FR2849701A1 - Optical signal polarization controller for vehicle, has electric field application unit with two three-dimensional electrodes to modify birefringence and/or orientation of optical axis based on field applied to liquid crystal cell - Google Patents

Abstract

The controller (20) has a liquid crystal cell with two parallel substrate plates with an electro-optical composite material of liquid crystal. An electrical field application unit applies an electrical field perpendicular to a part of content of the cell. The application unit has a pair of massive three-dimensional electrodes arranged in such a manner to modify birefringence and/or orientation of an optical axis, according to the electrical field.

Description

Device for controlling the polarization of a light signal using

  a liquid crystal material stabilized by a polymer.

  The field of the invention is that of the transmission of signals by optical fibers. More specifically, the invention relates to a device for controlling the polarization of a signal conveyed in the form of a light beam.

  Such a polarization controller applies in particular, but not exclusively, within a compensation system for the polarization modal dispersion. It should be recalled that the increase in throughput on optical fibers leads to taking into account phenomena hitherto considered negligible.

  This is the case of polarization modal dispersion (PMD), especially on old generation fibers. It will be recalled that by polarization modal dispersion (or PMD, for "Polarization Mode Dispersion" in English) is meant the fact that during the transmission, the optical pulses are split into two polarization states.

  To compensate for this dispersion, it is known to insert in series, between the optical transmission fiber and the photodetector, a compensation system comprising a polarization controller, a polarization maintaining fiber and means for measuring the degree of polarization. on the polarization maintaining fiber. In other words, and as illustrated in FIG. 1, the optical transmission fiber 1 is connected to the input of the polarization controller 2, and the output of the latter is connected to one end of the polarization-maintaining fiber 3, the other end of the latter being connected to the photodetector 4. The operation of this compensation system is as follows: using appropriate measurement means 5, the degree of polarization is measured on the fiber to maintain polarization, so as to quantify the importance of the dispersion, and the polarization controller is modified so as to minimize the dispersion.

  If we consider that the polarization modal dispersion phenomenon 30 becomes troublesome from 10% of the bit time, a dispersion of 10 ps is the tolerable limit for a bit rate of 10 Gbit / s.

  The compensation of the polarization modal dispersion imposes strong constraints on the polarization controllers, namely: rapid control (qq 10jts), dynamic and endless.

  Several known technologies of polarization controller, aimed at meeting these constraints, are known and proposed.

  A first is the Lithium Niobate (LiNbO3) technology, described in particular in the document entitled "Endless polarization control using integrated optic lithium niobate device" (in French: "optical device integrated with Lithium Niobate using endless polarization control" ), Electron. Letters Vol 24, 10 pp. 266-268, 1988, by N. Walker and G. Walker, JLT, Vol. 8 pp. 438-458, 1990.

  It relates to integrated optics components on which electrodes distributed on the guide make it possible to alternate TE / TM mode conversions with phase shifts. Three independent potentials allow to realize an endless dynamic controller. This first technology has several drawbacks, namely notably: high control voltages (more than 100 volts), residual birefringence outside the field, insertion losses (typically 3-4 dB) and a high manufacturing cost.

  A second known polarization controller technology uses opto-ceramic components (PLZT type) from the company Corning (registered trademark). Mention may in particular be made by reference of the French patent of Thomson CSF, No. FR 00 17226 of December 28, 2000, concerning a "polarization control device in an optical link".

  A third known technology of polarization controller, consists of a conventional combination of phase plates (two quarter-waves and half-waves) with variable axes. Reference may be made in particular to the article by Z. Zhuang et al. entitled "polarization controller using nematic liquid crystal" (in French: "polarization controller using a nematic liquid crystal"), Optics Letters, Vol. 24, pp 694-696, 1999. In theory, a single phase blade with variable axis and phase shift is sufficient.

