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Thread: Calling all physics geeks! :)

  1. #1
    Join Date
    Jan 2006
    Charleston, SC

    Question Calling all physics geeks! :)

    I've been thinking about laser fundamentals recently and I recalled a conversation I had with Sam Goldwasser almost 2 decades ago where we discussed how the cavity size (that is, the linear distance between the two mirrors that make up the Fabry-Perot resonator, not just the linear length of the gain medium) needs to be an integer multiple of the output wavelength to reach optimal gain in the cavity.

    Obviously with external mirrors you can adjust for this. And even with hard-sealed, non-adjustable optics (thinking HeNe tubes here), the tube length (and thus the cavity length) is still quite long when compared to the fundamental output wavelength, so even a tiny shift in the output wavelength (minute fractions of a nm, which would be well within quantum uncertainty limits for the electron energy levels involved) would still add up to enough of a total shift in length along the cavity to allow for a perfect standing wave to form between the mirrors. No problem so far...

    My question then, is thus: in the case of a low power single-mode direct-injection laser diode (which has an almost comically short cavity size), does this short cavity end up broadening the output spectrum between different diodes? And is there a point where the cavity becomes so short that lasing action can't happen because you can't get a standing wave to fit in the cavity at the desired output wavelength? (That is, the cavity is not a convenient multiple of the wavelength and you can't get enough waves in the cavity to allow for a tiny shift in wavelength to add up to a large enough distance change between the mirrors...)


  2. #2
    Join Date
    Nov 2008
    Cleveland Ohio


    Certainly anything less than the plank length is too short.

    i have not looked into this yet but what about the case of superlasing where there is no cavity.

    A laser is light Amplification by Stimulated Emision of Radiation so seems to me based on common sense so long as you can create an excited state and dump it by stimulation with a photon of the right energy you should be able to emit radiation that is amplified. I think the cavity for purposes of a must have is not a must have. Now that means the output will be lower than if it feedsback but you still have laser action? Make sense? Based on nothing but thought experiment so I could be totally wrong. For 3xapmle you might call the gain medium itself a cavity. I’m thinking in terms of a reflected signal forming the cavity rather than just the gain medium.

    Seems so long as you can form a pn junction you can lase so the answer is where the tunneling leakage is greater than the ability of the hole pairs to form such that a one way current can flow in the band gap. That is given by the lattice energy, the doping, geometry, band gap, tunneling,..... page 305 and on in the below
    Last edited by kecked; 01-05-2019 at 12:31.

  3. #3
    Join Date
    Oct 2012


    ... there already are some lasers built with/from "quantum-dots".

    Here some German articles with lasers in the sizes of some microns to nanometers:

    Aufruf zum Projekt "Müll-freie Meere" -
    Call for the project "garbage-free seas" -

  4. #4
    Join Date
    Dec 2018


    What I learned was that to boost power you have to fold the beam. The longer the cavity length the greater the potential power because a longer cavity length will capture multiple wave lengths (the lasing medium does not just produce one pure frequency of radiation). If you make the cavity length equal to only one mode then you are giving up all those other potential wave lengths that can add to the power. You would have an extremely pure output of just that one frequency but it would be extremely weak.

    This is something I want to research further -
    - Does a longer length also boost power because there is more gain medium to pump and because pumping is a semi random event the more medium there is the easier it would be to make a constant beam because you have tons of random excitation and relaxation happening at different times so you don't have to "pulse" the laser?
    - In the case of an antenna the frequency is determined by the power supply to the antenna and the antenna is designed to be a total length of 1/2 wave length in order to radiate at 90 deg. If I remember correctly if the antenna length is not 1/2 then you get radiation at some other angle (I would have to dust off my old mat lab models to see what the pattern looked like).

    Obviously making a 35' long laser cavity is impractical which is why beam folding is important. I am guessing this is what laser designers are attempting to do with fiber lasers because you can coil a fiber laser back and forth as long as the fiber does not exceed the critical angle.

    These are good questions and going back to the fundamentals is always a good idea, I am going to think about this some more. Optial frequencies and radio frequencies are the same EM radiation just a different wave length so the fundamentals should still apply.

  5. #5
    Join Date
    Jul 2008


    OK, maybe a little off topic but this is definitely physics geek material. Kecked mentioned something about Planck length and I came across this in the Wikipedia article on the Planck constant:

    "On 16 November 2018, the International Bureau of Weights and Measures (BIPM) voted to redefine the kilogram by fixing the value of the Planck constant, thereby defining the kilogram in terms of the second and the speed of light. Starting 20 May 2019, the new value is exactly

    h = 6.626 070 15 × 10 − 34 J ⋅ s "

    Holy crap, I must have missed Lester Holt's announcement that day...

    Announcing the upcoming retirement of the International Prototype of the Kilogram- that beloved platinum-iridium cylinder circa 1889 that sits under multiple bell jars in a vault near Paris.

    Click image for larger version. 

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    Once milk has been poured over corn flakes, the clock starts ticking.

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