Abstract
Four-wave optical parametric amplification (FWOPA) in a Raman-active medium is experimentally investigated by use of an air-filled hollow fiber. A femtosecond pump pulse shorter than the period of molecular motion excites the coherent molecular motion of the Raman-active molecules during the parametric amplification of a signal pulse. The excited coherent motion modulates the frequency of the signal pulse during the parametric amplification, and shifts it to lower frequencies. The magnitude of the frequency redshift depends on the pump intensity, resulting in intensity-dependent spectral characteristics that are different from those in the FWOPA induced in a noble-gas-filled hollow fiber.
Highlights
Nonlinear optical effects have been investigated in various media for broadband light amplification.In particular, the optical parametric amplification via the second-order nonlinearity of a birefringent crystal has allowed broadband light amplification supporting sub-10-fs pulses in the visible and infrared wavelength regions [1,2]
In the four-wave optical parametric amplification (FWOPA) induced with a gas-filled hollow fiber, it is possible to balance the material phase mismatch and the modal phase mismatch by properly selecting the core diameter of the hollow fiber [21]
The output beams projected on a white screen had Gaussian-like spatial intensity distributions, including the pump pulse and the signal pulses with and without the amplification by the FWOPA
Summary
Nonlinear optical effects have been investigated in various media for broadband light amplification. In the scheme of the FWOPA, self-phase modulation (SPM) and cross-phase modulation (XPM) simultaneously induced by an intense pump pulse significantly alter the phase matching condition by the induction of the nonlinear phase mismatch It modifies the wavelength dependence of the amplification gain to what is not predicted by the linear phase mismatch. When a Raman-active gas medium and a pump pulse that is shorter than the period of molecular motion (vibration or rotation) are utilized in the FWOPA, it is expected that the coherent molecular motion is impulsively excited by the pump pulse via impulsive stimulated Raman scattering [12]. The coherent molecular motion is expected to have effects on the amplification characteristics of the FWOPA It has, to our knowledge, not been experimentally investigated how the coherent motion alters the amplification characteristics of the signal pulse being amplified via the FWOPA induced in a Raman-active gas. The amplification characteristics in the Raman-active medium are different from those of the FWOPA induced in a noble-gas filled hollow fiber
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