  In this approach, liquid crystal solutions (nematic or smectic) are the most used. They indeed have strong electro-optical effects over short distances and allow endless rotation of the director. Reference may be made in particular to the article by T. Chiba et al. entitled "Polarization stabilize using liquid crystal rotatable waveplates" (in French: "polarization stabilizer using rotating liquid crystal waves"), JLT Vol. 17, pp. 885-890, 1999.

  Nematic liquid crystal solutions are unfortunately generally too slow (qq 10ms).

  Smectic liquid crystal solutions have therefore been proposed. Reference may be made in particular to the article by L. Dupont et al. entitled "Principle of 10 polarization mode dispersion controller using homeotropic electroclinic liquid crystal confined single mode fiber device" (in French: "principle of a polarization modal dispersion controller using a homeotropic and electroclinical liquid crystal confined in a single mode fiber device" ), Optics Communications, Vol. 176, pp. 113-119, 2000. and RNRT Copoldyn. Reference may also be made to American patent No. US 5,313,562 to Marconi GEC Ltd, having the title "Optical device with electrodes end-to-end with electric field causing homeotropic alignment of liquid crystal in space between ends" (in French: "optical device with electrodes facing each other to apply an electric field causing a homeotropic alignment of the liquid crystal in the space 20 between the ends of the electrodes"). The text of this Dupont article and the text of this US patent No. US 5,313,562 to Marconi GEC Ltd are inserted here by reference. However, these smectic liquid crystal solutions most of the time have high alignment quality constraints (need to use alignment layers) and low modulation angles.

  To overcome or simply limit the various problems mentioned above, a more recent technology replaces in the polarization controller, the liquid crystal by a heterogeneous system made up of droplets of liquid crystal of small diameter, dispersed in a polymer matrix. This heterogeneous system 30 is called "nano-PDLC", with reference to the English term PDLC (Polymer Dispersed Liquid Crystal ", for" liquid crystal dispersed in a polymer). In a heterogeneous PDLC "classic" system, the size of the droplets is comparable to the wavelength of the incident light, and there is a phenomenon of diffusion.

  Such a diffusion phenomenon no longer exists with the heterogeneous "nanoPDLC" system, because the liquid crystal droplets are small compared to the wavelength. We can again refer to the French patent of Thomson CSF n0 FR 00 17226 of December 28, 2000, on the one hand, and to the French patent application of the company OPTOGONE n0 FR 01 16341, of December 17, 2001, concerning a " device for controlling the polarization of a light signal conveyed in the form of a light beam ", on the other hand. In addition, it is also possible to cite for reference, the article by Matsumoto et al. entitled "Light processing and optical devices using nano-sized droplets of liquid crystal dispersed in polymer" (in French: Optical light processing device using nano-droplets of liquid crystal dispersed in a polymer ") and published in 1999 in the international journal: Journal of Intelligent 15 Systems and Structures Vol. 10, pp. 489-492.

  These various materials of the prior art are used in the polarization controller as a programmable phase plate (with variable delay, with rotating axis, or else a combination of the two). Used in the different approaches of the prior art, these materials on the other hand have different drawbacks. In particular: - nematic liquid crystals have poor response times (a few ms), often not very compatible with certain constraints in the telecommunications field; - PLZTs and nano-PDLCs, although clearly faster than 25 nematic liquid crystals, require the application of high voltages; - smectic liquid crystals are faster than the first two materials, under low voltage, but require in return a delicate alignment which forces small thicknesses which has the effect of limiting phase shifts.

  In addition, the technologies used according to the prior art, in particular the Thalès solutions (international patent application No. WO 02/054142) and Gec Marconi (American patent no US 5,313,562) offer the additional disadvantage that a polarization dependence, resulting from the inhomogeneity of the electric field lines, can occur. Such a drawback is major in the telecommunications field, a field which is non-exclusively and non-restrictively covered by the scope of the present invention.

  To date, no prior art is known of electro-optical materials making it possible to achieve a good compromise between speed (a few gs) and low control voltage. Neither are any known technological solutions 10 favoring the significant and homogeneous penetration of the electric field in the thickness of the cell, so as to obtain significant phase shifts, nor even even technological solutions making it possible to guarantee the homogeneity of the lines of field and thus avoid any polarization dependence potentially induced by the orientation of the directing vectors which follow the electric field lines.

