| Abstract [eng] |
APPLICATION OF PHASE-MISMATCHED THREE-WAVE INTERACTION FOR PULSE SPECTRAL BROADENING Experiments on three-wave interactions began shortly after the invention of the laser, while the efficiency of parametric processes increased significantly after the discovery of the phase-matching condition. This stimulated the development of widely tunable optical parametric amplifiers. In most femtosecond OPA systems, a white-light continuum generated through third-order nonlinear effects is used as the seed. However, spectral broadening can also be realized in second-order nonlinear crystals via phase-mismatched three-wave interactions. Therefore, in this work, a novel OPA scheme was investigated in which 1035 nm femtosecond laser radiation was used as the seed, while spectral broadening and parametric amplification occurred simultaneously in the same OPA crystal. An 8 mm type-II BBO crystal and a second-harmonic pump pulse (517.5 nm) were used for the three-wave interaction in a collinear geometry with the seed pulse. Under phase-mismatched conditions, the tuning range of the generated and amplified radiation extended from 850 nm to 1330 nm and showed good agreement with the theoretical phase-matching model, while the conversion efficiency from the pump to the idler wave reached up to 4%. Cross-correlation measurements of the 970 nm idler wave showed that increasing the pump energy caused the idler wave to be generated at different depths inside the nonlinear crystal, resulting in asymmetric spectral broadening due to group-velocity mismatch. M^2measurements revealed asymmetry of the idler beam along the x-axis (M_x^2\approx\ 1,78, Epump = 120 μJ), which is explained by transverse beam walk-off and the resulting nonuniform amplification across the beam profile. Pulse energy stability measurements showed only 2.05% amplitude fluctuations, while blocking the seed pulse also suppressed the idler wave, confirming that the observed radiation was not amplified parametric fluorescence. The observed processes were analyzed by studying changes in the signal-wave spectrum as a function of pump energy and phase mismatch. It was shown that increasing the pump energy at small Δk efficiently generates new frequency components, whereas increasing the phase mismatch enables phase matching for more distant wavelengths and significantly extends the tuning range. In addition, a short-pass filter inserted into the seed arm confirmed that the newly generated frequencies were absent from the initial seed spectrum. The obtained results demonstrate that phase-mismatched three-wave interactions can be effectively employed for simultaneous spectral broadening and amplification in an OPA crystal, enabling a simple and compact optical parametric amplifier configuration. |
