Abstract:
This specification describes technologies relating to optical fiber network monitoring. A monitoring system is provided. The monitoring system includes a fiber network including a plurality of branch fibers and a main station coupled to a main fiber of the fiber network to broadcast communications signals to a plurality of branch stations. The monitoring system includes a monitoring device configured to transmit a monitoring signal and detect reflected portions of the monitoring signal such that the received portions specifically identify a condition of specific branch fibers of the plurality of branch fibers and a plurality of filtering devices coupled to each respective branch fiber, each filtering device including a transmission window configured to pass a plurality of communication wavelengths and a distinct wavelength of the monitoring signal, where the distinct wavelength is not within the transmission window, and block the remaining wavelengths, where the distinct wavelength identifies the respective branch fiber.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119 to PCT Application Serial No. PCT/CN2008/000817, filed on Apr. 21, 2008, to inventors Tian Zhu, Pei-Ling Wu, and Peng Wang, and titled Fiber Network Monitoring. 
     
    
     BACKGROUND 
       [0002]    The present disclosure relates to fiber network monitoring. 
         [0003]    Optical fiber networks typically include a main fiber connected to a number of branch fibers. A signal can be broadcast from a source location to multiple destination locations through the fiber network. Typically, the condition of the fiber network is monitored. A monitor can be placed at a location in the network, for example, at the broadcasting location. The monitor remotely monitors, e.g., from the broadcasting location, the condition of the optical fiber network. 
         [0004]    Optical time domain reflectometry (“OTDR”) is typically used for inspecting a single fiber. A short pulse of light is transmitted into a fiber using an OTDR device. Backscattered light from the light pulse in the fiber is monitored using the OTDR device for abrupt changes indicative of a fault in the fiber. For a fiber network, since the light pulse splits and propagates to all branches, the detected backscattered light is contributed from all branches. Consequently, even when a fault is detected, the fault may not be able to be identified with reference to a specific branch fiber. 
       SUMMARY 
       [0005]    This specification describes technologies relating to optical fiber network monitoring. In general, one aspect of the subject matter described in this specification can be embodied in monitoring systems including a fiber network including multiple branch fibers and a main station coupled to a main fiber of the fiber network, the main station configured to broadcast communications signals to multiple branch stations coupled to the respective branch fibers of the multiple branch fibers. The monitoring system also includes a monitoring device configured to transmit a monitoring signal and detect reflected portions of the monitoring signal such that the received portions of the monitoring signal specifically identify a condition of specific branch fibers of the multiple branch fibers and multiple filtering devices coupled to each respective branch fiber, each filtering device including a transmission window configured to pass multiple communication wavelengths and a distinct wavelength of the monitoring signal, where the distinct wavelength is not within the transmission window, and block the remaining wavelengths, where the distinct wavelength identifies the respective branch fiber. Other embodiments of this aspect include corresponding methods and apparatus. 
         [0006]    These and other embodiments can optionally include one or more of the following features. The intensity of the monitoring signal can be modulated by a modulating function. The modulating function can be periodic. The monitoring device can include a circulator coupled between a signal source and a receiver. 
         [0007]    The monitoring system can further include a splitter configured to separate the monitoring signals into each of the multiple branch fibers. The monitoring system can further include multiple reflecting elements, each reflecting element being positioned in along a corresponding branch fiber, each reflecting element being configured to reflect the particular wavelength passed by the corresponding filtering device of the branch fiber. 
         [0008]    Each filtering device can include a first fiber, a first lens for collimating light exiting from the first fiber, a filter for partially transmitting one or more transmission wavelengths and reflecting one or more reflection wavelengths of the collimated light according to a particular transmission function and where the reflection wavelengths do not exit the filtering device, a second lens for focusing filtered light including the one or more transmission wavelengths transmitted by the filter, and a second fiber for receiving focused light focused by the second lens. 
