PPLN

PPLN, or periodically poled lithium niobate, refers to lithium niobate crystals in which the ferroelectric domains have been inverted in a periodic pattern. The resulting structure enables quasi-phase matching, a technique that compensates for the natural phase mismatch in nonlinear frequency conversion and thereby greatly enhances efficiency for a broad set of wavelengths.

By selecting the poling period Λ and the crystal orientation, designers can achieve quasi-phase matching for second-harmonic generation, sum- or difference-frequency generation, or optical parametric processes across visible to mid-infrared ranges. The strong nonlinear coefficient of LiNbO3, especially in the d33 tensor element for z-cut crystals, is exploited in many devices.

Fabrication and variants: PPLN is typically produced by electric-field poling of MgO-doped LiNbO3, with lithographically patterned electrodes to define the domain inversion. MgO doping enhances resistance to photorefractive damage and supports high-power operation. PPLN can be used as bulk crystals or fabricated into waveguides (PPLN waveguides) for integrated photonics, enabling compact, high-efficiency devices.

Applications: bulk PPLN crystals and PPLN waveguides are used in second-harmonic generation (e.g., 1064 nm to 532 nm), frequency mixing, optical parametric oscillators, and efficient generation of entangled photons by spontaneous parametric down-conversion, with a path to telecom wavelengths around 1.5 μm.

Properties and considerations: PPLN offers broad transparency roughly from 350 nm to 5 μm and high nonlinearity, with temperature tuning to maintain phase matching. Performance depends on poling quality, uniformity, and operating temperature; photorefractive effects are mitigated by MgO doping but remain a design consideration.