Backwards optics

[macona]

I went to check my yag tonight to see if a spare HR from another yag would fit in the holder. When I pulled out the old HR I found it was installed backwards some time in the past. What effect does this have on the laser?

[krazer]

As long as it was a planar optic (ie, not a curved mirror), there will be a little bit of loss due to reflections off the 'wrong' surface, but in a big flashlamp pumped yag there is so much gain even a 10% reflection won't completely kill operation. If the surfaces were completely parallel (ie, no wedge) it may have acted as an etalon, but I doubt that this was the case. If it was a curved optic (which is fairly likely for your laser welder system), then having it in backwards will change the modes in the cavity quite a bit (the radius of curvature becomes the negative of what you started with!), and as long as the original designers didn't completely screw up their design flipping it the right way round should give you a bit more power/better beam quality.

In any case its good news for you, as it will do a lot less damage compared to if they had flipped the OC for the HR, or put 2 HRs in

[macona]

As long as it was a planar optic (ie, not a curved mirror), there will be a little bit of loss due to reflections off the 'wrong' surface, but in a big flashlamp pumped yag there is so much gain even a 10% reflection won't completely kill operation. If the surfaces were completely parallel (ie, no wedge) it may have acted as an etalon, but I doubt that this was the case. If it was a curved optic (which is fairly likely for your laser welder system), then having it in backwards will change the modes in the cavity quite a bit (the radius of curvature becomes the negative of what you started with!), and as long as the original designers didn't completely screw up their design flipping it the right way round should give you a bit more power/better beam quality.

In any case its good news for you, as it will do a lot less damage compared to if they had flipped the OC for the HR, or put 2 HRs in

In the welding config it is in it goes for higher divergence and higher power and uses a short cavity with flat mirrors on both ends. In low power low divergence mode it replaces the oc with an etalon and the rear mirror is replaced with a water cooled aperture which extends the cavity path through an intracavity beam expander to the HR which has a 10m radius.

I think I am going to try installing the LD optics this weekend and see what happens.

[mixedgas]

On the scientific yag lasers I service, the OC "IS" the etalon. One less optic in the cavity that way...
On a high power yag, the OC had best have a AR coat on the output side.

Steve

[macona]

Yes, that is exactly how this is, the standard 60%R OC is removed for an etalon.