System for achieving optical communication via polarization modification

Abstract

Disclosed is an optical communication system (S) comprising: • an emitter of polarized light comprising a source (1) configured to generate at least one photon polarized with a determined polarization on a propagation path; • a complex polarization modifier (2), placed on the propagation path of the light generated by the emitter and configured to modify the direction of the polarization of said light and to introduce a phase shift between two perpendicular components of the electric field of the light; • a receiver (3), placed on the propagation path of the light, downstream of the complex polarization modifier (2), and comprising a measuring instrument (35) making it possible to measure the ellipticity and orientation of the polarization of the light as expressed using Jones calculus.

Description

[0001] Description

[0002] Title: Optical communication system by polarization modification

[0003] The present invention relates to optical communication methods, and more particularly to those using the polarization of light.

[0004] Prior art

[0005] The transmission of information by optical fiber is currently done mainly using trains of electromagnetic waves, or by sending photons transmitted through an optical fiber.

[0006] Multiplexing is a technique for transmitting multiple streams of information through the same light beam or optical fiber. This can be done by transmitting waves of different wavelengths, but also by polarization division, for example by transmitting information with a linearly polarized wave in a first direction of electric field polarization and another piece of information with a linearly polarized wave in the direction perpendicular to said first direction of polarization, or information with a circularly polarized wave in one direction and another piece of information with a circularly polarized wave in the other direction.

[0007] The amount of information that can circulate through multiplexing remains limited.

[0008] There is therefore a need to improve the transmission of large amounts of information.

[0009] Statement of the invention

[0010] The invention aims to meet all or part of this need and has as its subject, according to a first of its aspects, an optical communication system comprising:

[0011] • A polarized light emitter comprising a source configured to generate at least one photon polarized according to a predetermined polarization on a propagation path;

[0012] • A complex polarization modifier placed on the propagation path of the light generated by the emitter, configured to modify the direction of the polarization of said light and introduce a phase shift between two perpendicular components of the electric field of the light;

[0013] • A receiver placed on the light propagation path, downstream of the complex polarization modifier, comprising a measuring instrument for measuring the ellipticity and orientation of the polarization of the light according to the Jones formalism.

[0014] The system according to the invention makes it possible to transmit information of either discrete type or continuous values ​​between the polarization modifier and the receiver.

[0015] The photons emitted by the emitter are preferably not entangled.

[0016] The optical communication system preferably comprises a single receiver.

[0017] The polarization state of the photon is not necessarily binary like that of a spin. The polarization state can notably be represented on the Jones sphere which characterizes the orientation and ellipticity of the polarization: a polarization can in fact be linear, the electric field always being parallel to an axis perpendicular to the direction of propagation of the photon, or circular, the electric field rotating around this axis, or between the two: the electric field traveling an ellipse around the propagation axis during a period of the wave.

[0018] Several pieces of information can be communicated by the communication system, each piece of information being transmitted by a light flux polarized according to a specific polarization of the Jones formalism.

[0019] The luminous flux can be long, for example lasting a millisecond or a microsecond and have a significant power, for example several Watts. Alternatively, the luminous flux is a photon or a succession of photons, for example 1000 photons which, if they are each separated by about 10 periods, that is to say 10 times the wavelength, form a luminous flux with a power of about 0.1 mW lasting about 30ps.

[0020] By polarization modifier is meant a component allowing on the one hand to modify the direction of the polarization by a predetermined angle and on the other hand to introduce a phase shift between two perpendicular components of the electric field of each photon, for example along the specific axes of the polarization modifier.

[0021] Issuer

[0022] The emitter may be configured to emit light with a predetermined polarization, in particular with a linear polarization along a predetermined axis. The emitter may comprise a source configured to emit light with a random polarization, and, downstream of this source, a linear polarizer configured to select a polarization direction of the photon.

[0023] The linear polarizer may comprise an optically transparent birefringent material, for example lithium niobate or rutile.

[0024] The emitter may comprise a single photon source, configured to emit photons one by one separated in time by a duration of the order of a few light periods, for example 10 or 100.

[0025] The transmitter may be configured to successively generate a plurality of photons.

[0026] The transmitter may include a clock.

[0027] Polarization modifier

[0028] The polarization modifier is advantageously arranged so that its user can choose the polarization state in which the photon is sent to the receiver from any of the possible polarizations as defined by the Jones formalism, in particular from a set of elliptical polarizations (also called "ellipsoidal").

