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Principal Sponsors:
 | Dense Wavelength Division Multiplexing (DWDM) Testing |
6. The Wavelength Meter
The interferometer-based wavelength meter is used in the laboratory to make accurate, repeatable measurements of source wavelengths. Such measurements are often needed in DWDM systems, in particular to check the center wavelengths and the drift characteristics of each of the transmitted optical channels. Although the intrinsic accuracy of about 0.001 nm that such instruments attain in the laboratory is entirely adequate to characterize DWDM components, providing comparable capabilities in field instruments—including such features as internal wavelength-reference sources and fast-fourier transform (FFT) processing for deconvolution—is a design and engineering challenge of considerable magnitude (see Figure 8).
Nevertheless, the wavelength meter is expected to be the instrument of choice for such DWDM tasks as accurately measuring the center wavelengths of distributed feedback lasers and monitoring how they change with time (both short-term and long-term), temperature, and other environmental conditions. Among the characteristics that are particularly important in a field version of a wavelength meter are the following:
- absolute wavelength accuracy—The ability to measure the absolute wavelength of a channel accurately is the strongest attribute of this type of instrument. With the help of an interferometer, which is usually both precise and accurate in wavelength, the absolute accuracy of the wavelength meter should be better than about 0.005 nm, adequate to locate individual DWDM channel wavelengths.
- absolute power accuracy—The ability to measure the exact power in each DWDM channel is important to verify the power flatness throughout the link. Using a wavemeter, the actual power in each channel can be determined through use of the FFT calculation to an accuracy that is limited by the approximations inherent in that calculation. The resulting absolute power accuracy will usually be a little lower than that of the OSA (see Figure 9).
- dynamic range—This is the ability to measure weak signals in the presence of strong ones. Dynamic range is required—as it is in the OSA—to measure the noise floor in a multichannel transmission system. The wavelength meter can attain a dynamic range of 20 dB to 25 dB for characterization of DWDM channels.
- number of channels—The number of channels the instrument can extract depends on the mechanical precision of the interferometer and on the extraction capacity of the FFT algorithm used. Forty to a hundred channels should be attainable, enough to characterize DWDM systems properly.
The wavelength meter's strength in absolute wavelength measurement enhances the OSA and is an excellent complementary instrument for the complete characterization of DWDM systems. Furthermore, certain field operations—determining DFB center wavelengths and troubleshooting lasers, in particular—require the accuracy that the wavelength meter provides.
Once again, the challenge is to modify present wavelength meters, which are intended for use in the laboratory, to make them suitable for the demanding DWDM field environment. The new field wavelength meter will have to be rugged and portable and will have to offer simple and—to as great a degree as possible—automatic test procedures.
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