Abstract:
The invention pertains to optical fiber transmission systems, and is particularly relevant to transmission of high volume of data and voice traffic among different locations. In particular, the improvement teaches the use of a single optical transport system for both metropolitan area transport and long haul transport of data and voice traffic.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority to U.S. Provisional Patent Application Serial No. 60/377,085, entitled “OPTICAL TRANSPORT SYSTEM UTILIZING REMOTE TERMINAL CONNECTIVITY”, by Angela Chiu, filed Apr. 30, 2002. 
     
    
     
       TECHNICAL FIELD OF THE INVENTION  
         [0002]    The present invention relates, in general, to the field of optical communications, and in particular to, an optical transport system that uses distributed terminals. Characteristics of a distributed terminal architecture are described in co-pending U.S. patent application Ser. No. 10/402,840 entitled “Distributed Terminal Optical Transmission System” incorporated herein by reference. More specifically, this invention teaches the architecture to provide connectivity between remote terminals.  
         BACKGROUND OF THE INVENTION  
         [0003]    A goal of many modern long-haul optical transport systems is to provide for the efficient transmission of large volumes of voice traffic and data traffic over trans-continental distances at low costs. Various methods of achieving these goals include time-division multiplexing (TDM) and wavelength-division multiplexing (WDM). In time division multiplexed systems, data streams comprised of short pulses of light are interleaved in the time domain to achieve high spectral efficiency, high data rate transport. In wavelength division multiplexed systems, data streams comprised of short pulses of light of different carrier frequencies, or equivalently wavelength, co-propagate in the same fiber to achieve high spectral efficiency, high data rate transport.  
           [0004]    The transmission medium of these systems is typically optical fiber. In addition there is a transmitter and a receiver. The transmitter typically includes a semiconductor diode laser, and supporting electronics. The laser is often a DFB laser stabilized to a specified frequency on the ITU frequency grid. The laser may be directly modulated with a data train with an advantage of low cost, and a disadvantage of low reach and capacity performance. In many long-haul systems, the laser is externally modulated using a modulator. A single stage modulator is sufficient for a non-return-zero (NRZ) modulation format. A two-stage modulator is typically used with the higher performance return-to-zero (RZ) modulation format. An example of a modulator technology is the Mach-Zehnder lithium niobate modulator. Alternatively, an electro-absorptive modulator may be used. After binary modulation, a high bit may be transmitted as an optical signal level with more power than the optical signal level in a low bit. Often, the optical signal level in a low bit is engineered to be equal to, or approximately equal to zero. In addition to binary modulation, the data can be transmitted with multiple levels, although in current optical transport systems, a two-level binary modulation scheme is predominantly employed. The receiver is located at the opposite end of the optical fiber, from the transmitter. The receiver is typically comprised of a semiconductor photodetector and accompanying electronics.  
           [0005]    Typical long-haul optical transport dense wavelength division multiplexed (DWDM) systems transmit 40 to 80 channels at 10 Gbps (gigabit per second) across distances of 3000 to 6000 km in a single 35-nm spectral band. In a duplex system, traffic is both transmitted and received between parties at opposite end of the link. In a DWDM system, different channels operating at distinct carrier frequencies are multiplexed using a multiplexer. Such multiplexers may be implemented using arrayed waveguide grating (AWG) technology or thin-film technology, or a variety of other technologies. After multiplexing, the optical signals are coupled into the transport fiber for transmission to the receiving end of the link. The total link distance may, in today&#39;s optical transport systems, be two different cities separated by continental distances, from 1000 km to 6000 km, for example. To successfully bridge these distances with sufficient optical signal power relative to noise, the signal is periodically amplified using an in-line optical amplifier. Typical span distances between optical amplifiers are 50-100 km. Thus, for example, 30 100-km spans would be used to transmit optical signals between points 3000 km apart. Examples of in-line optical amplifiers include erbium doped fiber amplifiers (EDFAs) and semiconductor optical amplifiers (SOAs).  
