Abstract:
A portable apparatus for measuring optical powers of optical signals propagating concurrently in opposite directions in an optical transmission path between two elements, at least one of the elements being operative to transmit a first optical signal (S 1 ) only if it continues to receive a second optical signal (S 2 ) from the other of said elements, comprises first and second connector means for connecting the apparatus into the optical transmission path in series therewith, and propagating and measuring means connected between the first and second connector means for propagating at least the second optical signal (S 2 ) towards the one of the elements, and measuring the optical powers of the concurrently propagating optical signals (S 1 , S 2 ). The measurement results may be displayed by a suitable display unit. Where one element transmits signals at two different wavelengths, the apparatus may separate parts of the corresponding optical signal portion according to wavelength and process them separately.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of U.S. patent application Ser. No. 14/468,890 filed Aug. 26, 2014 as a Continuation of U.S. patent application Ser. No. 13/204,350 filed Aug. 5, 2011 as a Continuation of U.S. patent application Ser. No. 11/713,735 filed Mar. 5, 2007 as a Continuation-in-Part of U.S. patent application Ser. No. 10/538,768 filed Jun. 10, 2005 as a Continuation-in-Part of International patent application No. PCT/CA2004/001552 filed Aug. 23, 2004 which designated the United States of America, and claimed priority from U.S. Provisional patent application No. 60/511,105 filed Oct. 15, 2003. The entire contents of each of these prior related applications are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The invention relates to a method and apparatus for testing transmission lines normally propagating optical signals and is especially, but not exclusively, applicable to a method and apparatus for measuring optical powers of optical signals in optical transmission lines of passive optical networks. 
     BACKGROUND ART 
     As the cost of optical fiber and associated components decreases, new telecommunications network deployments increasingly use optical fiber from the edge of a core network to a location at or very close to the end user. Such so-called FTTX (Fiber-to-the-X; where X is the home, the office, the building, the premises, the curb, etc.) installations are usually based on a passive optical network (PON) architecture, where a terminal at the core-network edge (Optical Line Terminal—OLT) broadcasts signals downstream along a fiber-optic cable to a N-port splitter, and each of the ports then terminates at an optical network terminal (ONT) located at a respective one of the end users&#39; premises. Typically, downstream signals are at either of two wavelengths, vis. 1490 nm for the downstream transmission of digital data and 1550 nm for the transmission of cable television (CATV) signals, while each end user&#39;s optical network terminal (ONT) transmits upstream data signals at a wavelength of approximately 1310 nm. It should be noted that the CATV signals are often transmitted in analog format. 
     An asynchronous transfer mode (ATM) or similar protocol is often used to encode the downstream and upstream data signals. The OLT includes in the downstream 1490-nm signals synchronization signals which permit each of the ONTs to send its upstream (1310-nm) signals in its own unique time slot so as to avoid interference with signals from other ONTs connected on the PON. For this reason, as well as for reasons of eye safety, there is no 1310-nm transmission from the ONTs when the fiber link is disconnected, thereby preventing reception of the 1490-nm downstream-data signal. 
     Field maintenance of such FTTX installations requires low-cost and easy-to-use diagnostic test instruments to measure the signals. An example of such diagnostic test instruments is an optical power meter that can independently measure the power at the distinct downstream and upstream signal wavelengths (e.g. 1310 nm, 1490 nm, 1550 nm). During a repair call, the results of such a measurement could indicate the source of possible trouble in the network or in the end-user&#39;s connection. It is also known to use optical spectrum analyzers (OSA) to measure optical power at several wavelengths at the same time. 
     A disadvantage of each of these instruments is that it is a one-port device that only measures the power if the signals at the different wavelengths are propagating in the same direction along the fiber. In the case of the OSA, a further disadvantage is that the instrument is generally much too costly and complicated for routine field applications. 
     SUMMARY 
     The present invention seeks to eliminate, or at least mitigate, the disadvantages of the prior art instruments, or at least provide an alternative and, to this end, provides a portable instrument for measuring parameters, e.g. optical power, of analog or digital optical signals that are propagating concurrently in opposite directions in an optical transmission path between two elements, such as network elements of a passive optical network, at least one of which elements will not transmit its optical signals if it ceases to receive signals from the other of the two elements. 
