Noncritical Phase Matching
Noncritical phase matching (sometimes called temperature phase matching or 90° phase matching) is a technique for obtaining phase matching of a nonlinear process (usually nonlinear frequency conversion, e.g. frequency doubling or parametric amplification). The interacting beams are aligned such that they propagate along some axis of the birefringent nonlinear crystal. The phase mismatch is minimized by adjusting the crystal temperature. In some cases (e.g. type-I frequency doubling) that implies that the phase velocities of the interacting beams are equal.
As an example, Figure 1 shows the required crystal temperature vs. pump wavelength for frequency doubling in LBO. There are two available phase-matching schemes, requiring different polarization conditions of the input.
The attribute “noncritical” comes from the fact that this technique is relatively insensitive to slight misalignment of the beams. Another advantage is that the phenomenon of spatial walk-off is avoided. Therefore, the conversion efficiency can often be higher with noncritical phase matching, because the beams involved can be more tightly focused. The disadvantage, however, is that the crystal temperature is usually somewhat away from room temperature, so that a temperature-stabilized crystal oven is required. Also, both the crystal and any anti-reflection coatings which may be required must be able to withstand the operation temperature and also changes between that temperature and room temperature.
There are some cases where the phase-matching temperature is relatively close to room temperature. It is then often possible to find a critical phase matching configuration with similar nonlinear and dispersive properties, and exhibiting only a small spatial walk-off.
The technique of quasi-phase matching allows for noncritical phase matching with crystals which normally could only be critically phase matched.
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