EP1356344A1 - Device for controlling polarisation in an optical connection - Google Patents

Device for controlling polarisation in an optical connection

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Publication number
EP1356344A1
EP1356344A1 EP01989643A EP01989643A EP1356344A1 EP 1356344 A1 EP1356344 A1 EP 1356344A1 EP 01989643 A EP01989643 A EP 01989643A EP 01989643 A EP01989643 A EP 01989643A EP 1356344 A1 EP1356344 A1 EP 1356344A1
Authority
EP
European Patent Office
Prior art keywords
optical
electrodes
polarization
electro
electric field
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP01989643A
Other languages
German (de)
French (fr)
Inventor
Jean-Pierre Huignard
Daniel Dolfi
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Thales SA
Original Assignee
Thales SA
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Thales SA filed Critical Thales SA
Publication of EP1356344A1 publication Critical patent/EP1356344A1/en
Withdrawn legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/03—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on ceramics or electro-optical crystals, e.g. exhibiting Pockels effect or Kerr effect
    • G02F1/055—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on ceramics or electro-optical crystals, e.g. exhibiting Pockels effect or Kerr effect the active material being a ceramic
    • G02F1/0555—Operation of the cell; Circuit arrangements
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/0136—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  for the control of polarisation, e.g. state of polarisation [SOP] control, polarisation scrambling, TE-TM mode conversion or separation
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1343—Electrodes
    • G02F1/134309—Electrodes characterised by their geometrical arrangement

Definitions

  • the present invention relates to a device for controlling polarization in an optical link.
  • Polarization control in optical links is a priority objective for future very high-speed fiber networks implementing wavelength-division multiplexing techniques.
  • a certain number of active components of the optical fiber network are sensitive to the state of polarization of the wave, in particular the fiber or semiconductor amplifiers, the switches or external modulators LiNbO 3 .
  • the latter generally operate in the best conditions with a linear incident polarization of the wave and, moreover, this direction of polarization must be kept parallel to one of the electrooptic axes of the modulator.
  • another very important application relates to compensation for polarization dispersion.
  • Electro-optical polarization control devices are known, but these devices do not make it possible to carry out a total control of the polarization, that is to say a control of the rotation of the axes of polarization and of the birefringence for each axis direction.
  • known optomechanical devices do not allow total control and their reaction time is too high.
  • the subject of the present invention is a device for controlling polarization in an optical link, a device which makes it possible to control both the rotation of the polarization axes and the birefringence for each axis direction, which has a very short response time. (for example of the order of 1 to a few microseconds approximately), which is compact and causes negligible insertion losses.
  • the device according to the invention comprises in the optical link whose polarization it is desired to control at least one block of electro-optical material with variable birefringence under the effect of an electric field, electrodes being arranged on at least one face of this block and being connected to a circuit making it possible to vary the electric voltages applied to these electrodes as a function of the desired rotation of the axes of polarization.
  • Figures 1 and 1A are respectively a simplified perspective view of a control device according to the invention, and a diagram showing the characteristics of an optical wave polarization in the device of Figure 1;
  • Figure 2 is a plan view of a first embodiment of an electro-optical unit that can be used in the device of Figure 1;
  • Figures 3 and 4 are respectively a schematic front view and a schematic sectional view of the block of Figure 2, the latter showing, in a simplified manner, the path of the electric field lines inside the block;
  • Figure 5 is a set of three diagrams showing, in a simplified manner, the evolution of the electric field lines in the block of Figure 2, as a function of various voltages applied to its electrodes;
  • Figure 8 is a diagram showing the cascade mounting of three electro-optical blocks, according to the invention.
  • Figures 9 and 10 are respectively a sectional view and a plan view of an alternative embodiment of the electro-optical unit according to the invention, with electrodes deposited on two opposite faces of the unit;
  • Figure 11 is a plan view of another alternative embodiment of the electro-optical unit according to the invention, with six electrodes; and • Figures 12 to 14 are schematic sectional views of another variant of an electro-optical unit according to the invention, of “PDLC” type material.
  • FIG. 1 Schematically shown in Figure 1 the essential elements of the device 1 of the invention.
  • This device 1 is inserted in the path of an optical beam conveyed, in this case, by optical fibers: an optical fiber 2 through which the optical beam arrives, the polarization 2A of which is to be treated, and an optical fiber 3 through which distributes the optical beam treated 3A by the device 1.
  • This device 1 essentially comprises an electro block -optics 4 and electronic circuits 5 for addressing the electrodes of block 4.
  • Block 4 is a block, for example in the form of a rectangular parallelepiped of birefringent material which can compensate at any time, under the effect of an electric field, the drifts in the polarization state of the optical beam coming from the optical fiber 2.
  • the changes in the polarization state of the optical beam can be very fast (variations in a few microseconds or milliseconds) and are due to variations in many parameters, in particular the temperature, the mechanical constraints imposed on the optical fibers, in the reconfiguration of the network, etc.
  • the device 1, with an electro-optical unit as described below makes it possible to obtain a very short response time (of the order of 1 to a few microseconds) with respect to variations in polarization of the optical beam 2A.
  • the device 1 transforms any form of polarization 2A into another form of polarization 2B.
  • a form of elliptical polarization is characterized by two angles: ⁇ and ⁇ .
  • the angle ⁇ is that determined by the axes Ox and OA (diagonal of the rectangle circumscribed at the ellipse).
  • the device 1 independently controls the direction of the axis of the ellipse and its ellipticity, whatever the incident polarization 2A.
  • F. Heismann "Analysis of a reset-free polarization controller for fast automatic polarization stabilization in fiber optic transmission Systems", Journal of Lightwave Technique, 12, 690, 1994, as well as F.
  • the polarization control had to be done by association of birefringent plates whose rotation of the respective axes is controlled, which implies prohibitive response times.
  • the present invention uses an electro-optical unit 4, on the electrodes of which it suffices to apply appropriate electrical voltages to control the direction of the axis and the birefringence of its material.
  • This device 1 uses the free propagation of the optical beam.
  • the electro-optical material constituting the block 4 is preferably a material whose coefficient of KERR has a high value (for example of the order of 10.10 m V " ).
  • This material is for example a PLZT ceramic (Pb - La - Zn - TiO 2 ).
  • Pb - La - Zn - TiO 2 PLZT ceramic
  • a block 6 such as that shown in FIG. 2 is produced, for example.
  • This block is in the form of a thin rectangular parallelepiped, the large faces of which are squares .
  • four electrodes are printed or fixed, for example, identical 7 to 10. These electrodes have a “T” shape, and their “horizontal” branches delimit a square in the center of the large face.
  • These electrodes 7 to 10 are respectively connected to electrical potentials V1 to V4.
  • the phase blade function with rotating axis is obtained by application to the electrodes 7 to 10 of a rotating electric field (see for example: P.
  • FIG. 3 shows the trace 11 of the optical beam coming from the fiber 2, and in FIG. 4 the electric field lines produced by two opposite electrodes, for example 7 and 8. In FIG. 5, three examples of electric field lines created for three different combinations of potentials applied to electrodes 7 to 10.