           [0006]    At the receiving end of the link, the optical channels are demultiplexed using a demultiplexer. Such demultiplexers may be implemented using AWG technology or thin-film technology, or a variety of other technologies. Each channel is then optically coupled to separate optical receivers.  
           [0007]    Other common variations include the presence of post-amplifiers and pre-amplifiers just before and after the multiplexer and de-multiplexer. Often, there is also included dispersion compensation with the in-line amplifiers. These dispersion compensators adjust the phase information of the optical pulses in order to compensate for the chromatic dispersion in the optical fiber while appreciating the role of optical nonlinearities in the optical fiber. Another variation that may be employed is the optical dropping and adding of channels at cities located in between the two end cities. The invention disclosed applies in any of these variations, as well as others.  
           [0008]    Traditionally, optical transport systems are either long haul systems, for traffic between distant cities, or metropolitan (“metro”) systems for traffic in and around a city. Typically the terminals of a long-haul optical transport system are located in one location such as a central office, and all the channels in a DWDM system are terminated. The traffic is then sorted by electronic identification of data and routed to different parts of the metropolitan area using metropolitan optical transport systems. In many practical circumstances, there is a space, power and cost inefficiency in terminating the long haul signal and retransmitting over a second metro-system. For this reason, the concept of a distributed terminal architecture was invented, and is disclosed in co-pending U.S. patent application Ser. No. 10/402,840, hereafter referred to as Jaggi.  
           [0009]    As taught by Jaggi, there was no provision for duplex traffic between distributed terminals in the same metropolitan area. It would be highly desirable for a terminal in one section of a city to exchange traffic with a second terminal in a second section of the city while also providing scalable communication with cities a great distance away.  
         SUMMARY OF THE INVENTION  
         [0010]    In the present invention, improvements to an optical transport system with a distributed terminal architecture are disclosed. More specifically, this invention teaches the architecture to provide scalable duplex connectivity between multiple terminals and remote terminals.  
           [0011]    In one embodiment of the invention, an overlay for connections in a distributed terminal architecture is taught.  
           [0012]    In another embodiment of the invention, an architecture to provide scalable duplex connectivity between multiple terminals at a terminal city overlay is taught.  
           [0013]    In another embodiment of the invention, an architecture to provide scalable duplex connectivity between terminals at optical-add-drop multiplexed (OADM) sites is taught. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which:  
         [0015]    [0015]FIG. 1 is a schematic illustration of a scalable multiplexed optical transport system.  
         [0016]    [0016]FIG. 2 is a schematic illustration of a scalable multiplexed optical transport system with a distributed terminal architecture having connectivity between remote terminals.  
         [0017]    [0017]FIG. 3 is a schematic illustration of a scalable optical transport system a distributed terminal architecture having connectivity between remote terminals at a terminal city in accordance with a preferred embodiment.  
         [0018]    [0018]FIG. 4 is a schematic illustration of a scalable optical transport system with a distributed terminal architecture having connectivity among remote terminals at an intermediate optical add-drop multiplexed (OADM) city in accordance with a preferred embodiment.  
         [0019]    [0019]FIG. 5 is a flow chart of the method of combining short haul traffic with long haul traffic in accordance with this invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments described herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention.  
