3.4 Amplitude modulation

In contrast to phase modulation, (sinusoidal) amplitude modulation always generates exactly two sidebands. Furthermore, a natural maximum modulation index exists: the modulation index is defined to be one (m = 1) when the amplitude is modulated between zero and the amplitude of the unmodulated field.

If the amplitude modulation is performed by an active element, for example by modulating the current of a laser diode, the following equation can be used to describe the output field:

( ) E = E exp (iω t) 1 + m cos (Ωt) 0 0 ( ) (58 ) = E0 exp (iω0t) 1 + m- exp (iΩt ) + m-exp (− iΩt ) . 2 2
However, passive amplitude modulators (like acousto-optic modulators or electro-optic modulators with polarisers) can only reduce the amplitude. In these cases, the following equation is more useful:
( m ( )) E = E0 exp (iω0t) 1 − -2 1 − cos (Ωt) ( m m m ) (59 ) = E0 exp (iω0t) 1 − 2-+ 4-exp (iΩt) + 4- exp (− iΩt) .
View Image

Figure 17: Electric field vector E0 exp (iω0t) depicted in the complex plane and in a rotating frame (x′, y′) rotating at ω0 so that the field vector appears stationary.

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