WO2003013033A1 - Electro-optical device for channel equalization in a multichannel optical link - Google Patents

Electro-optical device for channel equalization in a multichannel optical link Download PDF

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Publication number
WO2003013033A1
WO2003013033A1 PCT/FR2002/002683 FR0202683W WO03013033A1 WO 2003013033 A1 WO2003013033 A1 WO 2003013033A1 FR 0202683 W FR0202683 W FR 0202683W WO 03013033 A1 WO03013033 A1 WO 03013033A1
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WO
WIPO (PCT)
Prior art keywords
channel
network
optical
lens
modulator
Prior art date
Application number
PCT/FR2002/002683
Other languages
French (fr)
Inventor
Jean-Pierre Huignard
Brigitte Loiseaux
Original Assignee
Thales
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Publication of WO2003013033A1 publication Critical patent/WO2003013033A1/en

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29379—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device
    • G02B6/29391—Power equalisation of different channels, e.g. power flattening
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29304—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by diffraction, e.g. grating
    • G02B6/29305—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by diffraction, e.g. grating as bulk element, i.e. free space arrangement external to a light guide
    • G02B6/2931—Diffractive element operating in reflection
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/29—Repeaters
    • H04B10/291—Repeaters in which processing or amplification is carried out without conversion of the main signal from optical form
    • H04B10/293—Signal power control
    • H04B10/294—Signal power control in a multiwavelength system, e.g. gain equalisation
    • H04B10/2941—Signal power control in a multiwavelength system, e.g. gain equalisation using an equalising unit, e.g. a filter
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/501—Structural aspects
    • H04B10/506—Multiwavelength transmitters
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/564—Power control
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/264—Optical coupling means with optical elements between opposed fibre ends which perform a function other than beam splitting
    • G02B6/266—Optical coupling means with optical elements between opposed fibre ends which perform a function other than beam splitting the optical element being an attenuator
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J14/00—Optical multiplex systems
    • H04J14/02—Wavelength-division multiplex systems
    • H04J14/0221—Power control, e.g. to keep the total optical power constant

Definitions

  • the present invention relates to an electro-optical device for equalizing channels in a multi-channel optical link.
  • WDM wavelength multiplexing
  • WDM wavelength multiplexing
  • This condition is difficult to achieve in practice, given the spectral characteristics of the passive and active optical components present on the transmission link (multi-electric layers, amplifiers with fiber doped with Erbium, Raman fibers, etc.). These components have a non-uniform frequency response in the transmission wavelength band (for example in the 1500-1600 nm band).
  • the characteristics of the transmission line change during the reconfiguration of the transmission network (number of amplifier modules crossed, optical components for multiplexing and demultiplexing, filters ).
  • the distance between adjacent channels in a WDM link is approximately 0.5 to 1 nm.
  • the distance between adjacent channels in a WDM link is approximately 0.5 to 1 nm.
  • Known solutions for equalizing optical links are limited to a small number of channels (generally 10 to 20 channels) and produce a function of selective attenuation of the channels using optical components with fixed characteristics (for example components of the Bragg grating type with a period varying from one end to the other of the grating).
  • the present invention relates to an electro-optical device for equalizing the channels in a multichannel optical link, device which can simultaneously process a large number of channels (for example at least 100 channels), in an adaptable manner (programmable or by servo), and this, for channels close to each other (0.5 nm apart for example) and whatever the respective polarization of the different channels.
  • the device according to the invention comprises a channel separator device, a selective, and preferably controlled attenuation device, channels and a channel recombination device.
  • FIG. 1 is a block diagram of a device equalization according to the invention
  • Figure 2 is a simplified embodiment of an equalization device according to the invention.
  • Figure 3 is a set of simplified sectional views of a spatial modulator used in the device of Figure 2, showing a cell of this modulator in the "non-passing" state and in the "passing" state;
  • FIG. 4 shows details of embodiment of the modulator used for the invention
  • FIG. 5 is a diagram showing the evolution of the light transmission characteristic of the cell in FIG. 4 as a function of the electric voltage applied to it, in the “direct” case (in solid line) and in the “reverse” case (broken line);
  • Figures 6 to 8 are diagrams explaining the operation of a compact structure of the modulator of the invention.
  • FIG. 1 shows the block diagram of an optical transmission link including the equalizer device of the invention.
  • This link comprises an optical fiber 1 which carries a WDM signal with a large number of channels (for example 100).
  • the signal 1A available at the output of fiber 1 comprises spectral components of different amplitudes, as a result of the crossing of optical components with different spectral responses.
  • This signal 1A passes through the equalizer 2 of the invention, described in detail below, at the output of which the signal 2A is not equalized, but has a form different from that of signal 1A, and this, to take account of the spectral response of the set 3 of amplifiers and optical components through which this signal passes.
  • this signal 2A is complementary to the spectral response of the set 3, so that at the output of the set 3, the signal 3A is equalized, that is to say that is, it has a constant amplitude for all the wavelengths contained in this signal.
  • a servo link is provided between the output of the assembly 3 and the equalizer 2 so that the latter can compensate for any drift, or variation in the spectral response of the assembly 3.
  • FIG. 2 is a diagram of an embodiment of an equalizer 5 according to the invention.
  • the point 10 of incidence of the beam 8 on the grating 9 is disposed at the focal point object of a converging lens 11. Downstream of the lens 11, there is a one-dimensional spatial modulator 12, so that the majority of the length of its surface active is intercepted by the beam spread by the network 9 and collimated by the lens 11.
