This study presents optical diagnostics results used to characterize the optics for our pump-probe laser system at SLAC, focusing on the compression of the 800 nm OPCPA laser, operating at 35 W and 33 kHz. The OPCPA output is transported across various endstations of LCLS for pump experiments with the X-ray beam from the LINAC.

Optical pulses are compressed utilizing commercial chirped mirrors, where we encountered challenges related to pulse compression, beam caustic, and observed beam divergence post-compression. The characterization of these mirrors involved measuring their group delay dispersion (GDD) using white light interferometry, revealing discrepancies in GDD specifications from some vendors.

One vendor's mirrors, while meeting original GDD specifications, exhibited deformation due to the coating process, resulting in a convex shape. Consequently, over 100 mirrors were returned for back-coating to relieve this stress, significantly improving flatness. Systematic characterization of four sample mirrors showed a peak-to-valley (PV)*λ (632.8 nm) of ~0.57 and RMS*λ of ~0.13 before back-coating, improving to PV*λ ~0.14 and RMS*λ ~0.02 post-coating.

For larger rectangular chirped mirrors (40 x 80 mm), enhancements were observed from PV*λ ~0.84 ± 0.15 to PV*λ ~0.14 ± 0.02 after back-coating. These findings underscore the importance of precise optics characterization in optimizing laser systems for advanced experimental setups.