EXFO FTB-5800 Chromatic Dispersion Analyzer for FTB-500 - Manual (Page 99)

Chromatic Dispersion Analyzer 91
B Measuring Chromatic
Dispersion: Theory
EXFO’s Chromatic Dispersion Analyzer applies the phase-shift method,
which consists of measuring the phase variation of a sinusoidal intensity
modulated signal at a specific wavelength to obtain the relative group
delay of a fiber. The derivative of the relative group delay calculated from
fitted equations on computed results gives the dispersion, the
zero-dispersion wavelength and the dispersion slope.
Method Behind the Chromatic Dispersion
Analyzer
In EXFO’s Chromatic Dispersion Analyzer, the light source is a broadband
source, which is intensity modulated at a high frequency. This light is
injected into the fiber under test and the modulated light travels along the
fiber. The high-frequency intensity modulation propagates at a speed that
depends on wavelength and polarization. Values are obtained for the
differences in travel time between modulation at different wavelengths.
The higher the number of wavelengths at which this is done, the more
accurate the chromatic dispersion results will be.
The first fixed filter in the receiver extracts the portion of the light that will
follow an optical path. A second filter, which is a time-tunable filter, will
extract a second part of the original signal and then follow a different
optical path.
Different path combinations are added using a chopper. The amplitude of
the high-frequency signals are measured and digitized. The phase
difference is then computed using the different amplitudes. This value is
stored along with the tunable filters position and a new measurement
cycle begins. A new time delay is computed and stored with wavelength
data. When sufficient data is accumulated, chromatic dispersion can be
computed.