Optical parametric amplifier

The output beams in optical parametric generation are usually relatively weak and have relatively spread-out direction and frequency.

This problem is solved by using optical parametric amplification (OPA), also called difference frequency generation, as a second stage after the OPG.

In the OPA, the pump and idler photons usually travel collinearly through a nonlinear optical crystal.

Noncollinear OPAs were developed to have an additional degree of freedom, allowing constant gain up to second order in wavelength.

The optimal parameters are 4 degrees of noncollinearity, β-barium borate (BBO) as the material, a 400-nm pump wavelength, and signal around 800 nm (and can be tunable in the range 605-750 nm with sub-10 fs pulse width which allows exploring the ultrafast dynamics of large molecules[1]) This generates a bandwidth 3 times as large of that of a Ti-sapphire-amplifier.

Multipass can be used for walk off and group velocity (dispersion) compensation; constant intensity with increasing signal power means to have an exponential rising cross section.

Since the direction of the beams is fixed, multiple passes cannot be overlapped into a single small crystal like in a Ti:Sa amplifier.

Unless one uses noncolinear geometry and adjusts amplified beams onto the parametric fluorescence cone produced by the pump pulse.