Figure 4: Phasor diagrams for amplitude (Left panel) and phase (Right panel) modulated light.
Carrier field is given by a brown vector rotating clockwise with the rate around the origin of the
complex plane frame. Sideband fields are depicted as blue vectors. The lower ( ) sideband
vector origin rotates with the tip of the carrier vector, while its own tip also rotates with respect to
its origin counterclockwise with the rate . The upper ( ) sideband vector origin rotates
with the tip of the upper sideband vector, while its own tip also rotates with respect to its origin
counterclockwise with the rate . Modulated oscillation is a sum of these three vectors an is given
by the red vector. In the case of amplitude modulation (AM), the modulated oscillation vector is
always in phase with the carrier field while its length oscillates with the modulation frequency .
The time dependence of its projection onto the real axis that gives the AM-light electric field strength
is drawn to the right of the corresponding phasor diagram. In the case of phase modulation (PM),
sideband fields have a constant phase shift with respect to the carrier field (note factor in
front of the corresponding terms in Eq. (22); therefore its sum is always orthogonal to the carrier field
vector, and the resulting modulated oscillation vector (red arrow) has approximately the same length
as the carrier field vector but outruns or lags behind the latter periodically with the modulation
frequency . The resulting oscillation of the PM light electric field strength is drawn to the right
of the PM phasor diagram and is the projection of the PM oscillation vector on the real axis of the
complex plane.
|