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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 opticsnonlinear optics, methodsmethods


Cite the article using its DOI: https://doi.org/10.61835/xsf

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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 (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.

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.

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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?

The author's answer:

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.


What is meant with Z-cut crystal, and what is difference between type 1 and type 2 phase matching?

The author's answer:

A Z-cut crystal is one where the crystal's Z axis is perpendicular to some optically relevant surface, e.g. the input and surfaces of the crystal.

For the other question, see the article on phase matching. There, it is called type-I and type-II phase matching.


Does the frequency doubling process based on non-critical phase matching definitely not lead to aberration of light spot due to walk-off effect

The author's answer:

No, there is no spatial walk-off for noncritical phase matching. Unless you consider finite beam divergence, but that can usually be neglected.

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