[Physics] Reflect a laser into itself


Concerning the reflection of a laser into itself I have heard quite different thing from my tutors and I am asking therefore if there is some kind of general outcome if one were up to do this.

I have heard the following:

  • Holograms are created e.g. TEM is not the same anymore.
  • The laser will be destructed
  • Some physicist even use this to extend the cavity for a better mode control

Has anyone done this and is there a different answer for every type of laser?

Best Answer

All three options can happen, depending how well you couple the light back to the cavity mode.

First of all, it is not easy to actually couple the light back to the laser. So, in case you don't observe anything, it is likely that you just haven't coupled well enough.

When you put a little more effort into the coupling, probably the first thing to notice would be power fluctuations. The better your mode-matching becomes, the more the distant reflected beam becomes part of the laser cavity. In case of Semiconductor lasers, this might indeed destroy the laser and therefore commercially available lasers often have isolators built in, to prevent this.

Especially lasers based on free-space cavities might change their mode-shape. And yes, adding an additional cavity gives you more control over the modes, allowing e.g. for single mode operation.

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