Spatial Beam Cleaning in Ytterbium Spatiotemporal Mode-Locked Fiber Laser

Spatiotemporal mode-locked (STML) lasers based on multimode fibers enable the simultaneous locking of lon-gitudinal and transverse laser modes [1]. These light sources permit scaling up the generated pulse energy and exhibit complex spatiotemporal dynamics [1]–[3]. <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{Yb}^{3+}$</tex>-doped STML fiber lasers are normal-dispersion ring cavities that support the propagation of multidimensional dissipative solitons: for this reason, their operation hinges on the use of spectral filters, isolators, and spatial filters. High beam quality output in an STML Yb fiber laser was obtained via intracavity Kerr beam self-cleaning. However, a grating pair was necessary to compensate for the normal cavity dispersion [4]. Here, we introduce and experimentally demonstrate a new laser scheme, leading to high-beam quality output from an STML Yb fiber laser. Our cavity setup is free of isolators, spatial/spectral filters, and dispersion-compensating elements. STML is based on the combined action of nonlinear polarization rotation (NPR) and spatial beam self-cleaning, which acts as Kerr-lens mode locking in bulk lasers. We generated ultrashort high-energy pulses with a near-Gaussian beam profile.