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"Unlike pulsed THz imaging, the CW imaging (...) only yields intensity data and does not provide any depth, frequency-domain or time-domain information (...)." [1]

Why does a pulsed signal provide more information than the CW signal? How is this achieved?

[1] Comparison between pulsed terahertz time-domain imaging and continuous wave terahertz imaging, Nicholas Karpowicz et al. Semicond. Sci. Technol. 20, S293,(2005)

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    $\begingroup$ Say you are making some excitation, which has a decay rate. In CW you get no info on what that rate is. $\endgroup$
    – Jon Custer
    Commented May 5, 2021 at 13:06
  • $\begingroup$ Just to see if I understand why: is it because a CW would just keep exciting the atom not allowing it to de-excite to a lower energetic state? I understand that would have an influence in time-domain info, but what about frequency? $\endgroup$
    – user7077252
    Commented May 5, 2021 at 13:23

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With pulses you can measure the time it takes for the pulse to fly to the target and come back after reflection. This allows to calculate the depth of the target. In practice you may get multiple reflections for the same pulse and this will result in various features in the image. And fir each feature you have the depth relative to the transducer surface. There is no matter of atom excitation. The same principle applies to ultrasound imaging and pulsed Doppler.

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  • $\begingroup$ So in which situations would a CW setup be preferable? Thank you for your help, your explanation makes perfect sense. $\endgroup$
    – user7077252
    Commented May 5, 2021 at 13:48
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    $\begingroup$ Well, I suppose the CW may require simpler/cheaper electronics. I don't know if there is really a case where it is "preferable". Thus is a subjective matter. In the case of ultrasound, to produce short, nice pulses requires more sophisticated transducers. However all usual medical imaging uses pulsed methods. $\endgroup$
    – nasu
    Commented May 5, 2021 at 14:11
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    $\begingroup$ It's too general to say what is preferable. CW is the limit of pulsed doppler with infinite PRF (sometimes)...so it depends on how you want to set your range & doppler window and also what range &doppler ambiguities you can tolerate. CW has a range ambiguity of 0 (sometimes), and hence much more doppler range. Remember: doppler is NOT a shift of the carrier frequency, it's a shift of the phase after mixing down the signal. $\endgroup$
    – JEB
    Commented May 5, 2021 at 14:13

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