         [0021]    In FIG. 1 is shown a block diagram of an optical transport system with a distributed terminal architecture as taught by Jaggi. The distributed terminal architecture of one preferred embodiment comprises master terminal  110 , terminal  112  and remote terminals  114  and  116 . A specific advantage of the present invention is scalability that allows additional terminals and remote terminals to be added to the architecture. In FIG. 1, master terminal  110  and terminal  112  comprise terminals separated by long haul distances. In a preferred embodiment, a plurality of spans  132  and in-line amplifiers  130  will enable total link distances that are measured in thousands of kilometers. As an example, master terminal  110  may be located in one metropolitan area, while terminal  112  may be located in a second metropolitan area located 6000 km away. Terminal  112  may function as a remote terminal where it is located. In addition to terminal  112 , there is also remote terminal  114  and second remote terminal  116  located in the second metropolitan area. In this example, terminal  112 , remote terminal  114  and remote terminal  116  comprise distributed terminals in the second metropolitan area. In a preferred embodiment, the fiber link pair  124  between terminal  112  and remote terminal  114  may be a distance of 50 km. In the preferred embodiment, the fiber link pair  126  between terminal  112  and remote terminal  116  may also be 50 km in length. In operation, duplex communication will occur between master terminal  110  and any of terminal  112 , remote terminal  114  or remote terminal  116 . In a preferred embodiment, one set wavelengths in a spectral band from master terminal  110  terminate in terminal  112 , a second set of wavelengths in a spectral band from master terminal  110  terminate in remote terminal  114  and a third set of wavelengths in a spectral band from master terminal  110  terminate in remote terminal  116 . In a preferred embodiment, the spectral band is the L-band, which extends from approximately 1565 nm to 1605 nm.  
         [0022]    It should be noted that master terminal  110  may also be replaced with a distributed architecture in the first metropolitan area.  
         [0023]    [0023]FIG. 1 depicts an optical transport system supporting duplex operation wherein each endpoint can both send and receive voice and data traffic. This is important to achieve a typical conversation. In FIG. 1, duplex operation is shown to use two distinct fibers, the both together often referred to as a fiber pair. For example, optical transport systems are sometimes deployed with bidirectional traffic providing duplex service on a single fiber.  
         [0024]    In FIG. 2 is shown a schematic illustration of a multiplexed optical transport system with a distributed terminal architecture having duplex connectivity  225  between terminal  112  and remote terminals  114  and  116 . The ellipses below remote terminal  116  indicate that any number of remote terminals can be accommodated. In a preferred embodiment, duplex connectivity  225  is a very high data rate optical link enabled by wavelengths not used in duplex communication with master terminal  110 . For example, if duplex communication with master terminal  110  uses optical signals in the L-band, then duplex connectivity between terminal  112  and remote terminal  114  may use signals in the C-band.  
         [0025]    In FIG. 3 is a block diagram of an optical transport system with a distributed terminal architecture having connectivity between remote terminals at a terminal city in accordance with a preferred embodiment. In particular FIG. 3 shows multiplexing and de-multiplexing arrangements in terminal  112 , remote terminal  114  and remote terminal  116  to enable duplex connectivity  225 . Shown also is fiber link pair  124  and fiber link pair  126 . The arrangement is shown relative to long haul fiber pair  132 .  
         [0026]    The arrangement comprises multiplexers  310 ,  312 ,  314 ,  316 ,  318 ,  350  and  351  as shown in FIG. 3. These multiplexers combine individual wavelengths or channels into bands of wavelengths or channels. Each multiplexer can be a n×1 multiplexer to accommodate differing requirements. In addition, the arrangement comprises de-multiplexers  311 ,  313 ,  315 ,  317 ,  319 ,  352  and  353 . These de-multiplexers subdivide a band of wavelengths, or channels, into particular wavelengths or channels. Examples of multiplexing and de-multiplexing technologies include thin film filters, array waveguides and interleavers, and combinations thereof.  
         [0027]    The arrangement further comprises wavelength selective couplers,  320 ,  322 ,  324 ,  326 ,  354  and  357  and wavelength selective de-couplers  321 ,  323 ,  325 ,  327 ,  355  and  356 . In a preferred embodiment, wavelength selective couplers may be C/L band couplers, which act to couple together C-band signals from one input port and L-band signals from a second input port, and combine them onto a single output port. One technology known in the art for this C/L band coupler is thin film filter technology. In a preferred embodiment, wavelength selective de-couplers may be C/L band de-couplers, which act to de-couple C-band signals and L-band signals from a single input port into C-band signals on a first output port and L-band signals on a second output port. One technology known in the art for this C/L band de-coupler is thin film filter technology. It is noted that a C/L band coupler using thin film filter technology may be used as a C/L band de-coupler by reversing the input and output designations on the ports.  