[0029] In one embodiment, the linear polarization of a luminous flux can be transformed into ellipsoidal polarization as represented by the Jones formalism by firstly modifying the orientation of the polarization of a luminous flux of linear polarization and known direction, thus distributing in a predetermined manner the electric field along an x ​​axis and a y axis perpendicular to the x axis, then, in a second step by modifying the phase of the electric field along one of two perpendicular directions, for example the y axis.

[0030] The polarization modifier may include a modifier of the ellipticity of the polarization, on the propagation path, between the transmitter and the receiver.

[0031] The polarization modifier preferably comprises a polarization direction modifier disposed on the propagation path, between the transmitter and the polarization ellipticity modifier or between the polarization ellipticity modifier and the receiver.

[0032] For example, if a linearly polarized light has its polarization rotated by an angle 9 with respect to the x axis and if is the phase shift between the electric field along the x axis and the electric field along the y axis, a> being the angular frequency of the electric field, the components E x summer y of electric field are:

[0033] E x = E Cos (0) Cos (mt)

[0034] E y = E Sin (0) Cos (mt + )

[0035] Polarization direction modifier:

[0036] The polarization direction modifier may include a half-wave plate, including a variable-orientation half-wave plate

[0037] Alternatively, the polarization direction modifier may comprise a first quarter-wave plate configured to modify the linear polarization of the light flux incident on the polarization direction modifier into circular polarization, preferably followed by a second quarter-wave plate transforming the circular polarization into linear polarization of the light flux oriented along an axis dependent on the direction of the axis of the second quarter-wave plate.

[0038] The orientation of at least one of the two quarter-wave plates can be variable.

[0039] Rotating the axis of the second quarter-wave plate changes the direction of the linear polarization of the light flux. If the plate is a half-wave plate, rotating the half-wave plate changes the direction of the linear polarization of the light flux.

[0040] Alternatively, the orientation of the first quarter-wave plate may be changed, with that of the second quarter-wave plate being fixed, or the orientations of both quarter-wave plates may be changed.

[0041] The rotation of a quarter-wave plate or a half-wave plate is obtained for example by mechanical control to a sensor or to an electrically controlled device allowing its rotation, for example by rubbing on an axis set in motion by a piezoelectric material or by an electric motor device, for example direct current.

[0042] Since the rotational inertia of a quarter-wave plate can be significant, the light fluxes can be sent successively to different quarter-wave plates whose orientations will have been adjusted beforehand, for example by a mechanical rotation device, which allows time to modify the direction of each of the quarter-wave plates between two passages of light flux. Alternatively, the polarization direction modifier can comprise twisted liquid crystals (twisted nematics) and the modification of the polarization direction can be obtained by passing the light flux through the liquid crystals above and below which are transparent electrodes which control the rotary power of said liquid crystals.

[0043] Alternatively, the polarization direction modifier may comprise chiral or rotatable materials. The polarization direction modifier may comprise a blade or prism of chiral or rotatable material affecting the rotation of the polarization by an angle depending on the location through which the light flux enters said chiral or rotatable material. The material may in particular be arranged between materials whose refractive index(es) are dynamically adjustable.

[0044] Alternatively, the polarization modifier may comprise several polarization direction modifiers, each allowing different polarization rotation angles. Said several polarization direction modifiers are for example fixed. Alternatively, the orientation of at least one, in particular all, of said polarization direction modifiers may be modifiable.

[0045] The light flux can be sent to various polarization direction modifiers rotating the polarization by predefined angles and then bringing the light flux output from these polarization direction modifiers back into a single flux.

[0046] The polarization modifier may comprise one or more first plates or one or more first prisms whose refractive index(es) are dynamically adjustable. The light flux may be sent, at a first point, onto the one or more first plates or the one or more first prisms, and the light flux may emerge from the one or more first plates or the one or more first prisms at a second point with an ordinate and / or abscissa different from the first point.

[0047] The polarization modifier may comprise, downstream of the first blade(s) or the first prism(s), an intermediate device, in particular comprising a material that is at least partly chiral or rotatable. The light flux emerges in particular from the first blade(s) or the first prism(s) by penetrating an intermediate device, rotating the direction of polarization of the light flux by an angle depending on the point through which the light flux enters said intermediate device. The intermediate device may comprise a blade or a prism.

