= Optical parametric chirped pulse amplification
= OPCPA
{c}
{synonym}
<Optical parametric chirped pulse amplification> combines the stretch–amplify–compress sequence of <chirped pulse amplification> with an <optical parametric amplifier>. A stretched signal pulse overlaps a strong pump pulse in a nonlinear crystal. <Three-wave mixing> transfers pump <energy> into the signal and an idler; subsequent <pulse compression> brings the amplified signal's frequencies back together. The pump supplies the added <energy>, rather than a stored <population inversion>.<Overview of Nonlinear Processes by Camacho Lab>[Sum and Difference Frequency Generation, parametric amplification]{ref}
<A. Dubietis>, <G. Jonušauskas> and <A. Piskarskas> reported chirped and stretched pulse parametric amplification in <beta barium borate> in 1992. Their paper establishes a concrete early implementation of the parametric route; its published title identifies the nonlinear crystal.<Powerful femtosecond pulse generation by chirped and stretched pulse parametric amplification in BBO crystal by A. Dubietis and others>[Optics Communications 88, 437–440, published title and byline]{ref}
For pump, signal and idler <angular frequencies>, the conversion obeys
$$
\omega_p=\omega_s+\omega_i.
$$
Efficient amplification also requires <phase matching>, so contributions generated along the crystal reinforce. For collinear propagation the corresponding wave-vector relation is $k_p=k_s+k_i$; a noncollinear geometry requires the vector relation. Crystal dispersion and the chosen geometry determine which signal bandwidth is efficiently amplified.
An original energy-accounting example uses a 515-nanometre pump and an 800-nanometre signal. Their vacuum wavelengths give
$$
\frac1{\lambda_i}=\frac1{515\,\mathrm{nm}}-\frac1{800\,\mathrm{nm}},\qquad
\lambda_i\approx1.446\,\mathrm{\mu m}.
$$
Suppose 40 percent of a 2-millijoule pump is converted into signal–idler pairs, with negligible seed energy and other losses in this chosen model. Each conversion divides photon energy according to frequency, so the added signal energy is
$$
\Delta U_s=0.40(2\,\mathrm{mJ})\frac{\omega_s}{\omega_p}
=0.80\frac{515}{800}\,\mathrm{mJ}=0.515\,\mathrm{mJ}.
$$
The idler receives the remaining 0.285 millijoule. Assigning all depleted pump energy to the signal would violate this accounting.
If a compressor transmits 80 percent of that signal energy into a 50-femtosecond <Gaussian pulse>, the surviving energy is 0.412 millijoule. The <chirped pulse amplification energy budget> gives
$$
P_{\rm pk}=\sqrt{\frac{4\ln2}{\pi}}\frac{U}{\tau}\approx7.74\,\mathrm{GW}.
$$
These are stipulated teaching values, not a reconstruction of the 1992 experiment. Stretching lowers <peak power> during amplification; it does not remove the need to manage pump intensity or optical damage.
A useful apparatus check varies pump–signal delay while recording signal and idler spectra and energies. This tests temporal overlap and the frequency-conversion relation. Measuring the compressed pulse separately checks whether amplified bandwidth and residual spectral phase support the claimed <pulse duration>. Gain alone does not demonstrate successful compression, and an energy measurement alone does not establish <peak power>.
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