Abstract:
An optical fiber breakage point may be located by coupling to the optical fiber an out-of-band optical test signal modulated at a periodic modulation pattern. A distance to the breakage point may be determined from a difference between modulation patterns of transmitted and received test signals.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present invention claims priority from U.S. Patent Application No. 61/894,552 filed Oct. 23, 2013, which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to optical communications equipment, and in particular to in-service monitoring of optical networks. 
     BACKGROUND 
     In a fiberoptic network, optical signals are encoded with digital streams of information and transmitted through a series of spans of optical fiber. At a receiver end, the optical signals are detected and decoded by a receiver. In Fiber to the Home (FTTH) applications, modulated optical carrier signals are used to relay broadband coaxial cable signals to and from end users. Similarly, in Fiber to the Antenna (FTTA) applications, modulated optical carrier signals are used to relay broadband radio frequency (RF) signals to and from antennas. In FTTA applications, a single optical fiber is used for bidirectional transmission, with a 1550 nm wavelength band being typically used for downstream transmission from a central station to an RF antenna, and a 1310 nm wavelength band used for an upstream transmission, that is, from the RF antenna back to the central station. 
     As passive fiberoptic links find an increasing use, so increase occurrences of fiber breakage, faulty fiber connections, open fibers, etc. A fiber breakage, or a mere deterioration of a fiber transmission, may occur during normal operation of a passive fiberoptic link. Thus, a need exists for monitoring fiber network integrity and performance level. 
     One drawback of prior art monitoring systems is that a precise location of the fiber breakage point in a fiberoptic network is unknown. Since fiberoptic network may span for tens and even hundreds of kilometers in some cases, it is desirable that a monitoring system have a fiber break locating functionality. 
     One known method of determining a distance to a fiber breakage is Optical Time-Domain Reflectometry (OTDR). In OTDR, a powerful laser pulse is launched at a proximal end of a fiberoptic link, and a time dependence of the reflected light power is monitored. Since speed of light in the fiberoptic link being tested is known, the distance to a fiber breakage point may be determined by measuring a time delay between the launched pulse and a pulse reflected from the fiber breakage point. OTDR, however, may disturb normal operation of a fiberoptic link, because the powerful laser pulse may interfere with optical data transmission. Furthermore, OTDR output data are rather complex, and require trained personnel to interpret. 
     SUMMARY 
     In one embodiment, an optical fiber breakage point may be located by coupling to the optical fiber an out-of-band optical test signal modulated at a modulation frequency that is periodically swept or ramped in time. A distance to the breakage point may be determined from a difference between a value of the modulation frequency of the optical test signal reflected from the breakage point, and a current value of the modulation frequency of the optical signal being coupled to the optical fiber. 
     In some implementations, there is provided a system for in-service monitoring of a fiberoptic network comprising a first fiberoptic link spanning between a test location and a first remote location, wherein the first fiberoptic link carries a first information signal at a first wavelength between the test location and the first remote location, the system comprising: 
     a transmitter comprising a light source for generating an optical test signal at a test wavelength different from the first wavelength, wherein the optical test signal is modulated at a modulation frequency periodically swept at a sweep period from a first modulation frequency to a second modulation frequency; 
     a first test channel comprising: 
     a first coupler for optically coupling the light source to the first fiberoptic link at the test location, for causing the optical test signal to propagate along the first fiberoptic link towards the first remote location; 
     a first wavelength-selective reflector for optically coupling into the fiberoptic link at the first remote location, for redirecting the optical test signal at the test wavelength to propagate back towards the test location, while propagating therethrough the first information signal at the first wavelength; 
     a first signal photodetector optically coupled to the first coupler, for detecting a first returning optical test signal at the test wavelength propagating in a direction from the first remote location towards the first coupler; and 
     a controller operationally coupled to the first signal photodetector and operable to: 
     determine a magnitude of the second returning optical test signal; 
     determine a modulation frequency offset of the second returning optical test signal relative to a current value of the periodically swept modulation frequency; and 
     detect a fault in the second fiberoptic link based on at least one of: 
     a comparison between the determined magnitude of the second returning optical test signal and a reference magnitude of the optical test signal redirected by the second wavelength-selective reflector; and 
     a comparison between the determined modulation frequency offset of the second returning optical test signal to a reference modulation frequency offset of the optical test signal redirected by the second wavelength-selective reflector. 
     In one embodiment, the transmitter comprises a linear frequency ramp generator operably coupled to the light source for modulating the light source so that the modulation frequency is ramped linearly during each sweep period. The system may be expanded or upgraded to monitor multiple fiberoptic links of the fiberoptic network, by providing a similar, separate test channel for each additional fiberoptic link to be monitored. 
     In accordance with the embodiments disclosed herein, there is further provided a method for in-service monitoring of a fiberoptic network comprising a fiberoptic link spanning between spaced apart a test location and a remote location, wherein the fiberoptic link carries an information signal at a first wavelength between the test location and the remote location, the method comprising: 
     (a) generating an optical test signal at a test wavelength different from the first wavelength, wherein the optical test signal is modulated at a modulation frequency periodically swept at a sweep period from a first modulation frequency to a second modulation frequency; 
     (b) optically coupling the optical test signal to the fiberoptic link at the test location, thereby causing the optical test signal to propagate along the fiberoptic link to the remote location; 
     (c) using a wavelength-selective reflector disposed at the remote location to redirect the optical test signal at the test wavelength to propagate back towards the test location, while propagating therethrough the information signal; 
     (d) detecting a returning optical test signal at the test wavelength propagating in a direction from the remote location towards the test location; 
     (e) determining a magnitude of the returning optical test signal, and determining a modulation frequency offset of the returning optical test signal relative to a current value of the periodically swept modulation frequency; and 
     (f) detecting a fault in the fiberoptic link based on at least one of: 
     a comparison between the magnitude of the first returning optical test signal determined in step (e) and a reference magnitude of the optical test signal redirected by the wavelength-selective reflector in step (c); and 
     a comparison between the modulation frequency offset of the returning optical test signal determined in step (e) and a reference modulation frequency offset of the optical test signal redirected by the wavelength-selective reflector in step (c). 
     Step (f) of the above method may include determining that a fiber break has occurred in the fiberoptic link when the modulation frequency offset of the returning optical test signal is smaller than the reference modulation frequency offset; and/or determining that a deterioration has occurred in the fiberoptic link when a magnitude of the returning optical signal at the reference modulation frequency offset is less than the reference magnitude. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments will now be described in conjunction with the drawings, in which: 
         FIG. 1A  illustrates a schematic view of a fiberoptic network having installed an in-service monitoring system; 
         FIG. 1B  illustrates a schematic view of the in-service monitoring system installed in the fiberoptic network of  FIG. 1A ; 
         FIG. 2  illustrates a block diagram of an embodiment of the monitoring system of  FIG. 1B ; 
         FIG. 3A  illustrates a time dependence of a modulation frequency of a test signal according to a preferred embodiment, in which the modulation frequency is linearly ramped; 
         FIG. 3B  illustrates a time dependence of the modulation amplitude when the modulation frequency is linearly ramped as shown in  FIG. 3A ; 
         FIG. 4  illustrates a frequency spectrum of a returning test signal measured by the monitoring system of  FIG. 2 ; 
         FIG. 5A  illustrates a frequency spectrum of a returning test signal during normal operation of the fiberoptic link being monitored; 
         FIG. 5B  illustrates a frequency spectrum of a returning test signal upon detecting an open fiber in the fiberoptic link being monitored; 
         FIG. 6  illustrates an example of a frequency spectrum of a returning signal measured by the system of  FIG. 2 ; 
         FIG. 7  illustrates a block diagram of an embodiment of a modulated light source of the monitoring system of  FIG. 1B ; 
         FIG. 8  illustrates a block diagram of an embodiment of a digital mixer of the monitoring system of  FIG. 1B ; 
         FIG. 9  illustrates a block diagram of an embodiment of a monitoring system having a reference photodiode; 
         FIG. 10  illustrates a block diagram of a multi-channel embodiment of a monitoring system; 
         FIGS. 11A to 11D  illustrate block diagrams of various illustrative embodiments of the monitoring systems of  FIG. 1B ,  FIG. 9 , and  FIG. 10 ; and 
         FIG. 12  illustrates a flow chart of a method embodiment for in-service monitoring a fiberoptic network. 
     