  • the modulator 12 is followed by a converging lens 13.
  • a dispersive network 14 identical to the network 9, which is arranged so that the image focal point 15 of the lens 13 is located on the active surface of the grating 14.
  • the spatially spread spectrum beam, which arrives from the lens 13 on the grating 14, is reflected on the latter by concentrating in a fine bundle 16.
  • This bundle 16 passes through a coupler 17 to an output fiber 18.
  • the equalizer is constituted by the modulator 12, and the optical elements 7, 9, 14, 17 (as well as any optical amplifiers, not shown) which constitute the assembly 3.
  • This device 5 operates from the following way.
  • the n different WDM channels contained in the incident optical signal arriving by the fiber 6 are spatially dispersed (or “spread out”) in a plane by the network 9 in an angular field (or sector) of angle at the apex ⁇ , which is determined by the width ⁇ of the spectral band of the incident signal and by the spatial period p of this network (or not of the network), and which is given by: ⁇ z (1 / p) ⁇ .
  • Each of these n channels, ⁇ i to ⁇ n intercepts an individual cell of the liquid crystal modulator 12.
  • Each of these cells is controlled so as to be more or less busy, in order to attenuate more or less the corresponding channel, to obtain, downstream of the coupler 17, the desired equalized signal.
  • This modulator comprises a bar made of material called "PDLC” ("Polymer Dispersed Liquid Crystal”), that is to say liquid crystal molecules 19 dispersed in a polymer material 20 (Figure 3).
  • PDLC Polymer Dispersed Liquid Crystal
  • This bar has on one side a continuous electrode 21 and on the other side a network of small electrodes 22 (like the teeth of a comb) individually connected to a corresponding individual voltage source (see FIG. 4).
  • FIG. 3 shows one of the cells of the bar.
  • a zero voltage is applied between the electrode 21 and an electrode 22.
  • the liquid crystal molecules 19 present between these electrodes are all oriented differently, and it follows that an incident light beam 23 is broadcast.
  • the molecules 19 are all oriented parallel to the direction of the electric field thus produced, that is to say perpendicular to the planes of these electrodes.
  • the incident beam 23 which is also perpendicular to the planes of the electrodes, is transmitted practically without losses and without being deflected. If the voltage applied between the electrodes 21 and 22 is varied from a zero value to the value V s , it is noted that the intensity of the beam collected downstream of the cell considered varies from a practically zero value up to at its maximum value (practically equal to the intensity of the incident beam).
  • This strip 24 is constituted by a layer 25 of “PDLC” material, of elongated rectangular shape, the thickness of which can be between 20 and 100 ⁇ m.
  • PDLC elongated rectangular shape
  • One of the faces of this layer comprises over its entire length and over a little less than half of its width, from one of its long sides, a continuous electrode 26.
  • the other face comprises a network 27 d 'individual electrodes, for example 100 electrodes, the number of which is equal to or greater than the number of WDM channels to be treated.
  • the outline of all of these electrodes is a rectangle extending parallel to the long sides of the strip and the short side of which is shorter than half the short side of the PDLC layer.
  • this contour rectangle occupies the part of the surface of the PDLC layer, which is not opposite the surface occupied by the electrode 26, being slightly offset from the long side of the electrode 26 which is closest to it so that there remains a space of width E (as seen in orthogonal projection on the plane of one of the faces of the PDLC layer).
  • the value of E is for example 50 ⁇ m.
  • Each of these elementary cells makes it possible to control the amplitude of a corresponding spectral component, provided, of course, that each spectral component arrives in the space between an individual electrode and the common electrode.
  • the thickness of the PDLC layer can be for example between 20 and 100 ⁇ m.
  • the PDLC material is of the “reverse PDLC” type, that is to say that for a zero voltage between the electrodes considered, the incident beam is practically not attenuated, while for an equal voltage or higher than a threshold voltage V s , the material is diffusing.
  • FIG. 5 shows an example of transmission curves relating to a “direct” PDLC material (curve 28) and to a “reverse” PDLC material (curve 29). These curves show the evolution of the transmission factor T as a function of the voltage V applied to the electrodes formed or attached to the PDLC material considered.
  • the networks 9 and 14 had a density of 1000 lines per millimeter, the PDLC modulator had a useful length of 10 mm and its threshold voltage V s was of the order of 50 V at most.
  • the spectral width of the incident signal was 100 nm, the focal distance of the lenses 11 and 13 was approximately 80 mm and the individual electrodes of the assembly 27 were 50 ⁇ m wide and their pitch was 100 ⁇ m.
  • the device 30 of these figures comprises a single dispersion network 31, a single lens 32 and a reflective spatial modulator 33.
  • the point of incidence 34 of the incident WDM beam 35 is located in the object focal plane (relative to the direction of propagation of the beam 35) of the lens 32.
  • the grating 31 forms with respect to the optical axis of the lens 32 an angle such that all the diffracted rays (spectral components of the WDM signal) by the grating 31 from the incident beam 35 pass through the lens 32 and are directed towards the modulator 33.
  • This modulator 33 being reflective, as described below with reference to FIG. 8, it returns the spectral components modulated in amplitude (in the same way as described above for the modulator 12) towards the lens, which concentrates around its focus 34 on the network 31.
  • the network 31 recomposes an output beam 36 antiparallel to the beam 35 and equalized.