  • these field lines are, at the center of the square delimited by the electrodes 7 to 10, substantially vertical (as seen in the drawing), substantially parallel to a diagonal of the square and substantially horizontal.
  • This field thus performs the function of a phase plate whose rotation ⁇ of the axis Ox follows the rotation of the field.
  • Block 6 can be called “modulator” of the polarization of the incident beam coming from fiber 2.
  • the values of optical indices n x and n y along the axes Ox and Oy (see FIGS. 1A and 2) of the optical beam in the plane of incidence on block 6 are:
  • - ⁇ is the angle of the axis Ox of the ellipse of the beam to be checked (or corrected) with the axis of block 6 passing through the centers of
  • - d is the distance between the electrodes 7 and 8 or 9 and 10 (assumed to be arranged symmetrically with respect to the center 0, on which the incident optical beam is centered);
  • - n 0 is the optical index of block 6 along Oy.
  • phase modulator function of block 6 which is exercised inside the square delimited by the electrodes 7 to 10 and more particularly in the vicinity of its center, is equivalent to a phase plate with axis rotating by an angle ⁇ and with variable ⁇ birefringence.
  • the axis of the phase plate thus obtained rotates at an angular speed ⁇ determined by the following relationships:
  • block 6 can be produced in several different ways.
  • a first embodiment consists in using a thin PLZT ceramic disc having a composition suitable for electro-optical applications.
  • This ceramic has a high KERR coefficient with negligible hysteresis.
  • two pairs of electrodes (7-8 and 9-10) are deposited from this ceramic (that of the input of the beam to be corrected), for example by metallic deposition under vacuum. These electrodes can be made of Au or Al for example.
  • the typical response time of electro-optical ceramic devices is around 1 ⁇ s.
  • the block is obtained from a polished PLZT substrate whose thickness is approximately 0.5 to 1 mm.
  • methods for depositing layers of PLZT by “sol-gel” techniques or by liquid epitaxy have been developed to produce components of large dimensions (for example greater than 5 cm 2 ).
  • two electro-optical functions can be realized, for example ⁇ / 2 and ⁇ / 4 blades with axes rotating on each face of the electro-optical ceramic substrate 13.
  • electrodes are printed which have, for example, the same configuration as that shown in FIG. 2. These electrodes are referenced 14 as a whole on one face of the substrate 13, and 15 as a whole on the other face.
  • a single mode optical fiber 16, ending with a focusing optic 17 sends an optical beam on the center of the face of the substrate 13 carrying the electrodes 14, while the beam coming from the the other face of the substrate is collected by the optics 18 (similar or identical to the optics 17) coupled to a single-mode optical fiber output 19.
  • the electrodes 14 and 15 are controlled by a circuit 20 so as to constitute, for example , on the side of the electrodes 14 a ⁇ / 4 blade with a rotating axis, and on the side of the electrodes 15 a ⁇ / 2 blade with a rotating axis.
  • a circuit 20 so as to constitute, for example , on the side of the electrodes 14 a ⁇ / 4 blade with a rotating axis, and on the side of the electrodes 15 a ⁇ / 2 blade with a rotating axis.
  • phase plates can thus be produced.
  • FIG 7 a compact variant of the device of Figure 6, this variant using components similar to those of Figure 6, and assigned the same reference numerals each followed by an "A".
  • the control circuit 20A like the circuit 20, controls the two modulators comprising the electrodes 14A and 15A respectively. to achieve endless operation of the two modulators (without stop for the rotating axis).
  • This device 21 receives an optical beam of an optical fiber 22 ending in a focusing optic 23 attached to a first modulator 24 which carries a set of electrodes 25.
  • the modulator 24 is followed by a second focusing optic 26, a second modulator 27 carrying electrodes 28, a third focusing optic 29, a third modulator 30 carrying electrodes 31 and a fourth focusing optic 32 coupled to an output optical fiber 33.
  • the modulators 24, 27 and 30 are for example of the type of the modulator of FIG. 2.
  • the electrodes 25, 28 and 31 are connected to a control circuit 34.
  • Each of the modulators 24, 27 and 30 acts as an electro-optical phase plate.
  • Each of these blades allows an electro-optical rotation of its axes and / or an electro-optical control of its birefringence for each axis direction, to constitute an electro-optical assembly with variable birefringence and orientation.
  • the control circuit 34 applies voltages to the various sets of electrodes 25, 28 and 31 making it possible, in a manner known per se, to perform the function of controlling the polarization of the incident beam.
  • the material constituting the electro-optical block of the invention can be not only PLZT, but any material having a high electro-optical coefficient (coefficient of KERR). It can be, for example, a ceramic such as PbSZT, BLTN, SBN, ... or else an electro-optical polymer layer, or a liquid crystal device (but it should be noted that liquid crystals have a response time too high, much greater than a few ⁇ s), or else PDLC (“Polymer Dispersed Liquid Crystal”), described below with reference to FIGS. 12 to 14.
  • FIGS. 9 and 10 show a variant of the modulator device of the invention, for which identical electrodes 35, 36 are arranged on the two faces of an electro-optical substrate 37.
  • This variant is used here not to combine two phase plates ( ⁇ / 4 and ⁇ / 2 by example), but to increase the efficiency of the modulator.
  • each configuration of electrodes 35, 36 is controlled by the same combination of voltages applied to these electrodes 35, 36 and has an "active thickness" (e1, e2 respectively in FIG. 9), that is to say say the thickness of electro-optical material, from the plane of the electrodes, for which the electric field created by these electrodes is effective with respect to controlling the polarization of the optical beam passing through the substrate 37.
  • FIG. 11 Another variant of the device of the invention is shown in FIG. 11.
  • the number of electrodes formed on one face of a substrate 38 is greater than four. In the embodiment of Figure 11, this number is six. These electrodes are referenced 39 to 44, and they are arranged regularly around the center of the face of the substrate 38, thus delimiting a hexagon. Thanks to this greater number of electrodes, one obtains, for a lower voltage applied to each electrode (lower than in the case of four electrodes), an electric field resulting, in the center of the hexagon, both more higher and more uniform.
  • the complexity of the electrode control device is greater than in the case of a four-electrode configuration. Of course, one could consider having an even larger number of electrodes on the face of an electrooptical substrate, but the complexity of the electrical control device would be even greater. A compromise must therefore be sought for each application between the complexity of the control device and the efficiency of the modulator.
  • the material constituting the electro-optical block of the PLZT type is replaced by a particular PDLC material, known as “nano droplets” (“nano droplets”).
  • This material comprises droplets 45 of liquid crystal included in a polymer matrix 46 (FIG. 12) by a rapid polymerization process, for example under UV illumination. It is then possible to obtain drops of liquid crystal whose size is much less than 1 ⁇ m. Although the medium thus obtained is inhomogeneous and the index of the liquid crystal and of the polymer are different, the medium is not however diffusing. Indeed, the drops 45 have in the present case a size much smaller than the wavelength of the optical beam passing through the medium. Everything therefore happens as if we were in the presence of an isotropic electro-optical ceramic. The electro-optical effect, in this case, results from the reorientation, under the effect of an electric field, of the liquid crystal molecules present in the drops.
