All optical materials used in high-energy and high-powered lasers are susceptible to laser-induced damage. Most laser damage is defect driven: an optic will tend to fail at isolated weak sites. Defects include inclusions that absorb radiation, causing thermal damage; and pits, scratches, or voids that concentrate radiation, causing dielectric breakdown damage. Impurities in the material can interact electronically, changing their oxidation state, which leads to absorbing color centers and bulk damage.
The damage threshold of an optic is basically the power density (irradiance) or energy density (fluence) it can withstand before it undergoes damage, which we typically define as a permanent, laser-induced change visible with dark-field and/or Nomarski microscopy at high magnification (150X). Exposure below this level does not cause damage. Exposure at higher levels causes damage with a probability that rises approximately linearly from zero at threshold to unity at some higher level. Here we concentrate on pulsed laser phenomena.
To measure a damage threshold, we expose a sample optic to different levels of laser fluence at the relevant wavelength and pulse duration. By using a TEM00 output, we can accurately determine the fluence at the sample from the beam properties. We use beam-profiling software to measure the spot size at the test piece. A fast photodiode and oscilloscope measure the pulse duration; numerical integration derives the true peak power, accounting for energy in the "tail" of the pulse; and a calibrated calorimeter measures the energy delivered, yielding the peak power density and energy density. If the pulse duration of the test laser varies moderately from the test requirement, we can apply the "root t" scaling rulescaling the fluence by t1/2, where t is the multiplication factor between the two times. If we measure a threshold of 5 J/cm2 at 20 ns, for example, we can infer that the damage threshold at 10 ns will be reduced by 2 (about 3.5 J/cm2). The power density threshold will increase by the same amount.
Figure 1: A plot of damage frequency versus fluence shows the threshold point at which damage accelerates.
Because of the defect-driven nature of damage thresholds, sufficient sample area must be irradiated to be representative. The damage-frequency method ensures this by irradiating about 10 different sites at one fluence level, then another 10 sites at the next level, and so on. A plot of the number of damage sites versus fluence reveals the threshold (see figure 1).
Figure 2: The least-fluence method involves testing a single site at each fluence. A smaller overlap region is indicative of better accuracy.
For limited sample areas, we use the least-fluence method. As before, fluence levels are distributed approximately uniformly over a range including the anticipated failure level. One site is irradiated at each level and we record the minimum fluence level required to cause damage as an estimate of the damage threshold. We plot whether or not a site damages versus fluence (see figure 2). The lowest fluence that causes a test site to damage is taken as the threshold. Although this method is less accurate than the frequency method, a greater number of available test sites and a small overlap in fluence between damaged and undamaged sites are indicative of better accuracy.
It is important to remember that damage testing takes place under near-ideal conditions. Real-world conditions include contamination, non-uniform spatial and temporal beam profiles, and instability. Real lasers need to operate well within the damage threshold of their optics to be reliable. oe
Ken Bell is director of sales and marketing at Big Sky Laser Technologies Inc., Bozeman, MT.