  • the modulator is made to rotate slightly around an axis passing through the optical axis of the lens 32, at a point 37.
  • This axis can either be perpendicular to the plane in which the spectral components of the incident beam are spread, as shown in FIG. 6, or be included in this plan, as shown in figure 7, the configuration of figure 6 being the preferred configuration (for optical reasons, ease of construction and compactness).
  • the modulator 33 is shown in the section view of FIG. 8.
  • the layer 38 of PDLC material has on one side a common reflecting electrode 39, which can occupy a large part of the width of this side, and on the other facing an array 40 of individual electrodes similar to the array 27 described above.
  • This network 40 occupies only part of the width of the layer 38, facing an edge of the electrode 39.
  • each individual electrode 40 and the common electrode 40 must take account of a double crossing of the layer 38 (or else, the characteristics of this layer 38 must be modified if one wishes to apply potentials of the same value as in the case of the modulator 12 of FIG. 2).

Abstract

The invention concerns an equalization device comprising a dispersive array (9) receiving the multichannel beam to be equalized (8), a convergent lens (11), a spatial modulator (12), another convergent lens (13) and a second array, identical to the first (14), for recombining the rays after they have been equalized. The invention is applicable to equalization of WDM links.

Description

DISPOSITIF ELECTRO-OPTIQUE D'EGALISATION DES CANAUX DANS UNE LIAISON OPTIQUE MULTICANAUX ELECTRO-OPTICAL DEVICE FOR EQUALIZING CHANNELS IN A MULTI-CHANNEL OPTICAL LINK
La présente invention se rapporte à un dispositif électro-optique d'égalisation des canaux dans une liaison optique multicanaux.The present invention relates to an electro-optical device for equalizing channels in a multi-channel optical link.
Dans les transmissions multicanaux d'informations par voie optique, en particulier les transmissions par multiplexage de longueur d'onde (dites « WDM », soit : « Wavelength Division Multiplexing »), il est important de maintenir un niveau de signal constant quel que soit le canal de transmission considéré parmi l'ensemble des canaux. Cette condition est difficile à réaliser en pratique, compte tenu des caractéristiques spectrales des composants optiques passifs et actifs présents sur la liaison de transmission (couches multidiélectriques, amplificateurs à fibre dopée à l'Erbium, fibres Raman, ...). Ces composants ont une réponse en fréquence non uniforme dans la bande de longueurs d'ondes de transmission (par exemple dans la bande 1500-1600 nm). De plus, les caractéristiques de la ligne de transmission évoluent lors de la reconfiguration du réseau de transmission (nombre de modules amplificateurs traversés, de composants optiques de multiplexage et de démultiplexage, filtres ...). Il est donc important de pouvoir mettre en œuvre un dispositif programmable ou asservi permettant d'assurer dans la liaison de transmission, et en particulier à la sortie des amplificateurs de cette liaison, une répartition uniforme de la puissance optique sur tous les canaux de cette liaison. De façon typique, la distance entre canaux adjacents dans une liaison WDM est d'environ 0,5 à 1 nm. Ainsi, par exemple dans la bande 1500-1600 nm, on peut loger environ 100 canaux, dont le niveau doit être égalisé de façon programmable ou asservie. Les solutions connues d'égalisation de liaisons optiques sont limitées à un faible nombre de canaux (en général 10 à 20 canaux) et produisent une fonction d'atténuation sélective des canaux à l'aide de composants optiques à caractéristiques fixes (par exemple composants du type réseau de Bragg à période variant d'une extrémité à l'autre du réseau). La présente invention a pour objet un dispositif électro-optique d'égalisation des canaux dans une liaison optique multicanaux, dispositif qui puisse traiter simultanément un grand nombre de canaux (par exemple au moins 100 canaux), de façon adaptable (programmable ou par asservissement), et ce, pour des canaux proches les uns des autres (distants de 0,5 nm par exemple) et quelle que soit la polarisation respective des différents canaux.In multichannel information transmission by optical means, in particular transmissions by wavelength multiplexing (called "WDM", that is to say "Wavelength Division Multiplexing"), it is important to maintain a constant signal level whatever the transmission channel considered among all the channels. This condition is difficult to achieve in practice, given the spectral characteristics of the passive and active optical components present on the transmission link (multi-electric layers, amplifiers with fiber doped with Erbium, Raman fibers, etc.). These components have a non-uniform frequency response in the transmission wavelength band (for example in the 1500-1600 nm band). In addition, the characteristics of the transmission line change during the reconfiguration of the transmission network (number of amplifier modules crossed, optical components for multiplexing and demultiplexing, filters ...). It is therefore important to be able to implement a programmable or controlled device making it possible to ensure in the transmission link, and in particular at the output of the amplifiers of this link, a uniform distribution of the optical power on all the channels of this link. . Typically, the distance between adjacent channels in a WDM link is approximately 0.5 to 1 nm. Thus, for example in the 1500-1600 nm band, it is possible to accommodate approximately 100 channels, the level of which must be programmable or slaved equalized. Known solutions for equalizing optical links are limited to a small number of channels (generally 10 to 20 channels) and produce a function of selective attenuation of the channels using optical components with fixed characteristics (for example components of the Bragg grating type with a period varying from one end to the other of the grating). The present invention relates to an electro-optical device for equalizing the channels in a multichannel optical link, device which can simultaneously process a large number of channels (for example at least 100 channels), in an adaptable manner (programmable or by servo), and this, for channels close to each other (0.5 nm apart for example) and whatever the respective polarization of the different channels.
