Source: http://aoot.osa.org/oe/abstract.cfm?uri=oe-25-26-32972
Timestamp: 2019-04-24 10:01:08+00:00

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We demonstrate two-color dispersive wave emission in the ultraviolet and near-infrared regions in an argon filled hypocycloid-core kagome fiber pumped by a femtosecond laser around 1 μm. These two dispersive waves show drastically distinct features in terms of bandwidth and tunability. The dispersive wave in the ultraviolet region has a bandwidth of tens of nanometers and can be widely tuned from at least 267 nm to 460 nm by changing the gas pressure, input pulse energy, and pump wavelength. In contrast, the dispersive wave in the near-infrared region has a narrower bandwidth of ~5 nm and is quite stably positioned near the edge of the fundamental transmission band even if the gas pressure or input pulse energy is significantly changed. An antiresonant tube model is applied to explain the experimental results and a good agreement is found between them. The dynamics show that the narrow-band dispersive wave in the near-infrared region originates from the steep slope of the dispersion near the edge of the transmission band.
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Fig. 1 Cross sections and fundamental mode profiles of the (a) Kagome fiber and (b) antiresonant tube. (c) The normalized z-Poynting vectors for the two structures. The blue and black curves correspond to the z-Poynting vectors of the Kagome fiber along the x- and y-axes, respectively and the red curve represents that of the antiresonant tube. (d) The effective modal indices for the Kagome fiber and antiresonant tube are shown by the blue and red curves, respectively. The black curve shows the modal index calculated from the capillary waveguide model . The gray blocks in the figure represent the resonant regions of the fiber.
Fig. 2 GVD curves of the antiresonant tube (solid curves) and the corresponding capillary waveguide model (dotted curves) at different argon pressures. The light-red (1), orange (2), green (3) and purple (4) regions show the transmission bands separated by the resonant regions.
Fig. 3 (a) Experimental set-up, L1-L4 are lenses. HWP is half-wave plate. OSA1 and OSA2 are optical spectrum analyzers. Power control is provided by VND (variable neutral density filter). Filter (FEL0850, Thorlabs) is used to block undesirable visible output wavelengths. The inset is the far-field beam profile. (b) Scanning electron microscope image of the cross section of the Kagome fiber (PMC-C-780, GLOphotonics). Superimposed in the center is the measured near-field beam profile. (c) The loss spectrum of the Kagome fiber.
Fig. 4 Experimentally measured spectral evolution with (a) pulse energy coupled into the fiber at a pressure of 11 bar and (b) gas pressure at a pulse energy of 1.51 μJ for pump at 1080 nm. Theoretical simulation results for Fig. 4(a) with Fig. 4(c) antiresonant tube model and (d) capillary waveguide model.
Fig. 5 Numerical simulation results for 1080 nm pump at a pulse energy of 1.51μJ and pressure of 11 bar. (a) Spectral evolution and (b) Temporal profile of the electric field along the propagation distance. Corresponding spectral (c) and electric field (d) profiles at typical distances. The labels (i), (ii), (iii) and (iv) in Figs. 5(a) and 5(b) represent the four different propagation stages.
Fig. 6 (a) Phase mismatch curves calculated at a pump wavelength of 1080 nm, pulse energy of 1.51 μJ, and argon pressure of 8 bar. The solid-red and blue curves represent antiresonant tube and capillary waveguide models, respectively. The two dashed black circles indicate phase matching points (i.e., the NIR-DW and UV-DW). (b) Phase-matching wavelengths as a function of pump wavelength at different gas pressures. The colored solid curves represent the UV-DW, and the colored dotted curves correspond to the NIR-DW. The three insets show the experimentally measured spectra of the NIR-DW at pump wavelengths of 980 nm (1.51 μJ, 11 bar), 1040 nm (1.05 μJ, 6 bar) and 1080 nm (1.51 μJ, 5 bar), respectively. The three vertical dashed gray lines represent different pump wavelengths used in the experiment.
Fig. 7 Experimental results (a) Output far-field beam spots of different spectral components at a pump wavelength of 980 nm, pulse energy of 1.51 μJ and argon pressure of 11 bar. (b) Tunable dispersive waves generated in the UV (VIS) wavelength range with different pump wavelengths, input pulse energies and argon pressures.

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