     According to one aspect of this invention, there is provided portable apparatus for connecting into (tapping) an optical transmission line normally carrying optical signals propagating concurrently in opposite directions, said apparatus comprising first and second connector means for connecting the apparatus into the optical transmission path in series therewith, and propagation means connected between the first and second connector means for propagating said at least one of said signals while extracting a portion of either said at least one of said signals or another of said signals. 
     The propagation means may also comprise means for detecting and processing said portion to determine the optical power of said at least one of said signals or another of said signals. 
     According to another aspect of the present invention, there is provided portable apparatus for measuring the optical power of optical signals normally propagating concurrently in opposite directions in an optical transmission path between two elements, at least one of the elements being operative to transmit a first optical signal (S 1 ) only if it continues to receive a second optical signal (S 2 ) from the other of said elements. The apparatus comprises first and second connector means for connecting the apparatus into the optical transmission path in series therewith, and propagating and measuring means connected between the first and second connector means for propagating at least the second optical signal (S 2 ) towards the one of the elements, and measuring the optical power of the concurrently propagating optical signals (S 1 , S 2 ). 
     The measurement results may be displayed by a suitable display unit. 
     Where one element transmits signals at two different wavelengths, the apparatus may separate parts of the corresponding optical signal portion according to wavelength and process them separately. 
     According to another aspect of the present invention, there is provided a portable test instrument for measuring an optical power of each of a first optical signal (S 1 ) and a second optical signal propagating concurrently in opposite directions in an optical transmission path ( 16 ,  16 / 1 , . . . ,  16 / 9 ) between two network elements ( 10 ,  14 / 1 , . . . ,  14 / 9 ), at least one ( 14 / 1 , . . . ,  14 / 9 ) of the network elements being operative to transmit the first optical signal (S 1 ) only if it continues to receive the second optical signal (S 2 ) from the other ( 10 ) of said network elements, the portable test instrument comprising first and second connector means ( 22 ,  24 ) for temporarily connecting the portable test instrument into the optical transmission path in series therewith, and propagating and measuring means ( 32 ,  38 ,  46 ) connected between the first and second connector means for propagating at least said second optical signal (S 2 ) towards said at least one ( 14 ) of the network elements, the propagating and measuring means ( 32 ,  38 ,  46 ) being operable to measure an optical power of each of the first optical signal (S 1 ) and the second optical signal (S 2 ). The propagating and measuring means comprises coupler means ( 32 ) having first and second ports ( 28 , 30 ) connected to the first and second connector means, respectively, so that a path therebetween within the coupler means completes the optical transmission path, a third port ( 36 ) for outputting a portion of each optical signal received via the second port ( 30 ) and a fourth port ( 34 ) for outputting a portion of each optical signal received via the first port ( 28 ) and detection means ( 38 ,  42 ,  44 ) coupled to the third and fourth ports for converting the optical signal portions into corresponding electrical signals. 
     Where said one of the network elements also normally receives via said optical transmission path a third optical signal (S 3 ) at a different wavelength from that of said second optical signal (S 2 ), the propagating and measuring means may further comprise means for measuring an optical power of the third optical signal (S 3 ). 
     Where said one of the network elements also normally receives via the optical transmission path a third optical signal (S 3 ) at a different wavelength to that of said second optical signal (S 2 ), the propagating and measuring means may further comprise a wavelength discriminator, for example a wavelength division multiplexer, connected to the coupler means for separating at least a portion (S 2 ′, S 3 ′) of the combined second and third optical signals (S 2 , S 3 ) according to wavelength to obtain corresponding separate parts (S 2 ″, S 3 ″) and supplying same to said detection means. 
     Alternatively, where said one of the network elements also normally receives via the optical transmission path a third optical signal (S 3 ) at a different wavelength to that of said second optical signal (S 2 ), the propagating and measuring means may further comprise a splitter connected to the coupler means for splitting corresponding optical signal portions (S 2 ′, S 3 ′) into two parts (S 2 ″, S 3 ″), and filter means coupled to the splitter for separating the two parts according to wavelength before supplying same to said detection means. 