  • FIG. 13 there is shown schematically some molecules of liquid crystal, which, in the absence of an electric field, are randomly oriented. When an electric voltage is applied to the electrodes 47, an electric field is created in the medium 46, and the molecules orient themselves parallel to the electric field lines 48.
  • the birefringence of this PDLC device is of the order of a few 10 ⁇ 3 for voltages applied to the electrodes 46 of the order of a few tens to a hundred volts, the inter-electrode space (d) having a dimension of l 'order of 100 ⁇ m
  • the response times obtained with this type of material are of the order of ten to a few tens of ⁇ s for material thicknesses of a few hundred ⁇ m.

Abstract

The invention concerns a device essentially comprising an electrooptic material plate (6), such as lead lanthanum zirconate titanate (PLZT) for example, whereon are printed electrodes (7 to 10). The electric field generated by applications of voltages (V1 to V4) on said electrodes produces a rotating-axis phase plate.

Description

DISPOSITIF DE CONTROLE DE POLARISATION DANS UNE LIAISON POLARIZATION CONTROL DEVICE IN A LINK
OPTIQUEOPTICAL
La présente invention se rapporte à un dispositif de contrôle de polarisation dans une liaison optique.The present invention relates to a device for controlling polarization in an optical link.
Le contrôle de la polarisation dans des liaisons optiques constitue un objectif prioritaire pour les futurs réseaux à fibres à très haut débit mettant en œuvre les techniques de multiplexage en longueur d'onde. En effet, un certain nombre de composants actifs du réseau à fibres optiques sont sensibles à l'état de polarisation de l'onde, en particulier les amplificateurs à fibres ou à semiconducteurs, les commutateurs ou modulateurs externes LiNbO3. Ces derniers fonctionnent généralement dans les meilleures conditions avec une polarisation incidente linéaire de l'onde et de plus, cette direction de polarisation doit être maintenue parallèle à l'un des axes électrooptiques du modulateur. Par ailleurs, une autre application très importante concerne la compensation de dispersion de polarisations.Polarization control in optical links is a priority objective for future very high-speed fiber networks implementing wavelength-division multiplexing techniques. Indeed, a certain number of active components of the optical fiber network are sensitive to the state of polarization of the wave, in particular the fiber or semiconductor amplifiers, the switches or external modulators LiNbO 3 . The latter generally operate in the best conditions with a linear incident polarization of the wave and, moreover, this direction of polarization must be kept parallel to one of the electrooptic axes of the modulator. Furthermore, another very important application relates to compensation for polarization dispersion.
On connaît des dispositifs électro-optiques de contrôle de polarisation, mais ces dispositifs ne permettent pas d'effectuer un contrôle total de la polarisation, c'est-à-dire un contrôle de la rotation des axes de polarisation et de la biréfringence pour chaque direction d'axe. De même, les dispositifs optomécaniques connus ne permettent pas de contrôle total et leur temps de réaction est trop élevé. La présente invention a pour objet un dispositif de contrôle de polarisation dans une liaison optique, dispositif qui permette de contrôler à la fois la rotation des axes de polarisation et de la biréfringence pour chaque direction d'axe, qui ait un temps de réponse très court (par exemple de l'ordre de 1 à quelques microsecondes environ), qui soit compact et provoque des pertes d'insertion négligeables.Electro-optical polarization control devices are known, but these devices do not make it possible to carry out a total control of the polarization, that is to say a control of the rotation of the axes of polarization and of the birefringence for each axis direction. Likewise, known optomechanical devices do not allow total control and their reaction time is too high. The subject of the present invention is a device for controlling polarization in an optical link, a device which makes it possible to control both the rotation of the polarization axes and the birefringence for each axis direction, which has a very short response time. (for example of the order of 1 to a few microseconds approximately), which is compact and causes negligible insertion losses.
Le dispositif conforme à l'invention comporte dans la liaison optique dont on désire contrôler la polarisation au moins un bloc de matériau électro-optique à biréfringence variable sous l'effet d'un champ électrique, des électrodes étant disposées sur au moins une face de ce bloc et étant reliées à un circuit permettant de faire varier les tensions électriques appliquées à ces électrodes en fonction de la rotation désirée des axes de polarisation. La présente invention sera mieux comprise à la lecture de la description détaillée de plusieurs modes de réalisation, pris à titre d'exemples non limitatifs et illustrés par le dessin annexé, sur lequel :The device according to the invention comprises in the optical link whose polarization it is desired to control at least one block of electro-optical material with variable birefringence under the effect of an electric field, electrodes being arranged on at least one face of this block and being connected to a circuit making it possible to vary the electric voltages applied to these electrodes as a function of the desired rotation of the axes of polarization. The present invention will be better understood on reading the detailed description of several embodiments, taken by way of nonlimiting examples and illustrated by the appended drawing, in which:
• les figures 1 et 1A sont respectivement une vue simplifiée, en perspective, d'un dispositif de contrôle conforme à l'invention, et un diagramme montrant les caractéristiques d'une polarisation d'onde optique dans le dispositif de la figure 1 ;• Figures 1 and 1A are respectively a simplified perspective view of a control device according to the invention, and a diagram showing the characteristics of an optical wave polarization in the device of Figure 1;
• la figure 2 est une vue en plan d'un premier mode de réalisation d'un bloc électro-optique pouvant être utilisé dans le dispositif de la figure 1 ;• Figure 2 is a plan view of a first embodiment of an electro-optical unit that can be used in the device of Figure 1;
• les figures 3 et 4 sont respectivement une vue schématique de face et une vue schématique en coupe du bloc de la figure 2, cette dernière montrant, de façon simplifiée, le parcours des lignes de champ électrique à l'intérieur du bloc ; « la figure 5 est un ensemble de trois diagrammes montrant, de façon simplifiée, l'évolution des lignes de champ électrique dans le bloc de la figure 2, en fonction de diverses tensions appliquées à ses électrodes ;• Figures 3 and 4 are respectively a schematic front view and a schematic sectional view of the block of Figure 2, the latter showing, in a simplified manner, the path of the electric field lines inside the block; "Figure 5 is a set of three diagrams showing, in a simplified manner, the evolution of the electric field lines in the block of Figure 2, as a function of various voltages applied to its electrodes;
• les figures 6 et 7 sont des schémas de variantes de réalisation du bloc de la figure 2 ;• Figures 6 and 7 are diagrams of alternative embodiments of the block of Figure 2;
• la figure 8 est un schéma montrant le montage en cascade de trois blocs électro-optiques, conformément à l'invention ;• Figure 8 is a diagram showing the cascade mounting of three electro-optical blocks, according to the invention;
• les figures 9 et 10 sont respectivement une vue en coupe et une vue en plan d'une variante de réalisation du bloc électro- optique conforme à l'invention, à électrodes déposées sur deux faces opposées du bloc ;• Figures 9 and 10 are respectively a sectional view and a plan view of an alternative embodiment of the electro-optical unit according to the invention, with electrodes deposited on two opposite faces of the unit;
• la figure 11 est une vue en plan d'une autre variante de réalisation du bloc électro-optique conforme à l'invention, à six électrodes ; et • les figures 12 à 14 sont des vues schématiques en coupe d'une autre variante d'un bloc électro-optique conforme à l'invention, à matériau de type « PDLC ».• Figure 11 is a plan view of another alternative embodiment of the electro-optical unit according to the invention, with six electrodes; and • Figures 12 to 14 are schematic sectional views of another variant of an electro-optical unit according to the invention, of “PDLC” type material.
