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Question about lasers

  1. Mar 11, 2013 #1


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    Lasers are known to emit a particular wavelength, and with a bandwith that is extremely narrow. I have been told that if a laser medium is white light, such as the light from HID lamps or flash lights, the laser will amplify just one wavelength. Is this correct, or is it possible for a laser to amplify several wavelengths at the same time? If not, why can"t a laser amplify white light (or several wavelengths)?
    I am thinking of if one placed parallell mirrors on each side of a tube containing the same gas as inside HID lamps, and power it up.

  2. jcsd
  3. Mar 11, 2013 #2
    The medium is the material that will be excited in the optical cavity.
    The length of the optical cavity will select the wavelength.
    (standing wave)
    If the medium emits on several wavelengths, AND the wavelengths are
    direct harmonics of each other, more than one wavelength can lase.
    Integrated energy from both wavelengths should be the same, as everything is coming from the same energy pool.
  4. Mar 23, 2013 #3


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    OK. So in a cavity with CO2 and Helium, typical mix for red lasers, the mirrors a few feet apart must be aliged precisely within a few nanometers, if not it will not work?

    I had once a very early version of a Yag laser which emitted green laser light. There was a concave mirror inside it. This mirror I could hold by hand, and still make the laser work. So I just question how precisely these mirrors must be for the active medium to emit a specific color.

  5. Mar 24, 2013 #4
    First, the medium must have optical gain at the wavelength(s) to be amplified. This usually involves one sort of trick or another, and ill not be found in a relatively high pressure gas mixture like that of a high intensity discharge lamp, which is in thermal equilibrium. The cavity mirrors are required to drastically reduce the number of EM field modes which have high quality factors (and so low losses per pass), so the gain is not lost into bazillions of modes. The higher the optical gain, the less critical the mirror alignment becomes. Pulsed lasers typically hav MUCH higher gains than CW systems like HeNe lasers.
  6. Mar 24, 2013 #5


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    This is basically correct. The gain medium absorbs light at the pump frequency, which is always higher than the laser frequency. You can see this by looking at a laser energy level diagram. The diagrams of three-level lasers like the one at the center of this page are the easiest to understand:
    The answer to this question is a little different than that of what pump color is absorbed. Laser media can amplify any wavelengths that fall within a narrow band of the nominal line.
    Because the laser medium operates with a single energy level combination consisting of an absorption line that decays rapidly to a long-lived metastable state which then decays to the ground state. (Four level lasers are more complicated.) The metastable state is absolutely essential to building up a population inversion, and the combination is relatively uncommon. Hence most materials are not lasers, and most lasers are monochromatic. Which, by the way, is very valuable because monochromatic coherent light is a powerful research and optical tool. White light, on the other hand, is incoherent and already very easy to generate without laser action.
    You won't have a laser.

    Regarding mirrors: they are required to provide multiple passes through the laser medium to provide the required gain. Their spacing is not as important as parallelism. Typical gas lasers like HeNe are therefore multimode.
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