    
    
     DETAILED DESCRIPTION 
     While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives and equivalents, as will be appreciated by those of skill in the art. In  FIGS. 1A ,  1 B,  2 , and  7  to  11 , similar reference numerals refer to similar elements. 
     Referring to  FIG. 1A , a system  100  for in-service monitoring of a fiberoptic network  102  may be provided as described below. The fiberoptic network  102  may include a network node  101  and a plurality of fiberoptic links  103  including a first fiberoptic link  104 . The fiberoptic links  103 ,  104  are connected to the network node  101 . The system  100  is shown installed into the first fiberoptic link  104  for monitoring the first fiberoptic link  104 , which spans between a test location  106  and a first remote location  108  of the fiberoptic network  102 . 
     During normal operation, the first fiberoptic link  104  carries a first information signal  110  at a first wavelength λ 1  between the test location  106  and an RF antenna  105  disposed at the first remote location  108 , for radio transmission of the first information signal  110 . Of course, the RF antenna  105  is only an example, and may be replaced with another terminal device, or another node, not shown, of the fiberoptic network  102 . Furthermore, the transmission on the first fiberoptic link  104  may be, and frequently is, bidirectional. 
     Turning to  FIG. 1B , the system  100  is shown in a greater detail. The system  100  may include a transmitter  112  including a light source  114  and a modulator/driver  128  operationally coupled to the light source  114 . The light source  114  (e.g. a laser diode) generates an optical test signal  116  at a test wavelength λ T  different from the first wavelength λ 1 . In accordance with one embodiment, the optical test signal  116  is modulated by the modulator/driver  128  at a modulation frequency f, which is periodically swept at a sweep period T from a first modulation frequency f 1  to a second, different modulation frequency f 2 . Depending on construction of the modulator/driver  128 , the modulation frequency may be swept linearly or non-linearly from frequency f 1  to frequency f 2, . The optical test signal  116  may also be amplitude modulated, phase modulated, or frequency modulated. An external modulator may also be used (not shown in  FIG. 1B ). 
     The system  100  further includes a first “test channel”  118  dedicated to monitoring the first fiberoptic link  104 . The first test channel  118  may include a first coupler  120  for optically coupling the light source  114  to the first fiberoptic link  104  at the test location  106 , for propagating the optical test signal  116  generated by the light source  114  along the first fiberoptic link  104  towards the first remote location  108 . In the embodiment shown, the first coupler  120  includes a wavelength division multiplexor (WDM)  130  for multiplexing the optical test signal  116  at the test wavelength λ T  and the first information signal  110  at the first wavelength λ 1 , and an optical splitter  132  optically coupled at its output port  151  to the WDM  130 . 
     The first test channel  118  may further include a first wavelength-selective reflector  122  ( FIG. 1B ) optically coupled into the first fiberoptic link  104  at the first remote location  108  ( FIG. 1A ), for redirecting e.g. reflecting the optical test signal  116  at the test wavelength λ T  to propagate back towards the test location  106 . At the same time, the first wavelength-selective reflector  122  may propagate through, e.g. transmit, the first information signal  110  at the first wavelength λ 1 , for reception at the RF antenna  105  or another terminal or intermediate communications device, as the case may be. 
     The first test channel  118  may further include a first signal photodetector  124  optically coupled to the first coupler  120 , for detecting a first returning optical test signal  117  at the test wavelength λ T  propagating in a direction from the first remote location  108  towards the first coupler  120  disposed at the test location  106 . More specifically, the first signal photodetector  124  is optically coupled to the optical splitter  132 , which is optically coupled to the WDM  130 , which is optically coupled to the first fiberoptic link  104  at the test location  106 . 
     In the embodiment shown in  FIG. 1B , the optical splitter  132  has first  141  and second  142  input ports and an output port  151 . The assignment of ports as “input” and “output” ports is for convenience only, because the optical splitter  132  is a bidirectional device. The first input port  141  is optically coupled to the light source  114  of the transmitter  112  for coupling the optical test signal  116  into the first fiberoptic link  104 , and the second input port  142  is optically coupled to the first signal photodetector  124 , for receiving the first returning optical test signal  117 . The output port  151  is optically coupled to the WDM  130  for launching the optical test signal  116  and for receiving the first returning optical test signal  117 . 
     The system  100  may further include a controller  126  operationally coupled to the first signal photodetector  124 . The controller  126  may be operable, e.g. via a software or hardware configuration, to determine a modulation frequency offset Δf of the first returning optical test signal  117  relative to a current value of the periodically swept modulation frequency f, and determining a magnitude M(Δf) of the first returning optical test signal  117  at the modulation frequency offset Δf. The current modulation signal at the periodically swept modulation frequency f may be obtained by the controller  126  from the modulator/driver  128  via a control line  127 . 