         [0028]    The arrangement further comprises optical coupler  340 , and optical de-coupler  341 . In a preferred embodiment, optical coupler  340  and optical de-coupler  341  may be splitters and combiners, in particular a 1×4 splitter and a 1×4 combiner. The ellipsis at  340  and  341  indicate that, in general, optical coupler  340  and optical  341  can be 1×n. A 1×n coupler allows for the invention to be easily scalable by adding additional signals from other remote terminals cheaply and effectively. In another preferred embodiment, AWG technology may be used to implement optical coupler  340  and optical de-coupler  341 . In this manner cyclic routing capability is provided. In particular, 4 port AWGs may be used for optical coupler  340  and optical de-coupler  341 . Shown in FIG. 3 is a unidirectional optical amplifier  345  to provide gain to the combined short haul signals. The use of a unidirectional optical amplifier further enhances the scalability of the invention by allowing multiple signals to be amplified without additional equipment or connections. Dispersion compensation may be included as part of the unidirectional optical amplifier to add additional capability as additional remote terminals are added.  
         [0029]    In another preferred embodiment wavelength selective de-coupler  321  and wavelength selective coupler  320  may be implemented via a splitter or combiner, in particular, a 1×4 splitter/combiner. Similarly, wavelength selective de-coupler  323  and wavelength selective coupler  322  may be implemented via a splitter or combiner, in particular, a 1×4 splitter/combiner. In general a 1×n splitter or combiner may be used. In this embodiment, optical coupler  340  may be implemented as a spectral band coupler and optical de-coupler  341  may be implemented as a spectral band de-coupler.  
         [0030]    The flow of signals through this arrangement may now be understood. Long haul traffic enters and departs the metropolitan area via fiber span  132 . Entering traffic is de-multiplexed in de-multiplexer  311 . The group of channels to be routed to remote terminal  114  proceeds to wavelength selective coupler  320 . At remote terminal  114 , the group of channels proceeds through wavelength selective de-coupler  325 , and are separated into particular channels via de-multiplexer  313 . The group of channels to be routed to remote terminal  116  proceeds from de-multiplexer  311  to wavelength selective coupler  322 . At remote terminal  116 , the group of channels proceeds through wavelength selective de-coupler  327 , and are separated into particular channels via de-multiplexer  317 . The group of channels to be routed to terminal  112  proceeds from de-multiplexer  311  to selective coupler  357 . The group of channels proceeds then through wavelength selective decoupler  355  and are separated into particular channels via demultiplexer  352 .  
         [0031]    Duplex communication between remote terminal  114  and master terminal  110  is enabled through a signal flow via multiplexer  312 , wavelength selective coupler  324 , wavelength selective de-coupler  321 , and multiplexer  310 . Duplex communication between remote terminal  116  and master terminal  110  is enabled through a signal flow via multiplexer  316 , wavelength selective coupler  326 , wavelength selective de-coupler  323 , and multiplexer  310 . Duplex communication between terminal  112  and master terminal  110  is enabled through a signal flow via multiplexer  350 , wavelength selective coupler  354 , wavelength selective decoupler  356  and multiplexer  310 .  
