RT&L FOCUS AREA(S): Directed Energy TECHNOLOGY AREA(S): Materials OBJECTIVE: Design chalcogenide window with a thermo-optic coefficient dn/dT = 0 and a low absorption coefficient at wavelengths near =1.064 m, while remaining optically transparent in the MWIR and LWIR regions. DESCRIPTION: Recently, chalcogenide optics have been developed with thermo-optic coefficients dn/dT near zero in the mid-wave infrared (MWIR) and long-wave infrared (LWIR) regions [Gleason], where n is the optical index of refraction and T is temperature. Currently, no such materials have been developed that accomplish this in the 1030 nm to 1070 nm, which covers Yb:YAG to Nd:YAG fiber lasers. We believe that the development of such a glass will have no thermal lensing at these wavelengths and thus, a higher damage threshold than chalcogenides currently available. Such optics will have to maintain a low absorption coefficient at 1 micron as well as good optical imaging quality like those chalcogenides currently on the market. The primary goal of this STTR is to develop methods for manufacturing chalcogenide glasses with optimal thermo-optical properties that can perform in harsh environments. The glass should have a temperature coefficient of refractive index dn/dT = 0 and low absorption coefficient around 1.064 m (1030-1070 nm), and optical transparency and high transmission in the MWIR and LWIR spectral regions while maintaining good optical imaging quality. The glass should be capable of handling high optical power densities without damage and should not degrade with exposure to light, humidity, etc. Fabrication techniques needed to realize the proposed chalcogenide designs should be clearly defined in the Phase I effort and an optical window manufactured. Such structures should be scalable for optics with a diameter up to 4 inches. It is expected that any such designs will require multiple glass compositions to be manufactured to fine tune to the appropriate composition. Key findings in Gleason et al. indicate that trends in refractive index and dn/dT were found to be related to the atomic structure present within the glassy network, as opposed to the atomic percentage of any individual constituent. Because of this additional degree of freedom, chalcogenide optics can be fabricated based on compositional design allowing for on demand glass properties including the thermal optical response as well as mechanical properties for environmental robustness. Such optics are useful for commercial applications that transmit over the MWIR and LWIR spectral regions. The chalcogenide optics will provide uninterrupted, enhanced force protection and day/night situational awareness. Military applications for this technology include laser safety devices for Mounted/Dismounted Ground System thermal sensors, and for thermal imaging systems on manned aircraft, unmanned aerial vehicles, and unattended ground sensors. PHASE I: Fabricate a chalcogenide glass that has a temperature coefficient of refractive index dn/dT = 0 around 1.064 m and zero or negative dn/dT in the range 1030-1070 nm. The glass must have transmission greater than 63.5%, in the MWIR and LWIR spectral regions and a low absorption coefficient, less than 0.05/cm, around 1.064 m. It must also maintain good optical imaging quality in the MWIR and LWIR spectral regions: 3-5 m and 7-10 m (ideally up to 14 m), respectively. Such glass should be capable of handling optical power densities up to 1 MW/cm2 in the range 1030-1070 nm without out damage, enabled by the low absorption coefficient and dn/dT of zero. The glass should be stable over time and not degrade or experience a change of properties with exposure to light, humidity, etc. [Frantz] Environmental specifications include an operating temperature of -40 C to +71 C, storage temperature and temperature shock from -51 C to +71 C, and an operating/storage humidity of -40 C to +71 C and 95% relative humidity (RH). It should be noted that designing such an optic will likely require the fabrication of multiple glass compositions to approach the desired material. The deliverables shall include a detailed design, fabrication plan, and multiple 1-inch diameter optical windows (coupons) with measurement results of the refractive index, absorption coefficient, thermal properties, transmittance and reflection spectra spanning the full spectral range (400 nm through 14 m), and dn/dT measurements. Coupons should be scalable to 4 inches in diameter. Designs that meet all of the specs, especially the damage threshold and transmission, but not the temperature coefficient of refractive index of zero, will be considered. PHASE II: Fabricate a chalcogenide glass, which meets all the phase 1 requirements, with an improved (higher) damage threshold targeting 10 MW/cm2 by focusing on other parameters (i.e. homogeneity enhancement, precision annealing profiles, reduction of absorption coefficient through different compositions or other methods) that may increase the damage threshold. The absorption coefficient and other parameters should meet phase 1 requirements. Damage testing will be conducted at the U.S. Army Research Laboratory with a 200 m to 900 m laser beam spot size. The expected deliverables are a write-up of the results of the fabrication study varying other parameters (and their effect on the various properties) and at least four one-inch chalcogenide windows with the improved damage threshold that meet the specs of phase 2. Such windows should be scalable to four inches in Phase 3. Additionally, potential commercial and military transition partners for a Phase 3 effort shall be identified. PHASE III DUAL USE APPLICATIONS: Chalcogenides such as these can be used in systems which use thermal systems. Potential commercial applications include thermal security cameras for use in Homeland Security applications (perimeter security at airports, coastal ports, nuclear power installations), UAV sensors, as well as satellite sensors. The possibility to incorporate these structures into current sensors could also be explored, for the potential use in both ground vehicles and aircrafts. Additionally, selenide-based chalcogenide glasses are already widely employed in the athermal imaging systems, especially in the low-cost IR system of auto navigation. [Lin] Finally, such optics may be used to increase the thermal stability of SWIR cameras. REFERENCES: B. Gleason, L. Sisken, C. Smith and K. Richardson, "Designing mid-wave infrared (MWIR) thermo-optic coefficient (dn/dT) in chalcogenide glasses," Proc. SPIE 9822, Advanced Optics for Defense Applications: UV through LWIR, 982207 (17 May 2016); https://doi.org/10.1117/12.2229056 J. Frantz, J. Myers, R. Bekele, C. Spillmann, J. Kolacz, H. Gotjen, V. Nguyen, C. McClain, and J. Sanghera, "Arsenic selenide thin film degradation and its mitigation," Opt. Mater. Express 8, 3659-3665 (2018). J. E. McElhenny, "Continuous Wave Laser Induced Damage Threshold of Ge28Sb12Se60 at 1.07 microns," in Frontiers in Optics / Laser Science, OSA Technical Digest (Optical Society of America, 2018), paper JW4A.5. J.E. McElhenny, N.K. Bambha, "Continuous wave laser-induced damage threshold of Schott IRG-24, IRG-25, and IRG-26 at 1.07 microns," Proc. SPIE 11173, Laser-induced Damage in Optical Materials 2019, 111731I (20 November 2019); https://doi.org/10.1117/12.2532062 Gleason, B., Richardson, K., Sisken, L. and Smith, C. (2016), Refractive Index and Thermo Optic Coefficients of Ge As Se Chalcogenide Glasses. Int J Appl Glass Sci, 7: 374-383. doi:10.1111/ijag.12190 C. Lin, C. R ssel and S. Dai, Chalcogenide glass-ceramics: Functional design and crystallization mechanism, Progress in Materials Science, 93, 1-44 (April 2018). B. Gleason, P. Wachtel, J. D. Musgraves, A. Qiao, N. Anheier, K. Richardson, "Compositional-tailoring of optical properties in IR transparent chalcogenide glasses for precision glass molding," Proc. SPIE 8884, Optifab 2013, 888417 (15 October 2013); https://doi.org/10.1117/12.2029215 KEYWORDS: high power, continuous wave, chalcogenide, 1 micron, optics, infrared, visible, high transmission, MWIR (mid wave infrared), LWIR (long wave infrared), absorption coefficient
