Electro-optical components for telecommunications:

Since the 1980s, with the emergence of innovative communications media (Internet, mobile telephony), combined with new information transmission media (near-infrared photons transported by optical fibers, microwaves), the development of new high-speed optoelectronic component chains has become a major challenge for the industries and services associated with these new technologies. To achieve data rates of several tens of Gbits/s, each type of component (sources, detectors, modulators, switches, multi/demultiplexers, amplifiers) has had to be optimized at every stage of its development. The first step is to select one or more materials offering an acceptable compromise in terms of manufacturing efficiency/cost, then to develop manufacturing techniques compatible with mass production and economically relevant, right through to the implementation of architectures compatible with the insertion of the component in an optical communications circuit.

For each of the main functions involved in an optical telecommunications network, specific materials have been selected, and an industrial sector has developed around them. For example, III-V semiconductors make up the bulk of laser sources and detectors at 1.55 μm; optical amplifiers use silica fibers doped with erbium ions, or semiconductor materials; optical modulators and switches can be made from semiconductors (involving the electroabsorption phenomenon) or non-linear materials using the electro-optical effect, such as lithium niobate or polymers.

While the current performance of each of the above-mentioned components is sufficiently proven to enable considerable development of optical and microwave communications, certain aspects still need to be improved: size (particularly for fiber amplifiers), manufacturing cost (for lithium niobate modulators, for example) and, more generally, the integration of all these elements into compact, multifunctional circuits.

The aim of this article is not to review all the technological solutions available for electro-optical modulation. We will confine ourselves to lithium niobate, which is currently used in industrial production for electro-optical modulation, and to polymer materials. Polymeric materials are proving most promising, thanks to their compatibility with very high modulation rates, combined with a high figure of merit, and the possibility of low-cost mass production, via optoelectronic circuit molding and printing techniques facilitated by the thermoplastic properties of these materials.

The first part will present the physical phenomena behind the modulation function, in particular electro-optics, and some examples of associated generic components. It will also outline the "specifications" required for the development...