ACCURACY CONSIDERATIONS
All Measurements
Keep optical connectors clean and in good condition. APC connectors
will generally provide improved power stability on single mode
systems.
For better loss testing accuracy, use an APC connector plugged into
the power meter. This will reduce line reflections and improve
repeatability.
To reduce the effect of polarisation changes, test leads should be neat,
coiled and physically stable.
In multimode systems, modal noise and general uncertainty are much
worse than in single mode systems and optimum measurement
repeatability will be obtained by use of a mandrel wrap on the source
patch lead.
Wavelength uncertainty affects power meter calibration. This is
significant with a Ge detector in the 1550 nm band (e.g. > 1560 nm in
cold weather). Or worse with InGaAs detectors @ 850 nm.
For general measuring from 660 to 1550 nm, Ge meters offer
adequate accuracy for many field users.
For better accuracy or linearity at wavelengths from 1000 – 1550 nm,
or wavelengths above 1550 nm, the InGaAs meter is preferred.
For high power testing, the H series meters offer excellent accuracy,
power handling, wavelength and connector reflections insensitivity.
Light source power may drift. When you have finished a test, go back
to the start position to check if the light source power is still within
acceptable limits. Specifications are for typical drift, warm up, and with
a specified level of return loss. Actual drift will vary between
instruments and test situations.
Most available laser sources are sensitive to reflections. Varying
reflections can induce laser source instability of around 0.3 dB. This is
very difficult to verify without a special test system, but can cause
errors. Reduced reflection will result in improved repeatability.
Due to emitter centre wavelength uncertainty (e.g. ± 20 nm), fiber
attenuation may vary with different light sources.
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