         [0032]    Duplex connectivity between remote terminals is now described through this arrangement. Signal flow from remote terminal  114  to remote terminal  116  proceeds via terminal  112  through multiplexer  314 , wavelength selective coupler  324 , wavelength selective de-coupler  321 , into optical coupler  340 , through unidirectional optical amplifier  345 , and into optical de-coupler  341  and on to wavelength selective coupler  322 . The desired path for signals continues through terminal  112  to remote terminal  116 , proceeds via wavelength selective coupler  322 , wavelength selective de-coupler  327 , and through de-multiplexer  319 . Depending on the implementation of optical de-coupler  341  there may also be a return path of signals from remote terminal  114 , back to remote terminal  114 . This return path proceeds via wavelength selective coupler  320 , and wavelength selective de-coupler  325 . If necessary, these signals are blocked in de-multiplexer  315 . Signal flow from remote terminal  116  to remote terminal  114  proceeds through multiplexer  318 , wavelength selective coupler  326 , wavelength selective de-coupler  323 , into optical coupler  340 , through unidirectional optical amplifier  345 , and into optical de-coupler  341  and on to wavelength selective coupler  320 . The desired path for signals to remote terminal  114  then proceeds via wavelength selective coupler  320 , wavelength selective de-coupler  325 , and through de-multiplexer  315 . Depending on the implementation of optical de-coupler  341  there may also be a return path of signals from remote terminal  116 , back to remote terminal  116 . This return path proceeds via wavelength selective coupler  322 , and wavelength selective de-coupler  327 . If necessary, these signals are blocked in de-multiplexer  319 . Duplex connectivity from terminal  112  to remote terminal  114  and from remote terminal  114  to terminal  112 , and from terminal  112  to remote terminal  116  and from remote terminal  116  to terminal  112  is provided in a similar matter. Also, similarly, there may be a return path of signals from terminal  112  back to terminal  112 . The invention provides scalability easily with the addition of optical coupler  340 , unidirectional amplifier  345  and optical decoupler  341  because additional remote terminals may be added without the need for duplicate amplification.  
         [0033]    Additionally, connectivity to other remote terminals can be added in a similar manner. The ellipses near couplers  340  and  341 , and de-multiplexer  311  and multiplexer  310 , show where additional connections to these terminals may be made.  
         [0034]    [0034]FIG. 4 is a schematic illustration of an optical transport system with a distributed terminal architecture with connectivity among remote terminals at an intermediate optical add-drop multiplexed (OADM) city in accordance with a preferred embodiment. The arrangement is shown relative to long haul fiber pair  132 , and in particular, at an optical add-drop multiplexing (OADM) site which deploys optical coupler  401  and optical de-coupler  402 . In a preferred embodiment, optical coupler  401  and optical de-coupler  402  are 50:50 or 3 dB splitters, and the OADM is configured in a broadcast and select mode.  
         [0035]    The architecture of the present invention comprises distributed terminals  403 ,  404  and  405 , and enables duplex connectivity among all distributed terminals, or between any two pairs of distributed terminals. Any or all of distributed terminals  403 ,  404  or  405  may also be remote terminals placed apart from the OADM site, potentially at different locations within a metropolitan area. In a preferred embodiment, short haul fiber pairs  406 ,  407  and  408  may be approximately 50 km from the OADM site. In will be understood by one skilled in the art, that the distances of short haul fiber pairs  406 ,  407  and  408  may be unequal, shorter, and, with appropriate optical amplification and dispersion compensation, much longer than 50 km from the OADM site.  
         [0036]    The arrangement further comprises wavelength selective coupler  410  and wavelength selective de-coupler  411 . In a preferred embodiment, wavelength selective coupler  410  may be C/L band couplers, which act to couple together C-band signals from one input port and L-band signals from a second input port, and combine them onto a single output port. One technology known in the art for this C/L band coupler is thin film filter technology. In a preferred embodiment, wavelength selective de-coupler  411  may be C/L band de-couplers, which act to de-couple C-band signals and L-band signals from a single input port into C-band signals on a first output port and L-band signals on a second output port. One technology known in the art for this C/L band de-coupler is thin film filter technology. It is noted that a C/L band coupler using thin film filter technology may be used as a C/L band de-coupler by reversing the input and output designations on the ports. The arrangement may also comprise optical amplifier  415 . As is well known in the art, this optical amplifier may be an erbium doped optical amplifier, or a semiconductor optical amplifier.  
         [0037]    The arrangement further comprises optical coupler  416  and optical de-coupler  417 . In a preferred embodiment, optical coupler  416  may be a 1×N combiner, and optical de-coupler  417  may be a 1×N splitter. The ellipses indicate that additional remote terminals may be included in other embodiments.  
