Spatial Mode Cleaning and Nonlinear Compression of an Innoslab-Based Laser in a Gas-Filled Multipass Cell

Ytterbium-based ultrafast lasers facilitate efficient and power-scalable femtosecond lasers with state-of-the-art fiber, Innoslab, and thin-disk architectures. Among them, Innoslab amplifier distinguished themselves for their outstanding performance in single-pass gain and compactness [1], [2]. A hybrid cavity is typically employed in the Innoslab geometry as a cylindrical thermal lens was established in the gain medium. In the fast axis, a self-reproducing laser mode is realized at each roundtrip through two plane mirrors along with the thermal lensing effect of the crystal. In the slow axis, the beam is expanded to balance the power increase, keeping the optics safe and B-integral small. A tight overlap between successive passes is necessary to achieve enough extraction efficiency and suppress parasitic lasing also. Consequently, the output beam shows sidelobes in the slow axis due to the diffraction at the output port, which deteriorate the beam quality. Other effects such as amplified spontaneous emission (ASE) or slight self-lasing will further degrade the beam quality if not eliminated. In the conventional Innoslab amplifier, a spatial filter was routinely applied to improve the output beam quality in the slow axis. The loss-afflicted spatial filter also demands precise alignment.