  The invention particularly aims to overcome these main drawbacks of the prior art.

  More specifically, an objective of the invention is to provide a device for controlling the polarization of a light signal having a good compromise between switching speed and low control voltages applied to the electrodes. Another objective of the invention is to provide a polarization controller having rapid response times (a few gs).

  A further object of the invention is to provide a polarization controller which is insensitive to polarization.

  Yet another objective of the invention is to provide a polarization controller having low insertion losses, that is to say which has optical pupils which are of sufficient width to facilitate collimation, alignment and limit losses, while requiring low control voltages.

  An additional objective of the invention is to provide a polarization controller which compensates for a possible phenomenon of double refraction.

  These objectives, as well as others which will appear subsequently, are achieved using a device for controlling the polarization of an optical signal conveyed in the form of a light beam comprising: a liquid crystal cell comprising two substrate plates essentially parallel to each other and between which is confined an electro-optical composite material of the liquid crystal type stabilized by a polymer (PSLC) present in low concentration; - Means for applying at least one electric field perpendicular to at least part of the contents of the cell. These 10 means comprise at least one pair of solid three-dimensional electrodes, so as to modify the birefringence and / or the orientation of the optical axis, depending on whether an electric field is applied or not.

  The general principle of the invention therefore consists in using a combined electro-optical material, consisting of liquid crystal (nematic, smectic, ferroelectric, anti-ferroelectric) stabilized by a polymer ("Polymer Stabilized Liquid Crystal" or PSLC). , as opposed to "Polymer Dispersed Liquid Crystal or PDLC"), and a system of solid electrodes of the three-dimensional type, so as to obtain in particular a good compromise 20 switching speed low control voltages between the electrodes.

  Preferably, the electro-optical composite material of the PSLC type is in the form of a gel. There is an advantage of the PSLC which is to impose a better mechanical stability of the compound while maintaining a privileged alignment. It therefore does not induce a problem of helix or chevron formation. The mechanical properties are therefore improved and the appearance of intrinsic or induced defects in the structure of the liquid crystal is limited. Furthermore, the dynamic properties (response time) linked to the nature of the liquid crystal used, namely rapid under low voltage when using smectics (smectics of type A or C, ferroelectric anti-ferroelectric), are preserved. with the PSLC. Relative to these latter advantageous aspects, there may be mentioned for further reference, the article by Furue entitled "The effect of the stabilization of a polymer on the alignment structure of a ferroelectric liquid crystal stabilized at the surface", or "The effect of Polymer stabilization on the alignment structure of surface stabilized ferroelectric liquid crystal", and published in 1998 in the journal Mol. Liquid Crystal (Vol. 317, pp. 259-271).

  The production of PSLCs requires the use of different monomers having specific properties, in particular: ability to be able to orient in the liquid crystal, preservation of the structure of the material during polymerization. For reference on the PSLC, we can cite chapter 8: "liquid crystals with dispersed or stabilized polymer" (in English: "Polymer10 Dispersed and Polymer-Stabilized Liquid Crystals"), written by Crawford and Zumer and belonging to the book entitled "Liquid Crystals Confined to Complex Geometries From Polymer to Porous Networks, (Taylor & Francis, London, 1996). In PSLC, liquid crystal is not encapsulated in the form of 15 droplets of variable sizes, as in PDLC The use of PSLCs thus makes it possible to avoid the creation of large anchoring forces and to promote the use of low electrical voltages. In addition, the application of a low voltage electrical current between the thick electrodes allows making and using cells with large pupils between the electrodes, the size of the latter being mainly determined by the voltage required to switch liquid crystal However, large optical pupils have the advantage of facilitating the assembly of such a polarization control device, in particular in terms of connection of optical fibers, while limiting insertion losses.

