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 
  • G02F1/0136—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

Definitions

• the field of the invention is that of the transmission of signals by optical fibers.
• the invention relates to a device for controlling the polarization of a signal conveyed, by optical fiber, in the form of a light beam.
• An ideal polarization controller is a birefringent plate which can control both the orientation of the own axes and the phase shift, and which also has endless control.
• Such a polarization controller applies in particular, but not exclusively, within a compensation system for the polarization modal dispersion.
• a compensation system comprising a polarization controller, a polarization maintaining fiber and means for measuring the degree of polarization on "polarization maintaining fiber
• 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 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 polarization maintaining fiber, 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 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.
• a first known technology of polarization controller aiming to meet these constraints, is the Niobate Lithium (LiNbO 3 ) technology. It is 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, 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 conversions of TE / TM modes with phase shifts. Three independent potentials allow to realize an endless dynamic controller.
• This first technology has several drawbacks, namely in particular: 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).
• a third known polarization controller technology which is the other most serious option, consists of a conventional combination of phase blades (two quarter-waves and half-waves) with variable axes.
• a single phase plate with an axis and variable phase shift is sufficient.
• liquid crystal solutions (nematic or smectic) are the most used because they have strong electro-optical effects over short distances and allow an endless rotation of the director.
• the invention particularly aims to overcome these various drawbacks of the state of the art.
• one of the objectives of the present invention is to provide a polarization controller allowing rapid control (a few tens of microseconds), that is to say having switching times (also called reconfiguration times) very low, compatible with the new high bit rates on optical fibers.
• the invention also aims to provide such a polarization controller allowing dynamic and endless control. Another objective of the invention is to provide such a polarization controller having a low manufacturing cost, in particular compared to those manufactured according to the aforementioned known technologies.
• An additional objective of the invention is to provide such a polarization controller which does not require any alignment layer.
• a device for controlling the polarization of a signal conveyed in the form of a light beam comprising: a cell made up of two substrate plates essentially parallel to each other and between which is confined a content comprising a polymer in which liquid crystal droplets are dispersed; first means for applying, to at least part of the contents of the cell, a first electric field substantially perpendicular to the direction of propagation of the light beam, so that, depending on whether the first electric field is applied or not, at least part of the contents of the cell constitute a birefringent or isotropic medium respectively.
• the size of the liquid crystal droplets is much smaller, and preferably at least in a ratio of ten, to the wavelength of the light beam.
• the general principle of the invention therefore consists in replacing, in the polarization controller, the liquid crystal by a heterogeneous system constituted by droplets of liquid crystal of small diameter dispersed in a polymer matrix.
• This heterogeneous system is called "nano-PDLC", with reference to the English term PDLC (Polymer Dispersed Liquid Crystal ", for” liquid crystal dispersed in a polymer).
• PDLC Polymer Dispersed Liquid Crystal ", for” liquid crystal dispersed in a polymer.
• 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 does not exist with the heterogeneous "nano-PDLC” system of the invention, because the droplets of liquid crystal are small compared to the wavelength.
• the advantages of the “nano-PDLC” are in particular: the ease of implementation, the absence of orientation layer for the liquid crystal, the stability over time of the structure, the rapid response time.
• the invention is also based on a completely new and inventive configuration of application of the electric field.
• the electric field is applied (substantially) perpendicular to the direction of light propagation, so that the light beam sees a birefringent material.
• the electric field is collinear with the direction of propagation of the light. It causes a reorientation of the director of the liquid crystal in the droplets.
• the directing vector of the liquid crystal in the droplets tends to align with the applied electric field but, statistically, the directions in each of the droplets are distributed on a cone whose axis of symmetry is the applied field.