[0048] The polarization modifier may comprise, downstream of the intermediate device, one or more second blades or one or more second prisms, and possibly one or more third blades or one or more third prisms, the refractive index(es) of which are dynamically adjustable and for which the refractive index(es) are in particular adjusted symmetrically with respect to those of the first blade(s) or the first prism(s). The light flux leaving the intermediate device may penetrate the one or more second blades or the one or more second prisms, and possibly the one or more third blades or the one or more second prisms.

[0049] The luminous flux may emerge from the second blade(s) or second prism(s) in the same direction as if the refractive index of the first blade(s) or first prism(s) and of the second blade(s) or second prism(s) both had a fixed and non-dynamic value.

[0050] The intermediate device is for example composed of two symmetrical prisms placed side by side, with the same refractive index, but having different chiral or rotating powers, the first of the two prisms having for example a chiral or rotating power and the second not having one or rotating the polarization in the opposite direction to the rotation imposed by the first prism.

[0051] One of the two prisms may comprise a chiral material, for example cadmium selenide (CdsE) nanoparticles with diameters ranging from 1.4 nm to 2.4 nm, as described in the article by Visheratina, Anastasia, and Nicholas A. Kotov. "Inorganic nanostructures with strong chiroptical activity." (CCS Chemistry 2.3 (2020): 583-604.).

[0052] The other of the two prisms is preferably non-chiral or alternatively is of inverse chirality to that of said first prism. The rotation of the direction of polarization of the luminous flux passing through a chiral material being proportional to the thickness crossed by said chiral material, the rotation of the direction of polarization of the luminous flux passing through the intermediate device depends on the point of penetration of the luminous flux into the intermediate device.

[0053] Alternatively, the intermediate device may comprise a "rotating" material, such as a quarter-wave plate followed by a juxtaposition of quarter-wave plates arranged such that a wave linearly polarized along the axis of the first slice of the material emerges from the second slice of the material with a polarization rotated by a predetermined angle.

[0054] Alternatively, the intermediate device may comprise a "rotating" material, such as a juxtaposition of half-wave plates arranged such that a wave linearly polarized along the axis of the first slice of the material emerges from the second slice of the material with a polarization rotated by a predetermined angle.

[0055] Alternatively, the intermediate device may comprise a "rotating" material, such as a juxtaposition of cylinders comprising chiral materials of different concentrations.

[0056] The blades or prisms whose refractive index(es) are dynamically adjustable include, for example, liquid crystals or one or two Pockels cells arranged between two transparent electrodes.

[0057] Since Pockels cells are birefringent materials whose birefringence is affected by the electric field, the first plate(s) or prism(s) comprising a Pockels cell are preferably oriented in such a way that the incident light flux is linearly polarized along the axis of the material most sensitive to the electric field. A single Pockels cell is then sufficient.

[0058] The second blade(s) or prism(s), and the third blade(s) or prism(s), comprising a Pockels cell, are preferably arranged such that their axes along which the polarization is most sensitive to the electric field are perpendicular to each other, such that the light flux incident on the second blade(s) or prism(s) emerges from the third blade(s) or prism(s) deflected in the same way, regardless of the direction of polarization of the electric field of the light incident on the second blade(s) or prism(s).

[0059] Pockels cells most often require large potential differences to operate as well as a thickness of the order of a centimeter. Preferably, an alternating voltage of 4V between the two electrodes is used per mm of thickness of the Pockels cell, at frequencies for example between 3.765MHz and 3.775Mhz for a Pockels cell containing lithium niobate, as described in the article "Longitudinal Piezoelectric resonant photoelastic modulator for efficient intensity modulation at megahertz frequencies" published in the journal Nature communications on March 22, 2022. The variation of the refractive index then depends on said frequency and said potential difference.

[0060] Polarization Ellipticity Modifier

[0061] The polarization ellipticity modifier allows the phase shift of the electric field component of the luminous flux by a predetermined angle along one of two fixed axes.

[0062] Preferably, the polarization ellipticity modifier comprises a first birefringent plate or prism arranged to split the beam into two electromagnetic waves of linear polarization, one along a first axis, the other along a second axis, and preferably a variable refractive index retardation plate arranged on the second axis.

[0063] The delay plate allows the electromagnetic wave oriented along the second axis to acquire a predetermined phase shift relative to the electromagnetic wave oriented along the first axis, before being mixed with it again by a new birefringent plate or prism allowing the two electromagnetic waves whose polarization fields are perpendicular to be brought together along the same axis.

