Noncritical Phase-Matching

If the refractive indices can be adjusted so that Θ_m = 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 n^o₁ and n^e₂ are nearly equal and d(n^o₁− n^e₂)/dT ≠ 0, noncritical phase-matching can be achieved by temperature tuning the crystal.
In addition, at Θ_m = 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).

مواضيع ذات صلة

الليزرات والسوائل التي تصعد إلى الأعلى

الليزر والميزر

Flashtube Explosion

Tube Failure

Plane–Parallel Resonator

Concave–Convex Resonator

Plano-Concave Resonator

Mirrors of Equal Curvature

Parasitic Oscillations

Prelasing

Depopulation Losses

The Three-Level System