Comparisons between laser damage and optical electric field behaviors for hafnia/silica antireflection coatings

Abstract

We compare designs and laser-induced damage thresholds (LIDTs) of hafnia/silica antireflection (AR) coatings for 1054 nm or dual 527 nm/1054 nm wavelengths and 0° to 45° angles of incidence (AOIs). For a 527 nm/1054 nm, 0° AOI AR coating, LIDTs from three runs arbitrarily selected over three years are ∼20 J/cm2 or higher at 1054 nm and <10 J/cm2 at 527 nm. Calculated optical electric field intensities within the coating show two intensity peaks for 527 nm but not for 1054 nm, correlating with the lower (higher) LIDTs at 527 nm (1054 nm). For 1054 nm AR coatings at 45° and 32° AOIs and S and P polarizations (Spol and Ppol), LIDTs are high for Spol (>35 J/cm2) but not as high for Ppol (>30 J/cm2 at 32° AOI; ∼15 J/cm2 at 45° AOI). Field intensities show that Ppol discontinuities at media interfaces correlate with the lower Ppol LIDTs at these AOIs. For Side 1 and Side 2 dual 527 nm/1054 nm AR coatings of a diagnostic beam splitter at 22.5° AOI, Spol and Ppol LIDTs (>10 J/cm2 at 527 nm; >35 J/cm2 at 1054 nm) are consistent with Spol and Ppol intensity behaviors.

Repair of a mirror coating on a large optic for high laser damage applications using ion milling and over-coating methods

“…The coating system uses planetary rotation, masking to maintain coating uniformity, and quartz crystal monitoring with a single crystal for layer thickness control. Each of the test substrates was prepared according to our standard cleaning method 10 immediately before they were loaded into the coating chamber. The coating chamber reached a base pressure of approximately 3e-6 Torr prior to deposition.…”

“…More specifically, when we learned that the coating on the 65-cm diameter optic was incorrect, we removed it from the coating chamber because we lacked experience in the repair of optical coatings and, therefore, needed to conduct tests to determine the best approach for repairs. After the 34-layer test optic was removed from the coating chamber, it was washed according to our protocol 10 and returned to the coating chamber for the over-coating process. The over-coating was a 35-layer mirror coating that was equivalent to the 34-layer coating except (1) the coating did not contain any Hf metal layers, and (2) the first layer was a quarter wave of SiO 2 to maintain the quarter-wave stack characteristics of the coating, since the outermost layer of the incorrect 34-layer coating was a half-wave of SiO 2 .…”

“…10 The pitting did not hamper our ability to clean the substrates in any discernable way. Moreover, the surface tension of the substrates as we washed them indicated they were quite clean, which is a testament to the effectiveness of ion milling as an in situ cleaning process.…”

How reduced vacuum pumping capability in a coating chamber affects the laser damage resistance of HfO2/SiO2antireflection and high-reflection coatings

“…1,2 Since 2005, the coating system has been in operation for the production of antireflection (AR), high reflection, polarizer, and dichroic coatings on meter-class optics, using mainly HfO 2 and SiO 2 coating materials. [3][4][5][6][7] Optics with high resistance to laser damage are essential for operating high-power laser systems such as the ZBacklighter lasers. However, the laser-induced damage threshold (LIDT) of an optical coating is generally lower than the LIDT of the optical substrate material.…”

“…We have reported on the LIDT results of the AR coating at 532 and 1064 nm in previous studies. 2,4 For this study, the AR and HR coatings were deposited in the coating chamber with one, two, or three cryo pumps in operation. The LIDT of each coating was then measured and analyzed to determine the negative effects of having only one or two cryo pumps in operation.…”