Specifications and footnotes: What does it all mean?
Many things in the fiber-optics world are defined by standards, regardless of whether they are set by the ITU, IEC, TIA, or other bodies. The same holds true for many types of test equipment and parameters. For example, one could argue that the standard for OTDRs is extremely well-defined. But...is it really?
The answer is yes under most circumstances, but not across the board. There are two parameters in OTDR measurement that are not defined and characterized in the standard: pulse width and reflectance. Because these two parameters influence both specifications and performance, they can lead to good specifications that yield poor performance, or vice versa.
It is therefore important to understand specifications, and what they do and do not mean.
Let’s start with pulse width. Generally speaking, a shorter pulse width on an OTDR will lead to the best spatial resolution, and the best event and attenuation dead zones needed to detect and measure very closely spaced events. The problem; however, is that the definition used to establish the width of the pulse is not standardized, and therefore, there is some divergence and liberties taken in the method used by each OTDR manufacturer to determine the measurement value.
Typically, bandwidth (or pulse width) is accepted to be full width at the half max (FWHM), or in other words, 3dB from the peak. This is the definition that EXFO uses. But, nothing prevents other manufacturers from using, for example, 1.5 dB down from the peak, where the pulse is necessarily narrower. Because only the pulse width is shown on the specification sheet, and not the method used to measure it, an OTDR may be mistakenly perceived as having better resolution than is actually the case. For example, a pulse width of 3 ns should, in theory, lead to better spatial resolution than one almost twice the size at 5 ns. This is a perfect example of a case in which a better spec is associated with worse performance. As such, a narrower specification higher in the peak does not provide suitable grounds for comparison. Such data is published for marketing reasons only (a better specification in the spec sheet blindsides users by hiding poor FWHM performance).
For the purpose of comparing OTDRs of equivalent performance, in addition to pulse width, receiver bandwidth is another crucial performance criterion that is not necessarily understood, specified or standardized.
EXFO uses a -45 dB reflectance to test dead zones. However, some manufacturers use a rather unknown value somewhere in the vicinity of –55 dB. Knowing the actual reflectance value for the attenuation dead zone is important, because this value represents the amount of light reflected back to the receiver. The higher the value (i.e., –45 dB vs. –55 dB), the longer it will take for the receiver to return to the reference level (fiber-only Raleigh backscattering level). As such, this represents another case in which a better specification (e.g., for the attenuation dead zone), yields inferior performance.
So, when shopping around for an OTDR, do not limit yourself to comparing specifications only. Instead, understand how they are measured and obtained, and if in doubt, put the units to the test by performing a head-to-head comparison using similar test fiber. The results will reveal the true situation.