Noncritical Phase-Matching
المؤلف:
Walter Koechner Michael Bass
المصدر:
Solid-state Lasers
الجزء والصفحة:
353
24-1-2021
1457
Noncritical Phase-Matching
If the refractive indices can be adjusted so that Θm = 90◦, by variation of a parameter such as the temperature or chemical composition of the crystal, the dependence of Δk on beam divergence or angular misalignment δθ is due to a much smaller quadratic term, instead of a linear relationship as is the case in critical phase-matching.
For example, second-harmonic generation in LiNbO3 occurs at 90◦ for 1.064 μm radiation provided the crystal is at a temperature of ≈ 107◦ C (for a crystal doped with 5% MgO). The allowable departure from the phase-matched direction is greater than 26mrad if the coherence length is to be 1 cm. For these reasons, 90◦ phase-matching is often called noncritical phase-matching. Provided that no1 and ne2 are nearly equal and d(no1− ne2)/dT ≠ 0, noncritical phase-matching can be achieved by temperature tuning the crystal.
In addition, at Θm = 90◦ there are no walk-off effects due to double refraction.
TABLE 1. Properties of important nonlinear materials (data are for 1064 to 532 nm conversion).
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