1. Field of the Invention
The invention concerns a frequency shifter for a medium infrared range wave, i.e. one with a wavelength of more than three microns. A device of this type has many applications in systems for data transmission by frequency modulation, systems for coherent detection by hetrodyning etc. Standard shifters are either of the acousto-optical type, using an interaction between an optical wave and a diffraction grating in motion in the propagation medium of the wave, or of the type with rotating birefringent plates, the rotation being mechanical or electro-optical.
2. Description of the Prior Art
The most commonly used devices in the medium infrared range are of the acousto-optical type. A device of this type has a crystal which is mechanically coupled with an acoustic transducer. The vibrations produced by the transducer are propagated in the crystal, creating local variations in its refractive index. The propagation of these local variations constitutes a diffraction grating which shifts at a constant speed. The wave which has to have its frequency shifted is projected to the surface of the crystal where it is diffracted in undergoing a frequency shift by Doppler effect. A drawback of this type of shifter is that the diffracted wave has an angle of emergence which is a function of the value of the frequency shift. However, to cope with this drawback, it is possible to use prior art shifters, associating them, for example, in pairs.
The shifted wave is then no longer deflected but this does not eliminate a shift by the beam parallel to itself. This fact raises a great many problems in systems since this shift depends on the frequency variation undergone by the beam. Furthermore, the association of these shifters increases the complexity of systems.
The aim of the invention is to propose a frequency shifter, operating in the medium infrared range, which does not have these drawbacks and can be easily integrated into an optical device integrated in a semiconductor substrate.