         [0009]    The filtering device can be configured to transmit particular wavelengths input to both the first fiber and the second fiber while blocking other wavelengths. The transmission function of the filter includes the transmission window and a defined width peak corresponding to a particular monitoring wavelength, where the transmission window is separated from the peak by a specified range of non-passed wavelengths. The transmission window can be substantially between 1250 nm and 1585 nm. A peak-width can be at a substantially 25% pass ratio of the defined width peak is less than 10 nm. The transmission function of the filter can cover substantially S-band and C-band, and can include a defined width peak substantially between 1561 nm and 1700 nm. The filter can be a thin films filter. The filtering device can be configured for coupling to a fiber connector selected from a group consisting of SC, LC, ST, and MU. 
         [0010]    In general, one aspect of the subject matter described in this specification can be embodied in methods that include the actions of receiving in a first direction one or more communications signals, the communications signals having wavelengths within a transmission window, receiving in the first direction a monitoring signal, the monitoring signal including one or more wavelengths distinct from the wavelengths of the transmission window, where the wavelengths of the transmission window and the wavelengths of the monitoring signal are separated by a specified range of wavelengths, passing the communications signals, passing a particular wavelength of the monitoring signal, and blocking all other wavelengths. Other embodiments of this aspect include corresponding systems and apparatus. 
         [0011]    These and other embodiments can optionally include one or more of the following features. The method can further include receiving from a second direction a reflected monitoring signal and passing the reflected monitoring signal. The intensity of the monitoring signal can be modulated by a modulating function. 
         [0012]    In general, one aspect of the subject matter described in this specification can be embodied in an apparatus that include a thin films filter having a specified transmission function including a transmission window covering an S-band and a C-band and a defined width peak at a specified wavelength corresponding to a particular monitoring signal and not within the transmission window. 
         [0013]    These and other embodiments can optionally include the following feature. The apparatus can be configured for coupling to a fiber connector selected from a group consisting of SC, LC, ST, and MU. 
         [0014]    In general, one aspect of the subject matter described in this specification can be embodied in a system that includes a source configured to provide an optical signal having multiple wavelengths; multiple filters disposed in distinct locations within an optical fiber network, each filter for partially transmitting one or more transmission wavelengths of the optical signal and reflecting one or more reflection wavelengths of the optical signal according to a particular transmission function, where the transmission function of each filter of the multiple filters includes a transmission window including one or more communication wavelengths and a distinct transmission peak corresponding to a respective monitoring wavelength for the respective filter; and a monitor configured to identify problems at particular locations in the optical fiber network according to wavelengths of the optical signal returned from the multiple filters. Other embodiments of this aspect include corresponding methods and apparatus. 
         [0015]    These and other embodiments can optionally include the following feature. An intensity of the optical signal can be modulated by a modulating function. 
         [0016]    Particular embodiments of the subject matter described in this specification can be implemented to realize one or more of the following advantages. A filtering device is provided for monitoring and identifying individual branches in a fiber network that is relatively inexpensive, easily installable, and simple to operate. 
         [0017]    The filtering device can include multiple ports that can be mated to various types of fiber connectors. Thus, an installer can easily add or change the filtering device in a fiber network. The filtering device can be used for identifying and monitoring individual branch in a fiber network at substantially the same time. The filter can be designed and manufactured to provide a transmission window for communication signals and a narrow transmission peak for a monitoring signal with a specific wavelength encoding a specific branch in a fiber network. Collimating optics for the filtering device can be designed and packaged to provide a very narrow width of the transmission peak such that the peak-width at substantially a 25% level can be 1 nm or less. Additionally, the packaging of the filtering device can take advantage of the matured technology for WDM device packaging, which can be stable in wide ranges of temperature and humidity. 
         [0018]    Accumulated leaking signals from all branches in the fiber network can generate a false alarm. The wavelength filtering device can filter the optical signal twice in both the forward and backward direction. Thus, the filter passes one specific composite wavelength and rejects other composite wavelengths of the monitoring signal in both directions. The leakage of other composite wavelengths can be suppressed. 
         [0019]    The intensity of a monitoring signal can be modulated to increase a signal-to-noise ratio. In the event of a fault including a broken or damaged optical fiber, the reflected intensity-modulated signal can provide information to infer the fault&#39;s location without using an expensive OTDR device. 
