Modulators and optical components integrated in lithium niobate - Applications

In [E 1 997] , we described in detail the operating principle of integrated electro-optic modulators using guided optics technologies in lithium niobate crystals. We provided the tools for designing single-mode optical waveguides and determined the conditions for light propagation in such a waveguide obtained by metal diffusion. Guided and confined propagation, coupled with coplanar electrodes on the surface of the material, enabled us to determine the applicable control voltages and, in particular, the modulation bandwidths that can be achieved. We have described in detail the manufacturing steps required to produce a high-performance, tested, and reliable industrial component. This description is based on a simple component, the phase modulator, which is the building block of a whole family of optical modulators with features that meet the specific needs of the targeted applications.

We will now turn to a second section, which describes the components currently available on the market that have been developed using these technologies. This description is not exhaustive, but is intended to be representative of the diversity of functions that can currently be achieved thanks to the high flexibility of lithium niobate and the richness of its electro-optic tensor and optical circuits.

In the first section, we will provide a detailed description of the Mach-Zehnder interferometer intensity modulator, which has played a major role in fiber transmission systems. Different versions will be proposed. We will describe its characterization and application for digital and analog modulation. We will focus in particular on an important parameter, the "chirp," or intrinsic phase jitter of the modulated signal, which plays an essential role in the behavior of signals transmitted in dispersive media such as silica optical fibers.

In a second section, we will study the case of the I&Q (In Phase & Quadrature) modulator, a more complex modulator that encloses two sub-Mach-Zehnder devices in a main Mach-Zehnder device, to which the signals are applied. The specific adjustment of the phases between the four arms of the circuit enables operation for specific encodings, in particular QPSK (Quadrature Phase Shift Keying) modulation, which is now widely used in ultra-high-speed optical networks (100-200-400 Gbit/s/channel). We will describe one of its original modes of operation with CS-SSB (Carrier Suppressed Single Side Band)...