Agilent Technologies 86038A User Manual page 44

Measurement Concepts
44
where L is the length of the fiber. The dispersion coefficient is usually
expressed in ps/nm·km.
Combining Equation 1 and
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
influence measurement noise. This relationship is expressed by adding a
phase noise term to Equation
∆φ
total
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
• Avoidance of phase wrapping when measuring highly dispersive devices.
• Avoidance of aliasing when measuring ripple.
Equation 5 warns us not to reduce modulation frequency more than
necessary.
∆τ
1
⋅
-- - ------- -
D =
∆λ
coeff
L
Equation 3
Equation 2, we obtain:
∆φ ⋅ ⋅ ⋅
= 360° D f
m
Equation 4
4:
( ⋅ ⋅ ⋅ ∆λ
= 360° D f
m
Equation 5
Agilent 86038A Optical Dispersion Analyzer, Third Edition
Description of the 86038A
∆λ
Equation 4
) ∆φ
+
noise