Because most nonlinear crystals are birefringent, beams that are collinear inside a crystal may not be collinear outside of it. The phase fronts (wave vector) do not point in the same direction as the energy flow (Poynting vector) because of walk-off. The phase matching angle makes possible any gain at all (0th order). In a collinear setup, the freedom to choose the center wavelength allows a constant gain up to first order in wavelength. 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. This generates a bandwidth 3 times as large of that of a Ti-sapphire-amplifier. The first order is mathematically equivalent to some properties of the group velocities involved, but this does not mean that pump and signal have the same group velocity. After propagation through 1-mm BBO, a short pump pulse no longer overlaps with the signal. Therefore, chirped pulse amplification must be used in situations requiring large gain amplification in long crystals. Long crystals introduce such a large chirp that a compressor is needed anyway. An extreme chirp can lengthen a 20-fs seed pulse to 50 ps, making it suitable for use as the pump. Unchirped 50-ps pulses with high energy can be generated from rare earth-based lasers. The optical parametric amplifier has a wider bandwidth than a -amplifier, which in turn has a wider bandwidth than an optical parametric oscillator because of white-light generation even one octave wide. Therefore, a subband can be selected and fairly short pulses can still be generated. The higher gain per mm for BBO compared to Ti:Sa and, more importantly, lower amplified spontaneous emission allows for higher overall gain. Interlacing compressors and OPA leads to tilted pulses. 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. 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.