Abstract:
A method and apparatus for low cost upgrading on demand of an optical fiber communication system without installing additional optical fiber and minimal installation of optical circuitry at destination and distribution terminals. The upgraded systems comprise an optical data loop of a plurality of destination terminals and a single intermediate terminal.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to methods and apparatus associated with broadband communications using optical fibers as the transmission media, and more specifically to methods and apparatus for on-demand upgrading of an existing optical network system with the capacity to service additional subscribers with broadband digital service with no installation of additional optical fibers and minimal replacement of existing infrastructure.  
           [0003]    2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98  
           [0004]    The telecommunications industry is using more and more optical or light fibers in lieu of copper wire. Optical fibers have an extremely high bandwidth thereby allowing the transmission of significantly more information than can be carried by a copper wire. The carried information includes broadband digital data carrying digital television signals, computer data, etc.  
           [0005]    Of course, modern telephone systems require bidirectional communications where each station on a communication channel can both transmit and receive. This is true, of course, whether the system uses electrical wiring or optical fibers as the transmission medium, and whether the information is simple analog voice or broadband digital signals. Early telephone communication systems solved this need by simply providing separate copper wires for carrying the communications in each direction. Some early attempts at using optical fibers as a transmission medium followed this example and also used two different optical fibers such as optical fibers  10  and  10 A in the prior art FIG. 1 for carrying the communications in each direction. As shown, in the prior art FIG. 1, fiber  10  is connected by an optical coupler  12  to an LED (light-emitting diode)  14  at one end and by optical coupler  16  to a PD (photodetection diode)  18  at the other end. Similarly, but in reverse, fiber  10 A is connected by an optical coupler  16 A to PD  18  at one end and by optical coupler  12 A to LED  14  at the other end.  
           [0006]    However, because of the extremely high bandwidths capable of being transmitted by an optical fiber, a single fiber is quite capable of carrying communications in both directions. One technique is WDM (wavelength divisional multiplexing) which is shown in the prior art FIG. 2 and uses different wavelengths for each direction of travel. Components in FIG. 2 and subsequent figures which operate the same as shown in FIG. 1 carry the same reference numbers. In the embodiment shown in FIG. 2, a central office  20  is connected to an immediate or RT (remote terminal)  22  by at least one pair of optical fibers  10 B. The remote terminal  22  may be further connected to a multiplicity of destination terminals by other pairs of optical fibers. As shown, the central office includes a light-emitting diode  14  optically connected to fiber optics  10  by optical coupler  12  for converting electrical signals to optical signals and a photodetection diode  18 A optically connected to optical fiber  10 A by a coupler  16 A for converting optical signals to electrical signals. The fiber optics  10  and fiber optics  10 A are each connected to a wavelength division multiplexer  24  which in turn is connected by optical coupler  26  to optical fiber  10 B. This arrangement is duplicated at the RDT  22  by light-emitting diode  14 A, photodetection diode  18 , and wavelength division multiplexer  24 A. It will, of course, be appreciated that although the figure is shown as providing communications between a central office  20  (station 1) and a remote terminal office  22  (station 2) prior to being further distributed to a multiplicity of destinations, the communications system could be used for providing communications between any two types of stations, examples include communication between two central offices, two remote terminal offices, or between a remote office and an individual user&#39;s location, etc. A typical communications system using an LED (light-emitting diode) and a PD (photodiode) with a single optical fiber is disclosed in U.S. Pat. No. 5,075,791 entitled “Method and Apparatus for Achieving Two-Way Long-Range Communication Over an Optical Fiber”, issued to Mark W. Hastings, and incorporated in its entirety hereby by reference.  
           [0007]    Yet another technique for using a single optical fiber  10  for telephone systems is illustrated in the prior art FIG. 3. The illustrated figure is referred to as TCM (time compression multiplexing). The system operates at a single frequency and uses a single optical fiber  10  and a single diode  30  and  30 A at each end connected by optical couplers  32  and  32 A, respectively, for both converting electrical signals to optical signals and for receiving optical signals and converting those optical signals to electrical signals. TCM systems have the obvious advantage of requiring fewer components.  
           [0008]    Still other and more advanced systems carry telephony communication (either analog or digital) at one wavelength of light and television signals (digital and/or analog) at another wavelength.  
