Undergrad Understanding Lasers: Questions and Answers

[Malamala]

I am sorry if these questions are basics, but I am not sure I totally understand the basics of how a laser works and the books I read are not totally clear for me, so any explanation would be greatly appreciated. So assuming we have a plane mirrors cavity and a gain medium in between, we create a population inversion in the gain medium, which initially creates photons by spontaneous emission and the photons that are produced parallel to the axis of the cavity gets amplified. The amplification is produced by stimulated emission due to the fact that the photons produced this way are in phase with the initial ones. So in the end from one spontaneous emitted photon you have a lots of photons. Here are my questions:

1) These spontaneous emissions can happen many times and at different locations inside the gain medium. But these spontaneously emitted photons are not in phase with each other (but they have the same frequency). So if 2 spontaneously emitted photon are out of phase by half a period, even if they get individually amplified a lot you would have 2 huge bunches of photon out of phase with each other. So the 2 electric fields would be huge, but they would cancel each other, so the output of the laser would be zero. Of course perfect cancelation is unlikely but I feel like overall, given the huge number of spontaneously emitted photons there would be lots of cancelations. So even if the amplification of one photon is super efficient overall it seems like this is an inefficient process due to this out of phase problem. What am I missing here?

2) Assuming there is just one bunch of photons (basically ignoring the problem in 1) ), the laser output appears when these photons hit one of the mirrors. So this happens every 2L/c seconds, where c is the speed of light in that medium and L the length of the cavity. So the laser created will appear as a pulse. However, I read that there are continuous wave lasers, too. How does that work? Doesn't the light still need to go back and forth between the 2 mirrors, how can you get a continuous output?

Thank you!

 

[BvU]

The idea of a laser (light amplification by stimulated emission of radiation) is that you populate the excited levels (by pumping) to such an extent that stimulated emission dominates spontaneous emission.

2) one of the mirrors transmits a fraction of the photons that go back and forth a large number of times. Pulse lasts as long as population inversion is maintained.

 

[Malamala]

The idea of a laser (light amplification by stimulated emission of radiation) is that you populate the excited levels (by pumping) to such an extent that stimulated emission dominates spontaneous emission.

2) one of the mirrors transmits a fraction of the photons that go back and forth a large number of times. Pulse lasts as long as population in version is maintained.

This doesn't answer my questions. For the first one, I know that you build it such that the stimulated emission dominates over spontaneous emission. But there will be lots of spontaneously emitted photons that gets amplified and even if the photons created by the amplification of one spontaneously emitted photon will be in phase, there is no reason for 2 initially spontaneously emitted photons to be in phase. For example, assume that you have 2 spontaneously emitted photons (emitted along the axis of the cavity), with the same frequency but having a phase difference of half a period. Each of them will get amplified a lot and the electric field associated to the amplification of each of them will be big, but they will be out of phase by half a period. So in the end you end up with 2 big electric fields, with the same frequency, but half a period out of phase. So they will destructively interfere so the total electric field leaving the cavity will be zero, even if the individual electric field are really big.

For the second question, I am not sure you understood what I asked. This is what I said in my post, too. You have a bunch of photons that go back and forth between the mirrors. When this bunch of photons hit one of the mirrors, part of this energy gets transmitted. But this is a pulsed energy i.e. the energy leaves the cavity only when the bunch of photons hits the mirror. However I read that there are also continuously wave lasers in which, the output of the laser is continuous and not in pulses as before. How can one create such a continuous laser?

 

[mfb]

1) These spontaneous emissions can happen many times and at different locations inside the gain medium. But these spontaneously emitted photons are not in phase with each other (but they have the same frequency). So if 2 spontaneously emitted photon are out of phase by half a period, even if they get individually amplified a lot you would have 2 huge bunches of photon out of phase with each other. So the 2 electric fields would be huge, but they would cancel each other, so the output of the laser would be zero. Of course perfect cancelation is unlikely but I feel like overall, given the huge number of spontaneously emitted photons there would be lots of cancelations. So even if the amplification of one photon is super efficient overall it seems like this is an inefficient process due to this out of phase problem. What am I missing here?

Energy can't disappear and you can't have two independent groups of photons like that. While the lasing process might start in two places with a specific phase shift this process can't happen in the same volume (and the stronger one will "win" quickly). Reducing the laser intensity would need a transition from the lower to the higher energy state, not the opposite. This transition happens, of course, but less often than the opposite.

2) Assuming there is just one bunch of photons (basically ignoring the problem in 1) ), the laser output appears when these photons hit one of the mirrors. So this happens every 2L/c seconds, where c is the speed of light in that medium and L the length of the cavity. So the laser created will appear as a pulse. However, I read that there are continuous wave lasers, too. How does that work? Doesn't the light still need to go back and forth between the 2 mirrors, how can you get a continuous output?

There is light everywhere in the cavity.