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What would happen in a hypothetical scenario in which a LASER strikes a near perfect reflector (like the ones made by Jason Valentine at Vanderbilt, which reflect 99.7% of visible infrared light). I understand that the LASER (machine that generates the beam, not the beam) would probably be damaged by the returning beam, if the angle of reflection was 0°, but also, wouldn't the photons strike and cause some sort of energy release? An explosion, maybe?

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I work with mirrors that reflect 99% of the laser wavelength; even higher rates are common for the mirrors used inside the laser cavity, though some method must be provided for the laser beam/pulse to escape, for example, by having the exit mirror at 90%.

The relative reflectivity of the cavity mirrors determines the average number of round trips for the stimulated emission photons; this is important to control so that the planned amplification is achieved, and for efficient use of pump power.

So what happens if some of the emitted light is returned to the laser cavity? There are several possibilities, but in the simplest case it messes up the mode structure of the cavity, and your previously nice beam has extraneous modes. This is because the coherent light entering the cavity is competing with the circulating photons for pump power. It's even been known to erratically increase the laser power.

The worst I have seen is with a pulsed laser where the output pulse was compressed external to the laser cavity, vastly increasing its power density. Due to an error in the Faraday isolator timing, which can rapidly change which polarization states are transmitted, this compressed pulse entered the laser cavity and burned out the Ti-sapphire crystal that was the heart of the laser.

Ooops! Not quite an explosion, but very expensive to repair.

So in general, don't let the laser light reenter the laser.

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