■ACCURACY CONSIDERATIONS
All measurements
Optical connectors should be kept clean and in good condition. Due to emitter center λ uncertainty (eg 1310 +/- 20 nm), fiber attenuation may vary with different sources.
This λ uncertainty can also affect power meter calibration. This is significant with InGaAs in the 850 nm band, and absolute errors in the order of 1 dB are possible. This issue affects absolute power measurements. It does not significantly affect attenuation measurements where the meter was referenced to the same source.
Source power may drift: when you have finished a test, go back to the start position to check if source power is still within acceptable limits. It is worth noting that most sources only quote typical drift after warm up. Actual drift may of course be significantly worse, and will be very variable between instruments. The N3974A Dual Laser Source is unusual in offering maximum drift specifications, without warm up.
Most available laser sources ( except the N3974A Dual Laser Source ) are sensitive to reflections. Varying reflections can induce source instability of around 0.3 dB. This is very difficult to verify without a special test system, but can be a source of errors.
Polarisation changes create disturbances in both the optical system and measurement equipment. To reduce this, keep your system physically stable. Keep patch leads neat, coiled and stable.
The N3970A Optical Power Meter has an InGaAs detector.
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