     The controller  126  may be further configured for detecting a fault in the first fiberoptic link  104  based on the determined modulation frequency offset Δf and the determined magnitude M(Δf) of the first returning optical test signal  117 . More specifically, the controller  126  may be configured for determining the fault in the first fiberoptic link  104  based on the determined modulation frequency offset Δf and the determined magnitude M(Δf) in relation to a “reference” modulation frequency offset Δf REF  and a “reference” magnitude M REF (Δf REF ), respectively, of the optical test signal  116  redirected by the first wavelength-selective reflector  122  to propagate back to the test location  106 . In other words, the first wavelength-selective reflector  122  may function as an amplitude and frequency reference for the detected first returning optical test signal  117 , allowing a length-to-fault and optical throughput calibration. During setting up of the monitoring system  100 , a technician may measure the reference magnitude M REF  and the frequency offset Δf REF  of the optical test signal  116  redirected (reflected) by the first wavelength-selective reflector  122  to propagate back to the test location  106 , and store the reference magnitude M REF  and the frequency offset Δf REF  in a memory of the controller  126 . During the subsequent monitoring, the controller  126  may perform a comparison between the determined magnitude M of the first returning optical test signal  117  and a reference magnitude M REF  stored in the memory of the controller  126 . The controller  126  may also perform a comparison between the determined modulation frequency offset Δf of the first returning optical test signal  117  to a reference modulation frequency offset Δf REF  stored in the memory of the controller  126 . 
     Referring to  FIG. 2 , a system  200  for in-service monitoring the first fiberoptic link  104  is an illustrative embodiment of the system  100  of  FIGS. 1A and 1B . The system  200  of  FIG. 2  includes a transmitter  212 , a WDM coupler  220  optically coupled to the transmitter  212 , a photodiode  224  optically coupled to the WDM coupler  220 , a transimpedance amplifier (TIA)  225  electrically coupled to the photodiode  224 , a controller  226  electrically coupled to the TIA  225  and the transmitter  212 , a detection control unit  239  electrically coupled to the controller  226 , and a wavelength-selective reflector  222  optically coupled to the first remote location  108  of the first fiberoptic link  104 . The transmitter  212  includes serially coupled a signal source  228 A, a laser driver  228 B, and a laser diode  214  emitting at the wavelength of 1625 nm in this illustrative example. 
     The signal source  228 A provides a modulation signal  229  for modulating the laser driver  228 B. The signal source  228 A functions as a frequency ramp generator, preferably a linear frequency ramp generator. In other words, it is preferable that the modulation signal  229  have the modulation frequency f M  periodically linearly ramped, or changing linearly with time e.g. in sawtooth-like fashion. 
     Referring to  FIGS. 3A and 3B , a time dependence  302  ( FIG. 3A ) of the modulation frequency f of the modulation signal  229  is shown. One can see that the modulation frequency f increases linearly in time from the first f 1  to the second f 2  modulation frequency, with a period T. A frequency sweeping range B ( FIG. 3A ) is equal to f 2 −f 1 .  FIG. 3B  illustrates a time dependence of the linearly frequency ramped modulation signal  229 . In  FIG. 3A , the modulation frequency f M  is ramped linearly during each sweep period T. The frequency f M  may also be periodically ramped down, or ramped up and then down, preferably in a linear fashion for simplified processing. 
     Referring back to  FIG. 2 , the modulation signal  229  may be applied to the laser driver  228 B for providing a driving current  231  that is amplitude modulated with the linearly ramped modulation frequency f as explained above. As a result, the optical test signal  116  is amplitude modulated with linearly ramped modulation frequency f Emission wavelength of the laser diode  214  may also be modulated in some types of the laser diode  214 . The amplitude modulated optical test signal  116  is coupled to the WDM coupler  220 , where it is multiplexed with the first information signal  110 . Both signals  110  and  116  co-propagate in the first fiberoptic link  104  towards the wavelength-selective reflector  222 . 
     The wavelength-selective reflector  222  may include an optical filter  245  and a mirror  246 . In operation, the optical filter  245  directs the first information signal  110  towards its intended destination, not shown. The optical test signal  116  at the wavelength of 1625 nm in this example is reflected by the mirror  246  to propagate back through the first fiberoptic link  104  towards the WDM coupler  220 , which is constructed to couple a reflected optical test signal  117  to the photodiode  224 . The transimpedance amplifier  225  amplifies the photocurrent of the photodiode  224 . The transimpedance amplifier  225  is usually disposed proximate the photodiode  224  to lower the noise figure. 
     In the embodiment shown in  FIG. 2 , the controller  226  includes an amplifier  234  and a mixer  252  electrically coupled together. In operation, the amplifier  234  amplifies a photocurrent  129  representing the reflected optical test signal  117 , and provides an amplified output signal  254  to the mixer  252 . A reference signal  229 A obtained from the linearly frequency ramped modulation signal  229 , e.g. a copy of the linearly frequency ramped modulation signal  229 , is provided to the mixer  252  via a control line  227 . The mixer  363  mixes the signals  254  and  229 A, providing a signal  258  at a differential frequency, that is, the frequency offset Δf. Since the reflected optical test signal  117  is delayed relative to the reference signal  229 A due to a finite speed of propagation of the optical test signal  116  and the reflected optical test signal  117  in the first fiberoptic link  104 , the frequency offset Δf corresponds to a time offset Δt, as shown in  FIG. 3A . Therefore, one may configure the controller  226  to determine a magnitude of the signal  258  at the frequency offset Δf, by obtaining a frequency spectrum of the mixed signal, and looking for peaks in the frequency spectrum. From the value of the frequency offset Δf and the corresponding time offset Δt, a distance to a reflective fault in the first fiberoptic link  104  may be determined. 