           [0009]    However, as mentioned above, optical fibers have extremely high bandwidths and use of an optical fiber for any of the above-mentioned existing systems is a very ineffective use of the fiber and, in fact, the available bandwidth of an optical fiber makes it possible to use both active and passive optical transmission techniques which can be used to carry a significantly-increased number of individual bidirectional broadband communication channels or signals.  
           [0010]    Of course, where early types of optical transmission systems have been installed, it is desirable to limit the time the operation of such systems is disrupted. Further, once an early type fiber-optic telephone system is installed, wholesale removal and replacement with a new system would normally be prohibitive from a cost point of view. Therefore, it would be advantageous to be able to upgrade on a demand basis an existing fiber-optic system to also carry a significantly increased number of broadband communication channels.  
         SUMMARY OF THE INVENTION  
         [0011]    It is an object of this invention to provide methods and apparatus for upgrading an optical communication transmission system so that it can carry a significantly-increased number of broadband bidirectional channels.  
           [0012]    It is another object of the invention to provide a method and apparatus to upgrade an existing optical communication transmission system without extensive installation of new optical fibers.  
           [0013]    It is still another object of the invention to provide methods and apparatus to upgrade a communication transmission system with minimal addition of new components.  
           [0014]    It is yet another object of the invention to allow upgrading of a optical fiber communication transmission system to occur on an on-demand-basis.  
           [0015]    The present invention accomplishes these and other objects in an existing optical fiber communication system which carries information between a multiplicity of destination terminals through a second location such as a remote or intermediate terminal to a primary terminal such as a central office. The optical fiber communication system includes a multiplicity of optical fiber pairs each of which has one end which terminates one each at the multiplicity of destination terminals and terminates at the other end at an intermediate terminal or distribution apparatus. Selected ones of the multiplicity of destination terminals include an OIU (optical interface unit) for extracting and injecting data with respect to light waves that are received at an input of the optical coupler OIU which is connected to one of the fibers of the pair associated with that terminal. Light waves receiving new data at the OIU are then connected to the other fiber of the pair by an output optical coupler. The communication system also includes at least a pair of optical fibers extending between a primary terminal or location and the intermediate terminal or distribution apparatus.  
           [0016]    To upgrade the existing optical network or communication system, an optical combining device such as a coupler/splitter located in the intermediate terminal includes a plurality of input optical connectors and an output optical connector. There is also an optical separating device which may also be a coupler/splitter included at the intermediate terminal having an input optical connector and a plurality of output optical connectors. A transmission loop comprising a plurality of the destination terminals which include an OIU and a like plurality of corresponding pairs of optical fibers is formed by selectively connecting the fibers of each pair at the intermediate terminal. To aid understanding of how the transmission loop is formed, it is worthwhile to arbitrarily designate that the end of a first fiber of each of the corresponding pair of fibers terminates at the intermediate terminal with a first optical connector and the end of the second fiber of each pair terminates at the intermediate terminal with a second optical connector. At the destination terminal each first fiber of the plurality of pairs are connected to the output optical connectors of the OIU and the second fiber of each plurality of pairs are connected to the input optical connectors of the OIU.  
           [0017]    At the intermediate or distribution terminal end, each first optical connector of the first optical fiber of a pair of fibers is connected to the second optical connector of the second optical fiber of another pair of fibers. This is true for each of the plurality of pairs terminating at the intermediate terminal except the first optical coupler of a first fiber of a selected “first” pair is connected to the input connector of the optical combining device, and the second optical connector of a second fiber of a “last” pair is connected to the output connector of the optical splitting device. Lastly, an output optical connector of the combining or coupler device is connected to a first fiber of a primary pair routed between the intermediate terminal and a primary terminal and the input connector of the optical separating or splitting device is connected to the second fiber of the primary pair.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    Features of the present invention will be more fully disclosed when taken in conjunction with the following Detailed Description of the Preferred Embodiment(s) in which like numerals represent like elements and in which:  
         [0019]    [0019]FIG. 1 is a block diagram of a prior art fiber optical communication system using two fibers to obtain bidirectional communication;  
         [0020]    [0020]FIG. 2 is a block diagram of another prior art bidirectional fiber-optic communication system using a single transmission fiber having a light-emitting diode and a photodetection diode at each end of the fiber;  
         [0021]    [0021]FIG. 3 is a block diagram of a prior art fiber optical communication system using a single fiber and a single transmit/receive diode at each end suitable for TCM;  
         [0022]    [0022]FIG. 4 is a schematic of a prior art Passive Optical Fiber distribution network suitable for being upgraded by the teachings of this invention.  
