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OTDRs Do It Out-of-Band
Methods for Testing Live Fibers
By Joseph Anello, Jr.

• Live testing
• Macrobend losses
• WDM basics
• Condition check
• In short
• Bottom Line

Since its development more than 20 years ago, the optical time domain reflectometer (OTDR) has become the industry workhorse for measuring the length and loss characteristics of optical fiber. Typically, the OTDR is used to determine these characteristics at the operating wavelength when there is no traffic on the glass.

But increasingly OTDRs are being used to perform measurements on "live fibers," fibers that are carrying active traffic, by using test wavelengths that differ from the operating wavelengths. These out-of-band (OOB) measurement techniques provide significant advantages over measurements made at the operating wavelengths.

Live testing

Live fiber testing requires the use of wavelength division multiplexers (WDMs) to inject the OTDRs light pulses into the fiber and to return the reflected light to the OTDR for measurement. A WDM is a special three-port optical coupler that will combine two specific wavelengths onto a single fiber. Used in reverse, it will separate those wavelengths from a single fiber to independent outputs. This is shown in Figure 1, where lO is the operating wavelength and lT is the test wavelength from the OTDR. It is important to put WDMs at both ends of the link to prevent lT from reaching the receiver. This also allows the OTDR to be connected to either end of the link for bidirectional testing.

For a typical single-mode link operating at 1,310 nm, 1,550 nm light from the OTDR could be used for testing and vice versa. This will allow OTDR testing without interrupting the traffic on the fiber. Usually, however, OOB testing implies the use of higher wavelengths, such as 1,625 nm, for testing. This allows testing on systems that already are carrying traffic at both 1,310 nm and 1,550 nm on a single fiberthe forerunner of the modern dense wavelength division multiplexing (DWDM) systems. However, testing at 1,625 nm has significant advantages over testing with shorter wavelengths.

Macrobend losses

Because longer wavelengths are more susceptible to bending losses, testing at 1,625 nm detects losses from macrobends that would not have been seen at 1,310 nm or 1,550 nm. Macrobends generally result during installation as the fiber-optic cable is routed. Macrobends also can result from any number of environmental factors after the cable is installed and in service.

Changes in tension because of thermal cycling, the movement of the surrounding soil and rocks (caused by frost heaves, seismic events and so on), and the stretching of aerial cable all can result in bending losses, which ultimately can affect communication on a fiber. By testing at 1,625 nm, the losses are detected before they induce loss at the operating wavelength, providing an early indication of potential traffic interruption.

In order to implement OOB testing on live fibers, special considerations must be made during the planning stages. Certainly, WDMs will need to be installed at both ends of the link to provide test ports for injecting OTDR signals and to prevent those signals from reaching the communications receiver at the other end. But the addition of in-line components to the traffic path must be planned to ensure that end-to-end loss budgets and other important link characteristics are not exceeded. This requires an understanding of the construction and specifications of the WDMs.

WDM basics

WDMs are made by partially melting two parallel fibers, which are brought together and stretched until their cores have the right amount of taper to couple the desired wavelength into and out of the common fiber. At the crucial point for OOB WDMs, all of the 1,625 nm light is coupled into the 1,550 nm fiber. Similarly, all of the 1,625 nm light entering through the common port is coupled to the 1,625 nm port. In reviewing WDM specifications, there are three key parameters that must be considered, as shown in Figure 2.

Insertion loss is the attenuation in decibels for a particular path through the device. Generally, WDMs will specify the insertion loss between each wavelength-specific port and the common port. Typical values are around 1.2 dB. It is important to note that this is the insertion loss for the WDM. If the WDMs will be connectorized and patchcords used to place them into the link, the connector and patch cord insertion losses must be added to the overall loss budget.

Isolation, which also is referred to as far-end cross-talk, describes the amount of light that enters through the common port, but exits through the wrong wavelength-specific port. Measured in decibels, it is expressed as an attenuation value of the undesired wavelength. Typical values for high-isolation WDMs, which must be used for OOB testing on live fibers, are around 50 dB, indicating that 1,550 nm light entering the common port will be attenuated by 50 dB before exiting the 1,625 nm port. Usually, each path has its own isolation value.

Directivity, which also is referred to as near-end cross-talk, describes the amount of light that enters through one wavelength-specific port and exits through the other. Measured in decibels, it also is expressed as an attenuation value. Typical values are around 60 dB, indicating that light entering the 1,625 nm port will be attenuated by 60 dB before exiting the 1,550 nm port.

Other important parameters that may affect communication on the link are the back-reflection or return loss, which describes the amount of input light reflected back along the path of transmission, and the bandpass, which describes the range of wavelengths around those specified that will be passed. All of these will need to be considered during planning with regard to the specifications of the terminal gear and the link characteristics to ensure that the OOB testing will not affect the active traffic. If more isolation is needed, WDMs can be cascaded or in-line optical bandpass filters can be used to increase the attenuation of unwanted signal components.

Condition check

Once these computations have been made and the appropriate WDMs have been added to the network, OTDRs can be used to measure the condition of the link without removing the active traffic. In fact, with a remotely controlled OTDR and an optical switch, many live fibers can be continuously monitored to watch for increased losses.

As mentioned earlier, the 1,625 nm measurement wavelength is more sensitive to losses from macrobends, which typically are caused by external forces acting on the cable. This provides an early indication of potential bit error rate (BER) increases because, as the macrobending continues to increase, it certainly will induce losses at the operating wavelength, causing communication interruption.

Such monitoring can be especially useful on links that have no redundancy, preventing service outages by providing early indications of cable degradation. On high security links, such OOB testing techniques can be used to detect unauthorized signal interception because bending is a key technique used to extract these signals from the fiber.

In short

Remember, OOB testing with long wavelength OTDRs can detect early degradation of optical fibers by measuring macrobend losses caused by external factors that place stresses on the fiber and cable. These stresses can be caused by any number of environmental elements such as thermal cycling, swaying, frost heaves, ground shifts, rodent intrusions and so on. Using special OOB WDMs permits measurements to be made on fibers carrying active traffic, allowing monitoring and maintenance to be planned before service interruptions occur. - TB

Joseph W. Anello, Jr. is chief engineer at GN Nettest. He may be reached via e-mail at .

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