Le dispositif conforme à l'invention comporte un dispositif séparateur de canaux, un dispositif d'atténuation sélective, et de préférence asservie, des canaux et un dispositif de recombinaison de canaux.The device according to the invention comprises a channel separator device, a selective, and preferably controlled attenuation device, channels and a channel recombination device.
La présente invention sera mieux comprise à la lecture de la description détaillée d'un mode de réalisation, pris à titre d'exemple non limitatif et illustré par le dessin annexé, sur lequel : • la figure 1 est un schéma synoptique d'un dispositif d'égalisation conforme à l'invention ;The present invention will be better understood on reading the detailed description of an embodiment, taken by way of nonlimiting example and illustrated by the appended drawing, in which: • FIG. 1 is a block diagram of a device equalization according to the invention;
• la figure 2 est un schéma de réalisation simplifié d'un dispositif d'égalisation conforme à l'invention ;• Figure 2 is a simplified embodiment of an equalization device according to the invention;
• la figure 3 est un ensemble de vues en coupe simplifiées d'un modulateur spatial utilisé dans le dispositif de la figure 2, montrant une cellule de ce modulateur à l'état « non passant » et à l'état « passant » ;• Figure 3 is a set of simplified sectional views of a spatial modulator used in the device of Figure 2, showing a cell of this modulator in the "non-passing" state and in the "passing" state;
• la figure 4 montre des détails de réalisation du modulateur utilisé pour l'invention ; • la figure 5 est un diagramme montrant l'évolution de la caractéristique de transmission de lumière de la cellule de la figure 4 en fonction de la tension électrique qui lui est appliquée, dans le cas « direct » (en trait continu) et dans le cas « inverse » (en trait interrompu) ; et • les figures 6 à 8 sont des schémas expliquant le fonctionnement d'une structure compacte du modulateur de l'invention.• Figure 4 shows details of embodiment of the modulator used for the invention; FIG. 5 is a diagram showing the evolution of the light transmission characteristic of the cell in FIG. 4 as a function of the electric voltage applied to it, in the “direct” case (in solid line) and in the "reverse" case (broken line); and • Figures 6 to 8 are diagrams explaining the operation of a compact structure of the modulator of the invention.
On a représenté en figure 1 le bloc-diagramme d'une liaison de transmission optique incluant le dispositif égaliseur de l'invention. Cette liaison comporte une fibre optique 1 qui véhicule un signal WDM à grand nombre de canaux (par exemple 100). Le signal 1A disponible à la sortie de la fibre 1 comprend des composantes spectrales d'amplitudes différentes, par suite de la traversée de composants optiques à réponses spectrales différentes. Ce signal 1A passe par l'égaliseur 2 de l'invention, décrit en détail ci-dessous, à la sortie duquel le signal 2A n'est pas égalisé, mais a une forme différente de celle du signal 1A, et ce, pour tenir compte de la réponse spectrale de l'ensemble 3 d'amplificateurs et de composants optiques par lesquels passe ce signal. La forme (en fonction de la longueur d'onde) de ce signal 2A est complémentaire de la réponse spectrale de l'ensemble 3, afin qu'à la sortie de l'ensemble 3, le signal 3A soit égalisé, c'est-à-dire qu'il ait une amplitude constante pour toutes les longueurs d'onde contenues dans ce signal. A cet effet, on prévoit une liaison d'asservissement entre la sortie de l'ensemble 3 et l'égaliseur 2 pour que ce dernier puisse compenser toute dérive, ou variation de la réponse spectrale de l'ensemble 3. On a représenté en figure 2 le schéma d'un mode de réalisation d'un égaliseur 5 conforme à l'invention. Une fibre optique 6, par laquelle arrive un signal WDM, est reliée à l'entrée de l'égaliseur 5, constituée par un coupleur 7 qui envoie obliquement un faisceau optique rectiligne 8 sur un réseau dispersif large bande 9. Le point 10 d'incidence du faisceau 8 sur le réseau 9 est disposé au point focal objet d'une lentille convergente 11. En aval de la lentille 11 , on dispose un modulateur spatial 12 à une dimension, de telle façon que la majorité de la longueur de sa surface active soit interceptée par le faisceau étalé par le réseau 9 et collimaté par la lentille 11. Le modulateur 12 est suivi d'une lentille convergente 13. On dispose en aval de la lentille 13 un réseau dispersif 14, identique au réseau 9, qui est disposé de façon que le point focal image 15 de la lentille 13 soit situé sur la surface active du réseau 14. Le faisceau à spectre étalé spatialement, qui arrive de la lentille 13 sur le réseau 14, se réfléchit sur ce dernier en se concentrant en un fin faisceau 16. Ce faisceau 16 passe par un coupleur 17 vers une fibre de sortie 18.FIG. 1 shows the block diagram of an optical transmission link including the equalizer device of the invention. This link comprises an optical fiber 1 which carries a WDM signal with a large number of channels (for example 100). The signal 1A available at the output of fiber 1 comprises spectral components of different amplitudes, as a result of the crossing of optical components with different spectral responses. This signal 1A passes through the equalizer 2 of the invention, described in detail below, at the output of which the signal 2A is not equalized, but has a form different from that of signal 1A, and this, to take account of the spectral response of the set 3 of amplifiers and optical components through which this signal passes. The shape (as a function of the wavelength) of this signal 2A is complementary to the spectral response of the set 3, so that at the output of the set 3, the signal 3A is equalized, that is to say that is, it has a constant amplitude for all the wavelengths contained in this signal. To this end, a servo link is provided between the output of the assembly 3 and the equalizer 2 so that the latter can compensate for any drift, or variation in the spectral response of the assembly 3. We have shown in FIG. 2 is a diagram of an embodiment of an equalizer 5 according to the invention. An optical fiber 6, through which a WDM signal arrives, is connected to the input of the equalizer 5, constituted by a coupler 7 which obliquely sends a rectilinear optical beam 8 over a broadband dispersive network 9. The point 10 of incidence of the beam 8 on the grating 9 is disposed at the focal point object of a converging lens 11. Downstream of the lens 11, there is a one-dimensional spatial modulator 12, so that the majority of the length of its surface active is intercepted by the beam spread by the network 9 and collimated by the lens 11. The modulator 12 is followed by a converging lens 13. There is downstream of the lens 13 a dispersive network 14, identical to the network 9, which is arranged so that the image focal point 15 of the lens 13 is located on the active surface of the grating 14. The spatially spread spectrum beam, which arrives from the lens 13 on the grating 14, is reflected on the latter by concentrating in a fine bundle 16. This bundle 16 passes through a coupler 17 to an output fiber 18.