         [0023]    [0023]FIG. 5 illustrates a first embodiment of the invention for upgrading the optical network of FIG. 4 to an active optical network with minimal new equipment and without the installation of additional optical fibers.  
         [0024]    [0024]FIG. 6 is an enlarged illustration of how the optical loop is formed between BOIU  66  and  68  and the corresponding pairs of optical fibers  50  and  52 .  
         [0025]    [0025]FIG. 7 illustrates another embodiment of the invention wherein the prior art passive optical system is upgraded to handle a large number of broadband subscribers by using  4  wavelengths of light, but continues to operate as a passive optical system.  
         [0026]    [0026]FIGS. 8A and 8B illustrate how the route protection switches operate so as to limit the number of customers or subscribers affected in the event of a failure of an OIU in one of the destination terminals of either of the embodiments shown in FIGS. 7 and 8.  
         [0027]    [0027]FIG. 9. illustrates the operation of the Route Protection switches which protect the system in the event the primary optical fiber failure between the intermediate terminal and the central office.  
         [0028]    [0028]FIG. 10 illustrates an upgrade similar to that of FIG. 7 but uses only two wavelengths of light and an additional pair of optical fibers between the intermediate terminal and the primary terminal.  
     
    
     DESCRIPTION OF THE INVENTION  
       [0029]    Referring now to FIG. 4, there is shown a bidirectional, passive optical network system. Elements of the system similar to elements discussed with respect to the prior art system of FIGS. 1, 2 and  3  carry common reference numbers. As shown, there is provided an intermediate distribution terminal  22  which is connected to optical communication equipment  40  at central office  20  by at least one primary pair of optical fibers  42 , and preferably by two primary pairs of optical fibers  42  and  44 . It is not uncommon for a spare pair of optical fibers to extend between an intermediate distribution terminal and a central office. Intermediate distribution terminal  22  is shown as including an optical splitter device  46  connected to one of the optical fibers  42   a  of fiber pair  42  and an optical combining device  48  connected to the other fiber  42   b  of fiber pair  42 . It should also be noted that, although the pair of fibers  42  are illustrated in the figure with the two individual fibers  42   a  and  42   b  traveling together in a common sheath, such an arrangement, although common, is not necessary. The two individual fibers could be completely separate and independent of each other. All that is necessary is that the two separate fibers start and end at the same location. As indicated in FIG. 4, optical splitter device  46  and optical combining device  48  may typically be devices having a ratio of 32:1. That is, the devices either receive light from or transmit light to thirty-two optical fibers, and this received or transmitted light is carried by a single fiber after either being split or combined, whichever is appropriate. For example, splitter  46  receives light carrying information from fiber  42   a  of fiber pair  42  and splits the light into, for example only, thirty-two portions which are coupled to one of the fibers of thirty-two different pairs of fibers such as pairs  50 ,  52 ,  54 ,  56 ,  58 ,  60 ,  62  and  64  between intermediate terminal  22  and thirty-two destination terminals such as the thirty-two OIUs (optical interface unit),  66 ,  68 ,  70 ,  72 ,  74 ,  76 ,  78  and  80  found in thirty-two destination terminals at thirty-two different locations. Likewise, combining device  48  located in intermediate terminal  22  receives light from the thirty-two OIUs on the other fiber of each of the fiber pairs  50  through  64 , combines the received light and couples it to the single fiber  42   b  of fiber pair  42  such that it is transmitted to optical communication equipment  40  at central office  20 . Thus, in the example shown in FIG. 4, there are thirty-two separate OIUs which may be installed at thirty-two distinct and separate locations including OIU  66  through  80  which are connected by one of the fibers of each of the thirty-two pairs of optical fibers  50  through  64  to the optical splitter device  46  in intermediate terminal  22 . The thirty-two OIUs are also connected by the other fiber of each pair to the optical combining or coupler unit  48  which is also located in intermediate terminal  22 . It will appreciated that the thirty-two OIUs, the thirty-two pairs of corresponding optical fibers and the 32:1 splitter unit  46  and 32:1 combining unit  48  represents a typical prior art passive optical network system. Also, as was discussed above with respect to individual fibers  42   a  and  42   b  which make up pair  42 , it is not necessary that the individual fibers of the pairs  50  through  64  or any other pair of optical fibers discussed herein, run side by side in a common sheath. It is only necessary that the individual fibers in a pair start and terminate at the same locations. Other prior art systems may use equipment which supports a number of destination terminals and corresponding pairs of optical fibers which is different than thirty-two.  