     Where the optical signals are analog, the measuring means may be arranged to extract the time-averaged optical power of the signal. 
     Where the optical signals comprise bursts of digital data alternating with lulls, the measuring means may be arranged to extract the optical power averaged over the duration of the individual bursts. More particularly, where the instrument is to be used for measuring power of optical signals comprised of “bursty” data streams (such as the ATM data signals), the measuring means may be arranged to extract the power only from the data bursts and not from any intervening series of digital zeros (i.e. lack of signal). Such bursty data streams are typical of both the upstream data sent by an optical network terminal (ONT) to a plurality of optical line terminals (OLTs) of a passive optical network (PON), and by the OLT to the plurality of ONTs. 
     The measuring means may comprise custom circuitry and/or a suitably-programmed microcomputer. 
     The test instrument may comprise display means for displaying measured values of optical power. 
     According to yet another aspect of the invention, there is provided a method of measuring an optical power of each of a first optical signal (S 1 ) and a second optical signal propagating concurrently in opposite directions in an optical transmission path ( 16 ,  16 / 1 , . . . ,  16 / 9 ) between two network elements ( 10 ,  14 / 1 , . . . ,  14 / 9 ), at least one ( 14 / 1 , . . . ,  14 / 9 ) of the network elements being operative to transmit the first optical signal (S 1 ) only if it continues to receive the second optical signal (S 2 ). The method comprises the steps of: temporarily connecting first and second connector means ( 22 ,  24 ) of a portable test instrument into the optical transmission path in series therewith, and using the portable test instrument to propagate at least said second optical signal (S 2 ) towards said at least one ( 14 ) of the network elements, and measure an optical power of each of the first optical signal (S 1 ) and the second optical signal (S 2 ). The portable test instrument comprises coupler means ( 32 ) having first and second ports ( 28 , 30 ) connected to the first and second connector means, respectively, so that a path therebetween within the coupler means completes the optical transmission path, a third port ( 36 ) for outputting a portion of each optical signal received via the second port ( 30 ) and a fourth port ( 34 ) for outputting a portion of each optical signal received via the first port ( 28 ) and detection means ( 38 ,  42 ,  44 ) coupled to the third and fourth ports for converting the optical signal portions into corresponding electrical signals. 
     Where said one of the network elements also receives via the optical transmission path a third optical signal (S 3 ) at a wavelength different from that of said second optical signal (S 2 ), the propagating and measuring step may also measure an optical power of the third optical signal (S 3 ). 
     Where said one of the network elements also receives via the optical transmission path a third optical signal (S 3 ) co-propagating with the said second optical signal (S 2 ) at a wavelength different from that of the said second optical signal (S 2 ), the measuring step may further include the steps of splitting a portion of the co-propagating optical signals into two parts, each comprising portions of the second and third optical signals (S 2 , S 3 ), separating each of the two parts according to wavelength, converting said parts into said second electrical signal and a third electrical signal, respectively, and also processing the third electrical signal to obtain a measured value of an optical power of the third optical signal (S 3 ). 
     Alternatively, where said one of the network elements also receives via the optical transmission path a third optical signal (S 3 ) co-propagating with the said second optical signal (S 2 ) at a wavelength different from that of the said second optical signal (S 2 ), said propagating and measuring step may employ a wavelength discriminator, for example a wavelength division multiplexer, connected to the coupler means for splitting a portion of the co-propagating optical signals into two parts each corresponding to a respective one of the second and third optical signals, converting the parts to said second electrical signal and a third electrical signal, and also processing the third electrical signal to obtain a measured value of an optical power of said third optical signal (S 3 ). 
     Where the optical signals are analog, the measurement step may derive the time-averaged optical power of the signal. 
     Where the optical signals comprise bursts of digital data alternating with lulls, the measuring step may derive the average of the optical power of said optical signal averaged over the duration of the individual bursts. More particularly, where the instrument is to be used for measuring power of optical signals comprised of “bursty” data streams (such as ATM data signals), the measuring step may extract the power only from the data bursts and not from any intervening series of digital zeros (i.e., lack of signal). 
