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Principal Sponsors:
 | Dense Wavelength Division Multiplexing (DWDM) Testing |
7. New Requirements for Traditional Fiber-Optic Test Instruments
In addition to instrumentation specifically designed for the maintenance of DWDM systems (i.e., the new OSAs and wavelength meters, the characteristics of which have been outlined elsewhere in this tutorial), conventional field installation and test equipment must also be considered because of the strong influence that some of the properties of fiber-optic links have on DWDM transmission. Although many of the basic attributes of these links are independent of the transmission mode used (TDM or WDM) and can thus be measured using conventional instruments, a few parameters are critical to proper DWDM operation, and special care must be taken in selecting field instruments to measure them.
Optical Loss Test Sets (OLTS)
Because of the use of several channels at different, precisely defined wavelengths, dedicated WDM power meters must be calibrated at specified wavelengths in the 1530 nm to 1565 nm band, to measure the power in individual channels at the output of demultiplexers. Optical loss test sets will also be used at the wavelengths used for optical supervisory channels (OSC)—1480 nm, 1510 nm, and 1625 nm, depending on the system design. Dedicated DFB light sources will be needed to verify the loss budget when the fiber is installed. The longest OSC wavelength, 1625 nm, requires particular attention, as this wavelength lies outside the range in which the fiber or cable manufacturer guarantees the performance of its product. Optical loss test sets specifically intended for this wavelength can be expected to reach the market soon.
Optical Time Domain Reflectometer
A clear tendency is emerging in the OTDR world to offer capabilities in the fourth window spectral region, at 1625 nm. In addition to the ability to test and troubleshoot the important 1625-nm optical supervisory channel, using this wavelength presents other important advantages. In particular, in many circumstances, live fibers may be tested at the 1625-nm wavelength, while normal DWDM transmission continues uninterrupted in the EDFA spectral region. Because optical losses due to fiber bending are more pronounced at 1625 nm than at the shorter DWDM operational wavelengths, OTDR testing at the long wavelength can reveal critical points in the installed fiber—places where the performance of the fiber is acceptable at the time of installation but could degrade over time (see Figure 10).
Back Reflection Meter
In a conventional (non–WDM) network, the optical return loss (ORL) can be determined with a single measurement using a back reflection meter at the operating wavelength. In DWDM systems there are two possibilities: an aggregate measure covering the entire wavelength band in use or a detailed one, giving results for each channel wavelength. Although the first is obviously quicker to perform and may provide enough information to satisfy a go/no-go acceptance test, ORL can vary considerably from channel to channel. This ORL variation with wavelength may be caused by defective Bragg gratings or, more often, from bad connectors at the output port of a multiplexer or demultiplexer. Excessive back reflection can cause instability in DFB source lasers, thereby affecting the overall system performance. As a result, an ability to perform the more complex wavelength-dependent measurement will often be needed.
An aggregate measurement is made with a broadband source and an independent power meter in the same way the measurement is carried out in a single-wavelength optical link. The measurement result is a single value—the total ORL power at the test point, over the entire transmission spectrum. The value of the ORL as a function of wavelength is often a more useful parameter intrinsically, and its determination may be essential if the simpler aggregate test should fail on a particular link. It is measured using a high-power broadband source, usually an erbium-amplified spontaneous emission (ASE) source. High power is needed to provide enough power in each measurement band, which may be as little as 0.1 nm wide, to give an adequate signal-to-noise ratio at the detector for the lowest ORL of interest. The detector is an optical spectrum analyzer of adequate resolution and sensitivity. The result, of course, is an individual ORL reading—often just the information needed to guide a troubleshooting session—for each DWDM channel (see Figure 11).
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