         [0030]    Referring now to FIG. 5, there is shown a first embodiment wherein an existing passive optical network such as was discussed with respect to FIG. 4 is suitable for being upgraded to an active optical network system for carrying broadband data signals. Those elements of FIG. 5 which are the same as those discussed with respect to FIG. 4 continue to carry the same reference numbers. As shown, a primary pair of optical fibers  42  having individual fibers  42   a  and  42   b  extends between optical equipment  40  in the central office  20 , and optical to electrical conversion equipment  82  in the intermediate distribution terminal  22 . Also similar to the optical network system shown in FIG. 4, there are included thirty-two corresponding pairs of optical fibers (including the representative eight pairs of optical fibers  50  through  64 ) which extend between intermediate terminal  22  and thirty-two separate destination terminals, each of which in the embodiment of FIG. 5 contains a BOIU (broadband optical interface unit) such as represented by BOIUs  66   a ,  68   a ,  70   a ,  72   a ,  74   a ,  76   a ,  78   a  and  80   a . In addition to optical/electrical data converting equipment  82  located in intermediate terminal  22 , there are also included optical communication units such as units  84  and  86  each of which includes an output optical connector  88  and an input optical connector  90 .  
         [0031]    As was discussed above with respect to FIG. 4, a pair of optical fibers extend between the intermediate terminal  22  and each of the BOIUs  66   a  through  80   a . As an example, the pair of optical fibers  50  include a first fiber  92  and second fiber  94 , and as a further example, and only for convenience, the first fiber  92  is shown carrying light from to intermediate terminal  22  to BOIU  66   a  whereas the second fiber  94  is shown carrying light in the opposite direction from the BOIU  66   a  to intermediate terminal  22 .  
         [0032]    Referring now to FIG. 6, there is shown a more detailed illustration of the connections between the optical equipment  84 , fiber optical pairs  50  and  52  and the BOIU  66   a  and BOIU  68   a . As shown, the first fiber  92  of optical pair  50  includes a “first” optical connector at the intermediate end of fiber  92  such as optical connector  96  at the end of optical fiber  92  which terminates in intermediate terminal  22 . There is also included optical connector  98  on the destination terminal end of fiber  92  which terminates at BOIU  66   a . Likewise, the second optical fiber  94  includes a “second” connector on the intermediate terminal  22  end of fiber  94  such as optical connector  100  at the end of optical fiber  94  and optical connector  102  on the other end which terminates at BOIU  66   a . It is also noted that BOIU  66   a  includes an input optical connector  104  and an output optical connector  106  which are connected to optical connectors  98  and  102 , respectively. Likewise, the optical pair  52  which extends between BOIU  68   a  and intermediate terminal  22  also includes a first optical fiber  108  having a “first” optical connector  110  at the end of fiber  108  which terminates in intermediate terminal  22  and an optical connector  112  at the end of fiber  108  which terminates at BOIU  68   a . Similarly, the second optical fiber  114  of optical pair  52  includes a “second” optical connector  116  on the end which terminates at intermediate terminal  22  and optical connector  118  on the end of optical fiber  114  which terminates at the BOIU  68   a . In the same manner as the BOIU  66   a , BOIU  68   a  also includes an input terminal  120  and an output terminal  122 .  