     According to still another aspect of the invention, there is provided a portable test instrument for measuring an optical power of each of a first optical signal (S 1 ) and a second optical signal propagating concurrently in opposite directions in an optical transmission path in a passive optical network ( 16 ,  16 / 1 , . . . ,  16 / 9 ) between an optical line terminal ( 10 ) and an optical network terminal ( 14 / 1 , . . . ,  14 / 9 ), the optical network terminal ( 14 / 1 , . . . ,  14 / 9 ) being operative to transmit the first optical signal (S 1 ) only if it continues to receive the second optical signal (S 2 ) from the optical line terminal ( 10 ), the portable test instrument comprising first and second connector means ( 22 ,  24 ) for temporarily connecting the portable test instrument into the optical transmission path in series therewith, and propagating and measuring means ( 32 ,  38 ,  46 ) connected between the first and second connector means for propagating at least said second optical signal (S 2 ) towards said optical network terminal ( 14 ), the propagating and measuring means ( 32 ,  38 ,  46 ) being operable to measure an optical power of each of the first optical signal (S 1 ) and the second optical signal (S 2 ). The propagating and measuring means comprises coupler means ( 32 ) having first and second ports ( 28 , 30 ) connected to the first and second connector means, respectively, so that a path therebetween within the coupler means completes the optical transmission path, a third port ( 36 ) for outputting a portion of each optical signal received via the second port ( 30 ) and a fourth port ( 34 ) for outputting a portion of each optical signal received via the first port ( 28 ) and detection means ( 38 ,  42 ,  44 ) coupled to the third and fourth ports for converting the optical signal portions into corresponding electrical signals. 
     Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description which describes embodiments by way of example only with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified block schematic diagram of a portion of a passive optical network; 
         FIG. 2  is a simplified block schematic diagram of a power meter according to a first embodiment inserted into a branch of the network; 
         FIG. 3  is a detail view illustrating a modification; 
         FIG. 4  is a simplified block schematic diagram of a power meter according to a second embodiment; 
         FIG. 5  is a simplified block schematic diagram of a power meter according to a third embodiment; and 
         FIG. 6  is a simplified block schematic diagram of a power meter according to a fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A portion of a passive optical network shown in  FIG. 1  comprises a first element in the form of a central office optical line terminal (OLT)  10  coupled by a 1:9 splitter  12  to a plurality of other elements in the form of optical network terminals (ONT)  14 / 1  to  14 / 9 , each coupled to a respective one of the nine ports of the splitter  12  by one of a corresponding plurality of optical waveguides  16 / 1  to  16 / 9 . (It should be noted that, although nine terminals and a nine-port splitter are shown for convenience of illustration, there could be more or fewer in practice.) The terminals use asynchronous transfer mode (ATM) or similar protocol to encode the downstream (OLT to ONTs) and upstream (ONTs to OLT) digital data signals. 
     OLT  10  broadcasts to the ONTs  14 / 1  to  14 / 9  a downstream data signal (S 2 ) at a wavelength of 1490-nm and a supplementary downstream signal (S 3 ) at a wavelength of 1550-nm and, in known manner, encodes the 1490-nm data signals (S 2 ) for synchronization purposes, the encoding being decoded by the ONTs and used to permit each of the ONTs  14 / 1  to  14 / 9  to send upstream, to the OLT  10 , 1310-nm digital optical data signals S 1  in its own unique time slot so as to avoid interference with signals from other ONTs connected to the same OLT  10 . The signal S 3 , generally carrying cable television (CATV) information, is supplied by CATV source  11  shown connected to the OLT  10  and combined with the data signals S 2  in known manner. (In practice, if signal S 3  is a CATV signal, it will be inserted later). 
     If they do not receive the downstream signal (S 2 ), and hence the synchronization information, the ONTs cannot normally transmit. For a field technician to make measurements of either two, or all three, of the signals, therefore, it is necessary for the ONTs  14 / 1  to  14 / 9  to continue receiving the downstream signals from the OLT  10 . 
