Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why LASER is monochromatic

  1. Mar 4, 2005 #1
    Hello



    Can any genius answer my question



    Q:I know that LASER is monochromatic.But i want to know WHY?


    Q:I also know that metastable state has greater life time .But i want to know why?
     
  2. jcsd
  3. Mar 4, 2005 #2
    1. Lasers are built around one specific transition = one pair of energy levels = one wavelength, thus the monochromaticity.

    2. You can see a metastable state as a local minimum. A relativily large bandgap has to be crossed for a subsequent transition to the next or fundamental state. Surely, one wouldn't expect every state to have the same lifetime. Some are extraordinarily long.

    Gonzolo, apparent genius-hood pending :approve: :zzz: :rolleyes:
     
  4. Mar 4, 2005 #3
    Actually, if you want to get technical, no laser is truly monochromatic. Some are closer than others and can be treated as such to a very good approximation, but the laser would have to be emitting light for all eternity to be considered purely mononchromatic.

    Another factor in creating a narrow-line laser is cavity design. Many solid-state laser media (ie. Ti:Sapphire) can lase over 100 nm (very broad band). If you place a Fabry-Perot Etalon inside the laser cavity, you can get just one wavelength to lase and get a narrow-line laser.
    One cause of extremely long-lived states is that the transition is technically forbidden by quantum mechanics. A good example of this is the ruby laser transition. How can the transition happen at all? Thermal effects nudge the electrons to a lower state. As you cool the ruby, the life time increases.

    Hmm..., where do you apply for that sort of thing? :biggrin:
     
  5. Mar 4, 2005 #4
    Yup. Femtosecond pulses are very wide. I think your explanation for metastable states is an alternative to my bandgap wording (only one cause). I tend to be vulgar.

    Usually, physicists don't call each other geniuses, so I didn't think it necessary to be too technical. The term "Genius" reminds me of Wile E. Coyote's business card in one of the classic cartoons :

    "Wile E. Coyote, Genius"

    Well, since I have a respectable corroboration now...

    Gonzolo, Apparent Genius :biggrin:
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Why LASER is monochromatic
  1. Monochromatic waves (Replies: 2)

  2. Monochromatic waves (Replies: 20)

Loading...