         [0033]    Therefore, referring to FIGS. 5 and 6, it is seen that lightwaves carrying data information is provided at connector  88  of optical equipment  84 . When optical connector  96  of fiber  92  is connected to optical connector  88  of optical equipment  84 , light is provided from the unit  84  through the “first” optical fiber  92  to the BOIU  66   a  through connector  98  on fiber  92  to input optical connector  104  on BOIU  66   a . As will be appreciated by those skilled in the art, data carried on “first” optical fiber  92  which is appropriate for or “addressed to” BOIU  66   a  will be extracted from the traveling lightwaves and put in suitable format for further transmission or use. In addition to extracting data from the light coming into BIOU  66  on optical fiber  92 , BOIU  66   a  also inserts new data onto the light traveling through the unit which exits BIOU  66   a  on connector  106  to connector  102  and onto “second” fiber  94  of pair  50 . Thus, new data inserted by BOIU  66   a  is now carried on “second” fiber  94  to connector  100  located in intermediate terminal  22 . However, it is noticed that connector  100  is not connected to the optical equipment  84 , but is instead connected to the “first” optical connector  110  on another “first” optical fiber  108  of fiber pair  52 . Then, in the same manner as was discussed above with respect to BOIU  66   a , light on “first” fiber  108  is connected through connector  112  at the destination terminal end to input connector  120  on BOIU  68   a  where the appropriate data for BIOU  68   a  is extracted and new data is injected onto the light and then the light is transmitted back out of output connector  122  on BOIU  68   a  to connector  118  of “second” fiber  114  of optical pair  52  to “second” connector  116  at the intermediate terminal end of optical fiber  114 . “Second” optical connector  116  is then connected to a first optical connector on a first optical fiber of optical fiber pair  54  which extends from intermediate terminal  22  to BOIU  70   a . After the data is extracted from the light on the first fiber of optical pair  54  and any new data is inserted onto the light traveling to the second fiber of optical pair  54 , it is again routed back to the intermediate terminal  22  and then to the first fiber of optical pair  56  to BOIU  72   a . The light coming from the output of BOIU  72   a  again travels back to the intermediate terminal  22  on the second fiber of pair  56  wherein the second fiber of optical pair  56  has a “second” connector at the intermediate end connected to the input terminal  90  of optical equipment  84 . Thus, it is seen that there has been described a transmission loop which extends initially from the output connector  88  of optical equipment  84  through BOIU  66   a  back to intermediate terminal  22  then out to BOIU  68   a  back to intermediate terminal  22  then out to BOIU  70   a  then back to intermediate terminal  22  and then to BOIU  72   a  and back to intermediate terminal  22  where it is connected to the input terminal  90  of optical equipment  84 .  
         [0034]    In the embodiment illustrated in FIG. 5, there are a plurality of units similar to optical equipment  84 , each of which is connected to a transmission loop with four separate BOIUs in the same manner as just discussed. For example, electrical equipment  86  in intermediate terminal  22  is part of the transmission loop made up by BOIU  74   a ,  76   a ,  78   a  and  80   a  along with corresponding optical fiber pairs  58 ,  60 ,  62  and  64 . It will also be appreciated, that although in the embodiment discussed, there are four BOIUs for every piece of optical equipment in intermediate terminal  86 , the number of BOIUs could be greater or less than four. It will also, of course, be appreciated that there are electrical connections between the optical to electrical equipment  82  and the optical equipment  84  and  86 . Thus, there has been described a transmission path wherein a plurality of BOIU units are connected to a single piece of optical equipment at the intermediate terminal  22  by means of a serial transmission loop. As will be appreciated by those skilled in the art, it would be possible that a single communication channel could be handled by each of the BOIU units or a large number of channels could be handled. When the equipment is initially installed, a smaller number of channels would be handled by each BOIU unit in a transmission loop and as new customers request service, the number of channels handled by each BOIU unit in the loop could increase. Eventually the number of channels being serviced by each BOIU unit could increase to such a level that optical equipment unit  84  at the intermediate terminal  22  could no longer handle the volume. In such a case, one of the BOIU units may necessarily have to be taken out of the loop so that there might be only three BOIU units in the loop because of the increased traffic. The BOIU unit taken out of the overloaded transmission loop would then be combined into another transmission loop and perhaps with a new piece of optical equipment similar to that of optical equipment  84 . It should be noted that each of the optical fiber pairs  50  through  56  are handling four times the number of channels because of the serial transmission loop than would be handled by each pair if each BOIU unit went to a separate piece of optical equipment such as optical equipment  84 . Thus, it can be seen that as more and more service is demanded and added at the BOIU units, it is a simple matter to rearrange the transmission loops and add equipment only as it is needed.  