[0064] The delay plate can include a Pockels cell or a non-linear material. The variable refractive index of the cell makes it possible to choose the phase shift imposed on the electromagnetic wave oriented along the second axis.

[0065] The modifier of the polarization ellipticity can be a Pockels cell.

[0066] The Pockels cell is for example made of monopotassium phosphate KDP (KH2PO4), potassium dideuterium phosphate DKDP (KD2PO4), or lithium niobate (LiNbO3). The Pockels cell can alternatively be made of other non-centrosymmetric media such as polymers polarized by an electric field, or placed between two electrodes perpendicular to the incident light beam, the incident light beam arriving linearly polarized and the birefringence of the cell varying according to the potential difference applied between the two said electrodes.As explained in the article published in the journal Nature Communication on March 22, 2022 entitled Longitudinal piezoelectric resonant photoelastic modulator for efficient intensity modulation at megahertz frequencies , the variation of the birefringence of the cell can be obtained by applying an alternating voltage of 2V applied between the two electrodes, at frequencies for example between 3.765MHz and 3.78Mhz for a 0.5mm thick Pockels cell made of lithium niobate, the choice of the frequency between these limits deciding the birefringence of said Pockels cell.

[0067] The phase shift between the polarization components of the photon along two perpendicular axes, phase shift introduced by the modifier of the ellipticity of the polarization, can be chosen from phase shifts spaced 9° apart between -90° and +81°.

[0068] Amplifier

[0069] The receiver may include, upstream of the measuring instrument, an optical amplifier making it possible to multiply the photon while preserving its polarization state.

[0070] Preferably, the optical amplifier is a doped fiber amplifier.

[0071] The optical amplifier is for example an erbium-doped fiber amplifier (EDFA), for example 4m long, into which the photon to be amplified is introduced at the same time as an amplifying wave of shorter wavelength, which makes it possible to amplify the Fonde corresponding to the introduced photon, with gains which can be of the order of 37db / m.

[0072] Alternatively, the optical amplifier is a doped fiber amplifier (DFA) using a dopant other than erbium.

[0073] Alternatively, the optical amplifier may be a vertical cavity amplifier (VCSOA) or a semiconductor type amplifier (S OA).

[0074] Measuring instrument

[0075] Measuring the average polarization of photons makes it possible to detect the polarization state of the photons and to deduce the information transmitted.

[0076] The receiver measuring instrument may comprise at least one photon detector arranged to measure the intensity of the luminous flux along two perpendicular axes and the phase shift of the light between these same two axes.

[0077] Preferably, the receiver measuring instrument comprises a succession of semi-reflecting blades, in particular arranged downstream of the optical amplifier, said blades preferably directing the luminous flux, in predefined proportions, towards instruments making it possible in particular to characterize the ellipticity of its polarization. Said instruments making it possible to characterize the ellipticity of the polarization are in particular configured to:

[0078] • measure the intensity of the component of the electric field of light along a first axis x,

[0079] • measure the intensity of the electric field component of the light along a second y axis perpendicular to the first x axis,

[0080] • measure the phase shift between the light along the x axis and the light along the y axis,

[0081] • and, preferably, measure the phase shift between the light in the two perpendicular directions x' and y', x' being the bisector of x and y.

[0082] The measurement of the intensity of the photon flux along the two perpendicular axes is done, for example, by separating the photon flux along the two perpendicular axes by a birefringent blade or prism, followed by two light intensity sensors placed at the output of said blade or prism, respectively on each of the two axes.

[0083] The measurement of the ellipticity of the polarization, i.e. the measurement of the phase shift of the light between its components along the two perpendicular axes, is done for example by the separation of the light along the two perpendicular axes by a birefringent plate or prism. This separation can be followed, for the light polarized along one of the two axes, by a rotation of this polarization axis of 90°, for example thanks to a rotating or chiral material or by the succession of two quarter-wave plates in order to generate two luminous fluxes of linear polarizations of the same direction then by a joint projection of these two luminous fluxes through Young's slits, on a screen, the interference of the two luminous fluxes drawing fringes whose positions depend on said phase shift.

[0084] Alternatively, the two light fluxes can interact with each other using another interferometer, for example an interferometer mixing the two light fluxes into a single flux, using a semi-reflecting plate crossed obliquely by one of the two fluxes and reflected obliquely for the other of the two fluxes, the single flux being projected onto a screen, or a camera, in order to measure its intensity, in the same way as a Michelson interferometer.