     In the embodiment shown in  FIG. 2 , the controller  226  further includes a low-pass filter (LPF)  235 , an analog-to-digital converter (ADC)  236 , a Fast Fourier Transform (FFT) unit  237 , and a detection unit  238  serially electrically coupled together. In operation, the LPF  235  filters the signal  258 , which is then digitized by the ADC  236 . The FFT unit  237  obtains the frequency spectrum of the signal  258 . The frequency spectrum may then be analyzed by the detection unit  238  for peaks, to determine fiber breaks, loose connectors, transmittivity loss, etc. The detection control unit  239  performs general control of the test system  200 , including scheduling, reporting, etc. 
     Turning to  FIG. 4 , an illustrative frequency spectrum  400  includes a reference peak  402  corresponding to the reflection of the optical test signal  116  from the wavelength-selective reflector  222 , as well as three reflection peaks  404 ,  406 , and  408  corresponding to open-fiber reflections of the optical test signal at distances of d 1 , d 2  and d 3 , respectively, from the test location  106  ( FIG. 1A ), where d 1 &gt;d 2 &gt;d 3 . For data processing purposes, the entire frequency spectrum  400  may be broken down into “frequency bins”  410  shown in  FIG. 4  with dashed rectangles. Each frequency bin  410  corresponds to a certain distance from the test location  106 . The detection unit  238  may be configured to search for peaks in each of the frequency bins  410 , e.g. the reflection peaks  404 ,  406 , and  408 , thus determining the magnitudes of reflections, as well as distances from the test location  106 , at which these reflections occur. This opens up a possibility of detection a fault such as fiber breakage. In other words, by measuring the magnitude of the differential frequency signal  258  in each frequency bin  410 , one can effectively determine an optical power level of the reflected optical test signal  117 , which has been reflected from a fiber segment of the first fiberoptic link  104  corresponding to each frequency bin  410 . The reflections detected in a particular fiber segment may indicate a fiber breakage in that segment. 
     The magnitude and the frequency offset of reference peak  402  provide the reference magnitude M REF  and the reference frequency offset Δf REF , which may then be used as references for magnitudes M and frequency offsets Δf of the reflection peaks  404 ,  406 , and  408 . In other words, the magnitudes M and the frequency offsets Δf of the reflection peaks  404 ,  406 , and  408  may be determined in relation to, e.g. as a percentage of the reference magnitude M REF  and the reference frequency offset Δf REF , respectively. For instance, a mere deterioration of the transmission of the first fiberoptic link  104  may be determined based on the detected magnitude of the reflected optical test signal  117  at the reference frequency offset Δf REF . 
     Referring to  FIG. 5A , a modulation frequency spectrum  500 A of the reflected optical test signal  117  corresponds to a normal operation of the first fiberoptic link  104 , that is, when no fiber breaks or transmission deterioration are present in the first fiberoptic link  104 . A noise floor  505  shows no pronounced peaks, indicating that no anomalous reflections are present. The reference peak  402  has the reference magnitude M REF , indicating that no transmission deterioration is present. In one embodiment, the detection unit  238  of the controller  226  is configured to determine that a deterioration has occurred in the first fiberoptic link when the magnitude M REF  of the first returning optical signal  117  at the reference modulation frequency offset Δf REF  is less than a pre-defined threshold  507 . 
     Turning to  FIG. 5B , a modulation frequency spectrum  500 B of the reflected optical test signal  117  corresponds to a case when a fiber break has occurred in the first fiberoptic link  104  between the first wavelength-selective reflector  122  installed at the first remote location  108 , at the test location  106 . The fiber break caused the optical test signal  116  to partially reflect from the fiber break point. Since the location of the reflection is closer than the location of the first wavelength-selective reflector  122 , the modulation frequency offset Δf of the first returning optical test signal  117  is smaller than the reference modulation frequency offset Δf REF . Accordingly, a reflection peak  508  has the modulation frequency offset Δf smaller than Δf REF . The detection unit  238  of the controller  226  may be configured for determining that the fiber break or cut has occurred in the first fiberoptic link  104  when the modulation frequency offset Δf of the first returning optical test signal  117  is smaller than the reference modulation frequency offset Δf REF . As explained above, the location of the break or cut may be determined from the magnitude of the modulation frequency offset Δf. 
     The monitoring system  200  has been used in an experiment aimed at locating an “open” or a broken or cut fiber in the first fiberoptic link  104 . The first fiberoptic link  104  has been simulated with 1 m long (“0 km fiber”), 10 km long, 25 km long, 35 km long, and 50 km long spans of an optical fiber. The data traffic at the center wavelength of 1530 nm, corresponding to the first information signal  110 , was propagated in the fiber spans of different lengths together with a tone at 1625 nm, corresponding to the optical test signal  116 . For each one of the fiber span lengths, two tests were performed, one with an open connector and one with the wavelength-selective reflector  222  optically coupled at the end of each fiber span. A frequency sweep range of B=5 MHz over a sweeping time period of about 2 ms, and a frequency bin size of about 5 kHz were used. The results of testing are summarized in the following Tables 1 and 2. In Table 1, the experimental data are organized in rows corresponding to different fiber span lengths. In Table 2, the experimental data are organized in rows corresponding to the optical power levels detected. 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Rx 
                 Tone 
                   
