HP 86037B, Instruction Manual

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Quick Start
Measuring Grating Devices
Without the benefit of curve fitting and since dispersion is the derivative of the
group delay, the effect of any noise is exaggerated. For example, large
amounts of noise will be measured at wavelengths in a filter’s cutoff region.
5 Use a 2 GHz RF modulation frequency with a small wavelength step size. Step
sizes down to 1 pm are possible. To set the step size,
refer to “Wavelength
Selection area” on page 2-6 .
Setting the RF modulation frequency
When selecting the RF modulation frequency, you’ll want to select the highest frequency
possible without introducing 360
°
phase shifts. This requires that you know the approxi-
mate group delay variation (between measurement wavelength steps) expected on the
device that you are measuring.
Set the RF modulation frequency to a value which has a period that is at least twice the
anticipated group delay between wavelength steps.
For example, when measuring a grating device that has a variation in group delay of as
much as 500 ps between wavelength steps, set the RF modulation frequency to 1 GHz or
less. If you specify a wavelength step size of 2 pm to 100 pm, it is unlikely that any but the
fastest gratings (such as tuning filters or dispersion compensation gratings) would require
less than the default 2 GHz frequency.
RF modulation frequency
1
2 group delay between wavelength steps
( )
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