BACKGROUND OF THE INVENTION 
   1. Field of the invention 
   The present invention relates to the structure of a fiber network, and more particularly to a detection system for identifying faults in a passive optical network. 
   2. Description of the Prior Art 
   In the prior art, when one of the fiber branches of a fiber communication network has a fault, it is very important to discover the broken fiber branch and the broken position of the fiber branch as fast as possible. Thereby, the fiber communication network can maintain the quality of signal transmission. 
   The detection system of the prior art is shown in  FIG. 1 . The detection system includes a main control section  1 , a fiber trunk connected to the main control section, a plurality of fiber branches connected to the fiber trunk via a splitter  2 , and a plurality of user stages respectively connected to a corresponding fiber branch. 
   The main control section  1  includes a plurality of signal sources, an optical circulator  15 , a first coupling unit that includes a wavelength division multiplexing (WDM) coupler  18 , and an optical coupler, a pumping source  16 , and a hookup survival detection unit  17 . The hookup survival detection unit  17  can be an optical spectrum analyzer (OSA). Each of the fiber branches is a single mode fiber and includes a fiber Bragg grating (FBG) such as an FBG  31 ,  41 ,  51 , or  61 , a thin film filter such as the thin film filter  33 ,  43 ,  53 , or  63 , and a user stage such as the user stage  35 ,  45 ,  55 , or  65 . 
   The first signal source  11  provides a data signal λ 1 . The second signal source  12  provides a data signal λ 2 . The third signal source  13  provides a data signal λ 3 . The fourth signal source  14  provides a data signal λ 4 . The coupler couples the data signals to the optical circulator  15 , and the optical circulator  15  transmits the data signals to the WDM coupler  18  for being further fed into the fiber trunk. Next, the splitter  2  transmits the data signals from the fiber trunk to each of the fiber branches, wherein each fiber trunk has the data signals λ 1  through λ 4  transmitted within. 
   Otherwise, the pumping source  16  simultaneously provides a pumping laser light λ 5  to the fiber trunk that includes a single mode fiber SMF and an Erbium-doped fiber EDF. The pumping laser light λ 5  pumps the Erbium-doped fiber EDF to emit an amplified spontaneous emission (ASE) with a band as detection signals such as the detection signals λ 51  through λ 54 . 
   Because each of the FBGs has its individual center wavelength (Bragg condition), the center wavelength of each FBG is set to equal the frequency of each detection signal. Because each of the thin film filters has its individual transmission frequency band, the transmission frequency band of each thin film filter is set to equal the frequency band of each data signal. 
   Hence, each of the data signals is transmitted to the corresponding user stage, and each of the detection signals is reflected to generate a corresponding reflected signal. The data signal λ 1  is transmitted to the first user stage  35 , and the detection signal λ 51  is reflected to generate the reflected signal λ′ 51 . The data signal λ 2  is transmitted to the second user stage  45 , and the detection signal λ 52  is reflected to generate the reflected signal λ′ 52 . The data signal λ 3  is transmitted to the third user stage  55 , and the detection signal λ 53  is reflected to generate the reflected signal λ′ 53 . The data signal λ 4  is transmitted to the fourth user stage  65 , and the detection signal λ 54  is reflected to generate the reflected signal λ′ 54 . 
   Finally, the reflected signals are transmitted back to the hookup survival detection unit  17 . The hookup survival detection unit  17  further uses the reflecting signals to identify whether each fiber branch has a fault or not. The spectrum of the signal transmitted in the broken fiber branch is different to the spectrum of the signal transmitted in an unbroken fiber branch. 
   However, the detection system of the prior art cannot discover the fail position of the broken fiber branch. This disadvantage causes difficultly to the user stages performing maintenance work on the fiber communication network. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention that the detection system can determine whether a fiber branch has a fault therein. 
   It is a second object of the present invention that the detection system can discover the fail position of the broken fiber branch. 
   In order to achieve the above objects, the present invention provides a detection system for identifying faults in a passive optical network. 
