On The Behaviour Of Microwave Signals In Optical Networks

In the last years, significant progress have been accomplished in the domain of optical communications such that the microwave transmission by optical carriers is now an existing system currently used. The technological advances in optical-to-electrical and electrical-to-optical conversion allows today to consider not only the simple optical transmission of microwave signals but also the processing of these signals using optical networks. For this purpose the propagation of the optical signal in a network with dimensions comparable to the wavelength of the modulating microwave signal should be studied. This requires to overcome some problems like the polarisation dependence, the dimensional effects on the modulating signals, the multimode properties of some optical components, etc., which makes the optical network fed by a microwave modulated laser beam a subject of great interest. The object of this work is to study the behaviour of the microwave modulating signal in two optical elements : the Fabry-Perot interferometer and the unbalanced Mach-Zehnder one. The two interferometers are constructed using single mode optical fibers and 3 dB directional couplers as shown in Fig. 1 a and b. The system is excited by an optical signal of wavelength λ=1.3 μm modulated by an electrical (microwave) signal of frequency between 0.3 MHz and 3 GHz. A suitable photodetector is used to extract the electrical output signal. The overall system (source, network and detector) may be considered as a microwave "device" for which we can define a microwave transfer function or, in general, microwave scattering parameters. Using the HP8702 lightwave component analyser with the HP83400A light source and the HF'83410A lightwave receiver, we have measured the characteristics of this system.