Choosing Setup Parameters - Agilent Technologies 86038A User Manual

Description of the 86038A
Agilent 86038A Optical Dispersion Analyzer, Third Edition

Choosing Setup Parameters

This section presents the mathematical relationships on which the group
delay and dispersion measurements are based. You will see how
wavelength resolution and measurement sensitivity depend on the
instrument setup parameters. This knowledge will help you to effectively
resolve group delay ripple, avoid phases wrapping errors, avoid aliasing
errors, and improve the repeatability (reduce the noise) of chromatic
dispersion and relative group delay measurements.
Background
The Agilent 86038A employs the modulation phase shift method. Light is
intensity modulated with an RF tone and applied to the device under test.
The transmitted (or reflected) signal is detected to recover the modulation
envelope, and the envelope phase is measured relative to the RF source.
Any change in the group delay ∆τ of the test device produces a
corresponding change in the modulation phase. In practice, the
wavelength is stepped or swept and the change in the group delay ∆τ for
each wavelength increment is calculated from the measured change in
phase according to:
where ∆φ is the phase change in degrees produced by a small wavelength
step, fm is the modulation frequency in Hz, and the subscript ∆λ indicates
that the change in group delay being measured was produced in response
to an incremental change in wavelength. The first term on the right side of
Equation 1 is the fraction of cycles of modulation phase shift produced by
the wavelength change. The second term is the time period of a single RF
cycle (1/freq = period). The product of the two terms has units of time.
The attribute called dispersion is defined by:
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
-12
1ps=10 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:
∆φ
1
∆τ = − − − − − ⋅
-----
( ∆λ ) °
360 f
m
Equation 1
∆τ
= − − − −
D
∆λ
Equation 2
Measurement Concepts
43