Modulators and photonic components integrated into lithium niobate - Principle and technology

The use of laser sources, with their spatial and temporal coherence properties and power densities, really opened up the field of photonics in optical telecommunications in the 1980s. This approach required the development of numerous optical and photonic functions for the active control of light. Among these, the optical modulation function has been the subject of particular attention. Prior to this, optical modulation  played an early role in the operation of the first generations of pulsed lasers. It was then implemented in triggered lasers, such as Q-Switch lasers, or in mode-locked lasers.

Electro-optic modulators, particularly in the lithium niobate version discussed in this article, are based on the Pockels effect and use nonlinear optical crystals. Among these crystals, lithium niobate, or ${\mathit{LiNbO}}_3$ , is particularly remarkable due to its electro-optical properties, which make it highly efficient compared to other materials.  The first bulk electro-optic modulators for lasers operate in free space and therefore require control voltages of several hundred volts to function. This is because the inter-electrode spacing is determined by the thickness of the crystal. These are referred to as bulk modulators.

Major changes will be brought about by the use of optical waveguides.  in order to confine light beneath the surface of the crystal. This technology, coupled with planar electrode transfer processes derived from microelectronics, enables transverse interactions between light and electric fields on a scale of a few microns rather than a few millimeters, as is the case with bulk modulators. This reduces control voltages by a factor of more than 100. This...