High Power Femtosecond Doubly Resonant Optical Parametric Oscillator

Abstract

Optical parametric oscillators (OPOs), which rely on an optical parametric amplification in a cavity, can extend the available optical frequency range of common laser sources. In a doubly resonant OPO (DROPO) two optical frequencies resonate in the cavity and with their ability to phase-lock to the pump, tailored multi-color electric fields can be generated. For example, such fields can be used for high harmonic generation or to generate THz radiation with MHz repetition rate. This thesis focuses on the design, characterization and stabilization of a high power femtosecond doubly resonant optical parametric oscillator (DROPO) that emits in the 2 µm wavelength range. The system is pumped by a femtosecond Kerr-lens modelocked Yb:YAG thin-disk laser, with a central wavelength around 1 µm, tens of watts of output power and a repetition rate in the MHz range. Due to the phase dependent amplification condition of the nonlinear process, a complex spectral behaviour arises, when the cavity length is detuned. It is strongly influenced by the dispersion in the cavity. The detuning behaviour is experimentally examined in this work and numerical models for its explanation are derived. DROPOs show characteristic on/off switching resonances when the cavity length is detuned. A new way to extend the number of detuning resonances in the degenerated regime is found and experimentally proven. In order to stabilize these systems on one of these resonances dither based schemes are often employed. These schemes induce additional noise in the oscillator which can disturb the experiment. In this work, a dither free approach is chosen which takes advantage of an asymmetry between a spectrally filtered sum frequency signal and the DROPO’s signal, when the cavity is detuned. Thus, it allows to use a proportional integral servo controller to stabilize the DROPO resonances for longer time scales. In addition to stability over time, some experiments also require high peak power. Passive enhancement cavities are well known for their ability to confine pulses and generate high peak powers to drive nonlinear processes. Before this work it was unknown if DROPOs can also achieve this enhancement. This thesis answers this question and shows that by choosing the right cavity parameters a strong enhancement is possible. The experimental results are underlined with a semianalytical model which elaborates that the theoretical limits of the maximum enhancement are similar to the passive enhancement cavity ones. To summarize, in this thesis a DROPO in the 2 µm wavelength range is presented, which will be used for the generation of single cycle THz pulses in the future.