   In the first embodiment of the present invention, the detection system includes a main control section, a fiber trunk connected to the main control section, a plurality of fiber branches connected to the fiber trunk via a splitter, and a plurality of user stages respectively connected to the corresponding fiber branch. The main control section includes a plurality of signal sources, a coupler connected to the signal sources, an isolator connected to the coupler, two proportion couplers, a fault position detection unit, and a hookup survival detection unit, wherein the hookup survival detection unit is an OSA. The fault position detection unit and the hookup survival detection unit is connected to one of the proportion couplers, while the other proportion coupler is installed on the fiber trunk. Each fiber branch also has a reflected and filtering unit that is connected to a corresponding user stage. 
   In the second embodiment of the present invention, the spectrum of the second embodiment is similar to the first embodiment, but it should be noted that the hookup survival detection unit includes a demultiplexer and a plurality of optical sensors, wherein each optical sensor corresponds to a corresponding fiber branch. 
   In the third embodiment of the present invention, the spectrum of the third embodiment is similar to the second embodiment, but it should be noted that the hookup survival detection unit includes a demultiplexer, a switch, and an optical sensor. 
   In the fourth embodiment of the present invention, the detection system includes a main control section, a fiber trunk connected to the main control section, an array waveguide grating connected to the fiber trunk, a plurality of fiber branches connected to the array waveguide grating, and a plurality of user stages respectively connected to the corresponding fiber branch. The main control section includes a plurality of signal sources, a WDM coupler, an optical coupler, and a fault position detection unit. 
   In the fifth embodiment of the present invention, the spectrum of the fifth embodiment is similar to the fourth embodiment, but it should be noted that the main control section further includes a hookup survival detection unit and each fiber branch further has a reflected unit. The hookup survival detection unit and the fault position detection unit connect to the proportion coupler which is in turn connected to the WDM coupler, and the WDM coupler connects in turn to the fiber trunk. 
   In the sixth embodiment of the present invention, the spectrum of the sixth embodiment is similar to the fourth embodiment, but it should be noted that the main control section further includes a hookup survival detection unit. The hookup survival detection unit connects to the fiber trunk via the proportion coupler, and the fault position detection unit connects in turn to the fiber trunk via the WDM coupler. 
   In the seventh embodiment of the present invention, the spectrum of the seventh embodiment is similar to the fourth embodiment, but it should be noted that the main control section further includes a switch unit. Each switch path of the switch unit corresponds to a corresponding fiber branch. 
   In the eighth embodiment of the present invention, the spectrum of the eighth embodiment is similar to the seventh embodiment, but it should be noted that the main control section further includes a hookup survival detection unit and the array waveguide grating is substituted by the DWDM demultiplexer. The hookup survival detection unit and the fault position detection unit connect to the switch unit in order to be further connected to the fiber trunk via the proportion coupler, wherein the hookup survival detection unit connects to the switch of the switch unit. 
   In the ninth embodiment of the present invention, the spectrum of the ninth embodiment is similar to the seventh embodiment, but it should be noted that the DWDM multiplexer is substituted for the array waveguide grating, wherein the number of channels of the DWDM demultiplexer is double of the user stages. Each two of the channels of the DWDM demultiplexer connect to one coupler in order to further connect to a corresponding fiber branch. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and further advantages of this invention may be better understood by referring to the following description, taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a structural diagram of the detection system of the previous invention for identifying faults; 
       FIG. 2  is a structural diagram of the first embodiment of the detection system of the previous invention for identifying faults; 
       FIG. 3  is a structural diagram of the second embodiment of the detection system of the previous invention for identifying faults; 
       FIG. 4  is a structural diagram of the third embodiment of the detection system of the previous invention for identifying faults; 
       FIG. 5  is a structural diagram of the fourth embodiment of the detection system of the present invention for identifying faults; 
       FIG. 6  is a structural diagram of the fifth embodiment of the detection system of the present invention for identifying faults; 
       FIG. 7  is a structural diagram of the sixth embodiment of the detection system of the present invention for identifying faults; 
       FIG. 8  is a structural diagram of the seventh embodiment of the detection system of the present invention for identifying faults; 
       FIG. 9  is a structural diagram of the eighth embodiment of the detection system of the present invention for identifying faults; and 
       FIG. 10  is a structural diagram of the ninth embodiment of the detection system of the present invention for identifying faults. 
