Optical: systems and elements – Optical modulator – Light wave temporal modulation
Patent
1996-08-12
1998-07-14
Rosenberger, Richard A.
Optical: systems and elements
Optical modulator
Light wave temporal modulation
G02F 101
Patent
active
057813280
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an electro-optical modulator. Such devices are used for modulating light beams, that is to say for modifying their polarization, their phase, their frequency or their intensity, by modifying the medium through which they propagate, by the action of an electric field.
By way of application of such modulators, mention may be made of: of different wavelength from an incident beam of given wavelength, by frequency modulation,
Electro-optical modulators are made either from bulk materials, in general single crystals constituting the electro-optical medium through which the light beam to be modulated passes, or from thin films or waveguides. The latter make it possible to obtain modulation in a wide frequency range (up to several gigahertz), which is very useful in telecommuniations in order to increase the data transfer rate. Such devices are of complex design and expensive.
Although limited to uses at lower frequencies (from several hertz to several hundreds of megahertz), modulators made from bulk crystals are widely used, in particular for intensity or phase modulation functions.
2. Description of the Related Art
Almost all crystal modulators currently marketed use as an electro-optical material a crystal of KDP (potassium dihydrogen phosphate) or ADP (ammonium dihydrogen phosphate). The document U.S. Pat. No. 5,157,539 describes an electro-optical modulator using a crystal of KDP. The electro-optical qualities of these materials are fully known. They have the advantage of being materials whose synthesis is fully mastered and inexpensive.
Some publications:
T. Tsukamoto et al., Japanese Journal of Applied Physics, Vol. 24 (1985), Supplement 24-3, pages 165-168,
R. Poprawski et al., Ferroelectrics, Vol. 79 (1988), pages 245-248,
A. Waskowska and Z. Cafzpla, Acta Cryst., Vol B38 (1982), pages 2017-2020, SeO.sub.4, and also its cyrstals in partly denterated, which properties do not disclose if it is possible to use it in an electro-optical modulator.
Among the materials which are used, mention may also be made of inorganic ferroelectrics (LiNbO.sub.3, KNbO.sub.3, BaTiO.sub.3), which generally have high electro-optical coefficients and refractive indices. However, their dielectric permittivity is high and their production is laborious and poorly mastered.
The direction of the electric field applied to the electro-optical material may be orthogonal to (this case is referred to as transverse configuration) or else collinear with (this case is referred to as longitudinal configuration) the direction of propagation of the light beam.
The case of a crystal in the form of a parallelepiped will be considered, with L and d the dimensions of the crystal, respectively in the direction of propagation of the beam to be modulated and in the direction of the electric field. L is equal to d in the case of a longitudinal configuration.
A dominant factor of a modulator is the half-wave voltage, defined by V.sub..pi. =(a*d)/(n.sup.3 *r*L), with
If L=d, then V.sub..pi. =V.sub..pi.* =a/(n.sup.3 *r). This is referred to as the reduced half-wave voltage, which is a factor depending only on the nature of the crystral.
The high-wave voltage V.sub..pi. is the voltage to be applied to the crystal in order to cause a phase shift of .pi. radians between the components of the polarization of a light beam passing through the modulator, that is to say the change from a maximum to a minimum in the intensity of the light transmitted through a suitably oriented polarizer. The modulation efficiency of a light beam depends greatly on V.sub..pi.. It is important for this voltage V.sub..pi. to be as small as possible.
The abovementioned hydrogen-bonded compounds (ADP, KDP) have electro-optical coefficients with non-negligible values. However, they have some drawbacks and actually require high control voltages.
Thus, for a wavelength a equal to 0.633 microns:
In order to limit the value of the electric voltage applied, the modulators comprise crystals of large dimension L
REFERENCES:
patent: 5157539 (1992-10-01), Beasley
T. Tsukamoto et al., Deflection of Light Induced by Ferroelectric-ferroelastic Crystals,Japanese Journal of Applied Physics, Supplement 24-3, vol. 24 (1985), pp. 165-168.
R. Popranski et al., Specific Heat of Hydrogen Selenate Crystals, Ferroelectrics, vol. 79, 1988, pp. 245-248.
A. Waskowska et al., Ammonium Deuterium Selenate and Rubidium Deuterium Selenate, ACTA Crystallographica, vol. B38, 1982, pp. 2017-2020.
J. Salvestrini et al., New Material With Strong Electro-Optic Effect: Rubidium Hydrogen Selenate (RbHSeO.sub.4), Applied Physics Letters, vol. 64, No. 15, Apr. 1994, pp. 1920-1922.
Fontana Marc
Salvestrini Jean-Paul
Rosenberger Richard A.
Universite de Metz
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