Dans ce dispositif 5, l'égaliseur est constitué par le modulateur 12, et les éléments optiques 7, 9, 14, 17 (ainsi que d'éventuels amplificateurs optiques, non représentés) qui constituent l'ensemble 3. Ce dispositif 5 fonctionne de la manière suivante. Les n différents canaux WDM contenus dans le signal optique incident arrivant par la fibre 6 sont dispersés spatialement (ou « étalés ») dans un plan par le réseau 9 dans un champ (ou secteur) angulaire d'angle au sommet Δθ, qui est déterminé par la largeur Δλ de la bande spectrale du signal incident et par la période spatiale p de ce réseau (ou pas du réseau), et qui est donné par : Δθ z (1/p) Δλ. Chacun de ces n canaux, λi à λn, intercepte une cellule individuelle du modulateur 12 à cristaux liquides. Chacune de ces cellules est commandée de façon à être plus ou moins passante, afin d'atténuer plus ou moins le canal correspondant, pour obtenir, en aval du coupleur 17, le signal égalisé recherché.In this device 5, the equalizer is constituted by the modulator 12, and the optical elements 7, 9, 14, 17 (as well as any optical amplifiers, not shown) which constitute the assembly 3. This device 5 operates from the following way. The n different WDM channels contained in the incident optical signal arriving by the fiber 6 are spatially dispersed (or “spread out”) in a plane by the network 9 in an angular field (or sector) of angle at the apex Δθ, which is determined by the width Δλ of the spectral band of the incident signal and by the spatial period p of this network (or not of the network), and which is given by: Δθ z (1 / p) Δλ. Each of these n channels, λi to λ n , intercepts an individual cell of the liquid crystal modulator 12. Each of these cells is controlled so as to be more or less busy, in order to attenuate more or less the corresponding channel, to obtain, downstream of the coupler 17, the desired equalized signal.
On va maintenant décrire un mode de réalisation du modulateur 12. Ce modulateur comporte une barrette en matériau dit « PDLC » (« Polymer Dispersed Liquid Crystal »), c'est-à-dire en molécules de cristaux liquides 19 dispersées dans un matériau polymère 20 (figure 3). Cette barrette comporte sur une face une électrode continue 21 et sur l'autre face un réseau de petites électrodes 22 (à la façon des dents d'un peigne) reliées individuellement à une source de tension individuelle correspondante (voir figure 4). On a représenté en figure 3 une des cellules de la barrette. A la partie gauche de cette figure, une tension nulle est appliquée entre l'électrode 21 et une électrode 22. Les molécules de cristaux liquides 19 présentes entre ces électrodes sont toutes orientées différemment, et il en résulte qu'un faisceau lumineux incident 23 est diffusé.We will now describe an embodiment of the modulator 12. This modulator comprises a bar made of material called "PDLC" ("Polymer Dispersed Liquid Crystal"), that is to say liquid crystal molecules 19 dispersed in a polymer material 20 (Figure 3). This bar has on one side a continuous electrode 21 and on the other side a network of small electrodes 22 (like the teeth of a comb) individually connected to a corresponding individual voltage source (see FIG. 4). FIG. 3 shows one of the cells of the bar. On the left side of this figure, a zero voltage is applied between the electrode 21 and an electrode 22. The liquid crystal molecules 19 present between these electrodes are all oriented differently, and it follows that an incident light beam 23 is broadcast.