   


   The drawings will be described further in connection with the following detailed description of the present invention. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The detection system of the present invention can be applied to a passive optical network (PON) and determines whether there is a fault on paths of a fiber network, or discovers the positions of faults on the paths of the fiber network. It should be noted that the detection system can be a tree topology, a ring topology, a star topology, a bus topology, etc. All methods are based on the needs of user stages. 
     FIG. 2  shows a structural diagram of the detection system of the first embodiment of the present invention. The main control section  10  includes a plurality of signal sources, an isolation device  180 , a coupling unit, a hookup survival detection unit  150 , and a fault position detection unit  160 . The signal sources can include a first signal source  110  through a fourth signal source  116 . The coupling unit can include the proportion couplers  142  and  143 . The fiber trunk can be a single mode fiber SMF. The fault position detection unit  160  can be an optical time domain reflector (OTDR). 
   The first signal source  110  provides a data signal λ 1 . The second signal source  112  provides a data signal λ 2 . The third signal source  114  provides a data signal λ 3 . And the fourth signal source  116  provides a data signal λ 4 . Simultaneously, the fault position detection unit  160  provides detection signals λ 51  through λ 54 . The proportion coupler  142  couples the data signals and the detection signals to the fiber trunk in proportion. The detection system further feeds respective data signals and respective detection signals into a corresponding fiber branch via the splitter  20 . 
   Next, each data signal is transmitted via the corresponding fiber branch for further transmission to a corresponding user stage via a corresponding fiber Bragg grating. Moreover, each detection signal is transmitted via the corresponding fiber branch for being further reflected by a corresponding thin film filter as a corresponding reflected signal. 
   The data signal λ 1  is transmitted via a first fiber branch for further transmission to the first user stage  350  via a fiber Bragg grating  310 . The data signal λ 2  is transmitted via a second fiber branch for further transmission to a second user stage  450  via a fiber Bragg grating  410 . The data signal λ 3  is transmitted via a third fiber branch for further transmission to a third user stage  550  via a fiber Bragg grating  510 . The fourth data signal λ 4  is transmitted via a fourth fiber branch for further transmission to a fourth user stage  650  via a fiber Bragg grating  610 . 
   The detection signal λ 51  is transmitted via the first fiber branch and is reflected by a thin film filter  330  to generate a reflected signal λ′ 51 . The detection signal λ 52  is transmitted via the second fiber branch and is reflected by the thin film filter  430  to generate the reflected signal λ′ 52 . The detection signal λ 53  is transmitted via the third fiber branch and is reflected by the thin film filter  530  to generate the reflected signal λ′ 53 . The detection signal λ 54  is transmitted via the fourth fiber branch and is reflected by the thin film filter  630  to generate the reflected signal λ′ 54 . 
   Finally, the reflected signals are transmitted in proportion to the hookup survival detection unit  150  and the fault position detection unit  160 . If a fiber has a fault (broken), the fault will cause Fresnel reflection in the broken section. The hookup survival detection unit  150  can use the reflected signals to determine whether each fiber branch has a fault or not. The fault position detection unit  160  can use the reflected signals to discover the position of the fault in each fiber branch. 
   The second embodiment of the present invention is shown in  FIG. 3 . It should be noted that the hookup survival detection unit  150  includes a demultiplexer  151  and a plurality of optical sensors in the main control section  10  of the second embodiment. The optical sensors can be photodiodes and are respectively the optical sensor PD 1  through PD 4 , wherein each optical sensor corresponds to a fiber branch. 
   When the reflected signals are transmitted to the hookup survival detection unit  150  and the fault position detection unit  160  via the proportion couplers  142  and  143 , the demultiplexer  151  respectively transmits the reflected signals to an optical sensor. The reflected signal λ′ 51  is transmitted to the optical sensor PD 1 . The reflected signal λ′ 52  is transmitted to the optical sensor PD 2 . The reflected signal λ′ 53  is transmitted to the optical sensor PD 3 . The reflected signal λ′ 54  is transmitted to the optical sensor PD 4 . 
   Hence, each optical sensor can use the received reflected signals to determine whether each fiber branch has a fault or not. 
