| Abstract [eng] |
Nonlinear crystals pumped by coherent laser sources enable the generation of tunable laser radiation across a wide infrared range. However, provided that specific conditions for the group velocities of the interacting waves are met, coherent radiation can be obtained in nonlinear crystals even if the pump light is incoherent. Backward-wave optical parametric oscillators (BWOPOs), in which the generated waves propagate in opposite directions, help meet these conditions and allow the generation of coherent waves over a much broader spectral range than conventional parametric devices. The aim of this work is to theoretically investigate the mechanism of coherent radiation generation in an incoherently pumped BWOPO. Finite-difference, split-step, and iterative methods were employed to model the interaction. First, it is shown that, due to improved group-velocity matching between the pump and the forward wave, the backward wave that is generated in a PPKTP crystal with a poling period of 427 nm is approximately 2.2 times closer to the transform limit when pumped by incoherent radiation at the wavelength of 1064 nm rather than at 532 nm. Second, it was found that, as the spectral width of the incoherent pump increases, the spectrum of the forward wave broadens proportionally to the pump, whereas the spectrum of the backward wave remains narrow. As a result, the spectral amplitude of the backward wave can exceed that of the pump. The model predicts that the spectral amplitude of the backward wave is approximately three times greater, while its spectral width is about 61 times narrower than that of the pump. Third, analytical iterative solutions for the amplitudes of the forward and backward waves were derived using the Riemann integration method. The obtained solutions reveal that the amplitude fluctuations of the incoherent pump directly modulate the temporal profile of the forward wave, whereas they are averaged out in the profile of the backward wave, resulting in the generation of a coherent backward-wave pulse. |
