Observation of transition between multimode Q-switching and spatiotemporal mode locking

Abstract

We report experimental observation of multimode <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m1"> <mml:mrow> <mml:mi mathvariant="italic">Q</mml:mi> </mml:mrow> </mml:math> -switching and spatiotemporal mode locking in a multimode fiber laser. A typical steady <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m2"> <mml:mrow> <mml:mi mathvariant="italic">Q</mml:mi> </mml:mrow> </mml:math> -switching state is achieved with a 1.88 μs pulse duration, a 70.14 kHz repetition rate, and a 215.8 mW output power, corresponding to the single pulse energy of 3.08 μJ. We find weak spatial filtering is essential to obtain stable <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m3"> <mml:mrow> <mml:mi mathvariant="italic">Q</mml:mi> </mml:mrow> </mml:math> -switched pulses, in contrast to the relatively stronger spatial filtering for spatiotemporal mode locking. Furthermore, a reversible transition process, as well as a critical bistable state, between multimode <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m4"> <mml:mrow> <mml:mi mathvariant="italic">Q</mml:mi> </mml:mrow> </mml:math> -switching and spatiotemporal mode locking, is achieved with specific spatial coupling and waveplates sets. We believe the results will not only contribute to understanding the complicated nonlinear dynamics in multimode, fiber-based platforms, but also benefit the development of promising high-pulse energy lasers.