Gas lasers have mirrors that are physically adjustable, as do many other types of lasers. They're not always easily adjustable, as in the case of the majority of HeNe tubes, but even these can be adjusted by using a jig designed for the purpose.
Unless the reflective surfaces of the mirrors in any laser system are perfectly aligned, the output will drop off and in extreme cases disappear altogether. With most larger gas laser systems the mirrors are both adjustable and replaceable, and they require regular alignments in order to maintain peak output.
The process of aligning gas lasers involves a laser power meter (preferably analog instead of digital since it's easier to read power peaks with a needle on a meter than with a digital number display) and a tool to adjust the angle of the mirrors. One begins making tiny adjustments to one of the mirrors while watching output. As the adjustment is made, the power will either rise or fall. Once this mirror has been adjusted to peak power, the other is adjusted the same way. Once that hits a peak, it's back to the other mirror, and so it goes, back and forth until the absolute highest power output is reached. This process is called "walking" the mirrors.
With diodes, the mirrors consist of the front and rear facets of a tiny piece of crystal, so they cannot be adjusted nor can they go out of alignment. They can, however, be blown completely apart by overvoltage/overcurrent.
