Materials for High-Energy Laser Windows: How Thermal Lensing and Thermal Stresses Control the Performance

摘要

The engineering of high-energy lasers (HELs) for applications such as the airborne laser (ABL) system requires optical windows capable of handling megajoule beam energies. The selection of a suitable window material involves considerations relating to thermal lensing, I.e., the beam distortion caused by thermally induced phase-aberrations, in addition to issues arising from the thermal stresses generated by beam-induced temperature gradients. In this paper we document analytical methods for evaluating the impact of both beam-induced optical distortions and beam-induced mechanical stresses, which may allow the designer to properly assess the performance of window-material candidates. Specifically, thermal lensing in conjunction with planar stresses control the allowable beam fluence, whereas the two axial-stress related failure modes (thermal-shock induced fracture and yielding in compression) control the allowable beam intensity. We illustrate these considerations in the light of an evaluation of the performance of three window-material candidates for operation at the 1.315-μm wavelength. Currently, fused SiO_2 is the window material of choice for contemplated HELs operating in the near infrared; it is, however, vulnerable to optical distortion, which renders this material unsuitable for applications that require transmitting large beam fluences. On assuming that stress-birefringence is of no concern, oxyfluoride glass outperforms SiO_2, but evidence of a poor thermal conductivity degrades this material's ability to transmit high-intensity beams. Fusion-cast CaF_2 emerges as the most promising "compromise" solution in the sense that this material combines superior optical features with acceptable thermomechanical properties; in effect, CaF_2 windows easily meet requirements as formulated for the first-generation ABL system.