   The third embodiment of the present invention is shown in  FIG. 4 . It should be noted that the hookup survival detection unit  150  includes a demultiplexer  151 , a switch  153 , and an optical sensor PD 1 . The optical sensor PD 1  can be an avalanche photodiode (APD) or any other type of sensing diode. 
   When the reflected signals are transmitted to the hookup survival detection unit  150  and the fault position detection unit  160  via the proportion couplers  142  and  143 , the demultiplexer  151  respectively transmits the reflected signals to the optical sensor PD 1  according to a switch of the switch  153 . That is, the switch  153  selects one of the reflected signals to transmit the selected reflected signal to the hookup survival detection unit  150  and the fault position detection unit  160 . Hence, the optical sensor PD 1  can use each received reflected signal to determine whether each fiber branch has a fault or not. 
   In the description of the above embodiments, the fiber Bragg gratings is associated with the fibers to act as a reflected and filtering unit that reflects and filters signals. Moreover, the fiber trunks and the fiber branches are single mode fibers. However, the present invention is not limited by the above description. The fiber trunks and the fiber branches can be single mode fibers, multimode fibers (MMF), or dispersion compensated fibers (DCF) in the first through the third embodiments. The reflected and filtering unit can be a device with an equivalent reflective capacities and an equivalent band, such as a reflective filter, a transflective filter, etc. 
   The fourth embodiment of the present invention is shown in  FIG. 5 . The detection system of the fourth embodiment includes a main control section  10 , a fiber trunk, a plurality of fiber branches, and a plurality of user stages. The main control section  10  connects to the fiber trunk and includes a plurality of signal sources, a coupling unit that includes a WDM coupler  144  and an optical coupler (not shown), and a fault position detection unit  160 . For example, the signal sources can include the first signal source  110 , the second signal source  112 , the third signal source  114 , and the fourth signal source  116 . The user stages are respectively installed in the railhead of each fiber branch and can include the first user stage  350 , the second user stage  450 , the third user stage  550 , and the fourth user stage  650 . 
   The first signal source  110  provides the data signal λ 1 . The second signal source  112  provides the data signal λ 2 . The third signal source  114  provides the data signal λ 3 . The fourth signal source  116  provides the data signal λ 4 . Simultaneously, the fault position detection unit  160  provides the detection signals λ 51  through λ 54 . The optical coupler couples the data signals with the detection signals for being further feed into the fiber trunk via the WDM coupler  144 . The detection system further respectively feeds the data signals and the detection signals into a corresponding fiber branch via the array waveguide grating (AWG)  22  installed between the fiber trunk and each fiber branch. 
   The data signal λ 1  and the detection signal λ 51  are transmitted via the first fiber branch. The data signal λ 2  and the detection signal λ 52  are transmitted via the second fiber branch. The data signal λ 3  and the detection signal λ 53  are transmitted via the third fiber branch. The fourth data signal λ 4  and the detection signal λ 54  are transmitted via the fourth fiber branch. 
   Furthermore, the data signals are respectively transmitted to a corresponding user stage. The data signal λ 1  is transmitted to the first user stage  350 . The data signal λ 2  is transmitted to the second user stage  450 . The data signal λ 3  is transmitted to the third user stage  550 . The fourth data signal λ 4  is transmitted to the fourth user stage  650 . 
   When a fiber has a fault, the fault causes Fresnel reflection within the broken section. Hence, if the first fiber branch has a fault, not only the detection signal λ 51  will be reflected, but also the data signal λ 1  will be reflected due to the failure to act as a reflected signal λ′ 51 . If the second fiber branch has a fault, not only the detection signal λ 52  will be reflected, but also the data signal λ 2  will be reflected due to the failure to act as a reflected signal λ′ 52 . If the third fiber branch has a fault, not only the detection signal λ 53  will be reflected, but also the data signal λ 3  will be reflected due to the failure to act as a reflected signal λ′ 53 . The same, if the fourth fiber branch has a fault, not only the detection signal λ 54  will be reflected, but also the data signal λ 4  will be reflected due to the failure to act as a reflected signal λ′ 54 . 