         [0020]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the invention will become apparent from the description, the drawings, and the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  shows a block diagram of an example optical fiber network using conventional monitoring. 
           [0022]      FIG. 2  shows a block diagram of an example fiber network including individual branch monitoring. 
           [0023]      FIG. 3  shows a flowchart of an example method for monitoring branches in an optical fiber network. 
           [0024]      FIG. 4  shows a display of an example transmission function of a filter for identifying and monitoring individual branches in a fiber network. 
           [0025]      FIG. 5  shows a block diagram of an example thin films filter. 
           [0026]      FIG. 6  shows an example transmission function for a filter. 
           [0027]      FIG. 7  shows an example filtering device. 
           [0028]      FIG. 8  shows an example filtering device mating to fiber connectors. 
           [0029]      FIG. 9  shows an example monitoring device. 
       
    
    
       [0030]    Like reference numbers and designations in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0031]      FIG. 1  shows a block diagram of an example optical fiber network  10  using conventional monitoring. The optical fiber network  10  includes a main fiber  20  coupled to multiple branch fibers, for example, four branch fibers  22 ,  24 ,  26 , and  28 . Each of the branch fibers  22 ,  24 ,  26 , and  28  is coupled to a respective branch station  32 ,  34 ,  36 , and  38 . Through the main fiber  20  and branch fibers  22 ,  24 ,  26 , and  28 , the network  10  joins a main station  30  and the branch stations  32 ,  34 ,  36 , and  38 . 
         [0032]    In some implementations, the optical fiber network  10  can be a passive optical network (“PON”) for “fiber to the x” (“FTTX”) applications. The main station  30  can be, for example, an optical line terminal (“OLT”), and branch stations  32 ,  34 ,  36 , or  38  can each be an optical network unit (“ONU”). 
         [0033]    A monitoring device  40  is positioned relative to the main station  30  for monitoring the condition of the network. For example, the monitoring device  40  can be part of the main station  30  or coupled to the main station  30 . Monitoring the condition of the network includes monitoring whether the connections between the main station  30  and the branch stations  22 ,  24 ,  26 , and  28  are in normal condition (i.e., no disconnections, unexpected losses, or other faults). However, the conventional monitoring device  40  using for example optical time domain reflectometry, only monitors the fiber network as a whole and can not monitor individual branch fibers. 
         [0034]      FIG. 2  shows a block diagram of an example optical fiber network  11  including individual branch monitoring. The optical fiber network  11  also includes a main fiber  20  connected to branch fibers  22 ,  24 ,  26 , and  28 , through an optical splitter  50 . Through the main fiber  20  and branch fibers  22 ,  24 ,  26 , and  28 , the network  11  joins a main station  30  and branch stations  32 ,  34 ,  36 , and  38 . In addition, the optical fiber network  11  includes wavelength filtering devices  42 ,  44 ,  46 , and  48  positioned along respective branch fibers  22 ,  24 ,  26 , and  28 . 
         [0035]    Similar to the network  10  of  FIG. 1 , the network  11  in  FIG. 2  can be a passive optical network (“PON”) for a FTTX application. The main station  30  can be an OLT, and one or more of the branch stations  32 ,  34 ,  36 , or  38  can be ONU&#39;s. 
         [0036]    A monitoring device  40  is positioned in or near the main station  30  for monitoring the condition of the optical fiber network  11 . The monitoring can include determining whether the connections between the main station and all branch stations are in normal condition (e.g., no disconnections, unexpected losses, or other faults occurring in the network). 
         [0037]    In some implementations, the monitoring device  40  can emit a monitoring signal  60  through main fiber  20 . The monitoring signal  60  can be composed of multiple wavelengths corresponding to a number of monitored branches, for example, four wavelengths, λ 1 , λ 2 , λ 3 , and λ 4  for monitoring branch fibers  22 ,  24 ,  26 , and  28 , respectively. The splitter  50  splits the monitoring signal  60  into each of the branch fibers  22 ,  24 ,  26 , and  28 . 