L'invention est décrite ci-dessous en référence au contrôle de la polarisation d'une onde optique se propageant dans la partie optique (en particulier dans des fibres optiques) d'un réseau de télécommunications à très haut débit, mais il est bien entendu qu'elle n'est pas limitée à cette seule application, et qu'elle peut être mise en œuvre dans de nombreuses autres applications où l'on désire modifier la polarisation d'une onde optique ou asservir cette polarisation. On a schématiquement représenté en figure 1 les éléments essentiels du dispositif 1 de l'invention. Ce dispositif 1 est inséré sur le trajet d'un faisceau optique véhiculé, dans le cas présent, par des fibres optiques : une fibre optique 2 par laquelle arrive le faisceau optique dont on veut traiter la polarisation 2A, et une fibre optique 3 par laquelle repart le faisceau optique traité 3A par le dispositif 1. Pour la clarté du dessin, on n'a pas représenté les éléments optiques de couplage du faisceau optique entre les fibres 2, 3 et le dispositif 1. Ce dispositif 1 comporte essentiellement un bloc électro-optique 4 et des circuits électroniques 5 d'adressage des électrodes du bloc 4. Le bloc 4 est un bloc, par exemple en forme de parallélépipède rectangle en matériau biréfringent pouvant compenser à chaque instant, sous l'effet d'un champ électrique, les dérives de l'état de polarisation du faisceau optique provenant de la fibre optique 2. Dans l'application considérée (ici, un réseau de télécommunications), les évolutions de l'état de polarisation du faisceau optique peuvent être très rapides (variations en quelques microsecondes ou millisecondes) et sont dues à des variations de nombreux paramètres, en particulier la température, les contraintes mécaniques imposées aux fibres optiques, à la reconfiguration du réseau, etc.. Le dispositif 1 , avec un bloc électro-optique tel que décrit ci-dessous permet d'obtenir un temps de réponse très court (de l'ordre de 1 à quelques microsecondes) vis-à-vis des variations de polarisation du faisceau optique 2A.The invention is described below with reference to the control of the polarization of an optical wave propagating in the optical part (in particular in optical fibers) of a telecommunications network with very high speed, but it is understood that it is not limited to this single application, and that it can be implemented in many other applications where it is desired to modify the polarization of an optical wave or enslave this polarization. Schematically shown in Figure 1 the essential elements of the device 1 of the invention. This device 1 is inserted in the path of an optical beam conveyed, in this case, by optical fibers: an optical fiber 2 through which the optical beam arrives, the polarization 2A of which is to be treated, and an optical fiber 3 through which distributes the optical beam treated 3A by the device 1. For the sake of clarity, the optical elements for coupling the optical beam between the fibers 2, 3 and the device have not been shown. This device 1 essentially comprises an electro block -optics 4 and electronic circuits 5 for addressing the electrodes of block 4. Block 4 is a block, for example in the form of a rectangular parallelepiped of birefringent material which can compensate at any time, under the effect of an electric field, the drifts in the polarization state of the optical beam coming from the optical fiber 2. In the application considered (here, a telecommunications network), the changes in the polarization state of the optical beam can be very fast (variations in a few microseconds or milliseconds) and are due to variations in many parameters, in particular the temperature, the mechanical constraints imposed on the optical fibers, in the reconfiguration of the network, etc. The device 1, with an electro-optical unit as described below makes it possible to obtain a very short response time (of the order of 1 to a few microseconds) with respect to variations in polarization of the optical beam 2A.
Le dispositif 1 transforme toute forme de polarisation 2A en une autre forme de polarisation 2B. Comme représenté en figure 1A, une forme de polarisation elliptique est caractérisée par deux angles : α et β. L'angle α est celui déterminé par les axes Ox et OA (diagonale du rectangle circonscrit à l'ellipse). En d'autres termes, le dispositif 1 contrôle de façon indépendante la direction de l'axe de l'ellipse et son ellipticité, quelle que soit la polarisation incidente 2A. Selon l'art antérieur (voir par exemple : F. Heismann : « Analysis of a reset-free polarization controller for fast automatic polarization stabilization in fiber optic transmission Systems », Journal of Lightwave Technique, 12, 690, 1994, ainsi que F. Heismann, M.S.W. Whalen : « Fast automatic polarization control », IEEE Photonics Tech.Lett. 4, 503, 1992), le contrôle de polarisation devait se faire par association de lames biréfringentes dont on contrôle la rotation des axes respectifs, ce qui implique des temps de réponse prohibitifs. Par contre, la présente invention fait appel à un bloc électro-optique 4, sur les électrodes duquel il suffit d'appliquer des tensions électriques appropriées pour contrôler la direction de l'axe et la biréfringence de son matériau. Ce dispositif 1 utilise la libre propagation du faisceau optique.The device 1 transforms any form of polarization 2A into another form of polarization 2B. As shown in FIG. 1A, a form of elliptical polarization is characterized by two angles: α and β. The angle α is that determined by the axes Ox and OA (diagonal of the rectangle circumscribed at the ellipse). In other words, the device 1 independently controls the direction of the axis of the ellipse and its ellipticity, whatever the incident polarization 2A. According to the prior art (see for example: F. Heismann: "Analysis of a reset-free polarization controller for fast automatic polarization stabilization in fiber optic transmission Systems", Journal of Lightwave Technique, 12, 690, 1994, as well as F. Heismann, MSW Whalen: "Fast automatic polarization control", IEEE Photonics Tech.Lett. 4, 503, 1992), the polarization control had to be done by association of birefringent plates whose rotation of the respective axes is controlled, which implies prohibitive response times. On the other hand, the present invention uses an electro-optical unit 4, on the electrodes of which it suffices to apply appropriate electrical voltages to control the direction of the axis and the birefringence of its material. This device 1 uses the free propagation of the optical beam.
