Optical: systems and elements – Optical modulator – Light wave temporal modulation
Patent
1990-02-08
1993-11-30
Hille, Rolf
Optical: systems and elements
Optical modulator
Light wave temporal modulation
307400, 385123, G02F 101
Patent
active
052670767
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a process for obtaining a structure generating non-linear electrooptical effects, i.e. a structure possessing a susceptibility somewhat greater than 1. It also relates to structures obtained from this process and in particular those enabling the generation of a second harmonic of an optical wave incident on such a structure. The invention further relates to possible applications for such structures that can be implemented in the form of planar layers, films or for producing optical guides.
DISCUSSION OF THE BACKGROUND
Since the development of intense light sources provided by lasers, electrooptical effects have been put into application in many fields such as optical telecommunications and optical signal processing. The electrooptical effects are induced in dielectric materials (i.e. bodies that are transparent to an electromagnetic wave) by the application of electric fields. These electric fields can be associated to light beams incident on the dielectric material. The refractive index of the dielectric material then becomes a function of the intensity of the incident light beam(s), so opening the field of non-linear optics (generation of harmonics, frequency transposition, optical memories, etc. . . ).
When an electric field E is applied to a dielectric, the electrically charged particles forming the matter of the dielectric (electrons, ionised atoms) find themselves displaced from their equilibrium position by a distance which is a function of: the intensity of the electric field E, the electrostatic binding forces to which these particles are submitted, and the frequency of the electric field applied to the arrangement of molecules of the dielectric. There results a polarisation of the matter denoted by the vector P and which is by definition equal to the product of the density of the displaced charges by the vector representative of that displacement. Between the electric field vector E and the polarisation vector associated to the matter P, there exists a matrix relation in the form: isotropic, .parallel..chi..parallel. reduces to a scalar. In the general case of anisotropic media, .parallel..chi..parallel. is a tensor.
However, if there is applied an intense electromagnetic field to the dielectric, at an optical frequency, the induced polarisation is no longer proportional to the applied optical field, but contains non-linear terms proportional to the square and cube of the applied fields. The range of effects produced for these non-linear polarisations has been studied in depth since the discovery by FRANKEN of the generation of second harmonics by a quartz crystal submitted to radiation from a ruby laser (P. A. FRANKEN, A. E. HILL, C. W. PETERS, G. WEIMREICH, Phys. Rev. Lett., Vol. 7, 1961, p. 118).
The response of a medium to the application of an electromagnetic field can be described developing the induced polarisation in a complete series of the applied electric field: .parallel..multidot..vertline.E.vertline..multidot.E+ . . .
The first term of this development represents the linear polarisation, whereas the n.sup.th term reflects the non-linear response of the n.sup.th order of the applied field. The coefficients .chi..sup.(n) are tensors of order (n+1) referred to as n.sup.th order non-linear susceptibility tensors. The different terms contained in P decrease very rapidly with the order n, and it was only with the appearance of intense luminous sources such as laser beams that non-linear electrooptical effects corresponding to second order susceptibility could be exploited.
For certain dielectrics and under certain conditions, a polarisation induced by an incident light beam and corresponding to a quantity of stored energy, gives rise to a radiation of a wave oscillating at a frequency double that or the incident luminous beam, which corresponds to the restitution of a part of the stored energy.
One of the conditions necessary for a dielectric to generate, by non-linear effect, a wave representing the second harmonic of an in
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Broussoux Dominique
Dubois Jean-Claude
Le Barny Pierre
Robin Philippe
"Thomson-CSF"
Hille Rolf
Ho Tan
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