Weak absorption and scattering losses from the visible to the near-infrared in single-crystal sapphire materials

Abstract

Durable window materials with minimal optical loss are important for future high-energy laser (HEL) systems that will operate at or near the megawatt level. Sapphire is recognized as a promising HEL window candidate due to its outstanding optical transparency and mechanical properties. However, its weak scattering characteristics and absorption levels near the 1-μm wavelength region need to be lowered further for future HEL applications. In our study, the weak absorption and scattering of sapphire samples provided by three vendors were measured. Ultraviolet–visible spectroscopy measurements were made in the wavelength range 190 to 600 nm and several absorption bands due to vacancies and impurities were detected. The bulk absorption of several samples at wavelengths of 355, 532, and 1064 nm were measured using photothermal common-path interferometry and the absorption coefficient values obtained were in the range of 10 − 5 to 10 − 2 cm − 1 and increased as the wavelength decreased. An empirical weak absorption tail model was used to fit the measured data. In addition, scattering measurements on all samples were made at 405, 532, 633, 1064, and 1550 nm using an instrument developed to assess the bidirectional scatterance probability distribution function. The total integrated scatterance was in the range of 10 − 4 to 10 − 2 and increased for all samples as the wavelength decreased. Surface roughness was found to contribute insignificantly to the scattering loss, while bulk defects along with subsurface damage have major impact. A simple single-scatter model was developed and applied to the measured bulk scattering data. The model suggests that impurity particles, porosity, and other density variations exist with a range of sizes that contribute to scattering. Overall, the measurements indicate that both weak absorption and scattering losses are strongly related to defect structures such as lattice disorder and impurities that were introduced during crystal growth or postgrowth processing. Understanding these defects and their contributions to optical loss can lead to improved manufacturing and processing methods.