Le matériau électro-optique constituant le bloc 4, est de préférence, un matériau dont le coefficient de KERR a une valeur élevée (par exemple de l'ordre de 10.10 m V" ). Ce matériau est par exemple une céramique PLZT (Pb - La - Zn - TiO2). De façon générale, sous l'application d'un champ électrique transverse généré par deux électrodes déposées sur un matériau électro-optique, on obtient une lame de phase biréfringente dont les indices nx, ny suivant deux axes orthogonaux Ox, Oy valent respectivement :The electro-optical material constituting the block 4 is preferably a material whose coefficient of KERR has a high value (for example of the order of 10.10 m V " ). This material is for example a PLZT ceramic (Pb - La - Zn - TiO 2 ). In general, under the application of a transverse electric field generated by two electrodes deposited on an electro-optical material, one obtains a birefringent phase plate whose indices n x , n y following two orthogonal axes Ox, Oy are worth respectively:
nx =n0 + n* RE2 ny = n0 expressions dans lesquelles R est le coefficient de KERR du matériau considéré, E la tension électrique entre les électrodes et n0 l'indice du matériau en l'absence de champ électrique.n x = n 0 + n * RE 2 n y = n 0 expressions in which R is the KERR coefficient of the material considered, E the electric voltage between the electrodes and n 0 the index of the material in the absence of an electric field .
Pour obtenir une fonction de lame de phase à axe tournant, on réalise par exemple, un bloc 6 tel que celui représenté en figure 2. Ce bloc se présente sous forme d'un parallélépipède rectangle de faible épaisseur, dont les grandes faces sont des carrés. Sur l'une des grandes faces du bloc 6, on imprime ou fixe quatre électrodes, par exemple, identiques 7 à 10. Ces électrodes ont une forme de « T », et leurs branches « horizontales » délimitent un carré au centre de la grande face. Ces électrodes 7 à 10 sont respectivement reliées à des potentiels électriques V1 à V4. La fonction lame de phase à axe tournant est obtenue par application aux électrodes 7 à 10 d'un champ électrique tournant (voir par exemple : P. Joffre : Mémoire de thèse « Microstructures électro-optiques à cristaux liquides et applications », INPG, 1991 ). Ce champ tournant est produit par application, d'une part, au couple d'électrodes opposées 7 et 8 d'une différence de potentiels V1-V2 et d'autre part au couple d'électrodes opposées 9 et 10 d'une différence de potentiels V3-V4, ces potentiels étant variables. On a représenté en figure 3, la trace 11 du faisceau optique provenant de la fibre 2, et en figure 4 les lignes de champ électrique produit par deux électrodes opposées, par exemple 7 et 8. En figure 5, on a représenté trois exemples de lignes de champs électriques créés pour trois différentes combinaisons de potentiels appliqués aux électrodes 7 à 10.To obtain a phase blade function with a rotating axis, a block 6 such as that shown in FIG. 2 is produced, for example. This block is in the form of a thin rectangular parallelepiped, the large faces of which are squares . On one of the large faces of block 6, four electrodes are printed or fixed, for example, identical 7 to 10. These electrodes have a “T” shape, and their “horizontal” branches delimit a square in the center of the large face. These electrodes 7 to 10 are respectively connected to electrical potentials V1 to V4. The phase blade function with rotating axis is obtained by application to the electrodes 7 to 10 of a rotating electric field (see for example: P. Joffre: Dissertation thesis “Electro-optical microstructures with liquid crystals and applications”, INPG, 1991). This rotating field is produced by applying, on the one hand, to the pair of opposite electrodes 7 and 8 of a difference in potentials V1-V2 and on the other hand to the pair of opposite electrodes 9 and 10 of a difference of potentials V3-V4, these potentials being variable. FIG. 3 shows the trace 11 of the optical beam coming from the fiber 2, and in FIG. 4 the electric field lines produced by two opposite electrodes, for example 7 and 8. In FIG. 5, three examples of electric field lines created for three different combinations of potentials applied to electrodes 7 to 10.
Respectivement, de gauche à droite sur la figure 5, on applique les potentiels suivants à ces électrodes 7 à 10 : (a) : 0, 0, -Vo, Vo (b) : -Vo, -Vo, -Vo, VoRespectively, from left to right in FIG. 5, the following potentials are applied to these electrodes 7 to 10: (a): 0, 0, -Vo, Vo (b): -Vo, -Vo, -Vo, Vo
(c) : -Vo, Vo, 0, 0(c): -Vo, Vo, 0, 0
Dans le premier cas, ces lignes de champ sont, au centre du carré délimité par les électrodes 7 à 10, sensiblement verticales (comme vu sur le dessin), sensiblement parallèles à une diagonale du carré et sensiblement horizontales. Ainsi, au voisinage du centre du bloc 6, on obtient avec une bonne approximation l'équivalent d'un champ électrique tournant par rapport à ce centre. Ce champ réalise ainsi la fonction d'une lame de phase dont la rotation θ de l'axe Ox suit la rotation du champ. Le bloc 6 peut être dénommé « modulateur » de la polarisation du faisceau incident provenant de la fibre 2. Les valeurs d'indices optiques nx et ny suivant les axes Ox et Oy (voir figures 1A et 2) du faisceau optique au plan d'incidence sur le bloc 6 sont :In the first case, these field lines are, at the center of the square delimited by the electrodes 7 to 10, substantially vertical (as seen in the drawing), substantially parallel to a diagonal of the square and substantially horizontal. Thus, in the vicinity of the center of block 6, the equivalent of a rotating electric field with respect to this center is obtained with a good approximation. This field thus performs the function of a phase plate whose rotation θ of the axis Ox follows the rotation of the field. Block 6 can be called “modulator” of the polarization of the incident beam coming from fiber 2. The values of optical indices n x and n y along the axes Ox and Oy (see FIGS. 1A and 2) of the optical beam in the plane of incidence on block 6 are:
avec Ex = — L Ey = - v3 -v2 d d with Ex = - L Ey = - v 3 -v 2 dd
Et tgθ = §And tgθ = §
Dans ces expressions : - θ est l'angle de l'axe Ox de l'ellipse du faisceau à contrôler (ou corriger) avec l'axe du bloc 6 passant par les centres desIn these expressions: - θ is the angle of the axis Ox of the ellipse of the beam to be checked (or corrected) with the axis of block 6 passing through the centers of
— > électrodes 7 et 8 (axe de référence OEx ) ;-> electrodes 7 and 8 (OEx reference axis);
- d est la distance entre les électrodes 7 et 8 ou 9 et 10 (supposées disposées symétriquement par rapport au centre 0, sur lequel est centré le faisceau optique incident) ;- d is the distance between the electrodes 7 and 8 or 9 and 10 (assumed to be arranged symmetrically with respect to the center 0, on which the incident optical beam is centered);
- R est le coefficient de KERR du matériau du bloc 6 ;- R is the KERR coefficient of the material of block 6;
- n0 est l'indice optique du bloc 6 suivant Oy.- n 0 is the optical index of block 6 along Oy.
