Figure 4. Relative timing between FEL and external laser.

An external laser is optically locked to the accelerator reference clock signal and used to generate a single-cycle THz pulse by optical rectification in lithium niobate (LiNbO₃). The resulting picosecond THz pulse and the FEL pulse are then overlapped in a neon (Ne) gas jet, where the FEL pulse profile and relative arrival time of the pulse with respect to the THz field is measured by streaking spectroscopy. As the THz pulse is phase-locked to the external laser, the arrival time with respect to the THz pulse is equivalent to the arrival time with respect to the external laser.