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If a few kHz modulation is imposed on the 80MHz pulse train of a laser by the means of either a Mch-Zehnder interferometer or a single acousto optical modulator triggered by an external function generator, what kind of modulation is taking place? Amplitude modulation or intensity modulation?

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  • $\begingroup$ modulation in amplitude leads to modulation in intensity, so is there some important difference that you are thinking about (nonlinearity)? $\endgroup$ – wcc May 16 at 3:32
  • $\begingroup$ "If a few kHz modulation" this is frequency modulation and it does not affect (much) the amplitude or the intensity ( which is a function of average amplitude) pbs.org/wgbh/aso/tryit/radio/radiorelayer.html em waves have the same mathematical description for all frequencies. $\endgroup$ – anna v May 16 at 3:41
  • $\begingroup$ @AmIAStudent Actually your answer better rephrases my question. "modulation of amplitude leads to modulation in intensity". So the question is still under normal circumstances is amplitude modulation same to intensity modulation or is there a fine difference between these two? $\endgroup$ – user188062 May 16 at 4:03
  • $\begingroup$ @annav actually this few kHz is significant. So much so that I am using this as a reference for my lock in amplifier to extract signals. $\endgroup$ – user188062 May 16 at 4:05
  • $\begingroup$ amplitude of electromagnetic waves is conventionally described by the E field, a time varying vector. en.wikipedia.org/wiki/Electromagnetic_radiation#Properties . intensity is given by the average of the amplitude as the Poynting vector hyperphysics.phy-astr.gsu.edu/hbase/Waves/emwv.html $\endgroup$ – anna v May 16 at 4:15
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The usage difference reflects the essentially noise-like nature of laser light relative to the modulation rate. The word intensity means average power, that is the variance of instantaneous amplitude, and is preferred when the underlying modulation is very slow relative to the natural fluctuation rate. The average instantaneous light amplitude is zero and in that sense it is just like RF thermal noise and it stays that way even when the mean square amplitude, ie., the intensity is modulated.

In amplitude modulation of a sinusoidal carrier there is a non-zero "amplitude" that is being changed at some rate much smaller than the carrier frequency. This also modulates the intensity, ie., the average power of the carrier but from an engineer's point of view it is usually done by a multiplicative process (heterodyning) instead of source bias modulation or by a variable attenuator as is more applicable for a laser source. Both bias modulation and variable attenuator can be used in RF but they are noisy and rather lossy when compared to mixing/heterodyning. Making a laser source somewhat noisier by bias modulation makes no difference in practice, not so for your AM radio...

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