• the polarization controller according to the invention is much faster. It is moreover of better quality and optical homogeneity (no alignment problem for large thicknesses, no alignment layers) than smectic liquid crystal solutions (cf. article by Dupont and American patent n ° US 5 313 562 cited above). In addition, unlike these, it does not present a problem of DC voltage. Finally, it has switching times as low as those of these known smectic liquid crystal solutions.
• the first means for applying the first electric field comprise at least one pair of electrodes disposed in a plane substantially parallel to the substrate plates.
• the first means for applying the first electric field comprise a plurality of pairs of electrodes, making it possible to arbitrarily orient the applied electric field.
• the plurality of pairs of electrodes is arranged in a star, so as to apply a first electric field rotating continuously.
• the first means for applying the first electric field comprise at least one pair of two-dimensional electrodes and produced on one of the faces of one of the two plates.
• the first means of applying the first electric field comprise: at least a first pair of two-dimensional electrodes, produced on the internal face, which is in contact with the contents of the cell, of one of the two plates; at least a second pair of two-dimensional electrodes, produced on the internal face, which is in contact with the contents of the cell, on the other of the two plates; and said at least first and second pairs of two-dimensional electrodes are complementary, so as to increase the penetration depth of the first electric field.
• the first means for applying the first electric field comprise at least one pair three-dimensional electrodes and having a thickness equal to at least a substantial part of the thickness of the contents of the cell.
• the aforementioned second embodiment with the two-dimensional electrodes (that is to say of small thicknesses), the depth of penetration of the electric field into the thickness of the cell remains low and inhomogeneous. It is therefore difficult to obtain significant phase shifts.
• the two substrate plates belong to the group comprising: glass plates and ends of optical fibers.
• the amplitude of the first electric field applied by said first application means is predetermined, so as to obtain a predetermined birefringence modulation as a function of said first electric field.
• said device also comprises second means for applying, to said at least part of the contents of the cell, a second electric field whose amplitude is predetermined, so as to obtain a birefringence modulation predetermined, function of the sum of said first and second electric fields.
• said device comprises means making it possible to vary the amplitude of the first electric field and / or of the second electric, so as to obtain a variable birefringence modulation.
• the invention also relates to the application of the aforementioned polarization control device to the implementation of a polarization modal dispersion compensation (PMD).
• PMD polarization modal dispersion compensation
• FIG. 1 already described previously, presents a simplified diagram of a compensation system for the polarization modal dispersion, comprising a polarization controller, a polarization-maintaining fiber and means for measuring the degree of polarization on the maintenance fiber polarization;
• FIG. 2 presents a simplified diagram of a particular embodiment of the polarization controller according to the present invention;
• FIG. 3 illustrates the application of an electric field perpendicular to the direction of propagation of the light beam;
• FIG. 1 already described previously, presents a simplified diagram of a compensation system for the polarization modal dispersion, comprising a polarization controller, a polarization-maintaining fiber and means for measuring the degree of polarization on the maintenance fiber polarization;
• FIG. 2 presents a simplified diagram of a particular embodiment of the polarization controller according to the present invention;
• FIG. 3 illustrates the application of an electric field perpendicular to the direction of propagation of the light beam;
• FIG. 1 already described previously
• FIG. 4 shows a simplified diagram of a first embodiment of the (two-dimensional) electrodes included in the polarization controller according to the present invention
• FIG. 5 presents a simplified diagram of a second embodiment of the electrodes (three-dimensional) included in the polarization controller according to the present invention
• FIGS. 6 and 7 each show a curve representing the voltages necessary to obtain a phase shift of ⁇ as a function of the thickness of the cell included in the polarization controller according to the present invention, for an inter-electrode space equal to 30 ⁇ m ( fig. 6) and 20 ⁇ m. (fig. 7) respectively.
• the invention therefore relates to a polarization controller which can in particular be implemented within a compensation system for polarization modal dispersion (PMD), as described above in relation to FIG. 1.
• PMD polarization modal dispersion
• 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 "nano-PDLC" material.
