Ultrafast two-color all-optical transverse mode conversion in a graded-index fiber

Abstract

We report on experimental conversion of picosecond probe pulses from the fundamental mode (LP01) to the next higher-order mode (LP11) by optically induced transient long-period gratings in a graded-index fiber using subpicosecond control pulses. In contrast to former experiments, the temporally synchronized control (1030 nm) and probe pulses (1250 nm) were easily distinguishable by their wavelengths, allowing for the first direct observation of such an ultrafast transverse mode conversion. Despite a nonperfect pulse duration ratio between control and probe pulses as well as an unavoidable excitation of unwanted higher-order modes, a clear energy conversion of 18% between the LP01-mode and the LP11-mode was observed at 2.3 times lower control pulse energies compared to previous experiments.

Several new directions for ultrafast fiber lasers [Invited]

“…Long-period gratings [177,218], photonic lanterns [219,220], multi-plane light conversion [221], and so on may be useful. A more inspired, but also more difficult, approach would be to rely on nonlinear mode conversion [222][223][224], and to design the pulse evolution so that the desired excitations happen automatically. Coupledcore multicore fibers may meanwhile make this task much easier compared to single-core, multimode fibers.…”

Spatiotemporal characterization of supercontinuum extending from the visible to the mid-infrared in a multimode graded-index optical fiber

“…This means that linear mode mixing can be effectively washed out by means of the Kerr effect, so that a cleaned multimode light beam remains effectively selfpreserved. Kerr self-cleaning (KSC) stems from nonlinear coupling among the fundamental mode and higher-order modes [3,4]. KSC occurs at power levels at least one order of magnitude lower than the critical power of catastrophic light self-focusing in a GRIN MMF [5].…”

“…For powers above the KSC threshold, nonlinear spectral broadening in MMFs results from a complex interplay between the spatial and temporal degrees of freedom [7][8][9]. Because of the self-imaging of the multimode beams in a GRIN MMF, the Kerr effect leads to a long-period intensity grating which induces mode conversion [4] and quasi-phase-matched (QPM) four-wave mixing (FWM) [10]. For temporal multimode femtosecond solitons in the anomalous dispersion regime [11], the nonlinear index grating produces an effective periodic nonlinearity which, in turn, induces a series of dispersive wave sidebands [12,13].…”

“…This means that linear mode mixing can be effectively washed out by means of the Kerr effect, so that a cleaned multimode light beam remains effectively selfpreserved. The mechanism for Kerr beam self-cleaning is the nonreciprocitity of the nonlinear coupling among the fundamental mode and the higher-order modes [3], which is induced by the dynamic longitudinal grating resulting from periodic mode beating and the Kerr effect [4]. It is important to note that Kerr self-cleaning occurs at peak pulse power levels which are at least one order of magnitude lower than the critical power associated with the well-known effect of catastrophic light self-focusing in a GRIN MMF [5], therefore the linear mode basis can still be considered for the expansion of the multimode field.…”

Spatial beam self-cleaning and supercontinuum generation with Yb-doped multimode graded-index fiber taper based on accelerating self-imaging and dissipative landscape

“…Spatial self-cleaning in GRIN MMFs is based on their characteristic spatial beam self-imaging effect, which introduces a longitudinal periodic modulation of the core refractive index, thanks to the Kerr effect. This index modulation acts as a dynamic long period grating, that phase-matches four-wave mixing interactions [18], leading to a complex power transfer between modes, or optical turbulence. This process leads to an irreversible depletion of intermediate modes, accompanied by energy flow into both LOMs and HOMs [19].…”