Agilent Technologies 86038B, User Manual

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62
Agilent 86038B Photonic Dispersion and Loss Analyzer, Second Edition
The attribute called dispersion is defined by:
Equation 2
where
∆τ
is the change in group delay in seconds corresponding
to a change in wavelength
∆λ
in meters. In practice, the
dispersion is expressed in units of picoseconds per nanometer
(ps/nm), where
1ps=10
-12
seconds.
The dispersion coefficient expresses the relationship of
dispersion to fiber length and is used to specify optical fiber. It
is obtained by dividing the dispersion value by the length of the
fiber:
Equation 3
where L is the length of the fiber. The dispersion coefficient is
usually expressed in ps/nm·km.
Combining Equation 1 and Equation 2 , we obtain:
Equation 4
Equation 4 shows that the amount of phase change measured in
response to a wavelength step is the product of device
dispersion, modulation frequency and wavelength step. This
equation provides several key insights into the capabilities of
the modulation phase shift measurement method.
Relationship of setup parameters to measurement noise
Here we examine how the terms on the right hand side of
Equation 4 influence measurement noise. This relationship is
expressed by adding a phase noise term to Equation 4 :
Equation 5
Equation 5 shows that as the product of dispersion, modulation
frequency and wavelength step grows smaller, the impact of
phase noise increases.
Three measurement applications sometimes require that
wavelength increment and/or modulation frequency be reduced,
possibly at the expense of increased noise. These are:
• Resolution of spectrally narrow group delay features
D
Δτ
Δλ
--------
=
D
coeff
1
L
----
Δτ
Δλ
--------
⋅ =
Δφ
360
°
D f
m
Δλ ⋅ ⋅ ⋅ =
Δφ
total
360
°
D f
m
Δλ ⋅ ⋅ ⋅ ( ) Δφ
noise
+ =