Suppose the RF frequency of the driver of an AOM is 40 MHz. I understand that it will Doppler shift the diffracted first order beam by 40 MHz but how does it modulate the amplitude of that first order beam by 40 MHz. I am just trying to understand that whether the Doppler shifting and the amplitude modulation both take place simultaneously or are in some way related phenomenon.


1 Answer 1


Doppler shift and amplitude modulation are two separate characteristics of the modulator.

For the first order, laser frequency is shifted by 40 MHz as you stated.

Laser amplitude is modulated by the RF envelope, not RF frequency.

Changes in RF envelope are slower than 40 MHz. They may be up to 30% of the RF frequency and usually much lower. This rate of envelope changes depends on the RF generator bandwidth, electro-acoustic transducer bandwidth, laser spot size and more.

  • $\begingroup$ Thank you very much for the response. I was wondering if AOMs are available which can amplitude modulate a laser beam below 4 MHz. Most of them are mentioned as having the driving RF frequency in range of tens of Megahertz . $\endgroup$
    – user188062
    Commented Sep 30, 2018 at 20:45
  • $\begingroup$ The speed at which an AOM can modulate a beam depends on the diameter of the beam and the time it takes for the sound wave to traverse the diameter of the beam. So for the fastest operation, you want a small beam located right next to the transducer, but you will be still limited by the speed of sound and the diameter of your beam. With a very small beam, you can probably manage 4 MHz but with reduced diffraction efficiency. $\endgroup$
    – wcc
    Commented Feb 20, 2019 at 17:11

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