High laser damage threshold surface relief micro-structures for anti-reflection applications

Microstructures built into the surfaces of an optic or window, are an effective replacement for thin-film coatings in anti-reflection (AR) and narrow-band filter applications. AR microstructures exhibit particularly noteworthy performance where an average reflection loss of less than 0.2% over a four-octave range (400-1800nm) has been demonstrated, and a loss of less than 0.03% is routinely achieved for narrow-band applications. Because AR micro-textures provide a gradual change in the refractive index at a material boundary, it is expected that light can propagate through the boundary without material damage at energy levels that are much higher than that found with thin-film interference coatings. Recently, it was shown that the laser induced damage threshold (LIDT) of an inexpensive borosilicate glass window containing AR microstructures was nearly 57 J/cm2 at 1064nm (20ns pulse). This LIDT is two to three times greater than the damage threshold of single-layer sol-gel AR coatings on fused silica often reported in the literature. The development of surface relief AR textures for use in high-energy laser applications is presented. Data from scanning electron microscope (SEM) analysis, reflection measurements, and LIDT testing, is shown for high performance AR microstructures fabricated in fused silica, and borosilicate glass. Results of LIDT testing at wavelengths ranging from the near ultraviolet through the near infrared confirm the initial result that AR microstructures can operate at pulsed laser power levels at least two times higher than thin-film coatings. For near infrared applications such as laser weapons and fiber optic communications requiring high performance AR, LIDT levels for AR microstructures in fused silica are found to be at least five times greater than conventional multi-layer thin film coatings. An initial surface absorption test at 1064nm shows that AR microstructures may also exhibit improved lifetimes within continuous wave laser systems.