Pulse stacking in the Stanford external cavity and photo-induced reflectivity in the infrared

We have investigated the use of pulse stacking in an external cavity in conjunction with the Stanford mid-infrared Free Electron Laser (FEL) to provide increased optical pulse energy to experimenters. Coupling the FEL into our external cavity, we achieved stored energies up to 80 times that of an individual incident pulse. The detailed dynamics of this process, including its dependence on cavity length, input steering, and input focussing, were investigated experimentally, and then compared with theory. We investigated several techniques for extracting energy from the cavity, including enhanced outcoupling and cavity dumping using photo-induced reflectivity. Enhanced outcoupling can extract only part of the stored energy, but at a repetition rate of 200kHz, while cavity dumping can extract all of the energy at a repetition rate of 1kHz. The critical need for high reflectivity to cavity dump efficiently led us to an independent and detailed study of photo-induced reflectivity. We made simultaneous measurements of the reflection and transmission of Ge and Si at several wavelengths and angles, and over two decades of pump laser fluence. Finally, we present a design for a permanent external cavity system optimized for the Stanford Picosecond FEL Center.