Because dispersion makes the fundamental and harmonic travel at different phase velocities, newly generated light drifts out of phase with what was made earlier and conversion oscillates instead of growing. Birefringent crystals fix this by angle tuning or temperature tuning so the two waves see the same index. Noncritical phase matching at 90 degrees, as with LBO near 150 degrees C for 1064 nm doubling, avoids walk-off and tolerates tight focusing.
Phase matching angle for type I in BBO at 800 nm is about 29 degrees, so rotate the crystal slowly and watch the blue.
Also heard as
- PM
Related terms
SHG
Second Harmonic Generation: converting light to exactly double its frequency, half its wavelength, in a nonlinear crystal.
Walk-off
The drifting apart of interacting beams in a crystal, either sideways from birefringence or in time from velocity mismatch.
Quasi-phase matching
Phase matching by periodically flipping a crystal's nonlinear orientation instead of relying on birefringence.
Oven
The temperature-controlled mount that holds a nonlinear crystal at its phase-matching temperature.
OPO
Optical Parametric Oscillator: a tunable source that splits pump photons into two longer-wavelength beams in a nonlinear crystal.