         [0035]    [0035]FIG. 7 illustrate two embodiments for upgrading an optical system which does not require active elements, and only incorporates passive elements at the intermediate or remote distribution terminal. For example, instead of the active elements  82 ,  84  and  86  which converted data from optical signals to electrical signals and from electrical signals to optical signals, and as was discussed with respect to FIGS. 5 and 6, the embodiment of FIG. 7 use passive elements such as an optical coupler/splitter to combine various wavelengths of light arriving on a plurality of optical fibers such that all of the optical signals can be carried on a single optical fiber. Similarly, an optical coupler/splitter with CWDM (continuous wave division multiplexing) may be used to separate the different wavelengths of light carrying the various signals, one each onto a plurality of different optical fibers. As an example only, a single fiber may be used to carry light having a wavelength of 1,310 nanometers as is typically used for telephony service as well as four different wavelengths, such as 1,510, 1530, 1,550 and 1,570 rather than a single nominal wavelength of 1,550 nanometers.  
         [0036]    More specifically, and as shown in FIG. 7, central office  20  is connected to intermediate or remote distribution terminal  22  by at least two primary optical fibers such as optical fiber pair  42  which has individual fibers  42   a  and  42   b . Intermediate terminal  22  is also connected to a plurality (such as thirty-two) of BOIU (broadband optical interface unit) by a like plurality of pairs of optical fibers. It should be noted that BOIU terminals  130  and  132  represent the first and eighth BOIUs forming a first optical loop of eight different BOIUs. The loop is connected by a corresponding eight pairs of optical fibers as represented by optical fiber pairs  134  and  136  in the same manner as the loop of four different BOIUs discussed with respect to FIGS. 5 and 6. Similarly, the ninth BOIU  138  and the sixteenth BOIU  140 , along with a first optical fiber pair  142  and an eighth optical fiber pair  144  represent a second optical loop of eight additional BOIUs and their corresponding eight pairs of optical fibers.  
         [0037]    Likewise, the seventeenth BOIU  146  and the twenty-fourth BOIU  148 , along with the seventeenth and twenty-fourth pairs of optical fibers  150  and  152 , respectively, represent a third optical loop of eight BOIUs. Finally, the twenty-fifth and thirty-second BOIUs  154  and  156 , respectively, with their corresponding pairs of optical fibers  158  and  160  represent a fourth optical data loop. In the example as shown, each of the four optical data loops carry light at slightly different wavelengths. For example, in the embodiment shown the optical loops 1, 2, 3 and 4 operate at 1,510, 1,530, 1,550 and 1,570 nanometers of light, respectively.  
         [0038]    As shown in FIG. 7, intermediate or remote distribution terminal  22  also includes an optical combination device or coupler  162  having its output side optically connected to optical fiber  42   a  of optical pair  42 . Also as shown, the four inputs of optical coupler  162  are fibers  134   a  from optical fiber pair  134 , optical fiber  142   a  from fiber pair  142 , optical fiber  150   a  from fiber pair  150  and optical fiber  158   a  from fiber pair  150 . Thus, it is seen that each of the four serial transmission loops has an input to the optical coupling device  162 . In a similar manner, there is an optical separation or splitter  164  in combination with a four-way optical filter  166 . The splitter/coupler  164  has its input  168  connected to optical fiber  42   b  of optical fiber  42 . Each of the four outputs are connected to one output of the four-way filter  166  and are in turn connected one each to the last fiber of each of the four loops. For example, fiber  136   b  from the first loop is connected to the filter  166  and then to splitter/coupler  164  and the optical fiber  144   b  from the second optical loop is also connected to filter  166  and then to coupler  164 . Likewise, optical fiber  152   b  from the third optical loop and optical fiber  160   b  from the fourth optical loop are connected through the filter  166  to the splitter/coupler  164 . Thus, it is seen that by using a  4 : 1  splitter/couplers  162  and  164 , and by putting eight BOIUs in each loop, all thirty-two of the BOIUs can be serviced.  
