Optoelectronics-microwave - Optical microwave signal generation

In the field of optical/microwave interactions, the strong duality between technologies developed for the expanding sector of very high-speed telecommunications and the processing of analog microwave signals of interest to the military (radar, electronic warfare and communications) is becoming increasingly apparent:

• duality in terms of active and, above all, passive optical components (couplers, isolators, etc.);

• duality of the problems of noise and linearity of the signals to be processed ;

• duality of certain functions (frequency multiplexing with high crosstalk – or crosstalk in optics –, low-noise optical amplification, etc.).

Today, optical fibers are the established medium for very high-speed digital terrestrial links, thanks to their very high bandwidth (in the THz range), low linear loss, small size, low weight and high immunity to electromagnetic radiation.

The possibility of multiplexing several dozen channels of different colors on a single optical fiber, together with the availability of erbium-doped fiber amplifiers, now makes it possible to install transoceanic links with a capacity of several tens of Gbit/s without a repeater. The digital domain is largely developed for, and by, civil telecommunications activities.

These properties of optical links can be used to advantage in microwave systems for the transmission of analog and digital signals.

It remains to be seen whether the performance of technologies developed for telecommunications meets the performance requirements of military systems, but the lower costs associated with the civilian market provide a good opportunity to integrate these technologies into military equipment.

However, there are fundamental differences between microwave photonics and digital optical telecommunications, linked to the nature of the signals to be transmitted and their field of application. In microwave photonics, the analog nature of the signals requires transfer functions that are as linear as possible, whereas in digital telecommunication, since the signal is binary-coded, transmission is much less sensitive to transfer function non-linearities. This analysis will have consequences for the definition of optoelectronic components.

Finally, given the availability of optoelectronic components operating at very high frequencies (> 20 GHz), it is now possible to envisage the direct generation of microwave signals by beating two optical waves. By using photodetectors with a bandwidth in excess of 100 GHz, this type of generation overcomes the limitations of current integrated optical intensity...