Slit apertures in BeamScan heads are made from a proprietary metallic alloy. The slit apertures are often blackened to reduce reflectivity and minimize reflections back into the laser cavity. Because customers have experienced slit damage, damage threshold tests were performed on various BeamScan slit apertures to establish general use guidelines for prevention of damage to slit apertures.

Blackened and unblackened apertures with nominal slit widths of 1 µm, 2 µm, 5 µm, and 25 µm were tested. Tests were made at laser wavelengths of 532 nm and 1.06 µm. Damage thresholds are defined here to be the average laser irradiance at which the onset of visual damage occurs. The average irradiance is defined as the average power divided by the beam area at the 1/e² beam diameter.

All tests were performed under normal BeamScan operating conditions with the aperture slits moving. (Damage to BeamScan apertures can occur at much lower power levels if the BeamScan head is off and the apertures are stationary in the laser beam.) The tests were performed at laser power levels <3 Watts for short time exposures - on the order of 5 minutes. The damage thresholds that were determined are therefore applicable only for short time exposures at these power levels. For high power lasers and long exposure times, the damage thresholds are likely to decrease due to excessive heating of the apertures and/or possible ablation that does not manifest itself as visual damage in short term exposure tests. These effects have not yet been quantified, so users are advised to exercise extreme caution when attempting to measure high power beams for long time intervals.

Note that for the case of blackened slit apertures, the onset of visual damage occurs when the black material begins to ablate. This type of damage does not affect the integrity of the slit but does increase the reflectivity of the aperture. Slit integrity is only compromised at the higher laser irradiance associated with damage to unblackened apertures. This damage takes the forms of wrinkling or creasing of the aperture due to thermal stress and scoring of the aperture due to melting of the metallic alloy. At higher irradiance and longer exposure times the apertures can be cut.

Recommended upper limits of average laser irradiance based on the results of the visual damage threshold tests for short time exposures (~5 minutes) at power levels less than 3 Watts are summarized in Table 1.

The values of average irradiance listed in Table 1 should be used as guidelines to determine if your operating conditions may cause damage to the apertures in your BeamScan head for short time exposure at power levels <3 Watts only. For long term exposures (<5 minutes) at higher power levels, the damage thresholds may be reduced.

Exercise Caution! Photon inc. does not warrant damage to slit apertures and detectors due to damage from high power lasers.