[0085] Receiver

[0086] The receiver preferably includes a clock, the clocks of the transmitter and receiver preferably being synchronized with each other. Between the amplifier and the measuring instruments, the receiver may include a succession of several lenses and / or mirrors to enlarge the cross-section of the light beam.

[0087] Selector

[0088] One or more selectors may be configured to send light fluxes to at least one polarization direction modifier.

[0089] One or more selectors can allow, at the output of the device(s) for modifying the direction of polarization, to combine the light fluxes along the same axis, for example in a guide.

[0090] One or more selectors may comprise a mirror whose axis direction is controlled, for example, by an electrical device. Alternatively, one or more selectors may comprise a prism or a blade, made of a material whose refractive index depends on an electric field, for example liquid crystals or a Pockels cell, or in a transparent material with a non-linear refractive index. An additional light flux, for example transverse and preferably of a wavelength different from that of the light flux transmitting information, may vary the refractive index of said non-linear material and may thus control the location and possibly the direction of exit of the light flux from said material.

[0091] One or more selectors can be integrated into single parts. For example, a selector can have a single input for light and several possible outputs for said light, the output taken by the light depending on the voltage used to operate said selector.

[0092] Photon transmission

[0093] Photons can be transmitted from the transmitter to the receiver in free or closed field, through space, the atmosphere, by an optical fiber or a combination of these means.

[0094] Lenses may be used for transmitting the luminous flux, in particular for transmission through space or the atmosphere. Where appropriate, anti-reflection coatings are preferably provided on said lenses. The size of the lenses used is preferably adapted to the length of the spatial or atmospheric transmission of the luminous fluxes.

[0095] To adjust the direction of emission of a signal-carrying light flux, particularly for spatial or atmospheric transmission, a phase-conjugate mirror can be used to reflect the emitted light back to the transmitter. For example, a laser light transmitter can scan a space to detect the receiver, the receiver reflecting the emitted light back to it using the phase-conjugate mirror, and the direction of the signal-carrying light flux is then adjusted to be parallel or coincident with the direction of the light reflected by the conjugate mirrors.

[0096] The laser light emitted by the laser light emitter may be of wavelength close to the wavelength of the light flux emitted by the emitter, transmitting information, and introduced into an objective lens used by the light flux transmitting information by a dichroic prism.

[0097] In a variant, a light flux used for aiming can be emitted parallel to the light flux transmitting information but distant, for example, by a few centimeters to be reflected by a phase conjugation mirror.

[0098] In another variant, it is possible not to use a phase conjugation mirror and to aim at the receiver or a target close to it, the information that the target is received being communicated by another means of communication, in particular by radio signal, or by optical means, the conjugate mirror then being arranged to dynamically modulate the reflection.

[0099] The area to be scanned can be identified by map recognition of the area in which the said receiver is likely to be located.

[0100] Anti-reflective coating

[0101] Anti-reflective layers may be placed at the interfaces between adjacent transparent media of different indices through which the light fluxes pass and / or at the interfaces of prisms and / or birefringent plates through which the light fluxes pass. The anti-reflective layers prevent loss of light flux.

[0102] The anti-reflection coatings are preferably adapted to the index(es) of the materials, and / or to the angle(s) of incidence and direction(s) of polarization of the luminous flux which must pass through it, and / or to the wavelength of the luminous flux. Dichroic filters

[0103] The receiver preferably comprises one or more dichroic filters allowing only light fluxes of a given wavelength to pass through, in particular a prism made of a transparent dispersive material.

[0104] The filter(s) are preferably placed in front of the measuring instrument(s), particularly if the refractive indices of non-linear materials are modified by the application of powerful light fluxes.

[0105] Photonic communication process

[0106] The subject of the invention, according to another of its aspects, is a photonic communication method transmitting coded information on a light flux, using the system described above, comprising the steps consisting of:

[0107] (1) Generating a polarized luminous flux of predetermined polarization from an emitter, the luminous flux being emitted towards a receiver,

[0108] (2) Encode the information on the luminous flux by introducing, using a polarization modifier placed between the transmitter and the receiver, a modification of the direction of the polarization of the luminous flux and a phase shift between the polarization components of the luminous flux along the two specific axes of the polarization modifier, the modification of the direction and the phase shift depending on the information to be transmitted,

[0109] (3) Measure the average phase shift between the polarization components of the luminous flux along two perpendicular axes as well as the proportion of the luminous intensity along these same axes, and determine according to these measurements the information transmitted by the transmitter.

