HP 86037B, Instruction Manual

Page: 14 / 204

1-8
Quick Start
Measuring Fiber Devices
Measuring Fiber Devices
In this section, you’ll learn how to optimize measurements on fiber devices.
You can, of course, skip this section and immediately begin making measure-
ments on your devices. However, be sure to read this material later to increase
your understanding of chromatic dispersion measurements.
By adjusting the RF modulation frequency, you can optimize measurement
accuracy. (Refer to the sidebar,
“Assumptions required for this optimization
method” on page 1-10 .) For information on changing the modulation fre-
quency, refer to “CD/Gain Meas command” on page 2-15 .
Because the system’s accuracy varies as a function of RF modulation fre-
quency, for optimum accuracy keep the modulation frequency above 1 GHz
and preferably at 2 GHz. This puts some restrictions on the wavelength step
size because too large of a step size results in a phase "wrap-around" where
the phase jump exceeds 360
°
as the wavelength is stepped. Use the following
formula to determine the maximum step size:
where:
(
∆λ ∆λ ∆λ ∆λ
)
MAX
is the maximum allowed step size (in nm) to keep the modulation
frequency at the desired value fm.
f
m
is the desired modulation frequency (in GHz).
S
0
is the dispersion slope in ps/(nm2×km).
λ
SPAN
is the spectral span (stop
λ
– start
λ
) of the measurement in nm.
L is the length of the system/fiber in km.
Suppose, for example, that you wanted to measure chromatic dispersion on a
70 km spool of dispersion shifted fiber between 1515 nm and 1575 nm. Select
the optimum modulation frequency of 2 GHz, and use the above equation to
determine the maximum allowed wavelength step size. Assuming the fiber’s
dispersion slope is 0.07 ps/(nm2 km), then:
5 0 0
S
o
ƒ
m
λ
S P A N
L
(∆λ)
M A X
=