         [0038]    In some implementations, the monitoring device  40  can emit a series of monitoring signals  60  sequentially, in which each signal has only one distinct wavelength, for example, λ 1 , λ 2 , λ 3 , and λ 4 . 
         [0039]    A wavelength filtering device can be positioned along the optical path of each respective branch fiber. For example, a wavelength filtering device  42  can be positioned in the optical path  22  between the splitter  50  and the branch station  32 . The wavelength filtering device  42  can include two ports. Each port is connected in-line with branch fiber  22 . The filtering device  42  transmits only one wavelength, e.g., λ 1 , of the four composite wavelengths λ 1 , λ 2 , λ 3 , and λ 4  in the monitoring signal  60 . The filtering device  42  blocks the other wavelengths (e.g., λ 2 , λ 3 , and λ 4 ). Therefore, the filtering device  42  passes a filtered signal  62  having only one wavelength, e.g., λ 1 . 
         [0040]    Similarly, each other branch fiber includes a respective wavelength filtering device transmitting a single wavelength of the monitoring signal  60 . Branch fiber  24  includes wavelength filtering device  44 , which transmits filtered signal  64  having wavelength λ 2 . Branch fiber  26  includes wavelength filtering device  46 , which transmits filtered signal  66  having wavelength λ 3  and branch fiber  28  includes wavelength filtering device  48 , which transmits filtered signal  68  having wavelength λ 4 . 
         [0041]    A reflecting element  52  is disposed in the optical path  22  between filtering device  42  and station  32 . In some implementations, the reflecting element  52  can be a device having two ports, which are also connected to fiber  22 . In some other implementations, the reflecting element  52  can be an additional coating on a surface of any element between filtering device  42  and the station  32 . The reflecting element  52  can either reflect the signal with any wavelength of λ 1 , λ 2 , λ 3 , and λ 4 , or one specific wavelength only, e.g., λ 1 , while passing optical communication signals of the fiber network. Communication signals will be discussed in greater detail below. 
         [0042]    When the branch fiber  22  is in normal condition, e.g., no fault in branch fiber  22 , the reflecting element  52  reflects the filtered signal  62 . The reflected signal passes back through the filtering device  42  and the splitter  50 . From the splitter  50 , the filtered signal  62  propagates back in main fiber  20  and is detected using the monitoring device  40  (e.g., at the main station  30 ). 
         [0043]    If there is a problem (e.g., a fault) in fiber  22  (optical path  22 ), the filtered signal  62  of λ 1  will not return to, and will not be detected by, the monitoring device  40 . Alternatively, the returned filtered signal  62  can have a large loss such that only a very weak signal is returned to the monitoring device  40 . Each branch reflects only a specific wavelength. Therefore, the detection of the reflected filtered signal having a specific wavelength allows monitoring of the condition of that specific branch from the main station  30 . Conversely, if there is a problem in a specific branch of the network, the signal of the corresponding wavelength will suffer from severe loss or be undetected. 
         [0044]    Since an optical fiber network is generally used for transmitting communication signals from one location to another location, these communication signals pass through the wavelength filtering devices  42 ,  44 ,  46 , or  48  without significant loss. For example, typical communications signals are transmitted in an S-band (1280-1350 nm) and C-band (1528-1561 nm). Therefore, in some implementations, the filtering devices  42 ,  44 ,  46 , and  48  have two transmission windows covering S-band and C-band, respectively. Alternatively, in some other implementations the filtering devices  42 ,  44 ,  46 , and  48  have a single transmission window covering substantially 1280-1561 nm. 
         [0045]      FIG. 3  is a flow chart of an example method  300  for monitoring branches in an optical fiber network. For convenience, the method  300  is described with respect to a device that performs the monitoring (e.g., monitoring device  40  of  FIG. 2 ). 
         [0046]    The monitoring device transmits  302  an optical signal having multiple distinct wavelengths. In some implementations, the monitoring device transmits an optical signal having a number of distinct wavelengths equal to the number of branch fibers to be monitored. The wavelengths of the optical signal can be outside the range of wavelengths used for data communication on the optical fiber network. 
