TECHNICAL GUIDE: PPLN Crystal Grating Design

Periodically Poled Lithium Niobate (PPLN) is a non-linear optical material widely used for efficient frequency conversion, such as second harmonic generation (SHG), sum frequency generation (SFG), difference frequency generation (DFG ) and optical parametric oscillation (OPO). The key to PPLN’s functionality is the use of quasi-phase matching (QPM), achieved through periodic poling of the ferroelectric domains within the lithium niobate (LiNbO3) crystal.

Covesion’s proprietary poling process enables the fabrication of high quality, periodically poled gratings with a wide range of designs.

This guide provides an overview of PPLN crystal grating designs and their suitability for different applications.

Quasi-phase matching (QPM)

QPM compensates for phase mismatches in non-linear interactions by periodically inverting the non-linear coefficient d₃₃ within the PPLN crystal. The phase-matching condition is given by^[1]:

where:

• k_p, k_s, k_i are the wave vectors of the pump, signal, and idler waves,
• ⋀ is the poling period,
• ∆k is the phase mismatch.

Proper selection of the poling period ensures efficient energy transfer in the desired frequency conversion process. The required poling period varies with temperature, wavelength, and material dispersion, necessitating precise control of the grating fabrication.

Types of grating designs

Covesion are able to fabricate a wide variety of grating designs. These are available as off-the-shelf and custom solutions.

Single-period gratings

A single periodic structure used for narrowband phase matching, typically optimized for a specific wavelength and temperature.

Multi-period gratings

Integration of multiple, single period grating structures within the same crystal, enabling phase matching across a wide wavelength range

Chirped and aperiodic gratings

A continuously varying poling period designed for broadband phase matching and pulse compression applications. The design can be linear or aperiodic to optimize conversion across multiple wavelengths and fine control the output spectrum.

Fan-out gratings

A spatially varying poling period that allows phase matching across a range of wavelengths by translating the beam across the crystal’s width. This enables the crystal to be wavelength tuned at a fixed temperature. This is often used in OPO systems for tuneable mid-IR generation via a combination of temperature and spatial tuning.

Summary

PPLN crystal grating design enables efficient frequency conversion through quasi-phase matching. By carefully selecting and optimizing the poling period, structure, and fabrication techniques, PPLN gratings can be tailored for a wide range of optical applications covering the entire lithium niobate transmission window from 390nm to 6µm. Contact us for more details of the standard and custom crystal designs that we offer.

References

1. J. A. Armstrong, “Interactions between light waves in a nonlinear dielectric”, Phys. Rev. 127 (6), 1918 (1962)