La fonction de modulateur de phase du bloc 6, qui s'exerce à l'intérieur du carré délimité par les électrodes 7 à 10 et plus particulièrement au voisinage de son centre, est équivalente à une lame de phase à axe tournant d'un angle θ et à biréfringence Δφ variable. Ces paramètres sont donnés par les relations suivantes, en fonction des tensions Vi à V appliquées aux électrodes 7 à 10 :The phase modulator function of block 6, which is exercised inside the square delimited by the electrodes 7 to 10 and more particularly in the vicinity of its center, is equivalent to a phase plate with axis rotating by an angle θ and with variable Δφ birefringence. These parameters are given by the following relationships, as a function of the voltages Vi to V applied to the electrodes 7 to 10:
θ = artg v4 -v3 v2 -v,θ = artg v 4 -v 3 v 2 -v,
L'axe de la lame de phase ainsi obtenue tourne à une vitesse angulaire Ω déterminée par les relations suivantes :The axis of the phase plate thus obtained rotates at an angular speed Ω determined by the following relationships:
V4 - V3 = Vo cos Ωt V2 - Vi = Vo sin ΩtV 4 - V 3 = Vo cos Ωt V 2 - Vi = Vo sin Ωt
En pratique, on peut réaliser le bloc 6 de plusieurs façons différentes. Un premier mode de réalisation consiste à utiliser un disque mince de céramique PLZT ayant une composition adaptée aux applications électro-optiques. Par exemple, la composition de cette céramique peut être Pbι-χLaxZryTiι-,yθ2 avec x = 0,09 et y = 0,65. Cette céramique possède un coefficient de KERR élevé avec un hystérésis négligeable. Sur une des faces de cette céramique (celle d'entrée du faisceau à corriger) on dépose deux paires d'électrodes (7-8 et 9-10), par exemple par dépôt métallique sous vide. Ces électrodes peuvent être en Au ou Al par exemple.In practice, block 6 can be produced in several different ways. A first embodiment consists in using a thin PLZT ceramic disc having a composition suitable for electro-optical applications. For example, the composition of this ceramic can be Pbι-χLa x Zr y Tiι-, yθ2 with x = 0.09 and y = 0.65. This ceramic has a high KERR coefficient with negligible hysteresis. On one side two pairs of electrodes (7-8 and 9-10) are deposited from this ceramic (that of the input of the beam to be corrected), for example by metallic deposition under vacuum. These electrodes can be made of Au or Al for example.
Dans un exemple de réalisation, les valeurs des paramètres cités ci-dessus étaient les suivantes :In an exemplary embodiment, the values of the parameters cited above were the following:
D = 100 μm, λ = 1 ,5 μm, R = 10.1016m2V2, n0 = 2,5. On a obtenu une lame demi-onde (Δφ = π) pour Vo = 100 V et une lame quart d'onde (Δφ = π/2) pour Vo = 50V.D = 100 μm, λ = 1.5 μm, R = 10.10 16 m 2 V 2 , n 0 = 2.5. We obtained a half-wave plate (Δφ = π) for Vo = 100 V and a quarter-wave plate (Δφ = π / 2) for Vo = 50V.
Le temps de réponse typique des dispositifs électro-optiques à céramique est de l'ordre de 1 μs. Le bloc est obtenu à partir d'un substrat PLZT poli dont l'épaisseur est de 0,5 à 1 mm environ. Récemment, des méthodes de dépôt de couches de PLZT par des techniques « sol-gel » ou par épitaxie liquide ont été mises au point pour réaliser des composants de grandes dimensions (par exemple supérieures à 5 cm2). Comme indiqué sur la figure 6, on peut réaliser deux fonctions électro-optiques, par exemple des lames λ/2 et λ/4 à axes tournant sur chaque face du substrat céramique 13 électro-optique. Sur chaque face de ce substrat, on imprime des électrodes qui ont, par exemple, la même configuration que celle représentée en figure 2. Ces électrodes sont référencées 14 dans leur ensemble sur une face du substrat 13, et 15 dans leur ensemble sur l'autre face. Une fibre optique monomode 16, se terminant par une optique de focalisation 17 (par exemple une microlentille à gradient d'indice) envoie un faisceau optique sur le centre de la face du substrat 13 portant les électrodes 14, tandis que le faisceau issu de l'autre face du substrat est recueilli par l'optique 18 (similaire ou identique à l'optique 17) couplée à une fibre optique monomode de sortie 19. Les électrodes 14 et 15 sont commandées par un circuit 20 de façon à constituer, par exemple, du côté des électrodes 14 une lame λ/4 à axe tournant, et du côté des électrodes 15 une lame λ/2 à axe tournant. Bien entendu, d'autres combinaisons de lames de phase peuvent ainsi être réalisées.The typical response time of electro-optical ceramic devices is around 1 μs. The block is obtained from a polished PLZT substrate whose thickness is approximately 0.5 to 1 mm. Recently, methods for depositing layers of PLZT by “sol-gel” techniques or by liquid epitaxy have been developed to produce components of large dimensions (for example greater than 5 cm 2 ). As indicated in FIG. 6, two electro-optical functions can be realized, for example λ / 2 and λ / 4 blades with axes rotating on each face of the electro-optical ceramic substrate 13. On each face of this substrate, electrodes are printed which have, for example, the same configuration as that shown in FIG. 2. These electrodes are referenced 14 as a whole on one face of the substrate 13, and 15 as a whole on the other face. A single mode optical fiber 16, ending with a focusing optic 17 (for example a microlens with an index gradient) sends an optical beam on the center of the face of the substrate 13 carrying the electrodes 14, while the beam coming from the the other face of the substrate is collected by the optics 18 (similar or identical to the optics 17) coupled to a single-mode optical fiber output 19. The electrodes 14 and 15 are controlled by a circuit 20 so as to constitute, for example , on the side of the electrodes 14 a λ / 4 blade with a rotating axis, and on the side of the electrodes 15 a λ / 2 blade with a rotating axis. Of course, other combinations of phase plates can thus be produced.
On a représenté en figure 7, une variante compacte du dispositif de la figure 6, cette variante utilisant des composants similaires à ceux de la figure 6, et affectés des mêmes références numériques suivies chacune d'un « A ». Le circuit de commande 20A, comme le circuit 20, contrôle les deux modulateurs comportant respectivement les électrodes 14A et 15A de façon à réaliser un fonctionnement sans fin des deux modulateurs (sans butée pour l'axe tournant).There is shown in Figure 7, a compact variant of the device of Figure 6, this variant using components similar to those of Figure 6, and assigned the same reference numerals each followed by an "A". The control circuit 20A, like the circuit 20, controls the two modulators comprising the electrodes 14A and 15A respectively. to achieve endless operation of the two modulators (without stop for the rotating axis).
