Abstract
A new method to generate multi-watt-level, octave-spanning, spectrally flat supercontinua stemmed from cascaded Raman scattering in graded-index multimode fibers is reported. Formation dynamics of supercontinua are investigated by studying the effect of fiber length and core size. High power handling capacity of the graded-index multimode fibers is demonstrated by power scaling experiments. Pump pulse repetition rate is scaled from kHz to MHz while pump pulse peak power remains same and ~4 W supercontinuum is achieved with 2 MHz pump repetition rate. To the best of our knowledge, this is the highest average power and repetition supercontinuum source ever reported based on a graded-index multimode silica fiber. Spatial properties of the generated supercontinua are measured and Gaussian-like beam profiles obtained for different wavelength ranges. Numerical simulations are performed to investigate underlying nonlinear dynamics in details and well-aligned with experimental observations.
Highlights
Supercontinuum generation in optical fibers is studied extensively in the past decades[1,2]
We report a novel technique to generate high average power spectrally flat octave-spanning supercontinua triggered by cascaded Raman scattering using a graded-index multimode fibers (MMFs) pumped with an all-fiber laser system
The highest supercontinuum output power of 3.96 W is achieved in graded-index MMF with 62.5 μm core diameter using picosecond pulses at MHz repetition rate
Summary
Supercontinuum generation in optical fibers is studied extensively in the past decades[1,2]. Nonlinear pulse propagation and modal interactions lead to discovery of unique phenomena such as Kerr self-cleaning[4,5], spatio-temporal solitons[6], ultrabroadband dispersive waves[7], spatio-temporal instability[8,9,10,11], quasi-phase matched intermodal four-wave mixing (FWM)[12] and stimulated Raman scattering (SRS)[13,14] With simultaneously exploiting these effects in graded-index MMFs various methods to generate supercontinua are presented in normal dispersion regime in the literature[15,16]. Krupa et al demonstrated the interplay between spatio-temporal instability and SRS while spatio-temporal instability peaks evolves to a supercontinuum[16] These studies achieve supercontinuum generation by benefiting from the combinations of the complex nonlinear dynamics such as harmonic generation and spatiotemporal instability by choosing pump pulses with high peak power and low repetition rates such as ~50 mW with 500 Hz and ~700 mW with 30 kHz, respectively. Our experimental setup is pump power limited, our observations suggest higher average output powers can be achieved in graded-index MMFs with higher repetition rate pump laser systems
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