     A test instrument  18  which allows the upstream and downstream optical signals to continue propagating, while measuring the power of the optical signals S 1 , S 2  and S 3  at all three wavelengths, will now be described with reference to  FIG. 2 , which shows the instrument  18  connected into branch waveguide  16 / 9  between the splitter  12  and ONT  14 / 9 . The test instrument  18  comprises a casing  20  having first  22  and second  24  bulkhead connector receptacles or ports shown coupled to the splitter  12  and ONT  14 / 9 , respectively, connector receptacle  24  being connected to the ONT  14 / 9  by a short jumper  26 . 
     Within the power meter casing  20 , the connector receptacles  22  and  24  are connected to first and second ports  28  and  30 , respectively, of a 2×2 optical coupler  32 , having an approximately 80:20 splitting ratio, which ratio is approximately the same at all of the wavelengths to be measured (i.e., 1310 nm, 1490 nm, 1550 nm). 
     Thus, coupler  32  splits each of the signals S 2 , S 3  and S 1  received at ports  28  and  30 , respectively into two parts with a ratio of 80:20. The 80 percent signal portions are each routed back to the other of the two connectors  22  and  24  while the 20 percent signal portions S 1 ′ and S 2 ′, S 3 ′ are each routed to one of the corresponding third and fourth ports  34  and  36 , respectively, of the coupler  32 . 
     Port  34 , which receives the 20 percent portion S 1 ′ of the signal S 1  from the ONT  14 / 9 , is connected by way of a filter  62 , conveniently a 1310 nm bandpass filter, to a first photodetector  38  for detecting light at wavelengths nominally at 1310 nm. Port  36 , which receives signal portions S 2 ′, S 3 ′ representing 20 percent of each of the 1490-nm and 1550-nm optical signals from the OLT  10 , is coupled to a 1×2 optical splitter  40 , having an approximately 90:10 splitting ratio that is approximately the same at all downstream wavelengths to be measured (i.e., 1490 nm, 1550 nm). 
     The 90 percent signal portions S 2 ″ from splitter  40  are routed via the corresponding output optical fiber from the optical splitter  40  to a second bandpass filter  64 , passing light within an approximately 15-nm wavelength band centered about 1490 nm and substantially attenuating light outside of this band (e.g. attenuation of greater than 40 dB at 1550 nm). The output S 2 ′″ of the second bandpass filter  64  is routed to a second photodetector  42 , which detects light nominally at 1490 nm. 
     The 10 percent signal portion S 3 ″ from splitter  40  is routed via the corresponding output optical fiber to a third bandpass filter  66 , passing light within an approximately 25-nm wavelength band centered about approximately 1550 nm and substantially attenuating light outside of this band (e.g. greater than 20 dB for analog CATV signals, greater than 40 dB for digital CATV signals). The output S 3 ′″ of the third bandpass filter  66  is coupled to the third photodetector  44 , which detects light nominally at 1550 nm. 
     The three photodetectors  38 ,  42  and  44  supply their corresponding electrical signals to an electronic measuring unit  46  which comprises a set of three similar amplifiers  48 ,  50  and  52  for amplifying the electrical signals from photodetectors  38 ,  42  and  44 , respectively. Power detectors  54  and  56  detect power of the amplified electrical signals from amplifiers  48  and  50 , respectively, and supply the power measurements to a processor unit  58  which, using an internal analog-to-digital converter, converts them to corresponding digital signals which it processes to obtain the required optical power measurements, specifically power, and supplies the measurement information to a display unit  60  for display of the measurements in a conventional manner. The amplified signal from amplifier  52 , corresponding to CATV signal S 3 , is supplied directly to the processor unit  58 , i.e., without power detection, to provide a measure of average optical power. 
     The actual power measurements made by measuring means  18  will depend upon the nature of the signals being measured. 
     Where the optical signals are analog, the measuring means may be arranged to extract the time-averaged optical power of the signal. 
     Where the optical signals comprise bursts alternating with lulls, the measuring means may be arranged to extract the optical power of the bursts. 
