Spatio‐Temporal Characterization of Pump‐Induced Wavefront Aberrations in Yb^{3 +} ‐Doped Materials (Laser Photonics Rev. 12(2)/2018)

Abstract

High Power Lasers In article number 1700211, Issa Tamer and co‐workers utilize Yb:CaF2 crystal and Yb:FP‐glass within the high power ultrashort POLARIS laser system to amplify near‐infrared laser pulses up to energies in the Joule regime. A comprehensive spatio‐temporal characterization of pump‐induced aberrations across the free aperture of the considered diode‐pumped active materials is presented. All relevant material constants are determined, allowing for accurate modeling of the aberrations. For the further development of next‐generation high power laser systems, extensive knowledge of the spatio‐temporal behavior of these aberrations is required.

Modeling of the 3D spatio-temporal thermal profile of joule-class -based laser amplifiers

“…The time-resolved detection of the temperature profile during the cooling phase of the pump cycle was possible for Yb:FP15 due to the low thermal conductivity, and consequently, the long thermal diffusion time [12] , that leads to a quasi-static state during the integration time of the thermal camera. For Yb:YAG (Figure 4(a)) and Yb:CaF 2 (Figure 4(b)), the simulated (dashed) front (black) and back (red) temperature profiles averaged over the pump cycle were compared to the measured (solid) profiles, with a maximum discrepancy between simulation and measurement of less than 0.3 • C. The maximum temperature difference between the measured and simulated temperature profiles of the pumped Yb:FP15 (Figure 4(c)) material throughout the 5 s pump cycle was determined to be below 0.2 • C. The modeling and validation principles described here are not only applicable to the comprehensive characterization of thermal effects -such as the inhomogeneous thermal contribution to pump-induced wavefront aberrations [12] in isolated setups, but can also be utilized to access the crucial 3D spatio-temporal thermal profile of a pumped active material in a high-energy amplifier under complex pumping configurations.…”

“…The time-resolved detection of the temperature profile during the cooling phase of the pump cycle was possible for Yb:FP15 due to the low thermal conductivity, and consequently, the long thermal diffusion time [12] , that leads to a quasi-static state during the integration time of the thermal camera. For Yb:YAG (Figure 4(a)) and (Figure 4(b)), the simulated (dashed) front (black) and back (red) temperature profiles averaged over the pump cycle were compared to the measured (solid) profiles, with a maximum discrepancy between simulation and measurement of less than .…”

“…The maximum temperature difference between the measured and simulated temperature profiles of the pumped Yb:FP15 (Figure 4(c)) material throughout the 5 s pump cycle was determined to be below . The modeling and validation principles described here are not only applicable to the comprehensive characterization of thermal effects – such as the inhomogeneous thermal contribution to pump-induced wavefront aberrations [12] – in isolated setups, but can also be utilized to access the crucial 3D spatio-temporal thermal profile of a pumped active material in a high-energy amplifier under complex pumping configurations.…”

“…A comparison of the thermal profile of Yb:FP15 glass with that of the two crystals in Section 3, Yb:YAG and , under the same pumping conditions within the A4 amplifier would result in an overall stronger thermal gradient and higher peak temperature for Yb:FP15, as the thermal conductivity – seen in Table 1 – is nearly an order of magnitude lower than for the cases of Yb:YAG and . The crystals would allow for a higher repetition rate of the amplifier; however, at the cost of narrower emission cross-sections and an increase in the impact of the thermal contributions to the spatial degradation of the amplified seed pulse, due to the significantly higher thermo-optic coefficients [12] .…”

“…Despite the reduced heat load within -doped materials, significant detrimental effects can still occur due to the pump-induced thermal profile. The influence of strong thermally induced [5] wavefront aberrations – in conjunction with electronic contributions [11] – may heavily distort [12] the amplified beam profile upon propagation. Additionally, the spatially varying depolarization of the amplified seed beam due to thermal stress [5] induced by the thermal gradient in a multi-hundred-joule pumped active material can directly lead to energy loss and possible spatial profile deformations after interacting with polarization-sensitive optical components within the laser system.…”