Agilent Technologies 86038A User Manual page 120

Function Reference
Curve Fit
120
The precision of chromatic dispersion measurements is greatly improved
by fitting an appropriate model equation to the measured relative group
delay data. Because direct differentiation of the raw group delay data
tends to amplify the effect of noise, the values λ
dispersion at any particular wavelength, are calculated from the fitted
curve. The model equation should be chosen according to the type of
device being measured.
Enable Curve Fit
When selected, applies the specified curve fitting algorithm to the trace.
Algorithm
Linear calculates single parameter least squares fit. The equation is the form:
y = mx + b. It is commonly used for dispersion-shifted fiber, in which waveguide
dispersion is dominant.
Quadratic curve is a least squares fit to the equation
a good approximation to any parabolic shape.
3rd Order Sellmeier curve fit is commonly used for dispersion-unshifted fiber,
in which material dispersion is dominant.
5th Order Sellmeier curve fit, although more affected by noise and instabilities
in the measurement path, provides more general purpose curve fitting. The five-
term Sellmeier fit can yield multiple zero-dispersion wavelengths. All the values
and their associated slopes are shown in the graphs. The system searches for
dispersion zeros in a wavelength range equaling approximately five times the
measurement span (2.5 times each side of the center wavelength). This allows
identification of zero-dispersion wavelengths which fall outside of the
measurement range. In some cases, due to the peculiarity of the five-term
Sellmeier fit, zero-dispersion wavelengths found outside of the measurement
range may not correspond to actual zero-dispersion wavelengths of the device
under test.
Equation Area
Displays the general form of the selected equation and additional
information specific to the selected curve fit.
Hide Original Trace
Removes the original trace from the graph so that only the curve fit trace is
visible.
Agilent 86038A Optical Dispersion Analyzer, Third Edition
Display Menu
, S and D , the value of
0 0 (λ)
2
. This is
y = Ax + Bx + C