I am using frequency tripled Nd: YAG laser but I am facing some problem of self focusing of beam behind the plano convex lens and it seems it is ionizing the air. Does it has anything to do with the coating of lens ?
Hi,
It may be inonization of atmospheric nitrogen since the triplicate frequency of the Nd: YAG laser is very close to the frequency of nitrogen inonization. But this is quite unusual in a non-instrumental level of perception unless the laser power is extremely high.
I can not to imagine selffocusing in the air because its low, near to1.000 index refraction not only in backward to beam propagation but even in forward one. But observation of discharge in air with plasma formation it is very known and studied effect since time of birth the laser cavity Q-switching. So I can propose that You have observed the same effect with UV picosecond lasing emission in backward direction. Moreover in backward direction the focusing lens forms two beams equal intensity which can interfere and gain intensity for the discharge level in air.
4 percent reflection from inside surface of glass gets focused behind the lens. This much focused power is sufficient for air breakdown.
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