         [0047]    The monitoring device detects  304  reflected wavelengths from the transmitted optical signal. The reflected wavelengths are returned, for example, after being filtered into individual branches of the fiber network, for example, using a splitter and filtering device (e.g., splitter  50  and filtering device  42  in  FIG. 2 ) and reflected back using a reflecting element (e.g., reflecting element  52  in  FIG. 2 ). 
         [0048]    The monitoring device determines  306  whether one or more wavelengths of the transmitted optical signal are not detected. Alternatively, the monitoring device can determine whether or not a received wavelength has a signal strength less than a specified threshold, indicating a high level of loss caused by a problem in a corresponding optical branch fiber. 
         [0049]    If all of the wavelengths are detected, then all the branches of the optical fiber network are functioning  308 . However, if one or more wavelengths are not detected, or are weakly detected, the monitoring device identifies  310  the branch fibers corresponding to the missing/weak wavelengths. Each branch fiber uses a filtering device to pass a particular wavelength of the signal transmitted from the monitoring device. The monitoring device can therefore identify which branch fiber corresponds to the missing or weak wavelengths. 
         [0050]    The monitoring device generates  312  an alert identifying a fault in branch fibers of the fiber network corresponding to the missing or weak wavelengths. In some implementations, the alert can be a signal to a network administrator, an alarm, logging the fault, or other action. 
         [0051]    In some implementations, the monitoring device can monitor the fiber network including transmitting the optical signal at various intervals. For example, the monitoring can be frequent or occasional. In some implementations, monitoring is triggered using some other indication of network performance, for example, weaker than expected signal strength at one or more branch stations (e.g., branch stations  32 ,  34 ,  36 , and  38 ). 
         [0052]      FIG. 4  shows a display of an example transmission function  400  of a filtering device (e.g., filtering device  42 ) in linear scale. The transmission function  400  is presented with respect to wavelength on the x-axis and transmittance on the y-axis. The filtering device transmits light in a transmission window from point A  402  (e.g., substantially 1280 nm) to B  404  (e.g., substantially 1585 nm or any wavelength between 1561 nm and 1585 nm). The window from point A  402  to point B  404  substantially covers the wavelengths used for communication signals. Additionally, light with a specific wavelength or narrow range of wavelengths at point C  406  (e.g., C=λ 1 =1602 nm with a width of 1 nm at 25% level) is transmitted. Light that is not transmitted from the filtering device (e.g., light wavelengths outside the transmission window) is blocked, e.g., reflected back off axis. 
         [0053]    In some implementations, the transmission function  400  covers an S-band (1280-1350 nm) and a C-band (1528-1561 nm) wavelengths. In some other implementations, the transmission function  400  includes a range of wavelengths from substantially 1350 nm to substantially 1528 nm, which is the gap between the S-band and C-band, can be any value, since there is no communication signal in this wavelength span. For example, a transmission function  410  (dashed line) in the interval of substantially 1350 nm to substantially 1528 nm can be a curved transmission function, or any other transmission function. 
         [0054]    In some implementations, the filtering device is configured to be applied to optical signals within a wavelength span from point A  402  to point D  408 . Consequently, only the transmission function  400  in the wavelength domain from point A  402  to point D  408  is of interest. The corresponding wavelengths of point A&lt;B&lt;C&lt;D, such that the wavelength λ 1  at point C  406  is not inside the transmission window between point A  402  and point B  404 . The window from point A  402  to point B  404  covers the S-band and C-band, and wavelength λ 1  at point C  406  corresponds to a wavelength of a particular monitoring signal (e.g., monitoring signal  60 ) including multiple wavelengths. 
         [0055]    The monitoring signal can be, for example, in an L-band (1561-1620 nm) having component wavelengths outside the transmission window from point A  402  to point B  404 . However, the monitoring signal can be composed of any wavelengths, as long as those wavelengths are not included in the transmission window from point A  402  to point B  404  while within the transmission window of a given fiber. In some implementations, the monitoring signal is substantially between 1561 nm and 1700 nm. 
