| Abstract [eng] |
Supercontinuum generation in solid-state bulk materials provides a simple and robust method to obtain broadband radiation, exhibiting high coherence and spatial quality, with spectrum ranging from the UV to far-IR. Supercontinuum is widely used in OPA and OPCPA systems, also in ultrafast spectroscopy and microscopy, optical metrology and many other fields. In some applications, particularly in high-repetition-rate laser systems, thermal effects and heat accumulation within the nonlinear crystal are critical factors for efficient supercontinuum generation. While thermal effects can negatively impact the stability and spectral extent of the supercontinuum, certain experimental variables, such as beam focusing geometry, can reduce the effects significantly. In this work, we investigated the impact of heat accumulation on supercontinuum generation in a YAG crystal. Supercontinuum generation was investigated using an Yb:KGW laser (\textit{CARBIDE, Light Conversion Ltd}), which produced 210 fs pulses with a central wavelength of 1030 nm at a repetition rate of up to 2 MHz. Experimental study is comprised of spectral measurements while varying beam focusing geometry and laser pulse repetition rate from 200 kHz to 2 MHz, nonlinear loss measurements, as well as pulse-to-pulse temporal measurements of red-shifted spectral shrinking. The experimental observations were backed-up with numerical simulations. The findings highlight the critical role of beam focusing geometry on blue-shifted, and, more significantly, on red-shifted spectral broadening. Entering the YAG crystal with a diverging pump beam produces the largest red shift, while a converging pump beam produces the largest blue-shifted spectral broadening. These results can be attributed to varying levels of nonlinear losses and heat accumulation within the material. Additionally, heat accumulation induces a thermal lensing effect, which is exacerbated at higher pulse rates and negatively impacts long-wavelength portion of supercontinuum spectrum. This is particularly evident when generating supercontinuum with a converging beam, in which case red-shifted spectrum shrinks dramaticaly with the increase of pulse repetition rate, and almost completely vanishes at 2 MHz. These changes occur on a microsecond time-scale, as justified by numerically simulated dynamics of heat accumulation and experimentally measured pulse-to-pulse dynamics of supercontinuum spectral amplitude in the long-wavelength range. However, due to significantly lower heat accumulation in the case of diverging pump beam, a stable supercontinuum is generated across the entire repetition rate range. |
