Observation of strong multi-pulse effects on spectral broadening of long-wavelength infrared pulses with 29.75 MHz repetition rate in a KRS-5 plate

Abstract Spectral broadening of femtosecond pulses is utilized to generate few-cycle pulses, which in turn enables the generation of isolated attosecond pulses. This phenomenon is also crucial for monitoring the carrier-envelope phase of femtosecond pulses using an f –2 f interferometer. A significant multi-pulse effect on the spectral broadening of long-wavelength infrared (LWIR) pulses (29.75 MHz micro-pulse repetition rate; 3.5 μs macro-pulse duration) was observed in a 3 mm thick KRS-5 plate positioned before the focal point of the LWIR pulses. Long-lifetime free carriers, induced by these LWIR pulses, cause nonlinear absorption in the KRS-5 plate and significantly affect the spectral broadening of the pulses. This effect is particularly pronounced for LWIR pulses with micro-pulse repetition rates exceeding 10 MHz, since the free carrier lifetime is longer than 100 ns.