Par contre, lorsqu'une tension suffisante Vs est appliquée entre les électrodes 21 et 22, les molécules 19 sont toutes orientées parallèlement à la direction du champ électrique ainsi produit, c'est-à-dire perpendiculairement aux plans de ces électrodes. Il en résulte, comme illustré sur la partie droite de la figure 3, que le faisceau incident 23, qui est, lui aussi perpendiculaire aux plans des électrodes, est transmis pratiquement sans pertes et sans être dévié. Si l'on fait varier la tension appliquée entre les électrodes 21 et 22 depuis une valeur nulle jusqu'à la valeur Vs, on constate que l'intensité du faisceau recueilli en aval de la cellule considérée varie depuis une valeur pratiquement nulle jusqu'à sa valeur maximale (pratiquement égale à l'intensité du faisceau incident). Il est donc possible avec une barrette interceptant les différentes composantes spectrales d'un signal d'agir individuellement sur l'amplitude de chacune de ces composantes, étant bien entendu que la dispersion spatiale des composantes au niveau de la barrette corresponde au pas (ou à un multiple de ce pas) des électrodes 22 de la barrette. Cette correction, qui est une diminution sélective d'amplitude des composantes, se fait, pour chacune des composantes, indépendamment de la correction affectant toutes les autres composantes. La correction peut se faire soit une fois pour toutes (si les conditions de fonctionnement des composants optiques actifs et/ou passifs en cause ne varient pas), soit de façon asservie, grâce à la mesure de l'amplitude des diverses composantes à la fin de la chaîne de transmission optique à contrôler. On a représenté en figure 4 des détails de réalisation de la barrette à cristaux liquides faisant fonction de modulateur spatial. Cette barrette 24 est constituée par une couche 25 de matériau « PDLC », de forme rectangulaire allongée, dont l'épaisseur peut être comprise entre 20 et 100 μm. L'une des faces de cette couche comporte sur toute sa longueur et sur un peu moins de la moitié de sa largeur, à partir de l'un de ses grands côtés, une électrode continue 26. L'autre face comporte un réseau 27 d'électrodes individuelles, par exemple 100 électrodes, dont le nombre est égal ou supérieur au nombre de voies WDM à traiter. Le contour de l'ensemble de ces électrodes est un rectangle s'étendant parallèlement aux grands côtés de la barrette et dont le petit côté est plus court que la moitié du petit côté de la couche de PDLC. En outre, ce rectangle de contour occupe la partie de la surface de la couche PDLC, qui n'est pas en vis-à-vis de la surface occupée par l'électrode 26, en étant légèrement décalée par rapport au grand côté de l'électrode 26 qui en est le plus proche de telle façon qu'il subsiste un espace de largeur E (tel que vu en projection orthogonale sur le plan de l'une des faces de la couche PDLC).On the other hand, when a sufficient voltage V s is applied between the electrodes 21 and 22, the molecules 19 are all oriented parallel to the direction of the electric field thus produced, that is to say perpendicular to the planes of these electrodes. As a result, as illustrated on the right-hand side of FIG. 3, the incident beam 23, which is also perpendicular to the planes of the electrodes, is transmitted practically without losses and without being deflected. If the voltage applied between the electrodes 21 and 22 is varied from a zero value to the value V s , it is noted that the intensity of the beam collected downstream of the cell considered varies from a practically zero value up to at its maximum value (practically equal to the intensity of the incident beam). It is therefore possible with a bar intercepting the different spectral components of a signal to act individually on the amplitude of each of these components, it being understood that the spatial dispersion of the components at the level of the bar corresponds to the pitch (or to a multiple of this step) of the electrodes 22 of the strip. This correction, which is a selective reduction in amplitude of the components, is done, for each of the components, independently of the correction affecting all the other components. The correction can be do this either once and for all (if the operating conditions of the active and / or passive optical components in question do not vary), or in a controlled manner, by measuring the amplitude of the various components at the end of the optical transmission to be checked. FIG. 4 shows details of the embodiment of the liquid crystal array acting as a spatial modulator. This strip 24 is constituted by a layer 25 of “PDLC” material, of elongated rectangular shape, the thickness of which can be between 20 and 100 μm. One of the faces of this layer comprises over its entire length and over a little less than half of its width, from one of its long sides, a continuous electrode 26. The other face comprises a network 27 d 'individual electrodes, for example 100 electrodes, the number of which is equal to or greater than the number of WDM channels to be treated. The outline of all of these electrodes is a rectangle extending parallel to the long sides of the strip and the short side of which is shorter than half the short side of the PDLC layer. In addition, this contour rectangle occupies the part of the surface of the PDLC layer, which is not opposite the surface occupied by the electrode 26, being slightly offset from the long side of the electrode 26 which is closest to it so that there remains a space of width E (as seen in orthogonal projection on the plane of one of the faces of the PDLC layer).
La valeur de E est par exemple de 50 μm. On définit ainsi entre chacune des électrodes élémentaires du réseau 27 et l'électrode commune 26 une cellule élémentaire dont le fonctionnement est celui exposé ci-dessus en référence à la figure 3. Chacune de ces cellules élémentaires permet de contrôler l'amplitude d'une composante spectrale correspondante, à condition, bien entendu, que chaque composante spectrale arrive dans l'espace compris entre une électrode individuelle et l'électrode commune. L'épaisseur de la couche de PDLC peut être comprise par exemple entre 20 et 100 μm. Dans un exemple de réalisation, le matériau PDLC est du type « PDLC inverse », c'est-à-dire que pour une tension nulle entre les électrodes considérées, le faisceau incident n'est pratiquement pas atténué, tandis que pour une tension égale ou supérieure à une tension de seuil Vs, le matériau est diffusant. On a représenté en figure 5 un exemple de courbes de transmission relatives à un matériau PDLC « direct » (courbe 28) et à un matériau PDLC « inverse » (courbe 29). Ces courbes montrent l'évolution du facteur de transmission T en fonction de la tension V appliquée aux électrodes formées ou rapportées sur le matériau PDLC considéré.The value of E is for example 50 μm. There is thus defined between each of the elementary electrodes of the network 27 and the common electrode 26 an elementary cell whose operation is that described above with reference to FIG. 3. Each of these elementary cells makes it possible to control the amplitude of a corresponding spectral component, provided, of course, that each spectral component arrives in the space between an individual electrode and the common electrode. The thickness of the PDLC layer can be for example between 20 and 100 μm. In an exemplary embodiment, the PDLC material is of the “reverse PDLC” type, that is to say that for a zero voltage between the electrodes considered, the incident beam is practically not attenuated, while for an equal voltage or higher than a threshold voltage V s , the material is diffusing. FIG. 5 shows an example of transmission curves relating to a “direct” PDLC material (curve 28) and to a “reverse” PDLC material (curve 29). These curves show the evolution of the transmission factor T as a function of the voltage V applied to the electrodes formed or attached to the PDLC material considered.