         [0056]      FIG. 5  shows a block diagram of an example thin films filter  500 . A substrate  502  is coated with a thin film  504 . A second thin film  506  is further coated on thin film  504 , and so on. A number of thin films, for example films  504 ,  506 ,  508 , and  510 , can be coated sequentially on the substrate  502 . Each thin film can have a different thickness. Additionally, two consecutive films can have different refractive indices. In some implementations, the thickness of each thin film layer ranges from substantially 100 nm to 1000 nm. Additionally, a given thin films filter can have between substantially 10 to 20 layers. 
         [0057]    When an input light  512  is incident to the filter  500 , the light is partially reflected at every interface of two films with different refractive indices. The partially reflected light from all interfaces are denoted by rays  514 ,  516 ,  518 ,  520 , and  522 . The reflected lights interfere to form a reflected light  524 . 
         [0058]    The selection of the thickness and refractive index of each thin film, which can be done using, for example, a computer program, results in a specific wavelength (e.g., λ 2 ) having a constructive interference at the reflected light  524 . Thus, effectively, light of the specific wavelength λ 2  will be fully reflected and contained in the reflected light  524 . The transmitted light  526  will have no component of the reflected wavelength, since the sum of the reflected light  524  and the transmitted light  526  is the same as the input light  512 . 
         [0059]    An individual can design a thin films filter (e.g., using some computer programs), which will reflect certain wavelengths and transmits other wavelengths. However, particular transmission curves can be difficult to design and construct. For example, a standard transmission curve has a band (window) only or a peak only, but not both band and peak (e.g., separated by some specified range of wavelengths). However, as shown in  FIG. 6 , a thin films structure for a filter can provide a unique transmission curve having a band and a peak. 
         [0060]      FIG. 6  shows an example logarithmic transmission function  600  of a thin films filter. The transmission function  600  can be calculated (e.g., using a computer), using numerical data associated with the thin films structure of the filter, for example, the thickness and refractive index of each film. A filtering device (e.g., filtering device  42  of  FIG. 2 ) includes a thin films filter having a particular transmission function. The transmission function  600  shows an example transmission pass ratio for a particular thin films filter of a filtering device. Note that 0 dB represents 100% passed, −6 dB is 25%, −20 dB is 1%, and −40 dB is 0.01%. 
         [0061]    For example, as compared with the transmission function  400  of  FIG. 4 , the filter is designed specifically to provide a transmission function in the wavelength span from point A  402  to point D  408  of  FIG. 4  (corresponding to points A  602  to point D  608  of  FIG. 6 ), where points A and D are positioned substantially at 1250 nm and 1620 nm, respectively. This corresponds to the range shown in the transmission function  600  of  FIG. 6 . Also, as shown in  FIG. 4 , the filter has a transmission window from point A  402  to point B  404  where point B  404  is positioned at substantially 1585 nm. In some implementations, the position of point B  404  is selected in a range from 1561 nm to 1585 nm. 
         [0062]    The transmission window of the transmission function  600  is shown as having a range of substantially 100% transmission ratio from  602  to  604 . In this example, point C  406  of  FIG. 4  is positioned substantially at 1602 nm, which corresponds to point C  606  in  FIG. 6 . In some implementations, the position of point C  606  is selected such that the corresponding wavelength of point B  604  is less than wavelength of point C  606  and the wavelength of point C  606  is less than the wavelength of point D  608 . A peak-width at substantially 25% (−6 dB) pass ratio level at point C  606  is substantially 1 nm. In some implementations, the peak-width has a value less than substantially 10 nm. 
         [0063]    The transmission function for thin films filters shown in  FIGS. 4 and 6  are examples. Other thin films filters of different transmission functions can be used, for example, having multiple transmission windows or peaks. 
