SHG   EFFICIENCY

    The second-harmonic conversion efficiency, defined as the ratio of the power of the second-harmonic output component to the power of the fundamental component, is directly proportional to the intensity of the electric field, I = P_fundamental/A. This explains why many harmonic generator crystals are placed inside the cavity of the laser where the intensity is highest. Incident beams may also be focused inside the crystal to increase the localized intensity in the crystal as the beam passes through it. To prevent damage in this case, one must ensure that the intensity does not exceed the damage threshold of the crystal.

       Efficiency is also proportional to the square of the length of the second harmonic crystal so that doubling the length of the crystal quadruples the second-harmonic output. We find that the relationship between incident power and second-harmonic output power is

 (1.1)

where K is a proportionality constant, l the length of the crystal, and A the beam area in the crystal.