We started again from the Phase Matching (PM) condition and the necessity of using birefringent nonlinear crystals to satisfy it. Specifically, we saw in detail the use of BBO crystals (negative uniaxial) to achieve Type I PM (ooe), in collinear and non-collinear configurations. We saw how the “magic angle” can be determined by means of a numerical script, by drawing the phase-matching curves as a function of the signal wavelength and crystal angle Ɵ for different values of the noncollinearity angle α. We saw that different pumping wavelengths (515 and 400 nm) feature different magic angles, corresponding to distinct values of Ɵ. We also evaluated the wavevector mismatch as a function of ʎ and Ɵ, at the magic angle, and quantified the deviation from perfect phase-matching caused by fixing a specific value of Ɵ.
Finally, we explored the NOPA architecture's technical layout, detailing the various optical components and the pivotal role of the delay line in tuning the pump-signal delay inside the nonlinear crystal. We noted that with a chirped seed, amplification is restricted entirely to the spectral components that temporally overlap with the pump. As a result, simply moving the delay line enables a tunable spectral shift of the amplified pulse across the phase-matching region.
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