Exfo FastReporter 2 User Manual download pdf (Page 367)

Post Processing Software 357

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 modulations 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.

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Exfo FastReporter 2 User Manual Page 367