Editing Optical parametric amplifier (section)

==Multipass OPA==
Multipass can be used for
walk off and [[group velocity]] ([[dispersion (optics)|dispersion]]) compensation;
constant intensity with increasing signal power means to have an exponential rising cross section. This can be done by means of lenses, which also refocus the beams to have the beam waist in the crystal;
reduction of OPG by increasing the pump power proportional to the signal and splitting the pump across the passes of the signal;
broadband amplification by dumping the idler and optionally individually detuning the crystals;
complete pump depletion by offsetting the pump and signal in time and space at every pass and feeding one pump pulse through all passes;
high gain with BBO, since BBO is only available in small dimensions. 
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.<ref name="Jarota 2019 192–204"/><ref>http://link.aip.org/link/?APPLAB/86/211120/1 Multipass bow type chirped pulse amplifier</ref>