     If the optical signals comprise bursty digital signals, the measuring means may be arranged to the extract the optical power of the bursts averaged over the duration of the burst. More particularly, where the instrument is to be used for measuring power of optical signals comprised of “bursty” data streams (such as the ATM data signals), the measuring means may be arranged to extract the power only from the data bursts and not from any intervening series of digital zeros (i.e. lack of signal). Such bursty data streams are typical of both the upstream data sent by an optical network terminal (ONT) to a plurality of optical line terminals (OLTs) of a passive optical network (PON), and by the OLT to the plurality of ONTs. 
     Typically, the field technician will disconnect the link  16 / 9  to ONT  14 / 9  at the home/premises etc. of the end-user at an existing “connectorized” coupling. The connector on the upstream part of the link  16 / 9  will then be connected to a specified one ( 22 ) of the two bulkhead connectors on the instrument, and the connector on the jumper  26  will be connected to the other. Of course, if a connectorized coupling between parts of the link  16 / 9  is available, the jumper  26  may not be needed. 
     While the link is disconnected, emission of the upstream data signals at wavelength 1310 nm by the ONT  14 / 9  will normally cease, and will then recommence when the two connectors are connected to their respective bulkhead connector receptacles  22 , 24  on the test instrument  18  and the ONT begins to receive the 1490 nm signal again. Measurements can then be taken. 
     The fact that there will be a temporary disruption in the line as the instrument  18  is inserted is not normally important, since the test instrument will normally be used in service calls where a problem has already been indicated by the customer. 
     Once the test instrument is inserted into the line, between the splitter  12  and the selected one of the ONTs  14 / 1  to  14 / 9  (see  FIG. 1 ), 80% portions of the downstream data and video signals (i.e. at 1490 nm and 1550 nm, respectively) will pass directly through to the ONT  14 / 9 . The ONT, thus synchronized via the received data signal, will then be able to emit its upstream (e.g. 1310-nm) data signal, an 80% portion of which will be sent upstream to the OLT  10 , the other 20% portion being diverted to the detector  38 . 
     It will be appreciated that the ratio of the coupler  32  need not be 80:20. Embodiments of the invention may employ different ratios. Generally, lower ratios entail more attenuation while higher ratios are more polarization-dependent. It should be noted, however, that preferred couplers are available commercially that have a particular band of wavelengths for which their ratios are substantially wavelength and polarization independent. 
     It will be appreciated that the invention is not limited to the measurement of optical power and to power meters, but could be applied to the measurement of other parameters, such as optical spectrum, bandwidth utilization in the transmission path or link, and so on. For example, the coupler  32  could be combined with an optical spectrum analyzer (OSA) which would replace the bandpass filters  62 ,  64 ,  66 , the detectors  38 ,  42  and  44 , the measuring means  46 , and the display  60 , and the optical splitter  40  would be replaced by a 2×1 coupler, preferably with a 50-50 splitting ratio, to couple the ports  34  and  36  of the 2×2 coupler  32  to the single input port of the OSA, thereby combining the two 20% signal portions. It will also be appreciated that the 2×1 coupler inherently will introduce a loss, typically of 50% or more. 
     Of course, instead of the OSA, an alternative single-port device coupled to a 2×1 coupler could replace the components  38 - 66  of  FIG. 2 . 
     The bandpass filter  62  serves as a discrimination filter and is desirable to avoid undesired effects caused by optical back reflection of the 1550 nm signal, which can be acute when measurements are taken close to the OLT  10 . It may be omitted, however, if the apparatus, e.g., test instrument, will normally be used close to the ONT terminal(s). 
     As illustrated in  FIG. 3 , which shows a modification to the instrument  18 , the splitter  40  and bandpass filters  64  and  66  may be replaced by a wavelength demultiplexer  68  (e.g., a low optical crosstalk WDM coupler) which separates the signals according to their respective wavelengths and supplies them to the detectors  42  and  44 , respectively. It will be noted that  FIG. 3  omits the bandpass filter  62 , but it may be included for the reasons discussed above. 
     The electronic measuring unit  46  may be digital rather than analog, in which case it could be a suitably programmed microcomputer. Such digital signal processing potentially is more efficient, but also likely to be more expensive. 
