Source: http://aoot.osa.org/josab/abstract.cfm?uri=josab-29-3-502
Timestamp: 2019-04-25 13:47:50+00:00

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We report in detail, both experimentally and using numerical simulation, the efficiency of generation of supercontinua in optical fiber driven by modulation instability of a continuous-wave (CW) pump source. It is shown that the degree of pump coherence has a dramatic effect on the resulting spectral expansion and it is discussed how this can be explained by having the proper conditions for efficient modulation instability to break the CW pump light into a train of fundamental solitons that subsequently undergo self-Raman shift to longer wavelengths. It is proposed that an optimal pump bandwidth exists corresponding to the optimal degree of pump incoherence, defined as a function of the modulation instability period.
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Fig. 1. Numerically modeled (a), (c), and (e) temporal and (b), (d), and (f) spectral intensity profiles of the input pump source for three pump bandwidths: (a) and (b) 0.13 nm; (c) and (d) 1.34 nm; (e) and (f) 4.02 nm. The time-averaged power is shown in (a), (c), and (e) with a dashed red line, and is constant to within 1% of a target value of 6.3 W.
Fig. 2. Relative peak power enhancement factor (where Ψenhancement=Ppeak/Paverage) as a function of the FWHM pump source bandwidth.
Fig. 3. (a) Components of the tunable ASE source. ASE seed: DF, Er-doped fiber amplifier; BPF, bandpass filter (Δλ=12 nm); Er-doped fiber preamplifier. TBPF, tunable bandpass filter (0.1<Δλ<15 nm). Power amp, 10 W Er-doped fiber amplifier. (b) Experimental setup. ISO, high-power fiberized optical isolator; HNLF, highly-nonlinear fiber. The cross denotes a permanent welded fusion splice between the output of the isolator and the HNLF for a fully fiber integrated format.
Fig. 4. Optical spectra and corresponding measured background-free (noncollinear) intensity AC trace for three increasing pump source bandwidths: (a) and (d) 0.36 nm; (b) and (e) 1.77 nm; (c) and (f) 5.28 nm.
Fig. 5. (a) Calculated dispersion curve (solid curve) and corresponding nonlinearity curve (dashed curve). The vertical dotted line corresponds to the experimental pump wavelength of 1.565 μm. (b) MI period (solid curve), and estimated energy of solitons emitted from MI (dashed curve) as a function of pump power for the given fiber parameters at the pump wavelength of 1.565 μm. The vertical dotted line denotes the average power of the CW laser source used in our experiments. Inset shows the estimated duration of the solitons emitted from MI.
Fig. 6. The −3 dB (or FWHM) pump source bandwidth as a function of the tunable bandpass filter bandwidth.
Fig. 7. Intensity AC traces of the input source for three pump bandwidths: (a) and (b) 0.36 nm; (c) and (d) 1.77 nm; (e) and (f) 5.28 nm. (a), (c), and (e) Experimental; (b), (d), and (f) numerical.
Fig. 8. Dependence of generated continuum width (10 dB) on the CW pump bandwidth (3 dB) for propagation of 6.4 W average power through 8 m of HNLF. (a) Experimental results; (b) numerical comparison.
Fig. 9. Temporal input field intensities for three pump bandwidths, computed using the CW laser model: (a) 0.33, (c) 2.58, and (e) 6.24 nm. The horizontal bar shows the modulation instability period, TMI, for the HNLF fiber parameters, pump wavelength (1.55 μm) and power (6.3 W) of the experiment. The corresponding computed single-shot spectrum after propagation in the 50 m length of HNLF: pump bandwidths: (b) 0.33, (d) 2.58, and (f) 6.24 nm. The spectral input pump line is shown with a dashed curve.
Fig. 10. Single-shot spectral evolutions as a function of propagation length in the HNLF for three input pump bandwidths: (a) 0.56, (b) 4.25, and (c) 38.66 nm.
Fig. 11. Single-shot spectrograms after the full 50 m propagation length of the HNLF for three input pump bandwidths: (a) 0.56, (b) 4.25, and (c) 38.66 nm.
Fig. 12. Output spectrum (averaged over 40 shots) obtained by pumping 20 m of fiber (γ=44 W−1 km−1, β2=−0.012 ps2 km−1) with a 10 W, 1065 nm CW source with the given pump bandwidth.
Fig. 13. (a) Obtained supercontinuum width (20 dB level, averaged over 40 shots) obtained by pumping 20 m of fiber (γ=44 W−1 km−1) with a 10 W, 1065 nm CW source, as a function of pump bandwidth. Each curve has β2 scaled to achieve the γ/|β2| values shown. (b) Optimum pump bandwidth extracted from (a) compared to the corresponding MI bandwidth, with corresponding linear fit.

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