               
               
                   
                 power 
                 frequency 
                 Calculated 
               
               
                 Test Condition 
                 (dBm) 
                 (MHz) 
                 distance (km) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 50 km 
                 With reflector 222 
                 −34.9 
                 1.259 
                 51.35 
               
               
                 fiber 
                 Open connector 
                 −42.9 
                 (tone is below 
                 N/A 
               
               
                   
                   
                   
                 noise floor) 
               
               
                 35 km 
                 With reflector 222 
                 −28.2 
                 0.894 
                 36.46 
               
               
                 fiber 
                 Open connector 
                 −40.1 
                 0.898 
                 36.63 
               
               
                 25 km 
                 With reflector 222 
                 −24.1 
                 0.63 
                 25.7 
               
               
                 fiber 
                 Open connector 
                 −36.6 
                 0.63 
                 25.7 
               
               
                 10 km 
                 With reflector 222 
                 −17 
                 0.264 
                 10.77 
               
               
                 fiber 
                 Open connector 
                 −30 
                 0.264 
                 10.77 
               
               
                 0 km 
                 With reflector 222 
                 −12.2 
                 0 
                 0 
               
               
                 fiber 
                 Open connector 
                 −25.9 
                 0 
                 0 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 50 km 
                 35 km 
                 25 km 
                 10 km 
                 0 km 
               
               
                 Optical Power 
                 fiber 
                 fiber 
                 fiber 
                 fiber 
                 fiber 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Optical power with 
                 −34.9 
                 −28.2 
                 −24.1 
                 −17 
                 −12.2 
               
               
                 reflector 222 (dBm) 
               
               
                 Open connector 
                 −42.9 
                 −40.1 
                 −36.6 
                 −30 
                 −25.9 
               
               
                 optical power (dBm) 
               
               
                 Optical power difference 
                 8 
                 11.9 
                 12.5 
                 13 
                 13.7 
               
               
                 (dB) 
               
               
                 Tone power difference 
                 16 
                 23.8 
                 25 
                 26 
                 27.4 
               
               
                 (dB) 
               
               
                 Detection margin (dB) 
                 +/−8 
                 +/−12 
                 +/12.5 
                 +/−13 
                 +/−13.7 
               
               
                   
               
             
          
         
       
     
