Figure 15 shows a time domain representation of an electromagnetic wave of frequency
, whose
amplitude or phase is modulated at a frequency
. One can easily see some characteristics of these two
types of modulation, for example, that amplitude modulation leaves the zero crossing of the
wave unchanged whereas with phase modulation the maximum and minimum amplitude of the
wave remains the same. In the frequency domain in which a modulated field is expanded into
several unmodulated field components, the interpretation of modulation becomes even easier: any
sinusoidal modulation of amplitude or phase generates new field components, which are shifted in
frequency with respect to the initial field. Basically, light power is shifted from one frequency
component, the carrier, to several others, the sidebands. The relative amplitudes and phases of these
sidebands differ for different types of modulation and different modulation strengths. This section
demonstrates how to compute the sideband components for amplitude, phase and frequency
modulation.
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