Pour satisfaire au mieux les contraintes opérationnelles qui obligent, dans les applications de télécommunications à haut débit par exemple, à suivre et à reconfigurer rapidement l'état de polarisation du réseau, on peut mettre en œuvre un dispositif tel que le dispositif 21 représenté en figure 8. Ce dispositif 21 reçoit un faisceau optique d'une fibre optique 22 se terminant par une optique de focalisation 23 accolée à un premier modulateur 24 qui porte un ensemble d'électrodes 25. Le modulateur 24 est suivi d'une deuxième optique de focalisation 26, d'un deuxième modulateur 27 portant des électrodes 28, d'une troisième optique de focalisation 29, d'un troisième modulateur 30 portant des électrodes 31 et d'une quatrième optique de focalisation 32 couplée à une fibre optique de sortie 33. Les modulateurs 24, 27 et 30, sont par exemple du type du modulateur de la figure 2. Les électrodes 25, 28 et 31 sont reliées à un circuit 34 de commande. Chacun des modulateurs 24, 27 et 30 agit en lame de phase électro-optique. Chacune de ces lames permet une rotation électrooptique de ses axes et/ou un contrôle électro-optique de sa biréfringence pour chaque direction d'axe, pour constituer un ensemble électro-optique à biréfringence et orientation variables. Le circuit de contrôle 34 applique aux différents jeux d'électrodes 25, 28 et 31 des tensions permettant de réaliser, de façon connue en soi, la fonction de contrôle de polarisation du faisceau incident.To best satisfy the operational constraints which oblige, in high-speed telecommunications applications for example, to rapidly monitor and reconfigure the state of polarization of the network, it is possible to implement a device such as the device 21 shown in FIG. 8. This device 21 receives an optical beam of an optical fiber 22 ending in a focusing optic 23 attached to a first modulator 24 which carries a set of electrodes 25. The modulator 24 is followed by a second focusing optic 26, a second modulator 27 carrying electrodes 28, a third focusing optic 29, a third modulator 30 carrying electrodes 31 and a fourth focusing optic 32 coupled to an output optical fiber 33. The modulators 24, 27 and 30 are for example of the type of the modulator of FIG. 2. The electrodes 25, 28 and 31 are connected to a control circuit 34. Each of the modulators 24, 27 and 30 acts as an electro-optical phase plate. Each of these blades allows an electro-optical rotation of its axes and / or an electro-optical control of its birefringence for each axis direction, to constitute an electro-optical assembly with variable birefringence and orientation. The control circuit 34 applies voltages to the various sets of electrodes 25, 28 and 31 making it possible, in a manner known per se, to perform the function of controlling the polarization of the incident beam.
Le matériau constituant le bloc électro-optique de l'invention peut être non seulement du PLZT, mais tout matériau présentant un coefficient électro-optique (coefficient de KERR) élevé. Ce peut être, par exemple, une céramique telle que PbSZT, BLTN, SBN, ... ou bien une couche polymère électro-optique, ou un dispositif à cristaux liquides (mais il faut noter que les cristaux liquides ont un temps de réponse trop élevé, bien supérieur à quelques μs), ou bien encore du PDLC (« Polymer Dispersed Liquid Crystal »), décrit ci-dessous en référence aux figures 12 à 14.The material constituting the electro-optical block of the invention can be not only PLZT, but any material having a high electro-optical coefficient (coefficient of KERR). It can be, for example, a ceramic such as PbSZT, BLTN, SBN, ... or else an electro-optical polymer layer, or a liquid crystal device (but it should be noted that liquid crystals have a response time too high, much greater than a few μs), or else PDLC (“Polymer Dispersed Liquid Crystal”), described below with reference to FIGS. 12 to 14.
On a représenté en figures 9 et 10 une variante du dispositif modulateur de l'invention, pour laquelle des électrodes identiques 35, 36 sont disposées sur les deux faces d'un substrat électro-optique 37. Cette variante est utilisée ici non pour combiner deux lames de phase (λ/4 et λ/2 par exemple), mais pour augmenter l'efficacité du modulateur. En effet, chaque configuration d'électrodes 35, 36, est commandée par la même combinaison de tensions appliquée à ces électrodes 35, 36 et présente une « épaisseur active » (e1 , e2 respectivement en figure 9), c'est-à-dire l'épaisseur de matériau électro-optique, à partir du plan des électrodes, pour laquelle le champ électrique créé par ces électrodes est efficace vis-à-vis du contrôle de la polarisation du faisceau optique traversant le substrat 37. Ainsi, au lieu d'avoir une seule épaisseur active e1 ou e2, les deux concourent au contrôle de la polarisation. Une autre variante du dispositif de l'invention est représentée en figure 11. Selon cette variante, le nombre d'électrodes formées sur une face d'un substrat 38 est supérieur à quatre. Dans le mode de réalisation de la figure 11 , ce nombre est de six. Ces électrodes sont référencées 39 à 44, et elles sont disposées régulièrement autour du centre de la face du substrat 38, délimitant ainsi un hexagone. Grâce à ce plus grand nombre d'électrodes, on obtient, pour une tension plus faible appliquée sur chaque électrode (plus faible que dans le cas de quatre électrodes), un champ électrique résultant, au centre de l'hexagone, à la fois plus élevé et plus uniforme. La complexité du dispositif de commande des électrodes est plus grande que dans le cas d'une configuration à quatre électrodes. Bien entendu, on pourrait envisager de disposer sur la face d'un substrat électrooptique un nombre encore plus grand d'électrodes, mais la complexité du dispositif de commande électrique serait encore plus grande. Il faut donc rechercher pour chaque application un compromis entre la complexité du dispositif de commande et l'efficacité du modulateur.FIGS. 9 and 10 show a variant of the modulator device of the invention, for which identical electrodes 35, 36 are arranged on the two faces of an electro-optical substrate 37. This variant is used here not to combine two phase plates (λ / 4 and λ / 2 by example), but to increase the efficiency of the modulator. Indeed, each configuration of electrodes 35, 36, is controlled by the same combination of voltages applied to these electrodes 35, 36 and has an "active thickness" (e1, e2 respectively in FIG. 9), that is to say say the thickness of electro-optical material, from the plane of the electrodes, for which the electric field created by these electrodes is effective with respect to controlling the polarization of the optical beam passing through the substrate 37. Thus, instead to have a single active thickness e1 or e2, both contribute to the control of the polarization. Another variant of the device of the invention is shown in FIG. 11. According to this variant, the number of electrodes formed on one face of a substrate 38 is greater than four. In the embodiment of Figure 11, this number is six. These electrodes are referenced 39 to 44, and they are arranged regularly around the center of the face of the substrate 38, thus delimiting a hexagon. Thanks to this greater number of electrodes, one obtains, for a lower voltage applied to each electrode (lower than in the case of four electrodes), an electric field resulting, in the center of the hexagon, both more higher and more uniform. The complexity of the electrode control device is greater than in the case of a four-electrode configuration. Of course, one could consider having an even larger number of electrodes on the face of an electrooptical substrate, but the complexity of the electrical control device would be even greater. A compromise must therefore be sought for each application between the complexity of the control device and the efficiency of the modulator.
Selon encore une autre variante du dispositif de l'invention, le matériau constituant le bloc électro-optique de type PLZT est remplacé par un matériau PDLC particulier, dit à « nano gouttes » (« nano droplets »).According to yet another variant of the device of the invention, the material constituting the electro-optical block of the PLZT type is replaced by a particular PDLC material, known as “nano droplets” (“nano droplets”).
