What is Your Loss Measurement Uncertainty?
September 15, 2015 / General, Installation and testing, Upgrading and troubleshooting, Best Practices
Testing structured fiber cabling to international standards such as ISO/IEC 14763-3 has been defined. In the last few years, the question of loss (attenuation) measurement uncertainty has been raised. Since 14763-3 specifies test limits, the user of the standard may need to know the amount of uncertainty in the measurement. Experts from IEC, well-versed in fiber metrology and uncertainty calculations, were requested to provide simplified guidelines, a table of sorts, that a user such as an installer might find useful. At the last “ISO” meeting (SC25) in Milan, and after some fine work by the experts, uncertainty values were provided.
Uncertainty of loss measurements using fiber test equipment has not been breached before commercially; optical power meter absolute power uncertainty has only been specified. There has been a need to provide attenuation measurement uncertainty (fixed values) for typical 14763-3 measurement configurations such as permanent links and channels using various reference methods, such as the 1-cord method, for both single-mode and multimode fiber cabling.
For single-mode attenuation measurements, the uncertainty is mostly determined by connector mating reproducibility, reference connector mating to the cabling under test, light source stability, and the light source wavelength uncertainty. These uncertainties tend to be fixed – the single-mode uncertainty is a non-variable value. That is, the uncertainty does not change with measured attenuation. From a graphical representation, the uncertainty remains relatively flat with varying measured attenuation.
For multimode attenuation measurements, the uncertainty contributions mentioned for single-mode apply but are not the greatest contributor to uncertainty. The largest amount of uncertainty is caused by the launch condition. Assuming the best case launch condition, encircled flux, the uncertainty varies as a function of measured attenuation. From a graphical representation, the uncertainty remains flat until approximately after a “deflection” point, then uncertainty increases linearly with measured attenuation. In other words, as attenuation increases so does the uncertainty.
In most cases, high loss measurements over short multimode distances may indicate a potential problem with the installed cabling. This means uncertainty will be high if attenuation is high and is cause for improving the condition, if possible, of the installed cabling or most likely the connector loss.
The important thing to remember is that uncertainty is a calculation based on a set of assumptions, tending to be less “firm” than one might expect. As a consequence, loss uncertainty can be misleading when fixed values are applied to test limits. There is consensus in standards that the methods used to calculate the fixed uncertainty values are valid. What is left to discuss in future standards meetings is how an installer might apply these uncertainty values to the attenuation measurements. For example, using an uncertainty value for an attenuation measurement should be avoided to qualify a “pass” or “fail” result that may be marginal. Fluke Networks will be at the forefront of this new development and plans to provide upcoming guidance.