     In Table 2, the “Tone power difference” corresponds to the difference in the optical power levels of the returning optical test signal  117 . One can see that the detection margin of +/−8 dB may be provided for fiber spans as long as 50 km. 
     Referring to  FIG. 6 , a typical measured differential modulation frequency spectrum  600  of the first returning optical test signal  117  is shown. Peak  601  at differential frequency of 630 KHz corresponds to reflections along the first fiberoptic link  104  from the wavelength-selective reflector  222 . The modulation frequency f has been ramped from 15 MHz to 20 MHz in this example. In one embodiment, the light source  112  has a peak optical power of no greater than 5 mW, to avoid impacting normal functioning of the fiberoptic network  102  ( FIG. 1A ). Also in one embodiment, a difference between the second f 2  and first f 1  modulation frequencies, that is, the modulation frequency ramp amplitude B ( FIG. 3A ) is at least 5 MHz. The optical test signal  116  may be amplitude modulated at no less than 80% modulation index. 
     Turning to  FIG. 7  with further reference to  FIG. 1B , an embodiment  712  ( FIG. 7 ) of the transmitter  112  ( FIG. 1B ) includes the light source  114  ( FIG. 7 ) and an electro-optical modulator  711  optically coupled to the light source  114 . The light source  114  is energized by a DC current supply. In this embodiment, a linear frequency ramp generator  728  may be operationally coupled to the electro-optical modulator  711  for modulating at least one of an amplitude, frequency and a phase of the optical test signal  116 . The linear frequency ramp generator  728  may provide a reference signal to the controller  126  ( FIG. 1B ), or the reference signal  229 A to the mixer  252  of the controller  226  of the monitoring system  200  ( FIG. 2 ). 
     Referring again to  FIG. 2 , the mixer  252  may be analog or digital. Turning to  FIG. 8  with further reference to  FIG. 2 , a fully digital controller embodiment  826  ( FIG. 8 ) may include an analog to digital converter  802  for digitizing the amplified output signal  254  of the first photodiode  224  ( FIG. 2 ) and the signal  229 A at the linearly ramped modulation frequency, to obtain respective digitized signals, and a digital signal processing (DSP) unit  804  for mixing the digitized signals to obtain a digitized test signal  858  at the differential frequency Δf. 
     Turning to  FIG. 9  with further reference to  FIG. 1B , a monitoring system  900  of  FIG. 9  is an embodiment of the monitoring system  100  of  FIG. 1B . Below, only differences between the monitoring systems  900  and  100  are described. An optical splitter  932  of the monitoring system  900  ( FIG. 9 ) includes first  941  and second  942  input ports and first  951  and second  952  output ports. A first test channel  918  of the monitoring system  900  further includes a reference photodetector  924  optically coupled to the second output port  952  for detecting a portion of the optical test signal  116  coupled to the first fiberoptic link  104  at the test location  106 , so as to obtain a reference signal  929 A. 
     A controller  926  of the monitoring system  900  may further include a mixer, not shown, for mixing a photocurrent  129 A generated by the first signal photodetector  124  with the reference signal  929 A to obtain a signal at the differential frequency, or the modulation frequency offset Δf. The controller  926  may further be configured for determining the magnitude M(Δf) of the returning optical test signal  117  at the differential frequency Δf, and a value of the differential frequency Δf. 
     The system  100  of  FIGS. 1A and 1B ,  200  of  FIG. 2 , and  900  of  FIG. 9  may be expanded to accommodate in-service monitoring of other fiberoptic links of the fiberoptic network  102 . Referring to  FIG. 10 , the fiberoptic network  102  includes a first fiberoptic link  104 A spanning between the test location  106  and a first remote location  108 A, and a second fiberoptic link  104 B spanning between the test location  106  and a second remote location  108 B. The first fiberoptic link  104 A carries a first information signal  110 A at a first wavelength between the test location  106  and the first remote location  108 A, and the second fiberoptic link  104 B carries a second information signal  110 B at a second wavelength between the test location  106  and the second remote location  108 B. 
     To monitor the first fiberoptic link  104 A, one embodiment of a multi-channel monitoring system  1000  includes a first test channel  118 A, which is similar to the first test channel  118  of the system  100  of  FIGS. 1A and 1B . Briefly, the first test channel  118 A includes a first coupler  120 A for optically coupling the light source  114  to the first fiberoptic link  104 A at the test location  106 , for causing the optical test signal  116  to propagate along the first fiberoptic link  104 A towards the first remote location  108 A. A first wavelength-selective reflector  122 A is provided for optically coupling into the first fiberoptic link  104 A at the first remote location  108 A, for redirecting the optical test signal  116  to propagate back to the first coupler  120 A at the test location  106 , while propagating the first information signal  110 A. A first signal photodetector  124 A is optically coupled to the first coupler  120 A for detecting a first returning optical test signal  117 A propagating in a direction from the first remote location  108 A towards the first coupler  120 A. 
