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I am very new to laser systems and have a quick question about the functionality of the amplifier.

To my understanding, the amplifier serves to increase the energy of the incoming photons alongside their population. However, my question is whether the gain they experience must be provided via electricity by the operator.

For example, if the beam gained 5 J of energy through the amplifier medium, would those 5 J need to come from a source of electricity provided by the operator?

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  • $\begingroup$ Any time you say, "energy...of photons," People will think you are talking about the energies of the individual photons, and not, for example, the total energy in a pulse of light. $\endgroup$
    – Solomon Slow
    Commented Nov 21, 2019 at 14:49
  • $\begingroup$ I may have misunderstood you, are you talkin about an amplifier for a laser, or a gain medium with which a laser amplification occurs ? $\endgroup$
    – DakkVader
    Commented Nov 21, 2019 at 14:59
  • $\begingroup$ @DakkVader The gain medium itself $\endgroup$
    – James Li
    Commented Nov 21, 2019 at 15:55
  • $\begingroup$ @JamesLi Then my answer below answers your question. Feel free to read it and vote however you'd like. $\endgroup$
    – DakkVader
    Commented Nov 21, 2019 at 16:16
  • $\begingroup$ @DakkVader Thank you, I think I understand now $\endgroup$
    – James Li
    Commented Nov 21, 2019 at 16:33

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The laser amplifier works by allowing incoming photons to stimulate emission from excited electrons in the gain medium.

The simplest gain medium one can describe is that of one with two energy levels, an upper and a lower energy level. Amplification occurs when an incoming photon allows an excited electron in the upper state to stimulate emission downwards and create an equal copy of the incoming photon. An incoming photon on the other hand may also be absorbed by the gain medium if there is a lot of un-excited electrons in the lower energy state. This is not good for our laser action.

enter image description here

So what is done to excite electrons is that in a conventional gain medium the gain medium is 'pumped'. We can now consider a three or four level system where we have a ground level, a higher one and one in between. Incoming photons may excite electrons from the bottom stage to the uppermost stage, where they will quickly de-excite to a middle energy state. Incoming electrons may then stimulate these middle-electrons to provide a laser behavior. In order for these systems to work, and for the medium to not absorb but to instead amplify we need to have more electrons in higher states than in the lower states. This is referred to as population inversion.

enter image description here

So to answer your question, in a conventional laser today the energy comes from another light source which in turn acts as the pump. Nowadays there are laser diodes and these are electrically pumped, meaning that a current or voltage through/over the diode provides the energy to move electrons into the right states. Previously with for example a HeNe-laser, a gas is pumped/excited with an electrical current. To summarize, yes, the energy comes from somewhere else.

This got longer than i expected, I hope I've done no error anywhere.

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  • $\begingroup$ Would it be safe to say that a laser amplifier basically is the same thing as a laser, but without the resonant cavity? $\endgroup$
    – Solomon Slow
    Commented Nov 21, 2019 at 14:44
  • $\begingroup$ @SolomonSlow Hold on, i may have misunderstood him, maybe he is actually talking about an amplifier and not the gain medium which I've not described. $\endgroup$
    – DakkVader
    Commented Nov 21, 2019 at 14:52
  • $\begingroup$ @SolomonSlow But essentially, yes, for these CW lasers. For pulsed lasers its a different story. $\endgroup$
    – DakkVader
    Commented Nov 21, 2019 at 14:59

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