1. Field of the Invention
The invention relates to a device for creating optical delays and its application to an optical control system for a scanning antenna. More particularly this application relates to a system for generating physical delays for the optical control of scanning antennas. It is a matter of simplifying the structure of the system generating the optical delays and of limiting the number of active elements allowing these delays to be selected.
2. Discussion of the Background
Systems are known which allow the control, by optical means, of the radiation pattern of an antenna. They allow control of the phase of the microwave frequency signal transmitted by each antenna element and whose frequency is included in the frequency band 1-12 GHz. The proposed device in particular provides the angular scanning function of the beam on the basis of the control of the phase of each element of the antenna.
The present scanning techniques are electronic and act on the respective phases of the controls of the various radiating elements. The phase-shifting elements are ferrite or PIN diode devices. These structures, apart from their weight and consumption, have the disadvantage of only allowing a narrow band functioning of the antenna. In fact, the phase shifters only act between 0 and 2.pi. and therefore, during a change in frequency in the transmitted signal, there is off-aiming of the antenna.
It is known, particularly from the French Patent Application N.degree. 87 05267 filed on the 14th Apr. 1987, to produce microwave frequency delay lines with programmable lengths and of small size by optical means. This consists in generating a microwave frequency signal by the beating of two collinear light waves, in making these two light waves travel a path whose length is adjustable by controlled reflectivity electro-optical modulators and in detecting the microwave frequency beat of the two collinear light waves at the end of the path by means of a photodetector.
According to the prior art, it is therefore a matter of encoding the microwave frequency signal, necessary for the control of the active elements of the antenna, on an optical carrier and to physically produce the delays which will allow the phase shifting of this microwave frequency in order to perform the electronic scanning appropriate to this type of radar. In order to produce P phase shifts, it is necessary to generate P optical paths having different lengths and to choose these delays either a priori or a posteriori.
The main problem encountered with the solutions proposed previously is the necessity of disposing in the system a number of optical switches (or light modulators) which is equal to the number of delays desired that is, each optical switch. In the case in which the delays are chosen a priori, this results in a high complexity of the system, in a high cost and a possible large attenuation of the optical wave (due to the different modulators). The same problems are also encountered when the choice of the delays is made a posteriori and is even more critical with regard to the attenuation of the optical wave.
The purpose of the invention is to provide an encoding system limiting the complexity of the system (but having an equivalent number of available delays as provided by prior systems,) while allowing a reduction in the attenuation of the optical wave and achieving these purposes at a lower cost.