         [0064]    In some implementations, the monitoring signals can be selected to have wavelengths that are within a window from 1585 nm to 1700 nm. When two adjacent monitoring signals are separated by 1 nm (the peak-width at 25% level), then a total number of 55 distinct monitoring signals can be used. As a result, up to 55 branches in an optical fiber network can be individually monitored. In some implementations, the number of monitoring signals can be increased. For example, the filter can be constructed with a narrower peak-width (i.e., the crosstalk is reduced optically), or the monitoring system can use a discriminatory detection circuit (i.e., the crosstalk is removed electronically). In a discriminatory circuit, all monitoring signals (e.g., λ 1 , λ 2 , λ 3 , and λ 4 ) can be detected, for example, an electronic processor can pick signals exceeding a specified threshold. 
         [0065]      FIG. 7  shows an example filtering device  700 . The filtering device  700  includes a ferrule  120 , first lens  128 , filter  130 , second lens  132 , and second ferrule  136 . The first ferrule  120  is configured to hold a first fiber  124 . The second ferrule  136  is configured to hold a second fiber  134 . 
         [0066]    Light  126  entering fiber  124  from outside the filtering device and then exiting fiber  124  is collimated using lens  128 . The collimated light is incident onto the filter  130 . The filter can be positioned at an angle relative to an axis of the incoming collimated light such that the filter  130  and the collimated light form an angle α (where a does not equal 90 degrees), so the collimated light is not normal to the filter  130 . 
         [0067]    For incoming light with transmitted wavelengths characterized in a transmission function, for example, as shown in  FIGS. 4 and 6 , the collimated light is transmitted through the filter  130 . The collimated light transmitted through the filter  130  is focused using lens  132  and enters the second fiber  134  held using the second ferrule  136 . Light  138  exits the filtering device  700  from fiber  134 . 
         [0068]    For incoming light with wavelengths not transmitted according to a transmission function (e.g., as shown in  FIGS. 4 and 6 ), the filter  130  reflects the collimated light. Since the collimated light is not normal to the filter  130 , reflected light  122  is off axis and thus does not re-enter the fiber  124 . 
         [0069]    Similarly, when light  140  enters the filtering device  700  through fiber  134 , the transmitted light (e.g., light in the transmission band of the filter  130 ) exits fiber  124  as light  142 . The light reflected from the filter  130  is off axis and does not re-enter fiber  134 . 
         [0070]    In some implementations, if the light incident onto the filter  130  in  FIG. 7  is not collimated, i.e., the incident angle of light is not uniform, the peak at point C ( 406  of  FIG. 4 ) can be broadened. The broadening is directly proportional to divergence of the light. However, the broadening of the peak at point C can increase the crosstalk among monitoring signals, e.g., λ 1 , λ 2 , λ 3 , and λ 4 , which, in turn, reduces the number of identifiable branches in an optical fiber network (e.g., fiber network  11  of  FIG. 2 ). 
         [0071]      FIG. 8  shows one implementation of the filtering device  700  joined with a first fiber  202  at a first side of the filtering device  700  and a second fiber  204  at a second side of the filtering device  700 . One end of the first fiber  202  is held within a first ferrule  206  in a first connector  210 . Similarly, one end of the second fiber  204  is held within a second ferrule  208  in a second connector  212 . Both first ferrule  206  of first fiber  202  and first ferrule  120  of the filtering device  700  are held and kept in position using a first adaptor  214 . In some implementations, the first adaptor  214  includes an alignment sleeve align and hold both ferrules. Similarly, second fiber  204  and the filtering device  700  are joined and held using a second adaptor  216 . Alternatively, first and second adaptors  214  and  216  can be included in a mechanical housing of the filtering device  100 . 
         [0072]    As shown in  FIG. 2 , without filtering devices included in fiber network  11 , branch fibers  22 ,  24 ,  26 , and  28  are often connected to splitter  50  through standard fiber connectors such as SC (subscriber connector or single coupling), LC (Lucent connector), ST (straight tip or stab and twist), and MU (miniature unit-coupling) type connectors. Thus, each branch fiber can be easily disconnected from and reconnected to the splitter such that an installation, upgrade, or repair to the branch fiber or network components can be easily conducted. 
