Research progress on periodically poled lithium niobate for nonlinear frequency conversion

Laser gain media offer a limited number of radiative wavelengths. With the continued development of laser applications, there has grown a need for a diversity of laser wavelengths. This is where the process of nonlinear wavelength/frequency conversion can be used as an effective means of extending the range of obtainable wavelengths from existing laser sources. Lithium niobate is a commonly used nonlinear optical frequency conversion crystal, particularly when structured via the periodic poling process (periodically poled lithium niobate (PPLN)). The application of periodic poling to achieve quasi-phase-matching (QPM) within a nonlinear optical crystal maximizes the nonlinear conversion efficiency throughout the length of the crystal, minimizes beam walk-off and depending on the poling configuration, can be used to convert a range of wavelengths. The diversity of PPLN is such that it has been used for wavelength conversion of laser fields across the visible, near-infrared and mid-infrared regions. This paper reviews research which has been conducted on PPLN and in particular examines the application of PPLN across a range of laser cavity structures.