A ray tracing model for thermal effects solid-state single-pass amplifiers is presented, which is able to simulate thermal lensing, depolarization and beam quality degradation. The ray tracing algorithm is based on alternatively evaluating the axial and radial gradient and thereby finding the trajectory in thermally influenced media. This enables to find the focal length distribution of the system. Additionally, to the position of the rays, its phase is also determined, which enables to reconstruct the wavefront of the beam after passing through the crystal. This wavefront is used for a Zernike polynomial analysis to determine spherical aberration, which is linked to the beam quality of the passing beam. Furthermode the total depolarization is obtained by finding the change of polarization for each ray separately. The simulation for thermal lensing is compared with a single-pass Nd:YVO4 system, the beam degradation is compared with a Nd:YVO<sub>4</sub>-MOAP system. Both show good agreement with the simulation data, as long as the gain of the system is homogeneous.
This work demonstrates a new simulation technique, which combines the previously published Dynamic Mode Analysis with the coupling of arbitrary sets of rate equations. Those arbitrary sets include the representation of gain media, which state population is influenced by interionic mechanisms. Especially the state population of Er:YAG is strongly influenced by upconversion and cross relaxation, which impacts the population inversion and the generated heat load. Therefore, the simulation of a resonator based on 50%-doped Er:YAG is performed and compared to experimental results. The accuracy of the presented technique is pointed out by a fine agreement between simulation and experiment with respect to gained cw output power and slope efficiency. Moreover, a finite element analysis of the heat load and the 3-dimensional population inversion in the crystal is illustrated.
The numerical simulation of a regenerative amplifier based on codoped Tm,Ho:YAG is presented. Within this work, a maximum pulse energy of 3.1 mJ is observed for 0.9 kW CW end-pumping at 785 nm. The simulation results demonstrate that interionic mechanisms such as upconversion and energy transfer significantly influence the population of states and consequently, the amplification. In detail, the most dominant mechanisms are identified by introducing the rate term k<sub>x</sub>N<sub>i</sub>N<sub>m</sub> as a quantity to compare the strength of all occuring interionic mechanisms. It can then be shown that the energy transfer mechanism E<sub>6512</sub> between Holmium and Thulium ions is the greatest source of population loss for the upper lasing state <sup>5 </sup>I<sub>7</sub> in Holmium. In summary, the presented model represents an efficient tool to characterize the influence of interionic mechanisms on the extractable energy in solid-state media under pulsed operation.
A numerical model for solid-state regenerative amplifiers is presented, which is able to precisely simulate the quantitative energy buildup of stretched femtosecond pulses over passed roundtrips in the cavity. In detail, this model is experimentally validated with a Ti:Sapphire regenerative amplifier. Additionally, the simulation of a Ho:YAG based regenerative amplifier is conducted and compared to experimental data from literature. Furthermore, a bifurcation study of the investigated Ho:YAG system is performed, which leads to the identification of stable and instable operation regimes. The presented numerical model exhibits a well agreement to the experimental results from the Ti:Sapphire regenerative amplifier. Also, the gained pulse energy from the Ho:YAG system could be approximated closely, while the mismatch is explained with the monochromatic calculation of pulse amplification. Since the model is applicable to other solid-state gain media, it allows for the efficient design of future amplification systems based on regenerative amplification.