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Published on August 31, 2011

Live Fiber Monitoring in CWDM Networks—Part 2

Part 1 of this article addressed the design and implementation of preventive monitoring on line coarse wavelength-division multiplexing (CWDM) traffic-intensive optical fiber links.

Out-of-Band 1650 nm OTDR Testing

Out-of-band testing for coarse wavelength-division multiplexing (CWDM) requires the use of a 1650 nm optical time-domain reflectometer (OTDR) instead of 1625 nm because the last CWDM channel at 1611 nm is too close to 1625 nm, which is traditionally used for live fiber testing of 1310/1550 nm circuits. For this, a three-port wavelength-division multiplexer (WDM) needs to be added on the line to inject the OTDR signal to the fiber carrying CWDM channels. If it is not inserted prior to CWDM commissioning, it will not be possible to test live without interrupting traffic – interruption to last long enough for device connection and test. Out-of-band testing adds a minimum of 1 dB penalty to the link budget, which needs to be taken into account at the circuit design stage. In cases where multiple sections need to be tested from a central site in one acquisition, then by-pass site(s) using DEMUX/MUX functions have to be planned, which increases link attenuation to another 1 dB.

Out-of-band OTDR testing at 1650 nm of CWDM links

Figure 1. Out-of-band OTDR testing at 1650 nm of CWDM links

 By-passing a node for testing end-to-end

Figure 2 By-passing a node for testing end-to-end.

At the end of the line, the 1650 nm test wavelength is usually rejected by the CWDM filter. Care must be taken to ensure that the CWDM filter at the end of the path is at least 30 to 35 dB isolation at 1650 nm. If not, the same WDM filter used for injection and by-pass can be installed at the end of line and act as a rejection filter for 1650 nm.

Testing out-of-band at 1650 nm has the advantage of quickly revealing unwanted bending of the cable or the individual fiber strand, as this wavelength propagates in the outer diameter of the fiber core. The difference in fiber attenuation between 1650 nm from 1550 nm is not larger than the difference between 1310 nm and 1550 nm. In most fibers seen and available today, the fiber attenuation at 1650 nm is about 0.25 dB/km (typically) compared to 0.2 dB for 1550 nm. For a 60 km link tested at 1650 nm, this is within 3 dB from what would be measured at 1550 nm on a fiber that has no excessive bending. It is therefore recommended to have a construction trace at 1625 nm, if possible at 1650 nm, in order to record the difference of the attenuation measured at both wavelengths for consistency in the measurement taken before and after the fiber goes live. Obviously, the implementation of a 1650 nm testing path throughout the network can, in some cases, be troublesome and adds costs and complexity, which is not always welcome.

In-Band Use of a CWDM Channel for OTDR Testing

The in-band method uses one or multiple CWDM channels for OTDR testing. This is a straight connection of a CWDM-enabled OTDR into the MUX/DEMUX CWDM filter used for transmission.

In-band OTDR testing using one unused CWDM channel

Figure 3. In-band OTDR testing using one unused CWDM channel.

Despite the fact that one channel becomes ‘dedicated’ to testing, in-band is a test method that can be implemented after the network has been commissioned, which is a great advantage compared to the out-of-band approach. It is also simple to establish a test route. When passive add/drops are used over a ring, the selected test channel will continue and follow the other express channels. When all channels terminate in a site and are regenerated for another section, it simply requires patching between the same channel ports on both sides of the route. Either the transmit or the receive port can be used.

In-band OTDR testing using 1470 nm

Figure 4. In-band OTDR testing using 1470 nm. For establishing end-to-end test, route patching is done in intermediate sites between 1470 nm ports of the CWDM MUX/DEMUX. Only one fiber, either transmit or receive is presented here. For fiber pair communication, two test ports of the test equipment have to be used.

It is interesting to note that service channel (SC), or expansion channel (EC) at 1310 nm or any other wavelength, if available and not in use, can also be selected for in-band testing, similarly to above with less stringent requirements in terms of channel bandwidth for the tolerance on central OTDR wavelength.

Testing through Broadband Tap Coupler (Monitor Port) of CWDM Optics

Some CWDM MUX/DEMUX/ADM devices are equipped with taps (1 to 10%) in order to monitor traffic. These monitor ports are broadband couplers, and extract a portion of traffic usually for testing with protocol analyzers. When testing in opposite direction than traffic, the CWDM signal power, even though attenuated by the tap, will be detrimental to perform a proper OTDR measurement—unless one selects a filter to isolate the OTDR wavelength band from active CWDM channels.

Broadband tap coupler as a monitor port can be used for OTDR testing

Figure 5. Broadband tap coupler as a monitor port can be used for OTDR testing but care must be taken to isolate the OTDR test wavelength from active channels.

The isolation of the filter to place between the tap/monitor port and OTDR must be at least 50 to 60 dB when testing against traffic. It must be a band-pass type if the test wavelength is in-band. If the OTDR is out-of-band, a high-pass filter for 1650 nm can be used. For the case of in-band, one or two cascaded optical add/drop multiplexers (OADMs) can be used as band-pass filters. If the fiber  transmits CWDM traffic  at the same wavelength selected for testing, OTDR will fail measuring.

Limitations with this approach are:

  • Major drop (seen in literature up to 23 dB or less than 1%) in optical attenuation, cutting large amounts of the available OTDR measurement range; if the monitor port ratio was 10% (10 dB), it could be used
  • The test direction is likely always against traffic—at least in case of fiber pair transmission, which doubles the number of test units to install, one at each end. For single fiber bidirectional transmission, this limitation does not apply
  • Additional care must be taken to ensure proper isolation of the OTDR from attenuated but unfiltered CWDM channels

Be sure to read Part 3 of this article as it examines practical examples of in-band remote testing and proactive monitoring.