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?
 A: 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.
A: In simple words there is no common answer to your question. I am not sure about the first one but second and third are absolutely possible depending on the class of lasers. There are other possibilities too.
In high power laser you use MOPA i.e. master oscillator power amplifier or CPA chirp pulse amplification techniques. In both these techniques the laser seed from oscillator is gradually amplified and expanded to decrease the power density. 
In this case even if a small part is back reflected (and that too not so critically aligned) the beam expanders will now act as beam reducer and the laser will decrease in size with each step. It can finally create damage in oscillator or in any of early amplifiers. This is a very serious issue and one has to deal with them using pockels cell or Faraday isolaters.
Second scenario appears in the oscillators when if you couple the laser back to the oscillator critically it can help in selecting the modes in a better way. 
I hope this will help
regards
A: I actually just did this and when done over small distances of 1-64 inches the laser creates concentric rings behind the lasers source caused by the laser passing through its own wake .
A: I have done both of the activities that I describe below . I used a common 5mw 650nm laser pointer.
A laser that is shone onto a distance mirror and reflected back onto itself, surprisingly, produces no noticeable results. I did this at a distance of 8o meters, and I used a lens to refocus the returning light. The beams passed through each other with no visible effects. 
Also, there is a method for measuring the speed of light that involves a rotating mirror and sending a laser beam to a distant mirror and sending the returning beam directly back to its source. The beams running against each other have no effect on the speed of light either. 
