Noncritical Phase Matching | previous | next | feedback |
Definition: a technique of phase matching which does not require a critical angular adjustment
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 so that they propagate along some axis of the birefringent nonlinear crystal. The phase mismatch is minimized by adjusting the crystal temperature so that the phase velocities of the interacting beams are equal.
Figure 1: Phase-matching temperature versus pump wavelength for noncritically phase-matched second-harmonic generation (frequency doubling) in LBO. The type-I or the type-II scheme may be more convenient, depending on the circumstances.
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 involved beams 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 stand the operation temperature as well as 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.
See also: critical phase matching, phase matching, quasi-phase matching, nonlinear frequency conversion
Categories: methods, nonlinear optics
