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Noncritical Phase Matching

Definition: a technique of phase matching which does not require a critical angular adjustment

More general term: phase matching

Opposite term: critical phase matching

German: unkritische Phasenanpassung

Categories: nonlinear optics, methods

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Noncritical phase matching (sometimes called temperature phase matching or 90° phase matching) is a technique for obtaining phase matching of a nonlinear process. 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 such that the phase velocities of the interacting beams are equal.

noncritically phase-matched second-harmonic generation in LBO
Figure 1: Phase-matching temperature versus pump wavelength for noncritically phase-matched second-harmonic generation (frequency doubling) in LBO.

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.

crystal oven
Figure 2: A compact crystal oven with electronic temperature controller for moderate temperatures up to 200 °C. The photograph has been kindly provided by EKSPLA.

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.

Questions and Comments from Users


Does operation in the NCPM regime also relax the spectral acceptance constraint, so that NCPM is useful in ultrafast frequency conversion?

Answer from the author:

No, this is a separate issue. If you have a nonlinear crystal which you can phase-match critically or non-critically, the spectral acceptance bandwidth will generally differ in those cases, but I don't think it is generally better for noncritical phase matching.

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See also: critical phase matching, phase matching, quasi-phase matching, nonlinear frequency conversion, crystal ovens
and other articles in the categories nonlinear optics, methods


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