Microwave optoelectronics - Modulation, links and switching

The first of the optical functions required for microwave signal processing involves the optical transmission of electrical information (signal), and requires dual frequency conversion: electrical/optical on transmission, and optical/electrical on reception, to restore the signal. The technological building blocks for these functions and optical architectures were presented in the article Microwave Optoelectronics - Components.

The electrical/optical conversion operation is called optical modulation.

As in the case of radio waves or microwave systems, signal modulation approaches are based on two main types of modulation: amplitude modulation and frequency modulation.

Given the maturity of the optoelectronic components available today, amplitude modulation is the predominant technology used.

Two approaches are used:

• external modulation. In this case, the optical intensity emitted at the laser output is modulated by an external electro-optical modulator;

• direct modulation, where, more simply, the current flowing through the laser is modulated, resulting in modulation of the light in optical intensity and frequency.

Analog optical links have the specificity of achieving very high performance in terms of phase noise, spectral purity and linearity. These parameters are taken into account in the specifications of the conversion components, and extend to the passive devices used in the composition of the said link, such as the optical isolator required to avoid triple-path phenomena, specific connectors, etc.

The second function highlighted in optical microwave signal processing is the realization of optical switching functions.

The objectives of these functions are summarized in the specifications of the critical parameters of the following switches and/or switch matrices:

• strong crosstalk ;

• low consumption ;

• low insertion losses ;

• equidistance of optical paths ;

• non-blocking matrix ;

• insensitivity to optical polarization.

A number of approaches are described in this article, aimed at defining optical switches and optical switch matrix architectures, on the one hand, and taking into account the above specifications in switch design, on the other.