Wavelength Conversion Technology - Periodically Poled Lithum Niobate (PPLN)
Periodically poled Lithium Niobate (PPLN) is a non-linear crystal that enables very high efficiency wavelength conversion. Classically it was thought that it was not possible to change the colour of light, however in modern physics it is well known that the non-linear properties of lithium niobate and other materials, allows wavelength conversion to occur. Many materials have these non-linear properties, however in lithium niobate this effect is much stronger than in conventional materials. In order to use the large non-linear effect in lithium niobate, Stratophase microstructure it using a process called periodic poling. This process is tailored to the particular application meaning that different periods are required for optimal operation at different wavelengths. Devices are available for frequency doubling, difference frequency generation, sum frequency generation, optical parametric oscillation and optical parametric amplification, in addition to custom crystals for any other nonlinear processes. The PPLN crystals are typically used by focusing the laser beam into the crystal, propagating along the length of the crystal. The light coming out of the other end of the crystal then contains the new wavelength and the remainder of the input light.
In
order for any non-linear process to function the optical fields within
the non-linear crystal must be “phase matched” ie
the
optical waves at the different colours must all be synchronised to each
other. In most non-linear crystals the different colours
produced
in any non-linear process will travel at different speeds through the
material causing them to drift out of phase as they propagate through
the crystal. In conventional frequency conversion materials such BBO or
LBO, this phase matching is achieved using the natural birefringence in
the material by propagating the light in a particular direction through
the crystal. Unfortunately this is not possible in Lithium Niobate,
although the nonlinear coefficient of Lithium Niobate is much larger
than these other crystals. The solution to phase matching in Lithium
Niobate is Periodic Poling, which
achieves "quasi"-phase matching and so enables the much higher
conversion
efficiency of lithium niobate to be used in practice.
The conversion efficiency attainable with PPLN is larger than with conventional crystals and as with all wavelength conversion crystals depends strongly on the properties of the laser beam. For instance single pass conversion efficiencies of over 80% have been demonstrated with an optimal pulsed laser but with a 1 watt continuous laser the efficiency would drop to just a few percent.
The key to producing PPLN is the poling process. This is the process that periodically inverts the crystal structure every few microns along the crystal. Lithium Niobate is a ferroelectric crystal which means that each unit cell in the crystal has a small electric dipole moment. This dipole is caused by slight offsets in the position of niobium and lithium ions in the unit cell. The application of an intense electric field can invert the crystal structure, rearranging the crystal at an atomic level. The electric field needed to invert the crystal is very large (~22kV/mm), and is applied for only a few milliseconds, after which the periodically reversed structure is permanently imprinted into the crystal structure.
To produce PPLN, Stratophase deposit a periodic electrode structure on a Lithium Niobate wafer and a voltage is applied to invert the crystal underneath the electrodes. The voltage is carefully controlled so that the poled regions are created with the desired shape. The design of the electrodes is also key to producing small period PPLN used for frequency doubling in to the visible wavelengths. Once poled, the crystals are permanently set into the new pattern.
Multiple patterns, with for example different periodicities, can be produced next to each other on a single device. In OPOs and other non-linear applications this enables broadband tuning of the devices by changing from one grating to the next.
PPLN is a robust, transparent crystal, although to gain maximum wavelength conversion efficiency its temperature must be carefully controlled. Uniform temperatures across the PPLN crystal are also important to maximise performance. Stratophase therefore supplies a range of miniature temperature controlled housing or ovens and associated temperature controllers to maximise wavelength conversion performance. Changing the PPLN crystal temperature can also be used for short range tuning of output wavelength in OPO and similar frequency conversion applications.
As a new non-linear optical element, PPLN extends the potential of existing laser-based systems by enabling efficient and robust access to new area of the colour spectrum from the mid infrared at >5um to the visible at 450nm and below. Stratophase also specialises in more complex crystal structures which integrate multi-functionality on a single optical chip.
PPLN finds itself at the centre of an expanding area of interest and a variety of applications in sectors such as aerospace, telecommunications and environmental monitoring.
Please explore our extensive range of PPLN products and accessories.