Optical material damage threshold plays an important role in high energy, high power laser systems. In many cases the achievable energy is limited by the failure of optical coatings, gratings, or even bulk materials. Hereby, the onset of early damage is of particular interest.
Our approach uses a micro channel plate (MCP) for collection of positive ions emerging from the substrate surface when damage occurs (see Fig. 1). This device is capable of detecting single ions and should therefore be highly sensitive in predicting the onset of damage while being insensitive to alignment.
Figure 1: Schematic of the MCP used for damage testing.
Damage testing was performed in a vacuum vessel at pressures below 5x10-6 Torr. This pressure provides a safe operating environment for the MCP and mitigates interferences of successive laser pulses with debris (see Fig. 2).
Figure 2: Laser damage setup using an MCP to detect the onset of damage.
For proof of concept we sent a 1053nm, 3 ns, and 4mJ beam onto a 2'' fused silica substrate. The 1.5 cm beam was focused to a spot size of 40 mm FWHM using a 40 cm focal length lens. The laser energy was slowly increased from 0.5 mJ to 4 mJ and the MCP signal was monitored while varying the bias voltage between -1000 and -2200 volts. Measurements showed that strong signals can be obtained at very low laser energy and a bias voltage of only -1000 volts (see Fig. 2).
Further details and actual damage testing results are discussed in the following paper:
Highly sensitive method for measuring the onset of damage using a micro channel plate
Schwarz, J; Ruggles, L; Rambo, P; Porter, J
Source: Proceedings of SPIE - The International Society for Optical Engineering; 2005; v.5647, p.379-384
Conference: 36th Annual Boulder Damage Symposium Proceedings: Laser-Induced Damage in Optical Materials: 2004; Sep 20-22 2004; Boulder, CO, United States