     To monitor the second fiberoptic link  104 B, the system  1000  further includes a second test channel  118 B similar to the first test channel  118 A. The second test channel  118 B may include a second coupler  120 B for optically coupling the light source  114  to the second fiberoptic link  104 B at the test location  106 , for causing the optical test signal  116  to propagate along the second fiberoptic link  104 B towards the second remote location  108 B. A second wavelength-selective reflector  122 B may be provided for optically coupling into the second fiberoptic link  104 B at the second remote location  108 B, for redirecting the optical test signal  116  to propagate back to the second coupler  120 B at the test location  106 , while propagating the second information signal  110 B. A second signal photodetector  124 B may be optically coupled to the second coupler  120 B for detecting a second returning optical test signal  117 B propagating in a direction from the second remote location  108 B towards the coupler  120 B. 
     In the embodiment shown, the system  1000  further includes a test signal splitter  1002  having an input port  1004  and first  1006 A and second  1006 B output ports. The input port  1004  is optically coupled to the light source  114  of the transmitter  112 , the first output port  1006 A is optically coupled to the first coupler  120 A of the first test channel  118 A, and the second output  1006 B port is optically coupled to the second coupler  120 B of the second test channel  118 B. 
     A controller  1026  of the system  1000  is similar to the controller  126  of the system  100  of  FIGS. 1A and 1B . The controller  1026  of the system  1000  is operationally coupled to first and second test channels  118 A and  118 B. Specifically, the controller  1026  is coupled the first signal photodetector  124 A and the second signal photodetector  124 B and configured for determining magnitudes M B  and M B  of the first and second returning optical test signals  117 A and  117 B, respectively, and determining modulation frequency offsets Δf A  and Δf B  of the first and second returning optical test signals  117 A and  117 B, respectively, relative to a current value of the periodically swept modulation frequency f. The controller  1026  may also be configured for determining a fault in the first and second fiberoptic links  104 A/B based on the determined magnitudes M B  and M B  and the determined modulation frequency offsets Δf A  and Δf B  of the first and second returning optical test signals  117 A and  117 B, respectively. The system  1000  may further be upgraded or expanded to include similar test channels  118 C,  118 D,  118 E, etc. for in-service monitoring other fiberoptic links as these are added to the fiberoptic network  102 . The controller  1026  may be time-shared. 
     Several non-limiting, illustrative implementations of the system  100  of  FIGS. 1A and 1B ,  200  of  FIG. 2 ,  900  of  FIG. 9 , and  1000  of  FIG. 10  will now be considered. 
     Referring to  FIG. 11A , a system  1100 A for in-service monitoring of the first fiberoptic link  104  may include a modulated light source  1112  and a test channel  1118 A. The modulated light source  1112  may include serially electrically coupled a linear frequency ramp generator  1128 A, a laser driver  1128 B, and the light source  114 . Function of these elements is similar to that of the system  200  of  FIG. 2 , specifically the signal source  228 A, the laser driver  228 B, and the laser diode  214 . An optical splitter  1102  may be provided for future addition of test channels. 
     Still referring to  FIG. 11A , the test channel  1118 A of the system  1100 A of  FIG. 11A  includes similar elements as the test channel  118  of the system  100  of  FIGS. 1A and 1B , specifically the WDM  130 , the optical splitter  132 , the first signal photodetector  124 , preferably an avalanche photodiode (APD). The TIA  225  may be provided to boost signal to noise ratio. An additional 1×2 coupler  1103  may be provided for coupling another test channel, not shown. An additional 2×2 coupler  1172  optically coupled to a pair of photodiodes  1174  may be provided for passively measuring power levels of λ 1 , the upstream bidirectional customer data being connected via an optical connector  1176 . 
     One distinctive feature of the first test channel  118 A is that the test channel  1118 A includes an internal signal processing circuitry including a difference detector  1152 , a time delay Δt/distance computation unit  1153 , and a power determination unit  1155  coupled to a DSP unit  1104  for performing comparison with corresponding reference values provided by a test signal reflecting from the wavelength-selective reflector  122 . Finally, an input/output circuit  1139  is provided for interfacing with external circuitry, not shown, for reporting link status/abnormal condition(s) detected. 
