Investigation of the transverse mode instability in an isolator-free quasi-unidirectional spatiotemporal self-mode locked fiber laser

A spatiotemporal (ST) self-mode locked (ML) Er-doped fiber laser based on the nonlinear multimode interference (NL-MMI) effect without an intracavity isolator has been proposed. Such a STML laser could operate in a quasi-unidirectional regime in both clockwise (CW) and counter-clockwise (CCW) directions. Different spatial dynamics of the STML solitons in the opposite directions are observed. For the STML solitons in the CCW direction, the beam profile is discretely distributed and the transverse mode instability (TMI) is observed. While for the STML solitons in the CW direction, the beam profile is relatively concentrated and can be partially controlled by adjusting the intracavity multimode (MM) fiber at the front-end of the MM coupler. These investigations would help to gain further understanding of the spatial and temporal dynamics and mechanism in the STML lasers.