Periodically-Poled Lithium Niobate (PPLN)Gooch & Housego PPLN nonlinear crystals allow for efficient conversion of wavelengths in the range from visible to 5 µm. The poling pattern is lithographically defined and highly customizable. This allows the crystals to be used in a wide range of applications using wavelength conversion. Magnesium doped PPLN is ideal for generating visible wavelengths while congruent PPLN is commonly used for generating infrared wavelengths for sensing or range finding.
G&H supplies PPLN chips made from un-doped or magnesium doped material. The traditional, un-doped PPLN is manufactured from Z-axis wafers such as are used in telecom applications. Congruently grown LN is very uniform, and phase-matching can be maintained over very long propagation lengths; we have produced parts as long as 80 mm. For applications where very low absorption in the 2-3 µm wavelength range is required, we recommend using treated wafers that are free of OH impurities. When visible light is generated, magnesium doping (MgO:PPLN) is the best choice because the 5 mol% doping ensures the material is above the threshold for photorefractive damage. Average powers of up to 1W can be produced at 532nm in such crystals without causing beam distortion.To achieve good conversion efficiency, the interacting waves need to be phase-matched by choosing the proper poling period. Typical periods range from about 7 µm for second harmonic generation (SHG) producing 532 nm to around 30 µm for optical parametric oscillators (OPOs) pumped by Nd:YAG lasers. The poling is done in wafer form before the chip is cut and polished. This allows a wide range of possible configurations by designing a poling mask specifically tailored to the application. A common OPO crystal geometry for pulsed lasers having moderate pulse energy is a chip with 1x11 mm aperture and a length of 10-20 mm. To allow continuous tuning, we can design fan-out gratings where the period changes smoothly as beam position is moved across the aperture. We have produced PPLN chips up to 80 mm in length to allow for maximum gain in singly-resonant continuous wave OPOs, and chips up to 40 mm in width to achieve fine tunability of the signal wavelength. In addition to side-by-side or fan-out gratings, patterns with multiple gratings along the beam direction can be manufactured to enable multiple nonlinear interactions in the same crystal. This can be used to increase the quantum efficiency of OPOs or to generate various different wavelengths at once. Other designs use chirped gratings to widen the wavelength acceptance or to compensate for group velocity walk-off in ultra-short pulses. Because of the large variety of poling patterns, our customers use PPLN in a wide range of applications.
- Pulsed lasers with a 1µm gain medium together with OPO crystals can generate eye-safe output useful for target illumination or range-finding.
- Difference frequency generation (DFG) to mix a fixed pump with a tunable laser allows generation of wavelengths suitable for trace gas detection in the 3-5 µm range.
- Visible laser light can be produced by second harmonic generation (SHG) of a near IR laser in MgO:PPLN.
Applications of PPLNChemical fingerprinting, compact laser displays, countermeasures, difference frequency generation (DFG), laser cooling, laser display, microscopy, range-finding, second harmonic generation (SHG), sensing, target illumination, telecommunications, wavelength conversion.