Temperature tuning is used when the pump ( laser ) frequency polarization is orthogonal to the signal and idler frequency polarization.
2.
At each coherence length the crystal axes are flipped which allows the energy to continue to positively flow from the pump to the signal and idler frequencies.
3.
Quasi-phase-matching ensures that there is positive energy flow from the pump frequency to signal and idler frequencies even though all the frequencies involved are not phase locked with each other.
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