• the latter comprises a polymer 8 in which liquid crystal droplets 9 are dispersed, the dimension of which (for example a few tens of nanometers) is less, at least in a ratio of ten, to the wavelength of the light beam; at least one pair of electrodes (see detailed discussion below, in connection with FIGS. 4 and 5) making it possible to apply, to at least part of the contents of the cell, an electric field E (see FIGS.
• the operating principle is as follows: depending on whether the electric field is applied or not, at least part of the contents of the cell constitute a birefringent or isotropic medium respectively.
• N Xn c l (E) + (l- ⁇ ) n P olymer (1)
• x denotes the relative proportion of liquid crystal compared to that of the polymer
• n ol (E) denotes the average liquid crystal index on all the droplets, which depends on the electric field E applied, and can be written:
• the graphs of FIGS. 6 and 7 summarize the voltages necessary to obtain a phase shift of ⁇ as a function of the thickness of the cell of "nano-PDLC" material, for an inter-electrode space d equal to 30 ⁇ m (fig.6) and 20 ⁇ m (fig. 7) respectively. It is assumed that the maximum index variations are of the order of 0.013 for applied electric fields of 20 V / ⁇ m. It can be seen that the phase shifts can be obtained from small thicknesses. This point is important because it makes it possible to envisage a device without collimation optics, the losses by beam divergence remaining very low (by example 0.1 dB for a conventional single-mode optical fiber, and insignificant for an extended core optical fiber).
• an electrode system is adopted for at least one of the two glass plates 6 and 7 comprising a plurality (three for example) of pairs of electrodes arranged in a star in a plane substantially parallel to the plate. of affected glass.
• 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.
• the electrode system of the glass plate referenced 6 comprises three pairs of two-dimensional electrodes (41a, 41b), (42a, 42b) and (43a, 43b). These electrodes are produced on one of the faces of the glass plate, preferably the internal face, which is in contact with the "nano-PDLC" material contained in the cell. These electrodes can be obtained by photolithography, either from a transparent conductive deposit (ITO), or from a metal deposit. For example, “LIGA” technology is used, which includes an electrolytic growth stage. By applying phase-shifted voltages to each of the electrodes of the same pair of electrodes (for example those referenced 41a and 41b in FIG. 4), an electric field E parallel to the glass plate is generated. The beam passing through the center of the electrode system then sees a birefringent material.
• the glass plates can both have complementary electrode systems, which makes it possible to increase the depth of penetration ((along the axis Oy) of the electric field.
• the electrode system (common to the two glass plates) comprises three pairs of three-dimensional electrodes (51a, 51b), (52a, 52b) and (53a, 53b) ("massive") very thick (a few tens of microns thick).
• These electrodes can be made of conductive materials (metals) or semiconductors (Silicon or others). They can be obtained either by photolitography of substrates, or by the use of micro-tips.
• the advantages of the polarization controller according to the invention are in particular the following: uniformity of the applied electric field; a dual function of director axis rotation and index modulation by means of a single tension control; a simplified electronic interface; great mechanical robustness; good confinement of the material allowing the expected effect on small thicknesses of active component (10-15 ⁇ m); a useful pupil of a few tens of ⁇ m (typically 30 ⁇ m) compatible with the use of single-mode or single-mode fibers with an extended core or comprising an external collimation micro-optic and allowing the use of reasonable voltages.
• the thickness of "nano-PDLC” is however small enough for the optical beam coming from a fiber does not diverge when passing through the material.
• the specifications for the optical compensation device are:
• the polarization controller comprises: in addition to the aforementioned (first) means for applying an electric field perpendicular to the direction of propagation of light, other (second) means for applying a second electric field.
• the birefringence modulation obtained is in this case a function of the sum of the first and second electric fields.
• a predetermined birefringence modulation is obtained.

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• Physics & Mathematics (AREA)
• Nonlinear Science (AREA)
• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)
• Liquid Crystal (AREA)

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• 2001
  • 2001-12-17 FR FR0116341A patent/FR2833720B1/fr not_active Expired - Fee Related
• 2002
  • 2002-12-17 WO PCT/FR2002/004422 patent/WO2003052496A2/fr not_active Application Discontinuation
  • 2002-12-17 US US10/499,213 patent/US20050007519A1/en not_active Abandoned
  • 2002-12-17 JP JP2003553325A patent/JP2005513528A/ja active Pending
  • 2002-12-17 CN CNA028251520A patent/CN1605040A/zh active Pending
  • 2002-12-17 EP EP02804941A patent/EP1456712A2/de not_active Withdrawn

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