Temporal prepulse contrast degradation in high-intensity CPA lasers from anisotropy of amplifier gain media.

We present a study of the temporal prepulse contrast degradation of high focused intensity pulses produced in CPA laser systems due to imperfections in amplifier design, alignment of amplifier components, and crystal inhomogeneity. Using an extended cross-polarized imaging technique, we demonstrate the presence of multiple crystal domains inside Ti:sapphire slabs with ≈10 diameter. The results of our numerical calculations show that crystalline c-axis orientation inhomogeneity caused by these crystal domains can lead to the generation of prepulses with a relative contrast of >10 within several picoseconds before the main pulse. In a multiple-slab amplifier head configuration sometimes used in high-repetition-rate systems, the misalignment of the crystalline c-axes of the amplifier slab with respect to each other can lead to the generation of prepulses with relative contrast as high as 10, depending on the magnitude of misalignment.