Characterization of spectrally-temporally multiplexed optical amplification of ultrafast laser pulses in few-mode fiber.

Few-mode fiber amplifiers have garnered significant attention for their applications in spatial division multiplexing (SDM) communication systems and ultrafast fiber lasers, as spatial modes provide additional degrees of freedom. While previous studies have predominantly focused on amplifying continuous-wave (CW) or quasi-CW signals in a single transverse mode, the amplification of ultrafast laser pulses with much higher peak power and broader spectral width in few-mode fiber remains largely unexplored, in which the involved fruitful spectrally-temporally nonlinear interplay can result in complicated dynamics and potentially deteriorate the spectral-temporal property of the ultrafast laser pulses. Here, we present a proof-of-concept demonstration of spectrally-temporally multiplexed (STM) amplification of few-mode ultrafast laser pulses using a few-mode erbium-doped fiber amplifier (FM-EDFA). We simultaneously characterize the few-mode ultrafast laser pulses in the temporal, spectral, and spatial domains, especially by generating temporally separated and spectrally correlated pulse trains. In the experiment, the ultrafast laser pulse is converted into a distinct spatial mode via photonic lantern, and subsequently recombined to enable simultaneous amplification. Simultaneous amplification of the LP01 and LP11 modes (Δt = 0 ns) results in higher gain for the LP01 mode, leading to nonlinear spectral broadening and modal competition. In this regard, introducing a 5 ns temporal delay between the two modes suppresses modal competition and gain saturation, enabling the LP11 mode to also achieve significant nonlinear spectral broadening. This temporally-separated amplification configuration reduces phase noise degradation and eliminates sidelobes in the autocorrelation trace, better preserving temporal pulse integrity. These results open a route toward scalable, high-dimensional ultrafast laser amplification, with promising potential of manipulating the spectral-temporal property of the ultrafast laser in parallel.