Spatiotemporal light-beam compression from nonlinear mode coupling

We experimentally demonstrate simultaneous spatial and temporal compression in the propagation of light pulses in multimode nonlinear optical fibers. We reveal that the spatial beam self-cleaning recently discovered in graded-index multimode fibers is accompanied by significant temporal reshaping and up to fourfold shortening of the injected subnanosecond laser pulses. Since the nonlinear coupling among the modes strongly depends on the instantaneous power, we explore the entire range of the nonlinear dynamics with a single optical pulse, where the optical power is continuously varied across the pulse profile.

Figure 1

Experimental setup: the laser beam is divided by a beam splitter (BS) to provide a trigger reference time. The beam directed to the fiber passes first through two half wave plates (HWP) and a polarization beam splitter cube (PBS). Input and output lenses have focal lengths $\text{f1}=50$ mm and $\text{f2}=8$ mm. The output beam is filtered by a band-pass filter (BPF) of 10 nm bandwidth centered at 1064 nm.

Figure 2

Experimental results: (a) output temporal waveforms vs input peak power. Vertical lines identify some peak power levels, whose corresponding waveforms are reported for ease of comparison in panels (b) and (c).

Figure 3

Experimental results. The curves in the top panel show the output peak power and the corresponding beam diameter upon input peak power. The markers (a)–(d) represent four characteristic points for which the lower panels report the spectrum, the spatial beam shape, and the corresponding temporal profile measured by photodiode 1.

Figure 4

Experimental temporal waveforms for different transverse positions ($x$). The input peak powers were 2 kW for panel (a) and 4 kW for panel (b). In both cases the color bar has been referred to the maximum signal level measured at the center of the beam. The vertical lines are guides to the eye to identify specific measurement points whose associated temporal waveforms are shown by the corresponding insets, with intensity normalized to its local maximum level.

Figure 5

Numerical results from Eq. (1) with 16 modes, where only the first 9 were excited at the input and the remaining modes set to zero. The curve represents the coefficient $T=|a_0{\left(L\right)|}^2/{\left|a_0\left(0\right)\right|}^2$ for different input peak power levels. This curve can be considered as a transfer function representing the output contribution of the fundamental mode at different input power values. The inset shows the evolution of the temporal waveform associated to the fundamental mode $a_0$ when assuming an input pulse duration of 1 ns.