         [0073]    As shown in  FIG. 8 , the first and second ferrules  120  and  136  of the filtering device  700  and their accompanying receptive parts (not shown) can be configured to mate to various types of connectors, for example, SC, LC, ST, MU, and others, in either PC (physical contact) or APC (angled polish connector) configuration. Therefore, an installer can easily include filtering devices  700  in the optical fiber network, for example, by first disconnecting branch fiber  22  from splitter  50  ( FIG. 2 ) and then connecting one side of filtering device  700  to splitter  50  and the other side of device  700  to fiber  22  through fiber connectors, respectively. 
         [0074]    In another embodiment, the filtering device  700  shown in  FIG. 7  can include two fiber pigtails instead of connector-ready first and second ferrules  120  and  136 . 
         [0075]    In yet another implementation, the filtering device  700  shown in  FIG. 7  can include another filter, instead of or in addition to, the filter having transmission characteristics as shown in  FIG. 4  or  6 . For example, a wavelength division multiplexing (WDM) filter or others can be used. For example, a connector-ready filtering device  700  can include a WDM filter as filter  130 . The device  700  can be a two-port WDM filter and connected to a receiver (Rx) in an optical fiber network. 
         [0076]    In further another implementation, the filter having transmission characteristics shown in  FIG. 4  or  6  is not necessarily disposed in an optical setup such as a filtering device shown in  FIG. 7  or  8 . For example, the filter can be used as a stand alone element or in combination with other elements in an optical setup or device. 
         [0077]    In some implementations, an OTDR device can also be used for detecting faults in a wavelength encoding fiber. 
         [0078]      FIG. 9  shows an example monitoring device  900 . The monitoring device  900  can be a particular type of monitoring device similar to the monitoring device  40  of  FIG. 2 . Monitoring device  900  includes a signal source  920 , a circulator  922 , and a receiver  924 . The signal source  920  transmits a monitoring signal  960  having multiple wavelengths. Alternatively, the signal source  920  transmits a series of monitoring signals  960  sequentially, in which each signal has only one distinct wavelength. 
         [0079]    The monitoring signal  960  is directed by the circulator  922  to a network through the main station  930  and a main fiber  932  corresponding, in some implementations, to the main station  30  and the main fiber  20  of  FIG. 2 . The reflected monitoring signal  961  from the network travels back to the circulator  922  through the main fiber  932  and the main station  930 . The circulator  922  directs the reflected monitoring signal  961  to the receiver  924 , where the signal is detected and processed. The receiver  924  can identify the wavelength of the reflected monitoring signal  961 . 
         [0080]    In some implementations, the intensity of the transmitted monitoring signal  960  can be modulated in the signal source  920 . The modulation function is preferably a sine function, although other functions, e.g., a sawtooth, square, or other periodic or non-periodic functions, can be used as the modulation function. The phase of the intensity modulation function—not the phase of the light wave, of the reflected monitoring signal  961  from a reflector, e.g., reflecting element  52  of  FIG. 2 , is known, since the distances from the signal source  920  to the reflector, and from the reflector to the receiver  924  are known. The signal source  920  and the receiver  924  are joined electronically by a communication channel  926 , so the processor in the receiver  924  can refer to the phase of the intensity modulation function at the signal source  920 . Consequently, the signal from the reflector can be extracted from other scattering or randomly-reflected signals in the network. The intensity modulation of monitoring signal will improve the signal-to-noise ratio for the signal detection. 
         [0081]    Furthermore, in the event of a fault in a particular fiber (e.g., a broken or damaged optical fiber), analyzing the phase of the intensity modulation function of the reflected monitoring signal allows the location of fault to be identified. Thus, the intensity modulation of monitoring signal will be able to identify fault&#39;s location without using an OTDR device. 
         [0082]    While this specification contains many specifics, these should not be construed as limitations on the scope of the invention or of what may be claimed, but rather as descriptions of features specific to particular embodiments of the invention. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
         [0083]    Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments. 
         [0084]    Thus, particular embodiments of the invention have been described. Other embodiments are within the scope of the following claims. For example, the actions recited in the claims can be performed in a different order and still achieve desirable results.