Temporal pre-pulse generation in high-intensity CPA lasers from imperfect domain orientation in anisotropic crystals

We present a study of the temporal pre-pulse contrast degradation of high focused intensity pulses produced in CPA laser systems due to imperfections in amplifier design, alignment of amplifier components, and crystal inhomogenity. Using a measurement technique we have developed, we demonstrate the presence of multiple crystal domains inside Ti:sapphire slabs with ≈10 cm diameter. The results of our numeric calculations show that crystalline c-axis orientation inhomogenity caused by these crystal domains can lead to generation of pre-pulses with relative contrast >10-10 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 amplifier slabs crystalline c-axes with respect to each other can lead to the generation of pre-pulses with relative contrast as high as 10-6, depending on the magnitude of misalignment.