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Operation
Checking Test Fiber Stability
Measurement theory
Stability is examined by measuring modulation phase at the output of the test fiber rela-
tive to the electrical modulation source, as a function of time. The change in group delay
required to produce a change in modulation phase is given by:
C h an g e in p ha s e
∆τ =
3 60 x M od ula tio n F req u en c y
In convenient units, this equation becomes:
C ha nge in phas e (degrees)
∆τ
=
(p s)
M odulation F requenc y (G H z)
An observed phase change of 0.36 degrees at a modulation frequency of 1 GHz is evi-
dence of a 1 ps change in the group delay of the test fiber. For convenience, you can set
the scale/div to 0.36 degrees so each vertical division corresponds to 1 ps of drift in the
group delay.
If you prefer to analyze the stability of the test fiber in terms of length, use the following
equation:
C h a n g e in p h a s e (d e g re e s )
∆
=
L ( m m )
n x M o d u la tio n F re q u e n c y (G H z)
Using this equation, an observed phase change of 1.45 degrees at a modulation frequency
of 1 GHz is evidence of a 1 mm change in the length of the test fiber, assuming a refrac-
tive index, n, of 1.45. For convenience, set the Scale/Div to 1.45 degrees so that each ver-
tical division corresponds to 1 mm of drift in the length of the test fiber.
2-38
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