  Advantageously, the electro-optical composite material of the liquid crystal type comprises a polymer concentration less than or equal to 10%. The interactions between the liquid crystal and the polymer make it possible to stabilize the structure of the liquid crystal and thus avoid the appearance of certain defects, preferably when the polymer is present in a concentration of less than 30%. It should however be noted that the presence of the polymer in the liquid crystal in a concentration substantially less than or equal to 10% makes it possible to optimize the stabilization of the liquid crystal and thus avoid the appearance of numerous defects, in particular related to the phases smectic. Obviously, other concentration values of the polymer in the liquid crystal may be suitable, depending in particular on the type of crystal used.

  Preferably, the liquid crystal included in the electro-optical composite material belongs to the group comprising: - nematics; - type A or type C smectics, chiral or non-chiral; - ferroelectric; - anti-ferroelectric.

  The dynamic properties (response time essentially) related to the nature of the liquid crystal used, namely, fast under low voltages, when using smectics (smectics C, smectics A, ferroelectric or antiferroelectric) are preserved with the PSLC. In the particular case of a smectic C, it is more advantageous to use the massive electrodes, to homogenize the response times. The advantage therefore results from the material-addressing combination. Preferably, the device according to the invention comprises means for compensating for the phenomenon of double refraction. Indeed, the PSLC is an electro-optical material mainly composed of liquid crystal, the polymer being present in reduced concentration. Certain modes of use can sometimes generate a splitting of the optical beam passing through it, characteristic of a phenomenon of double refraction incompatible with the constraints linked to telecommunications.

  Preferably, the means for compensating for the phenomenon of double refraction comprise at least one dielectric mirror on at least one output face of the device according to the invention, so as to compensate for the splitting of the optical beam and thus make the system insensitive to the phenomenon of double refraction. Advantageously, the solid three-dimensional electrodes are arranged in a plane substantially parallel to the substrate plates, so as to favor the addressing of the electrooptical material.

  Preferably, the three-dimensional solid electrodes are arranged in stars, so as to be able to generate a rotating electric field.

  The use of such three-dimensional electrodes makes it possible to operate with low voltages and thus to use larger optical pupils, with the advantageous and non-limiting consequences of eliminating many problems of collimation, centering and / or loss of insertion, incompatible with the current specifications of telecommunications standards, but specific to devices according to the prior art.

  Preferably, the solid three-dimensional electrodes are made of conductive or semiconductor materials. By conductive materials can be understood all the materials belonging to group 15 comprising metals and by semiconductor materials those belonging to group comprising silicon.

  Advantageously, the three-dimensional solid electrodes have a thickness of the order of ten microns. Preferably but not limited to, the thickness of the three-dimensional electrodes is between 10 and 20 15 microns. Of course, other values of the thickness of the three-dimensional electrodes may also be suitable depending on the desired use of the device according to the invention. In addition, the use of such electrodes offers the particular advantages of favoring the significant and homogeneous penetration of the electric field into the thickness of the cell, with the essential consequence, the possibility of obtaining significant phase shifts.

  Advantageously, the substrate plates are formed by glass plates or ends of optical fibers.

  The invention also relates to the application of the aforementioned polarization control device to the implementation of a compensation system for modal polarization dispersion (PMD).

  Preferably, the invention also relates to the application of the aforementioned polarization control device to the implementation of telecommunication systems. Other characteristics and advantages of the invention will appear more clearly on reading the following description of a preferred embodiment, given by way of simple illustrative and nonlimiting example, and the appended drawings, among which: FIG. 1 presents a simplified diagram of a compensation system for the polarization modal dispersion as already commented previously 10 in the description of the invention; - Figure 2 shows a simplified diagram of a particular embodiment of the polarization controller according to the present invention; - Figure 3 illustrates the application of an electric field perpendicular to the direction of propagation of the light beam; - Figure 4 shows a sectional view of the three-dimensional solid electrodes included in the polarization controller according to the present invention; - Figure 5 shows the general principle of the application to three-dimensional electrodes of a rotating electric field; - the figure 6 illustrates the differences of penetration of the electric field E (Z) in the cell, little important in the case of a use of two-dimensional electrodes of small thickness (figure 6-a), and important in the case of 'a use of thick three-dimensional electrodes according to the invention (figure 6-b).

