High Laser Damage Threshold, Broadband Anti-Reflection Treatment Based on Surface Relief Microstructures
ABSTRACT: In a Phase I project, the problems with optical performance and low threshold for damage limiting the advancement of mid-infrared (mid-IR) wavelength high power laser systems based on metal-ion doped chalcogenide materials, was addressed through the replacement of multi-layer thin-film coatings with optically functional surface relief microstructures. Anti-reflection (AR) textures, known as Motheye structures in the literature, were fabricated in ZnSe windows exhibiting reflection losses below 0.2% over a 1500nm wide spectral range in the mid-IR. AR microstructures (ARMs) were also demonstrated in chromium-ion doped ZnSe (Cr2+:ZnSe) and ClearTran ZnS. In standardized pulsed laser damage testing at a wavelength of 2.1µm, damage thresholds for ARMs treated Cr2+:ZnSe and ZnSe windows were found to be three to seven times higher than the 2J/cm2 often reported for thin-film AR coatings. Continuous wave (cw) laser damage testing at a wavelength of 1.94µm conducted by IPG Photonics indicates that ARMs-treated Cr2+:ZnSe windows can survive power densities up to 0.5 MW/cm2, a level equivalent to untreated material and about 50% higher than the damage threshold of thin-film AR coated Cr2+:ZnSe. The proposed Phase II project will further quantify the power handling and transmission advantages of ARMs technology in multiple chromium and iron doped material configurations through pulsed and cw laser damage testing and product integration trials in collaboration with IPG Photonics. A plasma-based etch process developed in the Phase I work that was found to be useful for removing residual sub-surface optical polishing damage, will be further investigated for its effectiveness at enhancing the damage resistance of all components in pulsed mid-IR lasers. Microstructure-based reflectors built in chalcogenide materials will also be designed and prototyped to serve as the high reflector (HR), output coupler (OC), dichroic splitter, and polarizer components needed to form a mid-IR laser. Multiple microstructure-based laser components will be delivered to AFRL for further evaluation. Commercialization efforts will include side-by-side comparison testing of microstructure-based components with thin-film coated components integrated within existing and planned laser products offered by IPG Photonics. BENEFIT: It is anticipated that a 5X increase in laser power handling capacity combined with enhanced operational lifetime will be achieved through the use of microstructure technology in place of thin-film coatings in solid state laser systems. In particular, the broad-band performance of ARMs will benefit tunable metal-ion doped ZnSe and ZnS lasers that find a wide range of applications throughout the mid-IR spectral region. Air Force platforms such as manned and un-manned aircraft requiring more reliable, higher power, wavelength agile mid-IR laser sources include laser communications, countermeasures (CIRCM), target designators, weapons, rangefinders, remote chemical sensors, and infrared scene projectors. Commercial applications include environmental chemical monitoring, industrial welding and cutting systems, and food processing. Significant advances in medical devices for surgery, noninvasive treatments, breath diagnostics, and imaging await higher power laser sources, as do astronomical instruments for spectroscopy, planetary exploration, earth and solar observations, and optical telecommunications. Although the proposed effort will immediately benefit metal ion doped ZnSe and ZnS laser materials, the robust nature of ARMs can be extended to any current or emerging laser technology such as quantum cascade lasers, zinc germanium phosphide lasers, and semiconductor microchip lasers, as well as silica and chalcogenide glass optical fiber laser delivery systems.
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