     Turning to  FIG. 11B , a system  1100 B for in-service monitoring of the first fiberoptic link  104  is similar to the system  1100 A of  FIG. 11A . One distinctive feature of the system  1100 B of  FIG. 11B , as compared with the system  1100 A of  FIG. 11A , is that a test channel  1118 B of the system  1100 B of  FIG. 11B  includes a reference photodetector  1124  for providing a reference electrical signal, similarly to the reference photodetector  924  of the monitoring system  900  of  FIG. 9 . 
     Referring now to  FIG. 11C , an system  1100 C for in-service monitoring of the first fiberoptic link  104  is similar to the system  1100 B of  FIG. 11B . One distinctive feature of the system  1100 C of  FIG. 11C , as compared with the system  1100 B of  FIG. 11B , is that the modulated light source  1112  is integrated into a test channel  1118 C. This allows the reference photodetector  1124  to be coupled to the optical splitter  1102 , simplifying overall construction. 
     Turning to  FIG. 11D , a system  1100 D for in-service monitoring of the first fiberoptic link  104  is similar to the system  1100 C of  FIG. 11C . The system  1100 D of  FIG. 11D  is further integrated into a single, standalone unit. In this embodiment, the light source  114  is directly optically coupled to a single splitter  132  and a single WDM  130 . The DSP unit  1104  may be implemented in field-programmable gate array (FPGA). System  1100 D may be implemented as a single small form pluggable (SFP) package, providing considerable cost and space savings. 
     Referring to  FIG. 12  with further reference to  FIGS. 1A and 1B , a method  1200  for in-service monitoring of the fiberoptic network  102  may be provided according to one embodiment. The method  1200  includes a step  1201  of generating the optical test signal  116  at the test wavelength λ T  different from the first wavelength λ 1 . The optical test signal  116  is modulated at a modulation frequency f periodically swept at the sweep period T from the first modulation frequency f 1  to the second modulation frequency f 2 . In a preferred embodiment, the modulation frequency f is ramped linearly between the first modulation frequency f 1  and the second modulation frequency f 2  during each sweep period T. 
     In a next step  1202 , the optical test signal  116  is optically coupled to the first fiberoptic link  104  at the test location  106 , thereby causing the optical test signal  116  to propagate along the first fiberoptic link  104  to the first remote location  108 . In a next step  1203 , the wavelength-selective reflector  122  disposed at the first remote location  108  is used to redirect the optical test signal  116  to propagate back towards the test location  106 , while propagating the first information signal  110  through the wavelength-selective reflector  122 . In a next step  1204 , the returning optical test signal  117  at the test wavelength λ T  is detected. 
     In a next step  1205 , the magnitude M of the returning optical test signal  117  the modulation frequency offset Δf of the returning optical test signal  117  is detected, and is compared to the current value of the periodically swept modulation frequency. Finally, in a step  1206 , a fault in the first fiberoptic link  104  is detected based on the determined magnitude M and the determined modulation frequency offset Δf of the returning optical test signal  117 . The detection step  1206  may include comparing the determined magnitude M and the determined modulation frequency offset Δf to a reference magnitude M REF  and a reference modulation frequency offset Δf REF , respectively, of the optical test signal  117  redirected by the wavelength-selective reflector  122  in the reflecting step  1203 , as explained above. 
     In one embodiment, the detection step  1206  may include determining that a fiber break has occurred in the first fiberoptic link  104  when the modulation frequency offset Δf of the returning optical test signal  117  is smaller than the reference modulation frequency offset Δf REF . Furthermore in one embodiment, the detection step  1206  may include determining that a deterioration has occurred in the fiberoptic link when the magnitude M of the returning optical signal at the reference modulation frequency offset is less than the reference magnitude M REF . To ensure that the first information signal  110  at the first wavelength λ 1  is not perturbed during transmission, the optical test signal  116  may have a peak optical power of no greater than 1 mW. Furthermore in one embodiment, the first fiberoptic link  104  may carry a second information signal at a second wavelength λ 2  from the remote location  108  to the test location  106 . The second wavelength λ 2  is typically different from the first wavelength λ 1 . The test wavelength is different from both the first wavelength λ 1  and the second wavelength λ 2 . 
     The hardware used to implement the various illustrative logics, logical blocks, modules, and circuits described in connection with the aspects disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but, in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. Alternatively, some steps or methods may be performed by circuitry that is specific to a given function. 
     The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments and modifications, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.