Selon un exemple de réalisation, les réseaux 9 et 14 présentaient une densité de 1000 traits par millimètre, le modulateur PDLC avait une longueur utile de 10 mm et sa tension de seuil Vs était de l'ordre de 50 V au maximum. La largeur spectrale du signal incident était de 100 nm, la distance focale des lentilles 11 et 13 était d'environ 80 mm et les électrodes individuelles de l'ensemble 27 avaient une largeur de 50 μm et leur pas était de 100 μm.According to an exemplary embodiment, the networks 9 and 14 had a density of 1000 lines per millimeter, the PDLC modulator had a useful length of 10 mm and its threshold voltage V s was of the order of 50 V at most. The spectral width of the incident signal was 100 nm, the focal distance of the lenses 11 and 13 was approximately 80 mm and the individual electrodes of the assembly 27 were 50 μm wide and their pitch was 100 μm.
On a représenté en figures 6 et 7 un mode de réalisation compact du dispositif de la figure 2. Le dispositif 30 de ces figures comporte un unique réseau de dispersion 31 , une unique lentille 32 et un modulateur spatial réfléchissant 33. Le point d'incidence 34 du faisceau WDM incident 35 est situé dans le plan focal objet (par rapport au sens de propagation du faisceau 35) de la lentille 32.There is shown in Figures 6 and 7 a compact embodiment of the device of Figure 2. The device 30 of these figures comprises a single dispersion network 31, a single lens 32 and a reflective spatial modulator 33. The point of incidence 34 of the incident WDM beam 35 is located in the object focal plane (relative to the direction of propagation of the beam 35) of the lens 32.
Le réseau 31 forme par rapport à l'axe optique de la lentille 32 un angle tel que tous les rayons diffractés (composantes spectrales du signal WDM) par le réseau 31 à partir du faisceau incident 35 passent par la lentille 32 et soient dirigés vers le modulateur 33. Ce modulateur 33 étant réfléchissant, comme décrit ci-dessous en référence à la figure 8, il renvoie les composantes spectrales modulées en amplitude (de la même façon que décrit ci-dessus pour le modulateur 12) vers la lentille, qui les concentre autour de son foyer 34 sur le réseau 31. A son tour, le réseau 31 recompose un faisceau de sortie 36 antiparallèle au faisceau 35 et égalisé. Pour que ce faisceau 36 ne soit pas confondu avec le faisceau 35 (c'est-à-dire pour que ces ceux faisceaux soient légèrement éloignés l'un de l'autre, tout en restant parallèles), on fait légèrement tourner le modulateur autour d'un axe passant par l'axe optique de la lentille 32, en un point 37. Cet axe peut soit être perpendiculaire au plan dans lequel sont étalées les composantes spectrales du faisceau incident, comme représenté en figure 6, soit être compris dans ce plan, comme représenté en figure 7, la configuration de la figure 6 étant la configuration préférée (pour des raisons optiques, de facilité de réalisation et de compacité).The grating 31 forms with respect to the optical axis of the lens 32 an angle such that all the diffracted rays (spectral components of the WDM signal) by the grating 31 from the incident beam 35 pass through the lens 32 and are directed towards the modulator 33. This modulator 33 being reflective, as described below with reference to FIG. 8, it returns the spectral components modulated in amplitude (in the same way as described above for the modulator 12) towards the lens, which concentrates around its focus 34 on the network 31. In turn, the network 31 recomposes an output beam 36 antiparallel to the beam 35 and equalized. So that this beam 36 is not confused with the beam 35 (that is to say so that these beams are slightly distant from each other, while remaining parallel), the modulator is made to rotate slightly around an axis passing through the optical axis of the lens 32, at a point 37. This axis can either be perpendicular to the plane in which the spectral components of the incident beam are spread, as shown in FIG. 6, or be included in this plan, as shown in figure 7, the configuration of figure 6 being the preferred configuration (for optical reasons, ease of construction and compactness).
