Generation of High-Energy Pulses by Managing the Kerr-Nonlinearity in Fiber-Based Laser Amplifiers

Abstract

Increasing the pulse energy of ultrafast laser systems is an important field of laser development. High pulse energies simplify and accelerate most applications, such as the stimulation of optical parametric effects or the processing of materials. The amplification of ultrashort pulses in glass fibers is a prominent method, as fiber amplifiers are inexpensive, flexible and highly-integrated. Resulting from the strong confinement in the fiber core and the high peak intensities of the laser pulses, the amplification is often limited by the onset of a nonlinear deterioration of the pulses.

Within this thesis, two methods of fiber-based pulse amplification by managing the Kerr-nonlinearity are presented. In the first method, the chirped-pulse amplification, nonlinear effects are suppressed by locally reducing the peak intensity. A chirped-pulse amplifier was realized that generated pulses with energies of 450 nJ and durations of 293 fs, limited by pump power. These pulse parameters were not sufficient for the intended application. In order to further decrease the pulse duration and increase pulse energy, the parameters of the amplified pulses had to be decoupled from the seed pulses. This is achieved in an amplifier based on the second approach. By enforcing the impact of Kerr-nonlinearity, the optical spectrum of self-similar pulses could be broadened by self-phase modulation during the amplification to generate pulses with 1 µJ pulse energy and a compressed duration of 50 fs at which level the amplification was limited by transverse mode instabilities.

This improvement of pulse parameters by nonlinear techniques is also exploited in a pulse regenerator. By feeding back a part of the amplified pulse into a second amplifier, a so-called Mamyshev oscillator is formed. Its principle of alternating spectral filtering between sections of gain and spectral regeneration allowed for the generation of mode-locked pulses. This Mamyshev oscillator was optimized for the generation of high-energy pulses by the analysis of optimum band-pass filter parameters and the implementation of a few-mode gain fiber. A pulse with a maximum energy of 650 nJ and a compressed duration of <100 fs was achieved. This was the highest pulse energy achieved by a Mamyshev oscillator based on standard Yb-doped fibers to date, even surpassing the performance of state-of-the-art Titanium:Sapphire lasers. A transfer of the Mamyshev oscillator concept to the regime of Thulium-doped gain fibers with the wavelength 2 µm is challenging due to the anomalous dispersion of the gain fibers which prevents parabolic pulse evolution. Nevertheless, a realization of this design is feasible. Mode-locked pulses with durations of <200 fs and pulse energies of 6.4 nJ were achieved. At this pulse duration it was the highest output power from a Thulium-doped fiber oscillator to date.

Due to the alteration of the pulse shape in the glass fibers, a characterization of the final pulses is necessary. A recently developed method for the required complete pulse analysis was transferred from the application in solid-state systems to fiber-based systems in this thesis, which involves the management of less precisely defined amounts of dispersion. The complete characterization by dispersion scans based on a grism compressor was achieved by the use of an adequate retrieval algorithm.