     It should be appreciated that, although each of the ONTs must receive the optical signal S 2  having a wavelength of 1490 nm, or it will not transmit its own optical signal S 1  of 1310 nm wavelength, it is not essential for the ONTs to receive the optical signal S 3  at 1550 nm transmitted by the OLT  10 . Accordingly,  FIG. 4  illustrates an alternative optical test instrument  18 ′ which conveys only the 1490 nm signal to the ONT  14 / 9 . 
     The optical power meter  18 ′ shown in  FIG. 4  comprises a wavelength-division-multiplexer  68 ′ having a “combined” port connected to receptacle  22  and two “divided” ports coupled to detectors  42  and  44 , respectively. The WDM  68 ′ separates the 1490 nm and 1550 nm optical signals S 2  and S 3  according to wavelength and supplies them to detectors  42  and  44 , respectively. The 1550 nm detector  44  is shown connected directly to the WDM  68 ′ (or via an optional bandpass filter  62 , shown in broken lines). The 1490 nm detector is shown connected by way of an 80:20 coupler  40 ′ which receives the 1490 nm signal from the WDM  68 ′ and splits it into first and second portions, namely an 80% portion and a 20% portion, conveying the 80% portion by way of a second coupler/splitter  32 ′ to receptacle  24  for transmission to the ONT and conveying the 20% portion to the detector  42  via bandpass filter  64 . The respective outputs of the detectors  42  and  44  are coupled to the processor  58  for processing of their corresponding electrical signals. 
     Second coupler/splitter  32 ′ receives the 1310 nm signal S 1  from ONT  14 / 9 , via the receptacle  24 , and splits the 1310 nm signal into two portions with a ratio of 80:20, conveying the 80% portion to the WDM  68 ′ by way of the coupler  40 ′ and the 20% portion to a detector  38  (or via an optional bandpass filter  66  shown in broken lines). The electrical signal from detector  38  is processed by the processor  58 ′, as before. 
     Thus, with this arrangement, the 1490 nm signal from the OLT  10  passes to the ONT  14 / 9  via the fiber branch  16 / 9 , the receptacle port  22 , the WDM  68 ′, the two couplers  40 ′ and  32 ′, receptacle  24  and fiber jumper/branch  26 . Providing it is receiving the 1490 nm signal, the ONT  14 / 9  transmits its own 1310 nm signal, which follows substantially the same return path to the OLT  10 . The couplers  32 ′ and  40 ′ extract respective small (20%) portions of the 1310 nm and 1490 nm signals for detection and processing, as required, by detectors  38 ,  42  and processor  58 ′. 
     It should be noted that, although the ONTs  14 / 1 - 14 / 9  need to receive the 1490 nm signal or they will not transmit their 1310 nm signals, it is not absolutely essential for the OLT  10  to receive those 1310 nm signals. Consequently, it would be possible for the WDM  68 ′ to be adapted to allow the 1490 nm signals to pass, but block the 1310 nm signals. Such an arrangement will now be described with reference to  FIG. 5 , which illustrates an alternative test instrument  18 ″ similar to that shown in  FIG. 4 , in that it includes a WDM  68 ″ connected between receptacle port  22  and detectors  42  and  44 , detector  42  being connected to WDM  68 ′ by way of a filter  64  and detector  44  being coupled, optionally, by way of a filter  62 , shown in broken lines. Likewise, receptacle port  24  is coupled to detector  38  by way of a coupler  32 ″. 
     The test instrument  18 ″ differs from that shown in  FIG. 4 , however, because it does not have a coupler/splitter  40 ′ coupling the signal S 2  (1490 nm) to receptacle port  24 , i.e., the signal S 2  is not simply conveyed through the power meter and a portion tapped off for measurement. Instead, the test instrument  18 ″ uses optical-electrical-optical (OEO) regeneration to regenerate the optical signal S 2  from the digital equivalent of the received signal S 2  supplied to the processor  58 ″ and then transmits the regenerated optical signal to the ONT as the signal S 2  it must receive in order to transmit its own signal S 1 . 
