Generation of multiband annular beams by femtosecond Bessel laser beam in silica glass

2018 
Single or double femtosecond Bessel laser beams are employed to generate multiple annular beams with different central wavelengths. A set of annular beams with bandwidths of several tens of nanometers and different peak wavelengths discretely distributed in the visible and infrared wavelength range are generated. No supercontinuum emission is observed. The propagation directions of these annular beams are wavelength-dependent and different from the propagation direction of the pump femtosecond Bessel beams, so four-wave mixing and secondary parametric process are considered to be the generation mechanism of these annular beams, which is further confirmed by numerical simulations. The energy conversion efficiency of single generated annular beam is in the order of 10-6. It is expected that specific resonant structures of the third-order nonlinear optical susceptibility of the sample used in experiments can enhance the energy conversion efficiency at certain wavelengths. These simultaneously generated colorful annular beams are actually composed of light pulses with nearly the same durations as the pump femtosecond pulses, which can find important applications where multi-wavelength ultrashort light pulses are needed, such as optical telecommunications, sensing, and pump-probe measurements. It is considered that stimulated four-wave mixing and corresponding cascaded parametric process are the main generation mechanism.
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