         [0038]    The arrangement further comprises wavelength selective couplers,  420 ,  422  and  424 , and wavelength selective de-couplers  421 ,  423 , and  425 . In a preferred embodiment, wavelength selective couplers may be C/L band couplers, which act to couple together C-band signals from one input port and L-band signals from a second input port, and combine them onto a single output port. One technology known in the art for this C/L band coupler is thin film filter technology. In a preferred embodiment, wavelength selective de-couplers may be C/L band decouplers, which act to de-couple C-band signals and L-band signals from a single input port into C-band signals on a first output port and L-band signals on a second output port. One technology known in the art for this C/L band de-coupler is thin film filter technology. It is noted that a C/L band coupler using thin film filter technology may be used as a C/L band de-coupler by reversing the input and output designations on the ports.  
         [0039]    The arrangement comprises multiplexers  430 ,  432 ,  434 ,  436 ,  438  and  440 . These multiplexers combine individual wavelengths or channels into bands of wavelengths or channels. In addition, the arrangement comprises de-multiplexers  431 ,  433 ,  435 ,  437 ,  439  and  441 . These de-multiplexers subdivide a band of wavelengths, or channels, into particular wavelengths or channels. Examples of multiplexing and de-multiplexing technologies include thin-film filters, AWGs and inter-leavers, and combinations thereof.  
         [0040]    The flow of signals through this arrangement may now be understood. Long haul traffic enters and departs the OADM via fiber span  132 . Entering traffic is split using optical de-coupler  402  and propagates through wavelength selective optical coupler  410 . Optical de-coupler  417  broadcasts the entering traffic to remote terminals  403 ,  404  and  405 . At remote terminals  403 ,  404  and  405 , the entering traffic proceeds through wavelength selective de-coupler  421 ,  423  and  425 , and is separated into particular channels via de-multiplexers  431 ,  435  and  439 .  
         [0041]    Traffic from distributed terminal  403  intended for transmission on fiber span  132  proceeds from multiplexer  430  to wavelength selective optical coupler  420  and optical coupler  416 . The signal proceeds to wavelength selective decoupler  411  to optical coupler  401  onto fiber span  132 . Traffic from distributed terminal  404  intended for transmission on fiber span  132  proceeds from multiplexer  434  to wavelength selective optical coupler  422  and optical coupler  416 . The signal proceeds to wavelength selective decoupler  411  to optical coupler  401  onto fiber span  132 . Traffic from distributed terminal  405  intended for transmission on fiber span  132  proceeds from multiplexer  438  to wavelength selective optical coupler  424  and optical coupler  416 . The signal proceeds to wavelength selective decoupler  411  to optical coupler  401  onto fiber span  132 .  
         [0042]    Connectivity among the distributed terminals is now described through this arrangement. Signals destined for remote terminals  404  and  405  that originates from remote terminal  403  proceeds via multiplexer  432 , wavelength selective optical coupler  420 , optical coupler  416 . From wavelength selective optical coupler  420  until wavelength selective optical de-coupler  411 , long haul traffic and short haul traffic propagates together. Wavelength selective optical de-coupler  411  decouples the long haul traffic from the short haul traffic. The short haul signal may proceed through optical amplifier  415 , and then into wavelength selective optical coupler  410  and optical de-coupler  417 . Optical de-coupler  417  routes the traffic to remote terminals  404  and  405 . Depending on the implementation, there may also be a return path to remote terminal  403 . Such traffic is blocked or otherwise sorted via de-multiplexer  433 . In remote terminal  404 , the traffic is routed via wavelength selective optical de-coupler  423  and optical de-multiplexer  437 . In remote terminal  405 , the traffic is routed via wavelength selective optical de-coupler  425  and optical de-multiplexer  441 .  
