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B Photon interactions in LASERs

  1. Jul 15, 2018 #1
    Hello, new here with lots of questions and minimal knowledge of physics.

    If I understand this correctly, when spontaneous emissions take place within a LASERs resonance chamber, they are not of the desired frequency nor are they paired with other coherent photons. So are they still desirable in helping to create a population inversion withing the gain medium or do they pose an overall negative effect?
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  3. Jul 16, 2018 #2

    Andy Resnick

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    IIRC, in a 2-level scheme, spontaneous emission simply reduces the 'efficiency' of pumping energy into the desired transition. Most (all) real lasers are 3- or 4-level systems, and in this case, spontaneous emission can be used to help build up an inverted population, by (for example) using a fast decay from the 'top' level to the upper laser level.
  4. Jul 16, 2018 #3
    Thanks for the info.

    When referring to an upper laser level, is that also whats called a metastable state or level?

    So in 3+ level lasers, if spontaneous emissions can aid in the population inversion, would it not be beneficial to mirror the sides of the resonance chamber as well as the ends, keeping more photons in play?
  5. Jul 17, 2018 #4

    Andy Resnick

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    The cavity is indeed mirrored on each end- one mirror is as close to 100% as possible, the other, the exit face, is about 95% or so. Note- it's called a resonant cavity for a reason- the cavity geometry helps ensure the desired transition has the highest gain.
  6. Aug 10, 2018 at 4:28 PM #5
    Putting mirrors around the laser tube (replacing the glass tube with a reflective tube?) would probably reduce the laser efficiency dramatically by causing more stimulated emissions that aren't aligned with the laser's intended gain profile. Ideally all incoherent photons (different phase, direction, wavelength, polarization, etc) get absorbed or destructively removed, because stimulated emission creates photons coherent with the incident photon. Those are my thoughts at least.

  7. Aug 11, 2018 at 9:52 AM #6
    Marisa - thanks for the response.

    I can understand the thinking that unwanted photons reflected from the chamber walls could be detrimental, but wouldn't that be offset by desirable photons being kept in play from mirrored sides?

    It seems like loosing desirable photons through the side walls only serves to lessen the overall amplification when the primary goal is to utilize as many as possible. They might have to cycle back and forth a while longer before properly aligning themselves, but ultimately you'd be increasing the lasers output - or a least I think it would.
  8. Aug 11, 2018 at 3:59 PM #7
    Photons that are incoherent aren't desirable, which includes any photons traveling in the wrong directions, especially because they can stimulate the emission of new photons that are coherent with them instead of the photons that are desired which reflect back and forth off of the mirrored ends. You would likely just be taking away from the pool of excited neon atoms that could emit the light you want. Remember that resonance in the cavity only occurs for standing waves, which means the photons have to be half-integer wavelengths of the laser cavity length. The photons you propose wouldn't meet that criteria. As the laser cavity expands in length due to things like thermal pressure as the gas heats up, the wavelength of light sweeping through the gain profile of the laser (or longitudinal mode) increases. If you aren't contributing photons that can fit the gain profile for the cavity length then you won't get lasing.
  9. Aug 11, 2018 at 5:04 PM #8
    When I try picturing an active resonance cavity, I end up with images of total chaos as photons are zipping around in every direction. When one does find a desirable photon that will eventually pass through the less reflective end, must it have been emitted exactly perpendicular to the mirrors?
  10. Aug 11, 2018 at 5:15 PM #9
    Because the only highly reflective surfaces are on the ends of the cavity, only photons that have the proper half integer wavelengths to be standing waves can resonate. Photons that aren't coherent with the resonant photons will destructively interfere with themselves and go away after a few passes on the mirrors, or get absorbed by the cavity/housing of the laser and go away. There are certainly photons going in all kinds of directions, but the ones that survive to stimulate the emission of new coherent light are resonating in the cavity. One of the mirrors is less reflective than the other so there's a probability that a photon will pass right through instead of reflecting, it is this leaked light that you see as laser light. The system is extremely collimated so any substantial amount of light leaking through will be in the intended direction of the laser beam, which is "perpendicular" to the mirrored ends. It would be more accurate to say "parallel with the laser's optical axis." It's possible some photons leak out of the output coupler (less reflective mirror) that aren't aligned with the optical axis, but their numbers are far fewer and there's little if not zero likelihood they'll pass the collimating lens or shutter where the beam comes out of the laser.

