Pulse-shaping strategies in short-pulse fiber amplifiers

Ultrashort pulse lasers are an important tool in scientific and industrial applications. However, many applications are demanding higher average powers from these ultrashort pulse sources. This can be achieved by combining direct diode pumping with novel gain media designs. In particular, ultrashort pulse fiber lasers are now delivering average powers in the kW range. However, the design of fiber lasers, producing pulses with high peak-powers, is challenging due to the impact of nonlinear effects. To significantly reduce these detrimental effects in ultrashort pulse fiber amplifers, the combination of chirped pulse amplification (CPA) and large mode area fibers is employed. Using these methods, the pulse energy of fiber lasers has been steadily increasing for the past few years. Recently, a fiber-based CPA-system has been demonstrated which produces pulse energies of around 1 mJ. However, both the stretching and the enlargement of the mode area are limited, and therefore, the impact of nonlinearity is still noticed in systems employing such devices. The aim of this thesis is the analysis of CPA-systems operated beyond the conventional nonlinear limit, which corresponds to accumulated nonlinear phase-shifts around 1 rad. This includes a detailed discussion of the influence of the nonlinear effect self-phase modulation on the output pulse of CPA-systems. An analytical model is presented. Emphasis is placed on the design of novel concepts to control the impact of self-phase modulation. Pulse-shaping is regarded as a powerful tool to accomplish this goal. Novel methods to control the impact of SPM are experimentally demonstrated. The design of these concepts is based on the theoretical findings. Both amplitude- and phase-shaping are studied. Model-based phase-shaping is implemented in a state-of-the-art fiber CPA-system. The influence of the polarization state is also highlighted. Additionally, existing techniques and recent advances are put into context

Optimization of high performance ultrafast fiber laser systems to > 10 GW peak power

We show that the peak powers of ytterbium-doped fiber chirped pulse amplification (CPA) can be scaled by at least 1 order of magnitude (in the transform limit) as compared to current systems by using a different spectral region of operation. A simple and fast model for saturated broadband fiber CPA systems is developed and applied to study the impact of the interplay between the spectrally dependent small signal gain and the saturation on the output bandwidth. The influence of self-phase modulation on the recompression of the pulse is discussed. It can be shown that the novel operation regime exhibits superior performance even if nonlinear effects are considered. The numerical results are significant for the design of the next generation of ultrafast high power fiber lasers

Towards the minimum pulse-duration from mJ-class fiber CPA-systems

The extension of the signal bandwidth of the amplified pulse is a key option to increase the peak-power from Yb-doped fiber CPA-systems. We analyze the spectral gain characteristics of state-of-the-art Yb-doped fiber amplifiers with regard to the broadest gain-bandwidth. While for input pulses with bandwidths < 5 nm a spectral center around 1035 nm is beneficial, longer central wavelengths (~ 1055 nm) are advantageous for broader signal bandwidths (~ 10 nm). It is shown that this spectral management is valid even if the impact of self-phase modulation is considered. The interplay of saturation-induced pulse-distortion and the spectral behavior of the gain are also discussed

The impact of spectral modulations of the contrast of pulses of nonlinear chirped-pulse amplification systems

A detrimental pulse distortion mechanism inherent to nonlinear chirped-pulse amplification systems is revealed and analyzed. When seeding the nonlinear amplification stage with pulses possessing weak side-pulses, the Kerr-nonlinearity causes a transfer of energy from the main pulse to side pulses. The resulting decrease in pulse contrast is determined by the accumulated nonlinear phase-shift (i.e., the B-integral) and the initial pulse-contrast. The energy transfer can be described by Bessel-functions. Thus, applications relying on a high pulse-contrast demand a low B-integral of the amplification system and a master-oscillator that exhibits an excellent pulse-contrast. In particular, nonlinear fiber CPA-systems operated at B-integrals far beyond p have to be revised in this context

Spectral-temporal management of Yb-doped fiber CPA-systems

To further scale the peak-power of state-of-the-art fiber CPA-systems, a careful optimization of the spectral as well as temporal dynamics is required. The wavelength dependence of the small-signal gain, as well as the saturation of the amplifier, strongly affect the signal bandwidth. For unsaturated amplifiers only a spectral optimization is required. It can be shown that both the spectral center and the width of the input spectrum strongly affect the output bandwidth. An optimization regarding these two parameters will be given. Design guidelines are presented. We develop a simple yet efficient model to simulate the impact of saturation in broadband Ytterbium-doped fiber CPA-systems. Using this model, we reveal that significant peak-power scaling up to 10 GW of current fiber CPA-systems is possible

Theoretical analysis of the gain bandwidth for noncollinear parametric amplification of ultrafast pulses

The choice of optimum phase-matching conditions for noncollinear optical parametric amplifiers is usually made on the basis of the linear spectral dispersion characteristics of the anisotropic nonlinear crystal. However, for high-peak-power operation, where pump depletion is involved, it is shown that the tolerance of the parametric gain with regard to k-vector mismatch is to change the optimum phase-matching parameters. Our calculations show that, with the revised parameters, an enhancement in peak power approaching 50% could be achieved

Transform-limited pulses from a mJ-class nonlinear fiber CPA-system by phase shaping

We experimentally demonstrate phase-shaping in fiber CPA-systems, providing pulse-energies at the mJ-level. The applied method is based on an analytical model describing the impact of SPM in CPA-systems. Using this phase-shaping technique nearly transform limited pulses are produced at B-integrals up to 10 rad. Compared to a nonlinear CPAsystem with the best performance being achieved by adjusting the compressor, operation of the same system using the phase-shaping method permits peak-power enhancement by a factor better than 2

Decrease of pulse-contrast in nonlinear chirped-pulse amplification systems due to high-frequency spectral phase ripples

It is analytically shown that weak initial spectral phase modulations cause a pulse-contrast degradation at the output of nonlinear chirped-pulse amplification systems. The Kerr-nonlinearity causes an energy-transfer from the main pulse to side-pulses during nonlinear amplification. The relative intensities of these side-pulses can be described in terms of Bessel-functions. It is shown that the intensities of the pulses are dependent on the magnitude of the accumulated nonlinear phase-shift (i.e., the B-integral), the depth and period of the initial spectral phase-modulation and the slope of the linear stretching chirp. The results are applicable to any type of laser amplifier that is based on the technique of chirped-pulse amplification. The analytical results presented in this paper are of particular importance for high peak-power laser applications requiring high pulse-contrasts, e. g. high field physics

Circular versus linear polarization in laser-amplifiers with Kerr-nonlinearity

In this contribution it is reported that circularly polarized light is advantageous if the Kerr-effect has to be minimized during laser-amplification. The experimental demonstration is based on a fiber CPA-system. The different polarization states result in different B-integrals, which are measured using phase-only pulse-shaping. The theoretical value of 2/3 for the ratio of the B-integrals of circularly and linearly polarized light is experimentally verified. In laser-amplifiers circularly polarized light reduces the detrimental impact of the Kerr-nonlinearity, and thus, increases the peak-power and the self-focussing threshold

Advantage of circularly polarized light in nonlinear fiber-amplifiers

We experimentally demonstrate that circular polarization state is beneficial if the Kerr-nonlinearity has to be lowered during the amplification of laser pulses. It can be shown that in a fiber-based chirped pulse amplification (CPA) system, the use of circularly and linearly polarized light result in different B-integrals, which are measured using phase-only pulse-shaping. The theoretical value of 2/3 for the ratio of the B-integrals of circularly and linearly polarized light is experimentally confirmed. Circularly polarized light facilitates peak-power scaling, moreover, the self-focussing threshold can be enhanced