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Principal Sponsors:
 | Dense Wavelength Division Multiplexing (DWDM) Testing |
5. OSA Characteristics
Characteristics essential to a field version of an OSA, while measuring the core parameters already identified, include the following:
- dynamic range—An adequate dynamic range—the ability to measure weak signals in the presence of strong ones—is needed to measure the power in a strong, nonsaturated signal and that of the adjacent noise floor (in a specified bandwidth). For example, in a system with 100 GHz (0.8 nm) channel spacing, an OSA must be able to measure an optical signal at a given wavelength and, just 0.4 nm away, an ASE noise level that may be 30 dB to 35 dB weaker (see Figure 5).
- optical sensitivity—The instrument sensitivity—the lowest signal level it can quantify—is generally determined by electronic considerations (the dark current of detectors, noise in detector preamplifiers, etc.). It must be low enough to permit the measurement of component insertion loss and assess the signal-to-noise ratio in all parts of a network.
- resolution bandwidth—The resolution bandwidth of an OSA determines its ability to deal with close optical channel spacing. It is measured as the width of the response curve at half peak power (i.e., 3 dB down) of the instrument to a monochromatic test signal. This specification is often called full-width half-maximum (FWHM) (see Figure 6).
- wavelength accuracy—This is without doubt the shortcoming of the optical spectrum analyzer. Good absolute wavelength accuracy requires the perfect positioning of the grating, which is difficult to do with rotational mechanisms. However, the precision of OSAs gives them the ability to detect unacceptable relative drifts in DFB laser sources. Outboard calibration options such as acetylene absorption cells can be used to improve absolute accuracy to a level acceptable for many other dense WDM test applications (see Figure 7).
Although complete redesign of the traditional OSA is needed before its capabilities can be offered to field personnel faced with the difficult task of maintaining and troubleshooting DWDM networks, its potential advantages in measuring appropriate parameters make the OSA the leading candidate to dominate the DWDM test field. The OSA offers, in a single package, virtually all the test capabilities needed, but many steps must be taken to simplify the hardware and make it rugged, as well as to provide the one-button test procedures, auto-diagnostic functions, and easy-to-follow approaches needed to take its undisputed advantages smoothly to the field, where they are badly needed.
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