     Thus, test instrument  18 ″ has a modulatable optical source  70 , such as a light-emitting diode (LED), driven by an electrical signal from the processor unit  58 ″ (produced by means of an internal digital-to-analog converter) that is the optical equivalent of the electrical signal supplied to the processor  58 ″ by detector  42 . The optical output from the LED  70  is applied to coupler  32 ′ which passes it to receptacle port  24  for transmission to the ONT  14 / 9 . In this case, there is no optical continuity through the power meter  18 ″, either upstream or downstream. Thus, the signals S 2  and S 3  are passed to detectors  42  and  44 , respectively, via WDM  68 ″ and the signal S 1  passes from coupler  32 ′ to detector  38 . 
     It should be appreciated that the splitter/coupler  32 ′ in the instruments of  FIGS. 4 and 5  could be replaced by a WDM coupler corresponding to the wavelengths 1310 nm and 1490 nm. 
       FIG. 6  illustrates yet another test instrument  18 ′″ which is similar to that shown in  FIG. 3 , in that it includes a WDM  68 ′, detectors  38 ,  42  and  44  for the 1310 nm, 1490 nm and 1550 nm signals, respectively, 1490 nm filter  64 , a processor unit  58 ′″ and, optionally, filters  62  and  66 . It differs, however, in that coupler  32  is omitted and replaced by a coupler  72  with a wavelength-selective reflective device  74 , such as a Bragg grating, written into the common path portion so as to reflect a portion, say about 5%, of the 1550 nm signal. The coupler  72  has ports  72 A and  72 B connected to the instrument ports  22  and  24 , respectively, and ports  72 C and  72 D connected to the WDM  68 ′ and detector  42 , respectively, the latter by way of 1490 nm filter  64 . 
     In operation, the coupler  72  receives the 1490 nm and 1550 nm signals from receptacle  22  and conveys a portion, about 95%, of the 1550 nm signal to port  24  for transmission to the ONT  14 / 9  and a portion, about 5%, to the detector  42 . The Bragg grating  74  reflects a portion of the 1550 nm signal, about 5%, which leaves the coupler  72  via port  72 C and is applied to WDM  68 ′. The 1310 nm signal from ONT  14 / 9  passes from receptacle port  24  to the coupler  72  and, leaving the coupler via port  72 C, is conveyed to the WDM  68 , along with the 5% portion S 3 ρ of the 1550 nm signal. The WDM  68 ′ separates the 1550 nm signal and the 1310 nm signal according to wavelength and conveys them to detectors  38  and  44 , respectively, optionally by way of bandpass filters  62  and  66 , respectively. 
     Thus, the test instrument  18 ′″ provides an optical path therethrough for all three of the optical signals S 1 , S 2  and S 3 . 
     It should be appreciated that the power meter  18 ′″ could be reconfigured to reflect a portion of the 1490 nm signal rather than the 1550 nm signal, i.e, by employing a different Bragg grating  74  and transposing the detectors  42  and  44 , with their filters, as appropriate. 
     It should also be appreciated that, in all of the above-described embodiments, the filters could be bandpass filters or a combination of low-pass and high-pass filters, and that the filters for the 1310 nm and 1550 nm signals are optional. 
     Moreover, it should be noted that, in the embodiments of  FIGS. 4, 5 and 6 , the programming of processor  58 ′,  58 ″ or  58 ′″ will be suitably modified as compared with that for the embodiment of  FIG. 2 . Such modification will not be described herein since it should be apparent to those skilled in this art. 
     It should be noted that, although  FIGS. 1 and 2  show the third signal S 3  inserted into the OLT  10 , it could be inserted later in the transmission path, for example between the OLT  10  and the splitter  12 . This is especially the case if the signal S 3  is a CATV signal. 
     It will be appreciated that, although the above-described embodiments are described as monitoring data signals S 1  and S 2  and CATV signals S 3 , the invention comprehends instruments and methods for monitoring other optical signals. 
     INDUSTRIAL APPLICABILITY 
     Portable apparatus embodying the present invention may be inexpensive and easy-to-use. Ease of use is especially critical when they are used for testing FTTX networks since the maintenance field technicians are generally the same personnel who maintain wire telephone connections and rarely have had significant training in fiber-optic technology. 
     Although embodiments of the invention have been described and illustrated in detail, it is to be clearly understood that the same are by way of illustration and example only and not to be taken by way of limitation, the scope of the present invention being limited only by the appended claims.