All-solid-state multipass spectral broadening to sub-20 fs. 
In this work, we present a nonlinear spectral broadening and compression scheme based on self-phase modulation in bulk media inside a Herriott-type multipass cell.  With this reliable approach, we achieved a spectral broadening factor of 22 while maintaining an efficiency of over 60% at an average input power of 100 W, and an excellent output beam quality with M2=1.2.  The output pulses were compressed to 18 fs, with the broadest spectrum supporting a Fourier-transform limit of 10 fs.  The high efficiency and approximately four-optical-cycle pulse duration mark an important milestone towards the realization of a compact, high power oscillator-based driver for XUV frequency combs and other nonlinear processes.