   Finally, the reflected signal is transmitted to the fault position detection unit  160  via the WDM coupler  144 . The fault position detection unit  160  can use the reflected signal to discover the position of the fault in the corresponding fiber branch. Moreover, if the fiber trunk has a fault, all of the data signals and the detection signals will be reflected by the fault. The fault position detection unit  160  uses the reflected signals to discover the position of the fault in the fiber trunk. 
   The fifth embodiment of the present invention is shown in  FIG. 6 . The structure of the fifth embodiment is similar to the fourth embodiment, but it should be noted that the main control section  10  includes a plurality of signal sources, a proportion coupler  142 , a WDM coupler  144 , a hookup survival detection unit  150 , and a fault position detection unit  160 . Moreover, the proportion coupler  142  is associated with the WDM coupler  144  to act as a coupling unit. 
   The hookup survival detection unit  150  and the fault position detection unit  160  connect to the proportion coupler  142  which is in turn connected to the WDM coupler  144  that is installed upon the fiber trunk. Moreover, each fiber branch has a reflected unit that includes a WDM coupler and a reflector. 
   Each data signal transmitted via a corresponding fiber branch is coupled to a corresponding user stage via the corresponding WDM coupler. Each reflected signal transmitted via the corresponding fiber branch is coupled to a corresponding reflector, which is then reflected by the reflector to act as a reflected signal. 
   Next, the WDM coupler  144  couples the reflected signals λ′ 51  through λ′ 54  to the proportion coupler  142  and is further coupled to the hookup survival detection unit  150  and the fault position detection unit  160 . Hence, the hookup survival detection unit  150  can use the reflected signal to determine whether the fiber trunk or each fiber branch has a fault or not. The fault position detection unit  160  can use the reflected signal to discover the position of the fault of the fiber trunk or each fiber branch. 
   If the fiber trunk or one of the fiber branches has a fault, the data signal and the detection signal will be directly reflected by the fault, but will not be transmitted to the corresponding reflector and the corresponding user stage. 
   The sixth embodiment of the present invention is shown in  FIG. 7 . The structure of the sixth embodiment is similar to the fourth embodiment, and it should be noted that the main control section  10  further includes a hookup survival detection unit  150  that connects to the fiber trunk via the proportion coupler  142  of the coupling unit. The fault position detection unit  160  connects to the fiber trunk via the WDM coupler  144  of the coupling unit. 
   When a fiber has a fault, the fault will cause Fresnel reflection at the broken section. Hence, if the first fiber branch has a fault, the detection signal λ 51  and the data signal λ 1  will be reflected by the fault to thereby generating the reflected signal λ′ 51 . If the second fiber branch has a fault, the detection signal λ 52  and the data signal λ 2  will be reflected by the fault to thereby generating the reflected signal λ′ 52 . If the third fiber branch has a fault, the detection signal λ 53  and the data signal λ 3  will be reflected by the fault to thereby generating the reflected signal λ′ 53 . If the fourth fiber branch has a fault, the detection signal λ 54  and the data signal λ 4  will be reflected by the fault to thereby generating the reflected signal λ′ 54 . 
   Finally, the reflected light is coupled proportionally to the hookup survival detection unit  150  via the proportion coupler  142 , and the reflected signal is coupled to the fault position detection unit  160  via the WDM coupler  144 . The hookup survival detection unit  150  can use the reflected signal to determine whether the fiber trunk or the fiber branch has a fault or not. The fault position detection unit  160  can use the reflected signal to discover the position of the fault in the fiber trunk or the corresponding fiber branch. 
   The seventh embodiment of the present invention is shown in  FIG. 8 . The structure of the seventh embodiment is similar to the fourth embodiment, but it should be noted that the main control section  10  further includes a switch unit that includes a DWDM multiplexer  175  and a switch  173 . The fault position detection unit  160  connects to the fiber trunk via the proportion coupler  142  of the coupling unit. Each of the switch paths corresponds to a corresponding fiber branch. 
   If the first fiber branch has a fault, the detection signal λ 51  and the data signal λ 1  will be reflected by the fault to generate the reflected signal λ′ 51 . If the second fiber branch has a fault, the detection signal λ 52  and the data signal λ 2  will be reflected by the fault to thereby generating the reflected signal λ′ 52 . If the third fiber branch has a fault, the detection signal λ 53  and the data signal λ 3  will be reflected by the fault to thereby generating the reflected signal λ′ 53 . If the fourth fiber branch has a fault, the detection signal λ 54  and the data signal λ 4  will be reflected by the fault to thereby generating the reflected signal λ′ 54 . 
