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A question of combining laser light

  1. Aug 9, 2010 #1
    Hi guys,

    I'm interested to know what would happen if you get a red laser and
    combine its light with green or a blue laser.

    What i would like to do is modulate the higher freq laser with a
    lower frequency light source and get a different colour output.

    Firstly is this possible? and secondly, if it's not possible,
    how would I get the same effect of what happens at radio frequecies
    when you 'mix' 2 different frequency radio waves?

  2. jcsd
  3. Aug 9, 2010 #2

    Andy Resnick

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  4. Aug 9, 2010 #3
    Hi Andy,
    Thanks for getting back to me.

    Practially, how easy is this to do?
  5. Aug 9, 2010 #4
    Actually, looking at what's around on the web, the hetrodyne seems to be more for signal analysis.

    I'm after a means of optically modulating the light from a 3W laser, not detecting via a sensor and interpreting a signal.
  6. Aug 9, 2010 #5
    You get an effective color that depends on the relative intensities of each beam.

    Based on what principle? How can frequency1 modulate frquency2? You can get interference if they are identical, similar (beats), or harmonics, but frequency modulation?

    Not sure what is meant by "mix", but if you mean beats, you need two frequencies that have to be almost the same, but not quite. This means they'll basically be the same color. This can cause intensity pulses, but not a frequency shift.

    If you use two frequencies where one is "almost" a harmonic of the other. You might get something interesting (a beat effect caused by very different frequencies), but still not frequency modulation.
    Last edited: Aug 9, 2010
  7. Aug 9, 2010 #6
  8. Aug 9, 2010 #7

    Andy Resnick

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    Light cannot directly modulate light, in general. What is usually done is to use light to modulate the optical properties of a material (Pockels cell, Kerr cell).
  9. Aug 9, 2010 #8
  10. Aug 9, 2010 #9


    Staff: Mentor

    Hi motorman, you may have a little bit of a misunderstanding about what modulation is and what it does.

    First, when you "combine" light that usually means that you simply add it together, e.g. shine both lights on the same spot. That is not what modulation is, modulation is when you multiply two signals together. So, if you had a laser, and you sinusoidally varied its power output at 1 Hz then you would be modulating the laser frequency by 1 Hz.

    When you modulate a signal using a sinusoidal waveform you generate two copies of the signal, one is shifted up by the modulation frequency, and the other is shifted down by the modulation frequency. So, if you were able to modulate a red laser (450 THz) by a blue laser (650 THz) you would get a -200 THz (near infra red) and a 1100 THz (near ultra violet) light signal.

    If you want to modulate a red laser (450 THz) and get a green laser (550 THz) then you would need to modulate using a 100 THz (near infra red) signal. I don't know of any practical way to do that. I would guess that you might be able to modulate using an RF signal, but that is not going to change the color visibly. In that sense, the visible spectrum is very broad band, which is why fiber optics can carry so much data.
  11. Aug 9, 2010 #10
    Photons, in free space, do not intermingle with respect to "change". Thus a red laser and a blue laser beam combined will NOT change.
    Your eyes might perceive this otherwise, but the laser light(s) themselves are immutably separated in this circumstance.
  12. Aug 10, 2010 #11
    Thanks for all the interesting replies!

    Dr-lots-o'watts: I know of the beat principle but I don't think it'll do what I want. I beginning to think simpley 'adding' the light source will not be suffificent as the resultant light might look mixed, but it will not be modulated as such.

    Andy & Arydberg: I think what you're suggesting is use something like a gain medium similiar to how a ruby rod laser works (if I remember correctly), and use an external modulated light source to mod the output? Is that right?

    DaleSpam: Mixing is what I want to achieve mathematically as per the link:

    It's the practical methods of do this that I'm looking for.

    Pallidin: Is there any way that if you had a photon of freq1 and and another of freq2, that you could get the 2nd photon to donate its energy to photon 1, to make photon2 more energetic?

    My thinking goes along the lines that if UV is more energetic than IR, by combining/colliding/or in some other way boosting the energy of a photon, the photon could increase in energy. So in theory, you could take an IR photon turn it into a UV photon.

    I know it may sound a bit far fetched, but I want to do a sanity check on this before I spend any serious time looking into this.
  13. Aug 10, 2010 #12


    Staff: Mentor

    Yes, that is exactly what I assumed you are interested in. So my comments above are all applicable. I don't know of any practical way to mix two signals which are each in the THz range, but in principle it could be possible.
  14. Aug 10, 2010 #13

    Andy Resnick

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    I can't speak for Arydberg, but I wasn't suggesting anything so exotic- certainly nothing active.

    Your other statement sounds a lot like 4-wave mixing, or optical parametric amplification.
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