Pulsed Laser Damage

The damage threshold of optics for use with true pulsed lasers such TEA CO2 lasers is expressed in units of Joules/cm2, at a given pulse length.

It is difficult to scale this figure for damage threshold to account for different pulse lengths, but there is some correlation that the figure varies with the square of the pulse length. So for a pulse twice as long the damage threshold in units of J/cm2 is four times greater.

Gold coated copper has a enormous pulsed laser damage threshold of 46.7J/cm2 for a 80ns pulse length. The reason for this is the amorphous coating. Laser damage initiates at grain boundaries on the surface of a crystalline material. If the surface is amorphous (glass like, with no crystalline features) this disruption of the crystal boundaries (“slipbanding”) is eliminated and the damage threshold leaps to a higher value associated with melting of the metal.

Measurment of pulsed laser damage threshold is described well in this paper from NASA

NASA-97-rp1395.PDF

CW Laser Damage

LBP in partnership with UK and European Universities have investigated the causes of Laser Damage, and measured the Laser Induced Damage Threshold (LIDT) of many CO2 laser optics.

This work has been published by SPIE

Proc. SPIE Vol. 3244, p. 188-198, Laser-Induced Damage in Optical Materials: 1997, Gregory J. Exarhos; Arthur H. Guenther; Mark R. Kozlowski; M. J. Soileau; Eds.

Proc. SPIE Vol. 2714, p. 281-281, 27th Annual Boulder Damage Symposium: Laser-Induced Damage in Optical Materials: 1995, Harold E. Bennett; Arthur H. Guenther; Mark R. Kozlowski; Brian E. Newnam; M. J. Soileau; Eds.

For an abstract of the work click here

The most important result is that Copper mirrors, and ZnSe lenses in a typical industrial laser operate well within their Laser Damage Threshold. The reason(s) optics fail is entirely due to external factors from their operating environment.

In particular mechanical forces from mounting, clamping, etc significantly reduce the lifetimes of ZnSe lenses. Increased absorption from external contamination can quickly lead to laser damage. The difference between a good lens and a poor lens can be an increase in absorption of just 1 part per thousand.

Fortunately the situation with copper mirrors is more forgiving, as their ability to ”sink” heat is so much better.

Gold coated copper mirrors are used on lasers of 40 KW power, and for 5 KW and above copper mirrors are the only realistic choice. Such a large power handling ability means that even when damaged or dirty copper mirrors carry on working with high power lasers.

For a laser with a 20mm beam diameter copper mirrors are working at just a fraction of their potential.

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