Optical: systems and elements – Holographic system or element – Having particular recording medium
Patent
1993-02-18
1996-07-23
Epps, Georgia Y.
Optical: systems and elements
Holographic system or element
Having particular recording medium
359 3, 359 7, 359248, G03H 102
Patent
active
055395422
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a photorefractive apparatus, i.e. an apparatus where a photorefractive effect is liable to occur.
Explanations of the photorefractive effect are provided in:
(1) Photorefractive effect, David Pepper, Jack Feinberg, Nicolai Kukhtarev, For the Science, No. 158, December 1990, pages 58 to 64.
Reference should also be made to:
(2) Topics in Applied Physics, Springer Verlag, Vol. 61, Photorefractive materials and their applications, Volume 1, Chapter 8, The Photorefractive Effect in Semiconductors, Alastair M. Glass and Jefferson Strait.
The apparatus according to the invention can be used as a rewritable holographic recording apparatus.
As a function of the components of the apparatus according to the invention, said rewritable holographic recording apparatus can be sensitive in the visible range or in the near infrared range.
The present invention has, inter alia, as applications all those of a rewritable holographic recording apparatus and in particular the deflection of light beams, the formation of reconfigurable optical interconnections, the production of phase conjugation mirrors and the provision of signal optical treatment or processing devices, which can be of an analog or digital nature.
In the case of analog signals, the present invention e.g. applies to the production of optical correlation devices and in the case of digital signals to the production of arrays of optical logic gates.
For some years now, photorefractive materials have constituted the most widely investigated class of rewritable holographic recording apparatuses for all dynamic holography applications.
The principle of the photorefractive effect is as follows. The charge carriers are photoexcited under the effect of a non-homogeneous illumination such as an interference pattern and are non-homogeneously redistributed, thus creating a space charge field. The latter induces by the electrooptical effect a variation of the refractive index, which is the "image", apart from a possible spatial displacement, of the initial interference pattern. The two fundamental elements of the photorefractive effect are consequently photoconductivity and the electrooptical effect.
Most frequently, the electrooptical effect is the Pockels effect and it is then necessary to use photorefractive materials in the form of solid monocrystals, which can be in the form of a parallelepiped, whereof each of the edges is a few millimeters.
Compared with several other non-linear optical effects, the photorefractive effect has an original characteristic. Thus, it is sensitive to light energy per surface unit and not to the light intensity of the write radiation.
This is due to the integration effect inherent in the formation of the space charge field, the number of displaced charges per time unit being proportional to the photon flux.
Thus, the variation of the refractive index dN increases, in a time called the response time and which is designated "tau", which is inversely proportional to the incident light intensity until it reaches a limit value dNmax.
Known photorefractive materials can be classified in two categories from the performance standpoint: LiNbO.sub.3, BaTiO.sub.3, KNbO.sub.3, (BaSr)Nb.sub.2 O.sub.6 (SBN), (Bi.sub.12 SiO.sub.20, Bi.sub.12 GeO.sub.20, Bi.sub.12 TiO.sub.20) and semiconductors (GaAs, InP, CdTe, GAP).
When account is taken of the refractive index variation per light intensity unit dN/I, which is a function of the figure of merit N.sup.3 .multidot.r/eps (in which I,N,r and eps respectively represent the intensity of the write radiation, the refractive index of the material, the effective linear electrooptical coefficient of the material and the dielectric constant of said material), it can be seen that all known solid photorefractive materials have identical characteristics to within a factor of 10.
The interest of the materials GaAs, InP and CdTe is of having a significant sensitivity to the wavelengths used in the field of optical telecommunications.
However, the known photoref
REFERENCES:
patent: 4037932 (1977-07-01), Haas et al.
patent: 4525687 (1985-06-01), Chemla et al.
patent: 5004325 (1991-04-01), Glass et al.
patent: 5222071 (1993-06-01), Pezeshki et al.
Journal of the Optical Society of America B., vol. 7, No. 11, Nov. 1990, New York, pp. 2217-2225; D. D. Nolte et al.: "Resonant photodiffractive effect in semi-insulating multiple quantum wells".
Gravey Philippe
Picoli Gilbert
Viallet Jean-Emmanuel
Epps Georgia Y.
France Telecom
Papalas Michael A.
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