Short Pulses in Engineered Nonlinear Media

University dissertation from Stockholm : KTH

Abstract: Short optical pulses and engineered nonlinear media is a powerful combination. Mode locked pulses exhibit high peak powers and short pulse duration and the engineered ferro-electric KTiOPO4 facilitates several different nonlinear processes. In this work we investigate the use of structured, second-order materials for generation, characterization and frequency conversion of short optical pulses.By cascading second harmonic generation and difference frequency generation the optical Kerr effect was emulated and two different Nd-based laser cavities were mode locked by the cascaded Kerr lensing effect. In one of the cavities 2.8 ps short pulses were generated and a strong pulse shortening took place through the interplay of the cavity design and the group velocity mismatch in the nonlinear crystal. The other laser had a hybrid mode locking scheme with active electro-optic modulation and passive cascaded Kerr lensing incorporated in a single partially poled KTP crystal. The long pulses from the active modulation were shortened when the passive mode locking started and 6.9 ps short pulses were generated.High-efficiency frequency conversion is not a trivial task in periodically poled materials for short pulses due to the large group velocity mismatch. Optimization of parameters such as the focussing condition and the crystal temperature allowed us to demonstrate 64% conversion efficiency by frequency doubling the fs pulses from a Yb:KYW laser in a single pass configuration. Quasi phase matching also offers new possibilities for nonlinear interactions. We demonstrated that it is possible to simultaneously utilize several phase matched second harmonic interactions, resulting in a dual-polarization second harmonic beam.Short pulse duration of the fundamental wave is a key parameter in the novel method that we demonstrated for characterization of the nonlinearity of periodically poled crystals. The method utilizes the group velocity mismatch between the two polarizations in a type II second harmonic generation configuration.The domain walls of PPKTP exhibit second order nonlinearities that are forbidden in the bulk material. This we used in a single shot frequency resolved optical gating arrangement. The spectral resolution came from ?erenkov phase matching, a non-collinear phase matching scheme that exhibits a substantial angular dispersion. The second harmonic light was imaged upon a CCD camera and with the spectral distribution on one axis and the temporal autocorrelation on the other. From this image we retrieved the full temporal profile of the fundamental pulse, as well as the phase. The spectral dispersion provided by the ?erenkov phase matching was large enough to characterize optical pulses as long as ~200 fs in a compact setup. The ?erenkov frequency resolved optical gating method samples a thin stripe of the beam, i.e. the area close to the domain wall. This provides the means for high spatial resolution measurements of the spectral-temporal characteristics of ultrafast optical fields.