[0110] Preferably, the photonic communication method comprises between step (2) and step (3), in particular if the light flux is composed of only one or a few photons: duplicating the photon into a flux of photons at the receiver, using an amplification device, the light thus created having retained the polarization state of the photon received at the receiver,

[0111] The same information can be coded on a predetermined number N of photons emitted successively by the transmitter. The phase shift between the polarization components of the luminous flux along two perpendicular axes can be chosen from phase shifts spaced 9° apart between -90° and +81°, and / or the direction of polarization of the luminous flux can be chosen from directions spaced 9° apart between -90° and +81°, the state of polarization of the luminous flux then being chosen from 361 distinct states of polarization.

[0112] The receiver can consider having received the information after having measured in step (4) a predetermined number n of photons carrying the same information, received by the receiver.

[0113] The transmitter can transmit a coded message comprising a plurality of information, in particular a plurality of letters, preferably each coded on one or more photons.

[0114] A transmitted piece of information, in particular a transmitted letter, encoded on one or more photons, may be separated from another transmitted piece of information, in particular another transmitted letter, by the transmission of a separating piece of information, in particular a separating letter, encoded on one or more photons. Preferably, the transmission of two identical pieces of information, in particular two identical letters, is separated by the transmission of a separating piece of information, in particular a separating letter.

[0115] Device for changing the direction of polarization

[0116] The invention relates, according to another of its aspects, to a device for modifying the direction of polarization of a luminous flux, the device belonging for example to an optical communication system, in particular to the optical communication system described above, the device comprising: one or more first plates or one or more first prisms whose refractive index(es) are dynamically adjustable, downstream of the one or more first plates or one or more first prisms, an intermediate device arranged to rotate the direction of polarization of the luminous flux by an angle depending on the point through which the luminous flux penetrates said intermediate device, and downstream of the intermediate device, one or more second plates or one or more second prisms, and optionally one or more third plates or one or more third prisms, whose refractive index(es) are dynamically adjustable.The polarization direction modification device allows the polarization direction of a linearly polarized light flux to be rotated under the effect of an electrical control generating electric fields.

[0117] The intermediate device may include a blade or a prism.

[0118] The intermediate device may comprise a material that is at least partly chiral or rotatable.

[0119] The refractive index(es) of the one or more second blades or of the one or more second prisms, and possibly of the one or more third blades or of the one or more third prisms, are in particular adjusted symmetrically with respect to those of the one or more first blades or of the one or more first prisms.

[0120] Device for changing the direction of polarization

[0121] The subject of the invention, according to another of its aspects, is a device for modifying the direction of polarization of a luminous flux, the device belonging for example to an optical communication system, in particular to the optical communication system described above, the device comprising several superimposed layers: a first layer being a lithium niobate crystal subjected to a first electric field Eci of intensity U in a direction y inducing a variation An x of its refractive index for luminous fluxes polarized in an x ​​direction perpendicular to the y direction, and a variation An yof its refractive index for light fluxes polarized in the y direction, a second layer being a second lithium niobate crystal oriented at 90° around the x axis relative to the first layer, subjected to a second electric field Ec2 of the same intensity U as the first perpendicular to the xy plane, the field EC2 inducing the variation An x of its refractive index for luminous fluxes polarized in the y direction and the variation An yof its refractive index for light fluxes polarized in the x direction, a third layer being a quarter-wave plate for the light flux from the second layer which enters the third layer at an entry point, the position of the entry point on the third layer depending on the intensity of the electric fields Eci and Ec2, a fourth layer being a succession of quarter-wave plates oriented in such a way that the light from the third layer enters circularly polarized and leaves linearly polarized in a direction depending on its entry point in the fourth layer, a fifth and a sixth layers of the same nature and orientation as the first and second layers respectively, and subjected to electric fields respectively in the y direction and in the direction perpendicular to the xy plane, the equal intensities U' of which are adjusted according to the electric fields Eci and Ec2,so that the light ray coming out of the sixth layer comes out at an invariant point whatever the value of the intensity U of the electric fields Eci and Ec2.,

[0122] The first, second, fifth and sixth layers (81; 82; 85; 86) may be liquid crystals or Pockels cells disposed between two transparent electrodes, or the refractive indices of the first, second, fifth and sixth layers (81; 82; 85; 86) may be modulated by the application of intense light and the first, second, fifth and sixth layers (81; 82; 85; 86) may be composed of non-linear refractive index materials.