         [0039]    It should also be noted that there is a route protection switch such as switches  170  and  172  located between each of the BOIUs and their corresponding fiber optical pair. For example, protection switch  170  is located between BOIU  130  and optical pair  134 . Likewise, route protection switch  172  is located between BOIU  132  and optical pair  136 . The purpose of the route protection switches is that in the event a single BOIU, such as for example BOIU  130 , were to fail, the route protection switch would operate to bypass that BOIU such that only the customers or subscribers associated with and receiving service through BOIU  30  would lose service. The fault protection switch simply bypasses BOIU  130  and couples the optical signal directly from the optical fiber  134   a  to optical fiber  134   b  of the optical pair  134 . FIGS. 8A and 8B illustrate the normal light path and the fault light path, respectively, through the fault protection switches. Thus, the seven remaining BOIUs can continue to cover and provide service without interruption.  
         [0040]    Also as shown, control office  20  includes an optical splitter/coupler  174  in combination with a CWDM filter  176  connected to optical fiber  42   a  of pair  42 . Similarly, optical coupler/splitter  178  connected to optical fiber  42   b  of pair  42 . Also as shown, there are four optical receivers and four optical transmitters such as receiver  180  and transmitter  182 . Each of the four receivers and transmitters are for receiving and transmitting light having one of the four different wavelengths. Thus, each receiver such as receiver  180  is coupled to the wave division multiplexer filter  176  such that only light of the proper wavelength is directed to the proper receiver. Similarly, each transmitter is connected to optical coupler  178 .  
         [0041]    In an alternate embodiment, there may be a second pair  184  of primary fibers made up of fibers  184   a  and  184   b . In the event there are two pairs of fibers extending between the intermediate or remote distribution terminal  22  in the central office  20 , redundancy may be provided such that if a fiber in the first primary pair  42  were to be cut or otherwise damaged, a fiber in the second fiber pair  184  can take over. This is accomplished by a pair of route protection switches  186  and  188  which are connected so that if, for example, fiber  42   a  of pair  42  were to be damaged or separated, switch  186  would activate such that the input of the optical coupler/splitter  174  would be connected to optical fiber  184   a  of fiber pair  184  rather than fiber  42   a  of pair  42 . Likewise, if optical fiber  42   b  were to be severed or damaged, then switch  188  would activate such that the output of optical coupler  178  is routed to fiber  184   b  of pair  184  rather than to optical fiber  42   b  of pair  42 . FIG. 9 illustrates the normal and fault positions of the route protection switches. It should be also be noted, however, that this alternate embodiment also requires that the optical coupler/splitter  162  and  164  discussed with respect to intermediate terminal  22  should have two outputs rather than a single output as was discussed before. That is, the optical coupler/splitters should be a 4:2 rather than a 4:1 splitter/coupler. Thus, it is seen there has been described a method of using existing fiber optical pairs to upgrade a system to a passive system with minimal change of equipment and no additional fibers required to be installed.  
         [0042]    Referring now to FIG. 10, there is shown still another alternate embodiment of the present invention where only two single wavelengths of light 1,550 and 1,310 are used. It is noted that the four optical loops are substantially the same as discussed with respect to FIG. 6. However, instead of a single pair  42  of fibers  42   a  and  42   b , the primary optical fiber bundle  186  is not made up of two fibers but is made up of four fibers  186   a ,  186   b ,  186   c  and  186   d . Further, if there is to be redundancy of the primary fiber  186 , it will be necessary to include a second four-fiber bundle  188  made up of fibers  188   a ,  188   b ,  188   c  and  188   d . In such an arrangement, it is not necessary to use the CWDM filters; it is only necessary to use a 2×2 optical coupler/splitter as indicated by optical coupler/splitters  190 ,  192 ,  194  and  196  in intermediate terminal  22 , and 2×2 optical coupler/splitter  198 ,  200 ,  202  and  204  in central office. Thus, in this arrangement, there is a fiber dedicated for each of the terminal loops each of which carries eight BOIUs. Likewise at the central office  20  in end of fibers  186  and  188 , each of the fibers are connected to its own receiver and transmitter, such as receiver  206  and transmitter  208 . To achieve redundancy in the event of a primary fiber bundle failure in this embodiment, there is also included four route protection switches such as switch  210  which operate similarly to the switches  186  and  187  with respect to FIG. 7 above. Thus, in the event of one of the primary fibers of optical bundle  186 , the appropriate switch, such as switch  210 , would switch positions such that the information is now routed through the appropriate fiber of fiber bundle  188  and then back to its appropriate optical splitter  190 .  
         [0043]    The corresponding structures, materials, acts and equivalents of all means or step; plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed.