G. Jonušauskas

G. Jonušauskas coauthored the 1992 beta barium borate experiment with A. Dubietis and A. Piskarskas, demonstrating parametric amplification of chirped and stretched femtosecond pulses.^[ref]

An optical parametric amplifier must satisfy both frequency relations and phase matching. For a pump, signal and idler with angular frequencies ${\omega }_p,{\omega }_s,{\omega }_i$, the three-wave process obeys ${\omega }_p={\omega }_s+{\omega }_i$. Coherent buildup also depends on their wavevectors: ideally ${\mathbf{k}}_p={\mathbf{k}}_s+{\mathbf{k}}_i$. Matching frequencies alone does not ensure that fields generated at different positions add constructively.

For an original undepleted-pump, weak-conversion illustration, suppose the driving amplitude is uniform along a crystal of length $L$. A scalar phase mismatch $\Delta k$ makes the generated field proportional toAt exact phase matching the limiting amplitude is proportional to $L$. At $\Delta kL=2\pi$ the idealized integral is zero because contributions cancel. This elementary coherent-sum model explains the need to control crystal orientation and optical frequencies; it is not the full high-gain solution or a reported setting from Jonušauskas's experiment.