On a représenté sur la vue en coupe de la figure 8 le modulateur 33. La couche 38 de matériau PDLC comporte sur une face une électrode réfléchissante commune 39, qui peut occuper une grande partie de la largeur de cette face, et sur l'autre face un réseau 40 d'électrodes individuelles similaire au réseau 27 décrit ci-dessus. Ce réseau 40 n'occupe qu'une partie de la largeur de la couche 38, en vis-à-vis d'un bord de l'électrode 39. Ainsi, un rayon incident 41 (une des composantes spectrales du faisceau incident) arrivant obliquement sur le modulateur, juste au-delà du bord d'une électrode individuelle du réseau 40 traverse la couche PDLC 38, se réfléchit sur l'électrode 39, re-traverse la couche 38 et repart vers la lentille 32 (rayon réfléchi 42). Bien entendu, le potentiel appliqué entre chaque électrode individuelle 40 et l'électrode commune 40 doit tenir compte d'une double traversée de la couche 38 (ou bien, les caractéristiques de cette couche 38 doivent être modifiées si l'on veut appliquer des potentiels de même valeur que dans le cas du modulateur 12 de la figure 2). The modulator 33 is shown in the section view of FIG. 8. The layer 38 of PDLC material has on one side a common reflecting electrode 39, which can occupy a large part of the width of this side, and on the other facing an array 40 of individual electrodes similar to the array 27 described above. This network 40 occupies only part of the width of the layer 38, facing an edge of the electrode 39. Thus, an incident ray 41 (one of the spectral components of the incident beam) arriving obliquely on the modulator, just beyond the edge of an individual electrode of the network 40 passes through the PDLC layer 38, is reflected on the electrode 39, crosses the layer 38 again and leaves towards the lens 32 (reflected ray 42) . Of course, the potential applied between each individual electrode 40 and the common electrode 40 must take account of a double crossing of the layer 38 (or else, the characteristics of this layer 38 must be modified if one wishes to apply potentials of the same value as in the case of the modulator 12 of FIG. 2).

Claims

REVENDICATIONS
1. Dispositif électro-optique d'égalisation des canaux dans une liaison optique multicanaux, recevant un faisceau multicanaux incident (8, 35), caractérisé par le fait qu'il comporte un dispositif séparateur de canaux (9, 31), un dispositif d'atténuation sélective de canaux (12, 33) et un dispositif de recombinaison de canaux (14, 31).1. Electro-optical channel equalization device in a multi-channel optical link, receiving an incident multi-channel beam (8, 35), characterized in that it comprises a channel separator device (9, 31), a device for selective channel attenuation (12, 33) and a channel recombination device (14, 31).
2. Dispositif selon la revendication 1, caractérisé par le fait que le dispositif séparateur de canaux est un réseau dispersif.2. Device according to claim 1, characterized in that the channel separator device is a dispersive network.
3. Dispositif selon la revendication 2, caractérisé par le fait que le réseau dispersif est associé à une lentille convergente (11 , 32) le réseau passant par le foyer de cette lentille (10, 34).3. Device according to claim 2, characterized in that the dispersive network is associated with a converging lens (11, 32) the network passing through the focal point of this lens (10, 34).
4. Dispositif selon l'une des revendications précédentes, caractérisé par le fait que le dispositif de recombinaison de canaux est un réseau dispersif (14, 31 ).4. Device according to one of the preceding claims, characterized in that the channel recombination device is a dispersive network (14, 31).
5. Dispositif selon la revendication 4, caractérisé par le fait que le réseau dispersif de recombinaison est associé à une lentille convergente (13,5. Device according to claim 4, characterized in that the dispersive recombination network is associated with a converging lens (13,
32) et qu'il passe par le foyer de cette lentille (15, 34).32) and that it passes through the focal point of this lens (15, 34).
6. Dispositif selon l'une des revendications précédentes, caractérisé par le fait que le dispositif d'atténuation sélective est un modulateur spatial. 6. Device according to one of the preceding claims, characterized in that the selective attenuation device is a spatial modulator.
7. Dispositif selon la revendication 6, caractérisé par le fait que le modulateur spatial comporte une couche de matériau « PDLC » sur une face de laquelle est disposée une électrode unique (26, 39) interceptant tous les canaux séparés par le dispositif séparateur, et sur l'autre face un réseau d'électrodes individuelles (27, 40) dont chacune intercepte une seule composante correspondante du faisceau incident, des potentiels individuels étant appliqués entre l'électrode unique et chacune des électrodes individuelles.7. Device according to claim 6, characterized in that the spatial modulator comprises a layer of “PDLC” material on one face of which is disposed a single electrode (26, 39) intercepting all the channels separated by the separator device, and on the other face a network of individual electrodes (27, 40) each of which intercepts a single corresponding component of the incident beam, individual potentials being applied between the single electrode and each of the individual electrodes.
8. Dispositif selon la revendication 6 ou 7, caractérisé par le fait que le modulateur spatial est du type à transmission (12). 8. Device according to claim 6 or 7, characterized in that the spatial modulator is of the transmission type (12).
9. Dispositif selon la revendication 6 ou 7, caractérisé par le fait que le modulateur spatial est du type à réflexion (33). 9. Device according to claim 6 or 7, characterized in that the spatial modulator is of the reflection type (33).
10. Dispositif selon la revendication 9, caractérisé par le fait que le dispositif séparateur et le dispositif de recombinaison sont un seul et même dispositif (31) associé à une unique lentille convergente (32). 10. Device according to claim 9, characterized in that the separator device and the recombination device are one and the same device (31) associated with a single converging lens (32).
PCT/FR2002/002683 2001-07-27 2002-07-26 Electro-optical device for channel equalization in a multichannel optical link WO2003013033A1 (en)

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FR0110124A FR2828036A1 (en) 2001-07-27 2001-07-27 Wavelength division multiplexing multichannel optical connection equalization method having multichannel input beacon channel separated and channel selective attenuation/recombination unit.
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