ABSTRACT
I. INTRODUCTION Second-harmonic generation (SHG) is a second-order nonlinear optical process (x(2) process) that creates an electromagnetic wave of twice the frequency (2w) of the original or fundamental wave. It has been employed as a technique to double the output frequency of lasers, generating coherent light in spectral regions where laser operation cannot be easily achieved. Diode-pumped solid-state lasers with frequency-doubled output in the blue and ultraviolet (UV) region are already commercially available. Currently, there is a strong interest in miniaturizing these lasers for low-power applications, i.e., as light sources for CD players. Due to the low power requirement, high SHG conversion efficiencies are necessary in the doubling device. Optical waveguides are favorable for that purpose, since they ultimately provide the best light confinement and therefore the highest possible SHG efficiencies. Figure 1 displays such an optical waveguide with a second-order nonlinearity, which is typically quantified in terms of the X(2) coefficient or the d(2) coefficient.