         [0043]    Signals destined for remote terminals  403  and  405  that originate from remote terminal  404  proceed via multiplexer  436 , wavelength selective optical coupler  422 , optical coupler  416 . From wavelength selective optical coupler  422  until wavelength selective optical de-coupler  411 , long haul traffic and short haul traffic propagates together. Wavelength selective optical de-coupler  411  decouples the long haul traffic from the short haul traffic. The short haul signal may proceed through optical amplifier  415 , and then into wavelength selective optical coupler  410 . Optical de-coupler  417  routes the traffic to remote terminals  403  and  405 . Depending on the implementation, there may also be a return path to remote terminal  404 . Such traffic is blocked or otherwise sorted via de-multiplexer  437 . In remote terminal  403 , the traffic is routed via wavelength selective optical de-coupler  421  and optical de-multiplexer  433 . In remote terminal  405 , the traffic is routed via wavelength selective optical de-coupler  425  and optical de-multiplexer  441 .  
         [0044]    Signals destined for distributed terminals  403  and  404  that originate from remote terminal  405  proceed via multiplexer  440 , wavelength selective optical coupler  424 , optical coupler  416 . From wavelength selective optical coupler  424  until wavelength selective optical de-coupler  411 , long haul traffic and short haul traffic propagates together. Wavelength selective optical de-coupler  411  decouples the long haul traffic from the short haul traffic. The short haul signal may proceed through optical amplifier  415 , and then into wavelength selective optical coupler  410 . Optical de-coupler  417  routes the traffic to remote terminals  403  and  404 . Depending on the implementation, there may also be a return path to remote terminal  405 . Such traffic is blocked or otherwise sorted via de-multiplexer  441 . In remote terminal  403 , the traffic is routed via wavelength selective optical de-coupler  421  and optical de-multiplexer  433 . In remote terminal  404 , the traffic is routed via wavelength selective optical de-coupler  423  and optical de-multiplexer  437 .  
         [0045]    Additional distributed terminals may be connected and traffic between terminals will flow in a similar manner to the above descriptions for terminals  403 ,  404  and  405 . The ellipses in FIG. 4 indicate additional distributed terminals and additional ports of coupler  416  and decoupler  417 .  
         [0046]    In FIG. 5 is shown a flow chart of the method of combining short haul traffic with long haul traffic in order to provide connectivity between distributed terminals which is a subject of this invention. In step  510 , short haul traffic is generated on a first spectral band. In a preferred embodiment, this first spectral band is the C-band. In step  512 , long haul traffic is generated on a second spectral band. In a preferred embodiment, this first spectral band is the L-band. In step  514 , the first spectral band and second spectral band are over-layed. In a preferred embodiment this step is accomplished using a wavelength selective optical coupler. A wavelength selective optical coupler may be a C/L band coupler. A thin film filter may be used to realize a C/L band coupler. In step  516  the combined traffic is propagated along a metropolitan fiber span. In step  518 , the short haul traffic is separated from the long haul traffic. In a preferred embodiment this step is accomplished using a wavelength selective optical de-coupler. A wavelength selective optical de-coupler may be a C/L band de-coupler. A thin film filter may be used to realize a C/L band de-coupler. At step  518 , the short haul and long haul traffic is also split into two directions. Long haul traffic is multiplexed at step  520 , followed by transmission on long haul optical fiber  522 . Short haul traffic is combined with other short haul traffic from other terminals in step  524 . It is amplified in unidirectional amplifier at  526  and then is separated into specific short haul traffic at step  528 . When separated, the short haul traffic is distributed at step  530 . In a preferred embodiment, this method provides half-duplex connectivity between two distributed terminals, and may be repeated in the opposite traffic flow direction to achieve duplex connectivity between the two distributed terminals.  
         [0047]    In an alternate embodiment, this method may be used to provide connectivity between a distributed terminal and a central location such as a master terminal or an OADM site. Additional routing from the central location is employed to further propagate the short haul traffic to a second distributed terminal. In a preferred embodiment, this additional routing may be achieved using an optical de-coupler. An optical splitter may be used to realize the optical de-coupler. In a preferred embodiment, this method provides half-duplex connectivity between two distributed terminals, and may be repeated in the opposite traffic flow direction to achieve duplex connectivity between the two distributed terminals.  
         [0048]    While this invention has been described in reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.