  The invention therefore relates to a polarization controller which can in particular and without limitation, be implemented within a compensation system for polarization modal dispersion (PMD), as previously described by way of illustration in relation to the figure 1.

  As illustrated in the simplified diagram of FIG. 2, in a particular embodiment of the invention, the polarization controller 20 comprises: - a cell made up of two glass plates 6 and 7 essentially parallel to each other and between which is confined a "PSLC" material 8 (see detailed description below) in the form of a gel; - at least one pair of electrodes (see detailed discussion below, in relation to FIG. 4) making it possible to apply, to at least part of the contents of the cell, a rotating electric field E (see FIG. 5) substantially perpendicular to the direction D of propagation of the light beam 10.

  The systems known as "polymer network stabilized liquid crystals", or PSLC belong to the family of composites integrating a polymer and a liquid crystal. In the family of composites, there are at least two main categories. In the first category, the liquid crystal is dispersed within a polymer matrix, the liquid crystal therefore not forming a continuous phase, unlike the polymer (case of PDLC type 15 composites - "Polymer Dispersed Liquid Crystal" in English - for which the liquid crystal droplets are dispersed in a polymer). In the second category, the liquid crystal and the polymer form a gel. The two phases are thus interconnected.

  The PSLC-type materials used in the present invention belong to the family comprising gels. They are produced with monomer concentrations typically less than 10%, while the PDLCs known according to the prior art require concentrations greater than 10%, so that they can ensure sufficient dispersion.

  PSLC type electro-optical materials are obtained by in-situ polymerization of a monomer, previously dissolved directly in the liquid crystal, with the optional addition of a polymerization initiator to the liquid crystal and monomer mixture. In general, the monomer is chosen so that it is soluble in sufficient quantity in the liquid crystal and in such a way that: - the network formed is stable after polymerization, - the structure of the liquid crystal phase is not or little modified by the presence of the monomer and of the network after polymerization, - the network, by anchoring effects, exhibits interactions whose nature makes it possible to obtain the desired effects such as the stabilization of the liquid crystal phase.

  Thus, depending on the nature of the monomer chosen, the polymer network is formed and organized into fibrils.

  In addition, the stabilization of the liquid crystal phase by the polymer network can take on different aspects. In particular and in a non-exhaustive manner, thanks to the interactions present between the polymer and the liquid crystal, the range of existence in temperature of a given liquid crystal phase can be extended, with the consequences: - improvement of the mechanical properties , - the elimination or limitation of defects, intrinsic or induced, in the structure of the liquid crystal, - the reduction of flow phenomena which may occur during the switching of the liquid crystal (reorientation of the liquid crystal molecules under the effect of an electric field), with the interesting consequence of reducing the response times.

  We now present, in relation to FIG. 4, a particular embodiment of the three-dimensional thick solid electrodes included in the polarization controller according to the present invention. The electrode system (common to the two glass plates) comprises three pairs of electrodes (41a, 41b), (42a, 42b) and (43a, 43b) three-dimensional ("massive") 25 of very thick (a few tens of microns). These electrodes can be made of conductive materials (metals) or semiconductors (Silicon or others). They can be obtained, for example, either by photolithography of substrates, or by the use of micro-tips.

  This electrode system has an axis of symmetry Oy orthogonal to the glass plate concerned. This axis of symmetry Oy coincides with the direction D of light propagation. Thus, it is possible to apply an electric field whose orientation is perfectly controlled and which can be rotated continuously and endlessly (see Figure 5).

