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Laser Power Increase?

  1. Jul 14, 2008 #1
    Has anybody ever tried adding mirrors around a laser tube cavity where a lot of spontaneous emission can be reflected back into the tube? The idea is like this - if there is a low probablitity of absorption, perhaps the emissions will be induced to occur down the main laser tube where the outside world can absorb the beam. A reference paper for this is signaldisplay <dot-com>/laser.pdf
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  3. Jul 15, 2008 #2


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    It's not clear, are these photons the same wavelength as the main laser transition? Pump photon wavelength? Other?

    If they are laser-transition photons, then I'd expect the population inversion to cause amplification, rather than absorption, of these photons. Amplification of the energy in these "off-axis" beams, I expect, would make for a decrease in power in the main output beam.
  4. Jul 15, 2008 #3
    These are laser transition photons. In a laser, there is initially spontaneous emission of photons and many of these are "lost" in terms of their contribution to the beam. Some are on the main axis of the mirrors and therefore can participate in stimulated emission. The purpose of the mirrors around the outside of the cavity (not the main axis mirrors, but mirrors placed around the laser tube where spontaneous photons would be directed) is not to reflect the photons back into the lasing medium, but to prevent them from ever being emitted in the direction of the outside mirrors in the first place. If photons really have a source and a sink as Wheeler-Feynman suggested in 1940s, then without the sink part of the equation, the photons will cease to be generated. That is, the highly reflective mirrors around the cavity (not on the ends, again, a new set around the cavity), reduce the sink to a very low level so that any absorption of spontaneous photons is extremely minimal. Then the photon does not emit in that direction, but rather in a direction where a sink is more likely - on the main axis where it can participated in stimulated emission (increasing the gain) and then out the laser tube on the main axis. The power goes up, assuming the photons behave this way. The random nature of spontaneous emission in QM has always been a curious item, but the source-sink nature of photons goes back even to Einstein's mention of the ideas of Tetrode, who we communicated with frequently on the source-sink possibility of photons. This model explains a lot of aspects of photon entanglement, non-locality, etc. The experiment then, is to put highly reflective mirrors around the laser tube cavity, in addition to the mirrors on the end. The loss from spontaneous emissions is high, and being able to force them to emit originally down the main beam axis adds to the stimulated emissions already taking place.
  5. Jul 16, 2008 #4


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    I'm not familiar with the Wheeler-Feynman work, though I have worked a lot with lasers. It seems to me that those photons would eventually leave the cavity, since one cannot construct perfect 100% mirrors and cannot make them completely surround the volume -- therefore, there would always be a sink for spontaneously emitted photons.

    But, you can always try building one and see what happens.
  6. Jul 16, 2008 #5
    Thanks for your reply. Quantum mechanics is mostly about energy and time - the amount of energy one needs to transfer and the amount of time required (hence, the uncertainty principle). If it takes longer for photons to find the sink (because they keep getting bounced around) then the preferred emission will be the more available sink in the shorter period of time. This is my theory. If it works the power increase may be small, or it may be.. big! That would be great, but it's a simple experiment to try..If you are interested and find some useful data I can publish your results (before November).

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