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Some questions of mine spawned upon reading my lecture notes about Q-switching:

"In a Q-switched laser we stop the laser from oscillating by altering some property of the laser cavity (otherwise known as changing the cavity quality – hence Q!). As an example, we might use a device that misaligns the beam in the cavity preventing lasing. The cavity now has a low Q. This allows the population of the upper laser level to build up and means that the signal is not being fed back into the oscillator so no lasing occurs. A fast switch then changes the alignment of the cavity to a high Q value. This allows the oscillation to take place and the pulse to be released."

  • Is my assumption right that, in order for this to be achieved, "stopping the laser from oscillating" refers to stopping photons from entering the gain medium, hitting a resonator, hitting the gain medium, hitting the other resonator? So, moving the resonator slightly will prevent that, as a photon will hit the resonator that's curved and not hit the other one since the trajectory doesn't allow for it (although that sounds classical.. and photons don't move classically). And what is the oscillator, in this case?
  • From what I gather, this is my interpretation: if we tilt a resonator, the photons don't gain properly. This causes emitted photons to hit the gain medium, (The emitted photons from the pump, I'd assume... which I'm not sure if is pumped constantly or pulsed here.. since I know this has to do with pulse emission but I'll explain why I'm not sure if it's pulsed pumping or constant pumping) and then not oscillate back. This means less photons hit the gain, causing less stimulated emission and more electrons sitting at a lasing band, with their high decay time. So, without any incident photons, they're more likely to stay there. Then.. I suppose.. from adjusting the "Q value", by aligning the mirror perfectly and fast, we get that needed amount of incident photons, and that cluster of lasing band electrons all undergo stimulated emission in close to at the same time? What gaps do I have in this?
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Is my assumption right that, in order for this to be achieved, "stopping the laser from oscillating" refers to stopping photons from entering the gain medium, hitting a resonator, hitting the gain medium, hitting the other resonator?

You can't stop photons from entering the gain medium, since some photons will be produced there by spontaneous emission.

However, once those spontaneous emission photons are produced, you want to prevent them returning to precipitate stimulated emission, or at least prevent enough of them doing so to start the system lasing.

And what is the oscillator, in this case?

The oscillator is the whole system. Optical resonator and gain medium.

This causes emitted photons to hit the gain medium, (The emitted photons from the pump, I'd assume ...

I'm not sure about the context, but usually we consider the pump energy to be coming in to the laser from somewhere else. From the point of view of the laser, pump photons would be absorbed (i.e., the opposite of emitted) in order to put the active species in its excited state. Probably they were emitted by some source somewhere, but that's not part of the laser proper.

Emitted photons likely refers to photons emitted by spontaneous emission in the gain medium.

if we tilt a resonator, the photons don't gain properly. This causes emitted photons to hit the gain medium, ... and then not oscillate back.

Again, the "emitted photons" start in the gain medium. Anything you do to keep them from being able to make a round trip (or half round-trip) through the cavity and get back to trigger a stimulated emission would be considered a Q reduction.

It could be de-aligning the cavity. It could be closing a shutter in the beam path. It could be adding a switched absorbing material in the beam path. Or any number of other things.

This means less photons hit the gain, causing less stimulated emission and more electrons sitting at a lasing band, with their high decay time. So, without any incident photons, they're more likely to stay there. Then.. I suppose.. from adjusting the "Q value", by aligning the mirror perfectly and fast, we get that needed amount of incident photons, and that cluster of lasing band electrons all undergo stimulated emission in close to at the same time?

This is all pretty much correct.

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