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Homework Help: Laser Wavelengths

  1. Apr 15, 2007 #1
    1. The problem statement, all variables and given/known data

    How many photons per second does a low power (1 mW) He-Ne laser ([itex]\lambda=336[/itex]nm) emit?

    At what He-Ne laser power do you expect quantum effects to become important?

    3. The attempt at a solution

    I got the answer to the first part, [itex]1.7\times 10^{15} [/itex] photons/second.

    But the second part makes no sense to me. The book doesn't talk about lasers at all in this chapter or the ones before it. I looked HeNe lasers up on google, and I saw statements like these:

    So apparently, the power does not effect the wavelength! And so for the HeNe laser in the question, the wavelength will always be [itex]\lambda=336[/itex]nm. So how could changing the power possibly effect the wavelengths and hence the relavence of quantum effects? Is this a trick question?

    Last edited: Apr 15, 2007
  2. jcsd
  3. Apr 16, 2007 #2
    It makes no sense to me either. Obviously it made no sense to anyone at PF or someone would have answered.
  4. Apr 16, 2007 #3
    Yeah, I'm not surprised. This books sucks horribly. It's littered with typos and the author doesn't even know how to use LaTeX! (For example, she uses a '.' (period) to represent dot products! And she randomly switches the usage of the Greek letters nu and upsilon, as if they were the same letter. I have no confidence in this text!)
  5. Apr 16, 2007 #4


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    I think... in general (?)... you need to look at when the photon energy becomes comparable to the bandwidth of the gain.
  6. Apr 16, 2007 #5
    I think the first major typo is the wavelength. A HeNe laser operates at
    633 nm and not 366 nm as indicated in your quote-box.
  7. Apr 16, 2007 #6
    Last edited by a moderator: Apr 22, 2017
  8. Apr 16, 2007 #7
    If you are searching for good books, I can recommend the following:

    1) Understanding Lasers: An Entry-Level Guide (IEEE Press Understanding Science & Technology Series) by Jeff Hecht
    This book is easy to read and serves good as an introduction.

    2) Principles of Lasers by Orazio Svelto
    This book has more details.

    3) Lasers (Hardcover) by A. E. Siegman
    This is the bible on lasers.

    4) Fundamentals of Photonics by Saleh, Teich
    Contains many topics besides lasers.
  9. Apr 17, 2007 #8


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    I'd recommend:

    Semiconductor-Laser Physics (Hardcover)
    by Weng W. Chow (Author), Stephan W. Koch (Author), Murray, III Sargent (Author)
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