   Finally, the reflected signal is coupled to the switch unit via the proportion coupler  142 , and then the switch  173  selects one of the switch paths for the reflected signal. Hence, the DWDM multiplexer  175  can transmit the reflected signal of the selected switch path to the fault position detection unit  160 . The fault position detection unit  160  can use the reflected signal to discover the position of the fault in the fiber trunk or the corresponding fiber branch. 
   The eighth embodiment of the present invention is shown in  FIG. 9 . The structure of the eighth embodiment is similar to the seventh embodiment, but it should be noted that the main control section  10  further includes a hookup survival detection unit  150  connected to the fiber trunk via the switch unit that includes the switch  173  and the proportion coupler  142 . 
   The reflected signal is coupled to the switch unit via the proportion coupler  142 , and then the switch  173  selects one of the switch paths for the reflected signal. For example, the present invention can first transmit the reflected signal from the first selected switch path to the hookup survival detection unit  150  and then transmit the reflected signal from the next selected switch path to the fault position detection unit  160  via the DWDM multiplexer  175 . The hookup survival detection unit  150  can use the reflected signal to determine whether the fiber trunk or the corresponding fiber branch has a fault. The fault position detection unit  160  can use the reflected signal to discover the position of the fault in the fiber trunk or the corresponding fiber branch. 
   The ninth embodiment of the present invention is shown in  FIG. 10 . The structure of the ninth embodiment is similar to the seventh embodiment, but it should be noted that the DWDM multiplexer  24  is used in substitution for the AWQ wherein the number of the channels of the DWDM multiplexer  24  is double the number of the user stages. 
   The DWDM multiplexer  24  respectively classifies the data signals and the detection signals into a plurality of groups according to the frequency of each data signal and the frequency of each detection signal. Each group is associated with a corresponding channel of the DWDM multiplexer  24 , which is in turn associated with a corresponding fiber branch and a user stage. Each group is coupled to the corresponding fiber branch via the second coupling unit that includes a plurality of 1×2 couplers such as the 1×2 coupler  810 ,  830 ,  850 , and  870 , wherein each 1×2 coupler corresponds to the corresponding group. 
   The data signal λ 1  and the detection signal λ 51  are coupled to the first fiber branch by the 1×2 coupler  810 . The data signal λ 2  and the detection signal λ 52  are coupled to the second fiber branch by the 1×2 coupler  830 . The data signal λ 3  and the detection signal λ 53  are coupled to the third fiber branch by the 1×2 coupler  850 . The fourth data signal λ 4  and the detection signal λ 54  are coupled to the fourth fiber branch by the 1×2 coupler  870 . 
   The reflected signal is coupled to the switch unit via the proportion coupler  142 , and then the switch  173  selects one of the switch paths for the reflected signal. Hence, the DWDM multiplexer  175  can transmit the reflected signal of the selected switch path to the fault position detection unit  160 . The fault position detection unit  160  can use the reflected signal to discover the position of the fault in the fiber trunk or the corresponding fiber branch. 
   In the description of the fourth through the ninth embodiments, the fiber trunks and the fiber branches are single mode fibers. However, the present invention is not limited to the above description. The fiber trunks and the fiber branches can be single mode fibers, multimode fibers, or dispersion compensated fibers in the fourth through the ninth embodiments. 
   Moreover, the fiber trunk has an isolator installed near the feeding position of the data signals for further preventing the data signals from being negatively affected by the reflected signals in each of the embodiments of the present invention. 
   The description above only illustrates specific embodiments and examples of the invention. The invention should therefore cover various modifications and variations made to the herein-described structure and operation of the invention, provided they fall within the scope of the invention as defined in the following appended claims.
A detection system that performs in a passive optical network is disclosed. The detection system uses a central office to provide detection signals to corresponding fiber branches for obtaining different reflected signals based on different optical network models. Hence, the central office can determine whether fiber branches in the passive optical network has a fault and where the fault is according to the reflected signals.