Solving Measurement Problems - Agilent Technologies 86038A User Manual

Troubleshooting and Upgrades
If problems occur testing narrowband devices
If results of different curve fit models do not agree
If fitted curve does not follow the actual data
If the Group Delay measurement appears to be noisy
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Solving Measurement Problems

The five-term Sellmeier fit is generally inappropriate for very narrowband
devices, such as Fabry-Perot filters. The result can be roughness in the
fitted curve and multiple values of lambda zero. This is not a problem when
testing fibers.
The three curve fit types differ in their ability to conform to the actual
dispersion curve of the tested fiber. As a result, a variation on the order of a
few hundredths to a few tenths of a nanometer can be observed in the
value of a lambda zero. The three-term Sellmeier curve fit tends to yield a
lower lambda zero value than the other two curve fitting algorithms.
Although the five-term Sellmeier curve fit has greater ability to fit the
measured data, it is also more easily affected by noise. Therefore, the
quadratic fit will generally give a more repeatable value of lambda zero. For
most DSF, the mean values of lambda zero returned by the five-term and
quadratic curve fits are in good agreement while the three-term fit yields a
lower value.
The shape of the curve fitted to the relative group delay data depends upon
the device under test and the type of fit selected. Conditions of high DUT
insertion loss, low DUT dispersion or narrow wavelength range can also
affect the quality of the fit. The system will always try to fit the selected
curve to the data. Because the 5-term Sellmeier curve fit has more
freedom to follow the shape of the data, it may display multiple zero
dispersion wavelengths under some conditions.
Make sure that the environment in which the system is placed is stable.
Also keep in mind that the instrument requires an 80 minute warm-up.
During that time, measurements will show a lot of drift. How the
instrument is performing in its environment can be observed by attaching a
patch fiber and running the Real-Time Port Monitor to check the power
stability. To run the Real-Time Port Monitor, attach a mouse and click on
System->Real Time Port Monitor (F12).
Solving Measurement Problems
Agilent 86038A Optical Dispersion Analyzer, Third Edition