths. The other OLT-side wavelength converter circuits (A 2 -A 8 ) likewise communicate with MUX  76 , but with each operating with its own assigned, unique intermediate wavelength pair. MUX circuit  76  is operable to aggregate the downstream data traffic at the various intermediate downstream wavelengths and inject them as a multiplexed downstream optical signal through its common port into fiber  52 . MUX  76  is also operable to receive a multiplexed upstream optical signal on fiber  52 , which bears information from the ONUs at the various intermediate upstream wavelengths. MUX  76  demultiplexes this signal and routes the information conveyed at each wavelength to the appropriate one of optical fibers  74  for upstream processing by the appropriate one of wavelength converters A 1 -A 8 . One channel and branching port of MUX  76  may be used for the optional auxiliary channel described below. Suitable components for constructing MUX  76  are known in the art. 
     ONU-side interface Y ( 56 ) includes DEMUX circuit  92  and plural ONU-side wavelength converters, e.g. B 1 -B 8 . DEMUX  92  is a counterpart of MUX  76  and is operable in a complementary fashion. Multiplexed, downstream traffic carried at the plurality of downstream intermediate wavelengths is coupled from fiber  52  into DEMUX  92  at its common port. DEMUX  92  separates the colors and routes each to the appropriate wavelength converter through its branching ports. In the exemplary implementation shown, 1450 nm downstream light is sent to converter B 1  via fiber  82 , which converts it to light at the expected base wavelength of 1490 nm. DEMUX  92  also receives BM upstream data at the various upstream intermediate wavelengths from the various ONU-side wavelength converters  80 , and multiplexes them for injection into fiber  52 . Circuitry and a branching port of DEMUX  92  may be provided for the optional auxiliary channel. 
     The circuitry  80  of exemplary wavelength converter B 1  of  FIG. 5B  carries out functions complementary to those of wavelength converter A 1 , converting 1450/1270 nm data traffic back to 1490/1310 nm. Downstream CNT optical data in fiber  82 , e.g. at 1450 nm for converters A 1  and B 1 , is received at the wavelength converter&#39;s demultiplexer port and transformed by CNT RX circuitry in CWDM Transceiver B 1  of circuit  84  to electrical pulses on one wire of differential pair  86 . A CDR circuit  226  (see  FIG. 7 ) is optionally included to reform and re-time the downstream data pulses. These pulses drive CNT TX of OLT TRX circuit  88  to produce 1490 nm data injected into fiber  90  for delivery to the ONU through the wavelength converter&#39;s ONU port. Upstream BM data at 1310 nm, coming through fiber  90  from the ONU, is received by burst-mode receiver circuitry BM RX in circuit  88 . These data are converted to electrical pulses on the other wire of pair  86  connected to FBV BM TX circuitry in CWDM Transceiver B 1 , which converts them to optical pulses at the desired intermediate upstream wavelength, e.g. 1270 nm for converter B 1 , that are carried by fiber  82 . Use of FBV valid arrangement for the upstream burst mode is preferred in circuit  80  for the same reasons as in the OLT-side wavelength converter. 
     The remaining converters, designated as A 2  to A 8  on the OLT-side and B 2  to B 8  on the ONU-side, are similar in function and structure to A 1  and B 1 . They connect on one side to fiber carrying data at the same single pair of base wavelengths (λ BD , λ BU ), but function with different intermediate wavelengths drawn from other available pairs preselected within the CWDM arrangement [(λ 2D , λ 2U ) . . . (λ 8D , λ 8U )]. 
     A representative CWDM protocol useful in some implementations of the present system is defined by the ITU-T G.694.2 standard, which identifies channels having center wavelengths of 1270 to 1610 nm, spaced at 20 nm intervals. The ITU-T G.694.2 standard is incorporated herein in the entirety by reference thereto. In practice, not all the channels in the CWDM protocol are equally attractive and usable, because some of the wavelengths coincide with the absorption peaks in typical silica fiber ( FIG. 2 ). The present system is preferably implemented using intermediate wavelength pairs, one being selected for each pair from the group of 1270, 1290, 1310, 1330, 1350, 1370, 1390, 1410, and 1430 nm for upstream and one being selected from the group of 1450, 1470, 1490, 1510, 1530, 1550, 1570, 1590, and 1610 nm for downstream. More preferably, up to eight wavelength pairs are chosen, with 1390 nm and 1610 nm being excluded. The absorption at the remaining wavelengths is low enough to allow acceptable Gbps-rate transmission over the desired distance. Optionally, two of the wavelengths (e.g., 1390 nm and 1610 nm) are used for an auxiliary channel, which is discussed in greater detail below. Other selections of multiplexed intermediate wavelengths are also possible, generally limited only by restriction to wavelengths for which optical attenuation in the selected fiber is low enough for the required transmission distance and by maintaining a channel spacing compatible with the wavelength selectivity of optical components that are feasible for a given application. 
     For example, dense wavelength division multiplexing (DWDM) uses narrower spacing between channels, and thus could provide many more channels and higher net bandwidth. One representative definition of a DWDM protocol is provided by ITU-T standard G-694.1, which is incorporated herein in the entirety by reference thereto. However, the close channel spacing in DWDM necessitates use of much more expensive components to generate, detect, and demultiplex optical signals at precisely defined wavelengths. Such precision is needed to prevent cross-talk between channels. The tight spectral purity requirements inherent in the close channel spacings of DWDM render components for its implementation more expensive and difficult to use, since wavelength drift resulting from temperature variations must be carefully limited. Nevertheless, in some circumstances the substantial increase in bandwidth afforded by using more multiplexed wavelengths in the long-distance fiber connection outweighs the extra cost and complexity of the hardware needed to implement a more highly multiplexed protocol such as DWDM in the present system. 
