Question about back reflection...

[MechEng3]

May be a tricky question....but could the same hold true for dpss in some cases? I have always wondered that the cheaper 532 units with less than nice optical coatings could fall victim to this....specially if they emitted a lot 1064.....

[buffo]

DPSS lasers are normally not subject to failure due to back-reflection because of the optical coatings inside the cavity. 808 nm light is the one that would have the best chance of making it back through all the components in the cavity and hitting the pump diode face, but in theory there shouldn't be any 808 in the output beam in the first place. Likewise, any 532 nm light that gets reflected back into the cavity should never get past the KTP crystal because of the HR 532 coating just upstream of it.

Finally, remember that direct injection red diodes are normally run very close to their maximum rated output (because we all want as much red as possible), and it's the high optical power density at the face of the diode that causes the damage. Now, IR pump diodes are not normally run really close to their maximums, so you have some breathing room there. Also, the emitter face on an IR pump diode is larger, which lowers the average power density. (This is why you need beam-shaping optics between the pump diode and the vanadate.)

Bottom line: unless you're talking about a multi-watt DPSS unit, you don't need to worry too much about back-reflection with DPSS lasers. But direct injection visible diodes are *very* susceptible to it. (Note that this goes for blue-ray diodes as well.)

Running the lasers at low power during alignment does two things: It reduces the risk of killing a diode due to back-reflection, and it makes it much easier to see the alignment because the beams (and thus the spots they make) are dimmer. When you run the lasers at full power, sometimes the splash from the mirrors (or dichros) is so bright that you can't tell for sure if the beams are intersecting perfectly or not.

Adam