Ce matériau, schématiquement illustré en figures 12 à 14, comporte des gouttelettes 45 de cristal liquide incluses dans une matrice polymère 46 (figure 12) par un procédé de polymérisation rapide, par exemple sous éclairement UV. Il est alors possible d'obtenir des gouttes de cristal liquide dont la taille est nettement inférieure à 1 μm. Bien que le milieu ainsi obtenu soit inhomogène et que l'indice du cristal liquide et du polymère soient différents, le milieu n'est pas pour autant diffusant. En effet, les gouttes 45 ont dans le cas présent une taille largement inférieure à la longueur d'onde du faisceau optique traversant le milieu. Tout se passe donc comme si on était en présence d'une céramique électro-optique isotrope. L'effet électro-optique, dans ce cas, résulte de la réorientation, sous l'effet d'un champ électrique, des molécules de cristal liquide présentes dans les gouttes. Ce phénomène est illustré en figures 13 et 14. En figure 13, on a représenté schématiquement quelques molécules de cristal liquide, qui, en l'absence de champ électrique, sont aléatoirement orientées. Lorsque l'on applique une tension électrique aux électrodes 47, on crée dans le milieu 46 un champ électrique, et les molécules s'orientent parallèlement aux lignes de champ électrique 48.This material, schematically illustrated in FIGS. 12 to 14, comprises droplets 45 of liquid crystal included in a polymer matrix 46 (FIG. 12) by a rapid polymerization process, for example under UV illumination. It is then possible to obtain drops of liquid crystal whose size is much less than 1 μm. Although the medium thus obtained is inhomogeneous and the index of the liquid crystal and of the polymer are different, the medium is not however diffusing. Indeed, the drops 45 have in the present case a size much smaller than the wavelength of the optical beam passing through the medium. Everything therefore happens as if we were in the presence of an isotropic electro-optical ceramic. The electro-optical effect, in this case, results from the reorientation, under the effect of an electric field, of the liquid crystal molecules present in the drops. This phenomenon is illustrated in Figures 13 and 14. In Figure 13, there is shown schematically some molecules of liquid crystal, which, in the absence of an electric field, are randomly oriented. When an electric voltage is applied to the electrodes 47, an electric field is created in the medium 46, and the molecules orient themselves parallel to the electric field lines 48.
La biréfringence de ce dispositif PDLC est de l'ordre de quelques 10"3 pour des tensions appliquées aux électrodes 46 de l'ordre de quelques dizaines à une centaine de Volts, l'espace inter-électrodes (d) ayant une dimension de l'ordre de 100 μm. Les temps de réponse obtenus avec ce type de matériau sont de l'ordre d'une dizaine à quelques dizaines de μs pour des épaisseurs de matériau de quelques centaines de μm. The birefringence of this PDLC device is of the order of a few 10 −3 for voltages applied to the electrodes 46 of the order of a few tens to a hundred volts, the inter-electrode space (d) having a dimension of l 'order of 100 μm The response times obtained with this type of material are of the order of ten to a few tens of μs for material thicknesses of a few hundred μm.

Claims

REVENDICATIONS
1. Dispositif de contrôle de polarisation dans une liaison optique, caractérisé par le fait qu'il comporte, dans la liaison optique dont on désire contrôler la polarisation, au moins un bloc de matériau électro-optique (6, 13, 13A, 24, 27, 30, 37, 38, 46) à biréfringence variable sous l'effet d'un champ électrique, des électrodes (7 à 10, 14, 15, 14A, 15A, 25, 28, 31, 35, 36, 39 à 44, 46) étant disposées sur au moins une face de ce bloc et étant reliées à un circuit (5, 20, 34) permettant de faire varier les tensions électriques appliquées à ces électrodes en fonction de la rotation désirée, des axes de polarisation. 1. Device for controlling polarization in an optical link, characterized in that it comprises, in the optical link whose polarization is to be controlled, at least one block of electro-optical material (6, 13, 13A, 24, 27, 30, 37, 38, 46) with variable birefringence under the effect of an electric field, electrodes (7 to 10, 14, 15, 14A, 15A, 25, 28, 31, 35, 36, 39 to 44, 46) being arranged on at least one face of this block and being connected to a circuit (5, 20, 34) making it possible to vary the electric voltages applied to these electrodes as a function of the desired rotation, of the axes of polarization.
2. Dispositif selon la revendication 1, caractérisé en ce que le matériau électro-optique est isotrope en l'absence de champ électrique appliqué, l'orientation des axes neutres dudit matériau et la biréfringence dudit matériau étant commandables par champ électrique.2. Device according to claim 1, characterized in that the electro-optical material is isotropic in the absence of an applied electric field, the orientation of the neutral axes of said material and the birefringence of said material being controllable by electric field.
3. Dispositif selon la revendication 2, caractérisé par le fait que le matériau électro-optique est l'un des matériaux suivants : PLZT, PbSZT,3. Device according to claim 2, characterized in that the electro-optical material is one of the following materials: PLZT, PbSZT,
BLTN, SBN, couche polymère électro-optique, cristaux liquides, PDLC à « nanogouttes ».BLTN, SBN, electro-optical polymer layer, liquid crystals, PDLC with "nanodrops".
4. Dispositif selon l'une des revendications précédentes, caractérisé par le fait que le bloc électro-optique (13, 13A) comporte sur chacune de ses faces principales des électrodes (14, 15, 14A, 15A) déterminant sur chacune de ces faces une lame de phase à axe tournant.4. Device according to one of the preceding claims, characterized in that the electro-optical unit (13, 13A) comprises on each of its main faces electrodes (14, 15, 14A, 15A) determining on each of these faces a phase blade with a rotating axis.
5. Dispositif selon l'une des revendications précédentes, caractérisé par le fait qu'il comporte plusieurs blocs en cascade sur le même trajet optique (26, 29, 32), chacun de ces blocs étant muni d'électrodes (25, 28, 31 ) de manière à réaliser un contrôle sans butée de la polarisation.5. Device according to one of the preceding claims, characterized in that it comprises several blocks in cascade on the same optical path (26, 29, 32), each of these blocks being provided with electrodes (25, 28, 31) so as to carry out a control without polarization stop.
6. Dispositif selon l'une des revendications précédentes, caractérisé par le fait que le contrôle de polarisation est un asservissement d'une direction de polarisation d'un faisceau optique. 6. Device according to one of the preceding claims, characterized in that the polarization control is a control of a direction of polarization of an optical beam.
EP01989643A 2000-12-28 2001-12-20 Device for controlling polarisation in an optical connection Withdrawn EP1356344A1 (en)

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FR0017226 2000-12-28
FR0017226A FR2819061B1 (en) 2000-12-28 2000-12-28 POLARIZATION CONTROL DEVICE IN AN OPTICAL LINK
PCT/FR2001/004114 WO2002054142A1 (en) 2000-12-28 2001-12-20 Device for controlling polarisation in an optical connection

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WO2002054142A1 (en) 2002-07-11

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