All rods do degrade very slowly, and in extreme high power apps or apps that are water cooled with direct cooling of the rod, the outside certainly degrades very slowly. . With lamp pumping the outer part of the rod can get bleached or become adsorbent. ND:YAG is long lived while For example ND:YLF rods in water cooled systems last about three or four years before the outer surface of the rod in contact with cooling water dissolves so bad as to reduce power by 30-40%.
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For most modern apps, at modest pump powers, other parts of the laser will die first. For example the mirrors might degrade because of the very high electric fields that can be created by the intracavity light. KTP and LBO crystals have a finite lifetime, as well.
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ND:YAG usually gets repolished and re-used if the ends are damaged, can suffer from ion migration, end coating damage, or thermal cycling causing cracks, etc.
But in general, in a sealed cavity CW or Q-Switched laser, it is extremely rare to replace a rod unless the cooling system corrodes or freezes. . For a very high power laser running in "Giant Pulse, Sub-Nanosecond, Amplifier, or Mode Locking services", replacing a rod may not be so rare..
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Clean cooling water of the proper pH is important here in the case of wet rods. With good cooling water a rod may only degrade 1-2% in terms of power in 10-20 years. I've seen quite a few 20 year old rods in service.
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Usually you can assume 5-7 years minimum for Nd:YAG, less for ND:GLASS, etc. Modern crystal growing techniques are amazingly good, and post grind inspection will easily catch 99% of possible defects.
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I have one ND:YLF diode pumped rod that has defects in it at the pump diode focal points, but other then that, I rarely see defects. The user simply moved the rod over to get around the defects a few times.
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small slices or wedges of rod in Green pointers are where I see the most gain medium defects or death. They are often very thin and grossly overdriven.
Steve
Last edited by mixedgas; 04-19-2018 at 12:41.
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