    Another feature of gas lasers worth mentioning is that they have a preferred linear polarization axis due to asymmetries in the construction of the tube and mirrors, as well as from some types of mirror coatings. Lasers that are "unpolarized" have nothing intentionally done to control their polarization, and so they have an inherent polarization due to those features I just mentioned. Polarization is a part of coherent light because light needs to have the same polarization to cause interference. This polarization stuff adds even further preference to light resonating in the cavity under the gain profile of the laser. Neon wants to return to ground state so it's fighting hard to emit photons and modes in the gain profile are a good bet to do just that.
  11. Aug 11, 2018 at 5:48 PM #10
    Okay, um so do you ever feel like your brain has a Charlie-Horse?

    Sorry for all of the questions and I really appreciate the help as I try and let this all sink in.

    Within the resonance cavity you have photons of different wavelengths traveling in different directions, but a photon is a photon right? So what determines the likelihood of a photon exiting the cavity, assuming it’s aligned with the optical axis and reaches the output coupler.
  12. Aug 11, 2018 at 6:09 PM #11
    Haha no charlie horses here. Don't apologize for asking questions, gas laser systems are very complex. I haven't worked with them for very long so I'm still piecing everything together. It just takes time and consideration of the role(s) each component plays.

    Yes there are photons of different wavelengths due to things like doppler broadening (an atom moving along a velocity vector emitting a photon in its direction of motion results in a blueshifted photon, emitting away from its direction of motion results in a redshifted photon) as well as other electronic transitions occurring than just the one being lased. Sure, photons are photons, not sure what you're implying here.

    The likelihood of reflectance vs transmittance at the output coupler would, I assume, be related to the uncertainty principle. I'm not exactly sure though.

  13. Aug 11, 2018 at 6:30 PM #12
    The big problem with muscle cramps in ones head is that you can’t massage them away.

    Depending on the type of gain medium and it’s atomic structure, is it possible to have more then one electron sitting at excited energy levels at the same time, and if so, can they become stimulated and emit photons simultaneously if the right conditions occure?
  14. Aug 11, 2018 at 6:52 PM #13
    I can't think of anything that makes that situation impossible, I just imagine it would be very improbable because there would need to be collisions between neon and two metastable helium atoms before the excited electron from the first collision decays. The excited state of neon for the 632.8 nm photon emission favored in HeNe lasers lasts ~ 10-7 seconds. Transfer of energy through collisions is made even more complicated by the relative velocities all the atoms in the gas share between each other. Remember that due to quantum mechanics, the energy of the excited helium has to exactly match the energy of the transition of interest in neon, otherwise that transition won't occur. I can't find any sources that discuss your suggestion as something commonplace so I'm not 100% sure how to feel about it.
  15. Aug 12, 2018 at 9:06 AM #14
    I’ve got to apologize again for all of my questions, I’m a teachers worst nightmare because I’m never happy with the answer “just because”. Okay for that matter I drove my parents crazy as well.

    There are times a red flag starts waving in the back of my head giving me a feeling that something doesn’t add up and I can’t always make sense of it, that’s usually when it comes out in the form of odd questions.

    When a photon of the proper energy level excites and electron, elevating it to a higher level, it requires a ‘physical’ collision for that to happen. But the proximity of an electromagnetic interference from a photon is all that’s needed to cause stimulated emission, or the electron returning to its rest state. So I guess to better clarify my previous question, could a photon pass through an electron cloud of an atom containing an excited electron and instead of the EM interference resulting in stimulated emission, could it passes right through due to there being too great of a distance from the excited election, thus allowing it to decay normally?

    Now that’s an ugly run on sentence.
  16. Aug 12, 2018 at 12:32 PM #15


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    What sort of physical collision were you thinking of?
  17. Aug 12, 2018 at 3:33 PM #16
    I'm sure close calls happen all the time and some excited neon atoms decay to ground state normally, although the photon will very likely be incoherent to the resonating photons. The idea is to have as many stimulated emissions as possible which is why there are so many more helium atoms in the gas than neon atoms. Helium is excited by electric discharge or xenon flash lamp so that there are more excited helium atoms than ground state helium atoms. This is called population inversion. Then, the neon atoms are much more likely to collide with excited helium atoms, so that more neon atoms are in an excited state than the ground state. Another population inversion. Laser cavities generally contain a smaller tube within the cavity that holds most of the excited atoms along the optical axis of the laser, so stimulated emission is the much more likely occurrence vs the normal decay to ground state. Look at some schematics and think about what the atoms are doing to see how the gas laser system is designed to make lasing the primary event within it by minimizing the amount of undesirable decays.
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