     Whatever the intermediate wavelengths chosen, pairs of counterpart ONT and ONU side converters must be employed that are capable of converting the chosen intermediate wavelength pairs to and from the base wavelengths. As seen in  FIG. 4 , the present system also includes an optional auxiliary, bidirectional communications channel that operates in a manner similar to that of data channels 1-8. Wavelength converters AS and BS, respectively situated in OLT-side interface X and ONU-side interface Y and shown in  FIGS. 6A and 6B , effect conversion of data carried on a base optical wavelength pair (λ BD , λ BU ) to a pair of intermediate auxiliary wavelengths (λ SD , λ SU ), using techniques and circuitry comparable to those in converters A 1 -A 8  and B 1 -B 8 . However, the auxiliary channel is ordinarily and preferably implemented with continuous-mode transmission in both directions instead of the burst-mode preferably used for the upstream transmission in the data channels, with data rates in both directions lower than those used for data channels 1-8. For the converters of  FIGS. 6A and 6B , λ SD =1610 nm and λ SU =1390 nm. 
     The auxiliary channel may be used for any suitable purpose, such as surveillance and monitoring of the integrity of the optical fiber. In many instances, these functions do not require as much bandwidth as is desired for the data links. As a result, the intermediate auxiliary wavelengths may be chosen from wavelengths in which optical attenuation is too high to sustain Gbps-level data rates, such as those used in GPON. For example, it has been found that the attenuation of 1390 and 1610 nm wavelengths is too high for these to be used reliably at the 1.25/2.5 Gbps rates of GPON systems, but low enough that less demanding communication at a 100 Mbps rate is still feasible. This distinction is believed to arise from the greater sensitivity of receivers operable at 100 Mbps than at 1.25/2.5 Gbps. As a result, propagation of 100 Mbps data at 1390/1610 nm, though more strongly attenuated, can coexist with the transmission of 1.25/2.5 Gbps data at the other frequencies in the present CWDM implementation, as discussed above. 
     A system-level depiction of a GPON implementation of the present optical network is depicted generally at  200  in  FIG. 7 . 
     In a conventional GPON system, only the components of sections T and D of  FIG. 7  would be used. Section T comprises 8 standard OLT transceivers  58   a  . . .  58   i , each interfaced with a conventional media access controller (MAC)  220 . A burst-mode clock-data recovery (CDR) circuit  222  is used to recover the timing needed to process 1.25 Gbps data in the upstream channel. The requisite reset signal for CDR circuit  222  is provided from the BM RX in the OLT, and CDR  222  in turn resets MAC  220 . The OLTs  58  also include a CNT TX circuit operative at 2.5 Gbps for downstream transmission. On the ONU side of a conventional GPON system, a conventional ONU multiplexer/splitter  202 , as illustrated in section D, would be provided for each of the 8 channels shown. Each ONU multiplexer/splitter  202  includes one ONU splitter common port and plural ONU splitter branching ports. Individual optical fibers carrying data bidirectionally at base wavelengths λ BD  and λ BU  would directly connect each ONU multiplexer/splitter  202  with its corresponding OLT  58   a  . . .  58   i . Individual ONUs, e.g. ONU  60   a   1 , would be connected to one of the multiplexer/splitters  202  by an optical fiber through intervening ONU repeater  204 , which is optionally included if necessary to amplify and reform the optical signal. 
     In the full system of  FIG. 7 , intervening circuitry and components, such as those detailed above in  FIGS. 4-6 , substitute for the direct pair-wise fiber connection of OLTs and ONUs or ONU nodes in the conventional system. 
     On the OLT side of the  FIG. 7  system, OLT-side wavelength converters A 1 -A 8  (previously described and detailed in  FIG. 5A ) and optional converter AS ( FIG. 6A ) are shown within wavelength conversion block  206 . Each of data channel converters A 1 -A 8  is connected in a conventional manner by a relatively short, single-mode optical fiber to one of the OLTs  58   a  . . .  58   i . The two major circuits of each converter (the ONU (T) and OLT (A)) typically are separate components connected by a pair of conductive wires in differential mode, though an integrated construction is also possible. A clock-data recovery (CDR) circuit  224  is optionally placed in the downstream electrical connection between the downstream receiver  66  and the downstream transmitter of circuit  70 . Because of the minimal spacing between OLTs  58  and circuits  66 , the 1.25 Gbps λ BU  transmitter section of circuit  66  can operate at lower power and the 2.5 Gbps λ BD  receiver can have lower sensitivity than would be needed for a normal configuration an ONU is a long distance from the OLT with which it is in communication. The transmitter section also must be a first-bit valid type, since the upstream signal is in burst mode. The auxiliary channel converter AS operates with transmit and receive capabilities, both in CNT mode and at a lower 100 Mbps data rate. 
     On the ONU side, wavelength conversion block  208  includes ONU-side wavelength converters B 1 -B 8  and optional auxiliary channel converter BS. Just as in OLT-side conversion block  206 , the two major groups of circuits are the ONUs of section B and the OLTs of section C. Each ONU group is served by one of the ONU-side wavelength converters, typically using a conventional multiplexer/splitter  202  and optional ONU repeater  204 . For each ONU multiplexer/splitter  202 , the ONU splitter common port is connected to the ONU and each end user device is connected to one of the ONU splitter branching ports 
     Having thus described the invention in rather full detail, it will be understood that such detail need not be strictly adhered to, but that additional changes and modifications may suggest themselves to one skilled in the art, all falling within the scope of the invention as defined by the subjoined claims.