<sod>The [[Nonlinear optics#Phase matching|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 laser|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 anyways.  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.<eod>
<sod>The [[Nonlinear optics#Phase matching|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 laser|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.<eod>