  The principle of the controller according to the invention resides in the electrooptical material used, of the PSLC type, in its addressing and in the functional advantages which result therefrom, in particular compared to PDLCs (conventional or nanodrops). The PSLCs differ from the liquid crystal by the polymer chain, which due to the interactions of polymeric liquid crystal surface, prevents the formation of certain defects (in particular in the smectic phases), without loss of the electro-dynamic properties of the liquid crystal (time of switching of the liquid crystal used) which composes it, unlike the PDLC according to the prior art. They differ from these in that the liquid crystal is not encapsulated in the form of droplets (unconnectedness of the liquid crystal) of variable sizes thus creating inking forces at the origin of the high switching voltages. PSLC materials are gels (interconnected liquid crystal phase). The monomers used for their production have the particularity of being able to orient themselves in the liquid crystal, without altering its structure during the polymerization. Furthermore, the interactions between the liquid crystal and the polymer make it possible to stabilize a given structure of the liquid crystal and thus avoid the appearance of defects for example. Compared to PDLCs characterized by a high concentration of polymer greater than 10%, PSLCs have a lower concentration (less than 10% polymer) and require monomers with specific properties. The advantages of the polarization controller according to the invention are in particular the following: - uniformity and homogeneity of the lines of the applied electric field; - dual function of director axis rotation and index modulation by means of a single tension control; - simplified electronic interface; great robustness and mechanical stability; - elimination of faults such as declination lines, formation of rafters, large propellers; reduction in the risks of loss of material potentially caused by the polarization dependence of the latter; - optimization and homogenization of response times (a few microseconds), functional results of better intrinsic addressing to the electro-optical material PSLC used; - insensitivity of the material to polarization and low insertion losses; - assembly made easier, especially in terms of optical fiber connection, by the production and use of high pupil cells (central space between electrodes) authorized by the possibility of using low electrical voltages - insensitivity to double refraction .

Claims (13)

  1. Device for controlling the polarization of an optical signal conveyed in the form of a light beam, characterized in that it comprises: a liquid crystal cell comprising two substrate plates essentially parallel to each other and between which is confined an electro-optical composite material of the liquid crystal type stabilized by a polymer (PSLC) present in low concentration; means for applying at least one electric field perpendicular to at least part of the content of said cell, said means comprising at least one pair of solid three-dimensional electrodes, so as to modify the birefringence and / or the orientation of the optical axis, depending on whether said electric field is applied or not.   2. Device according to claim 1, characterized in that the electro-optical composite material of PSLC type is in the form of a gel.   3. Device according to any one of claims 1 and 2, characterized in that the electro-optical composite material of liquid crystal type comprises a concentration of said polymer less than or equal to 10%. 4. Device according to any one of claims 1 to 3, characterized in that the liquid crystal included in the electro-optical composite material belongs to the group comprising: - nematics; - type A or type C smectics, chiral or non-chiral; - ferroelectric; - anti-ferroelectric.   5. Device according to any one of claims 1 to 4, characterized in that it comprises means for compensating for the phenomenon of double refraction.   6. Device according to claim 5, characterized in that said means for compensating for said phenomenon of double refraction comprise at least one dielectric mirror on at least one exit face of said device.   7. Device according to any one of claims 1 to 6, characterized in that said solid three-dimensional electrodes are arranged in a plane substantially parallel to the substrate plates, so as to favor the addressing of the electro-optical material.   8. Device according to any one of claims 1 to 7, characterized in that the three-dimensional solid electrodes are arranged in stars, so as to be able to generate a rotating electric field.   9. Device according to any one of claims 1 to 8, characterized in that said solid three-dimensional electrodes are made of conductive or semiconductor materials.   10. Device according to any one of claims 1 to 9, 15 characterized in that said solid three-dimensional electrodes have a thickness of the order of ten microns.   11. Device according to any one of claims 1 to 10, characterized in that said substrate plates are formed by glass plates or ends of optical fibers.   12. Application of the polarization control device according to any one of claims 1 to 11 to the implementation of a compensation system for the polarization modal dispersion (PMD).   13. Application of the polarization control device according to any one of claims 1 to 12 to the implementation of telecommunication systems.