[0123] Device for receiving a luminous flux

[0124] The subject of the invention, according to another of its aspects, is a device for receiving a luminous flux, arranged to measure the ellipticity and the orientation of the polarization of a light according to the Jones formalism, the device belonging for example to an optical communication system, in particular to the optical communication system described above, the device comprising: an amplifier, for example a doped fiber optical amplifier, configured to amplify the light, thus creating a luminous flux, an optical device comprising: two semi-reflecting mirrors, a mirror, three prisms, two photosensitive sensors, two cameras and three polarization direction modifiers, the first semi-reflecting mirror being configured to reflect and deflect a part of the luminous flux, for example a third, at the output of the amplifier,towards a first birefringent prism separating the polarized deflected light into two light fluxes along the two proper axes of the prism, the two light fluxes being sent respectively to sensitive photo sensors which measure their respective intensity, the second semi-reflecting mirror being configured to reflect a part of the light flux, for example half, not reflected at the output of the first mirror, and deflect this reflected flux towards a second birefringent prism separating the polarized deflected light into two light fluxes along the two proper axes of the second prism, the first light flux then being sent towards a first camera to illuminate a surface Si, the second flux being sent towards a first polarization direction modifier which modifies the polarization direction of the second flux by 90° before sending this flux towards the same surface Si of the first camera, interference fringes then appearing on the camera,whose position makes it possible to measure the phase shift between the fluxes oriented along the two proper axes of the second prism, the second polarization direction modifier being configured so that the light flux not reflected by the second semi-reflecting mirror passes through the second polarization direction modifier which modifies the polarization direction of the flux by 45°, the third mirror being configured to reflect the light flux having passed through the second polarization direction modifier, and deflect this reflected flux in the direction of a third birefringent prism separating the deflected light polarized into two light fluxes along the two proper axes of the third prism, the first light flux being sent towards a surface Si' of a second camera,the second flux being sent to a third polarization direction modifier which turns the polarization direction of the second flux by 90° before sending this flux to the same surface Si' of the third camera, interference fringes then appearing on the camera, which testifies to the phase shift between the fluxes oriented along the two proper axes of the third prism.,

[0125] Brief description of the drawings

[0126] The invention may be better understood by reading the detailed description which follows, non-limiting examples of its implementation, and by examining the appended drawing, in which: [Fig 1] figure 1 represents, schematically and partially, a communication system according to the invention,

[0127] [Fig 2A] Figure 2A represents, schematically and partially, a linearly polarized photon,

[0128] [Fig 2B] Figure 2B represents, schematically and partially, two circularly polarized photons,

[0129] [Fig 3] Figure 3 represents, schematically and partially, a modifier of the direction of the polarization according to the invention,

[0130] [Fig 4] Figure 4 shows, schematically and partially, a device rotating the polarization axis of linearly polarized light under the effect of an electrical command,

[0131] [Fig 5] Figure 5 schematically and partially represents a modifier of the ellipticity of the polarization according to the invention, and

[0132] [Fig 6] Figure 6 schematically and partially represents a receiver according to the invention.

[0133] Detailed description

[0134] Figure 1 shows an optical communication system S according to the invention comprising a light flux transmitter, a polarization modifier 2 and a receiver 3.

[0135] The light flux emitter comprises a source 1 configured to emit unpolarized light, towards a polarizing filter 4, linearly polarizing, for example vertically, the light. Alternatively, the source 1 can emit linearly polarized light towards the polarization modifier 2, without there being a polarization filter 4 in the system S.

[0136] Downstream of the polarization filter 4, on the path of the light, the polarization modifier 2 introduces a modification of the direction of the polarization and a phase shift between the polarization components of the light along two specific axes of the polarization modifier 2, the angle of rotation of the direction of polarization and the value of the phase shift introduced corresponding for example to a letter A.

[0137] At the output of the polarization modifier 2, the light is sent into an optical fiber 5, in the direction of the receiver 3. The receiver 3 comprises an optical amplifier 31 and one or more measuring instruments 35, to measure the intensity of the electric field corresponding to the luminous flux along two perpendicular axes as well as the phase shift between the components of said electric field along these axes, and to deduce the transmitted information, for example the letter A. The receiver 3 may further comprise an optical device 36 making it possible to orient the amplified luminous flux in the direction of the measuring instrument(s).

[0138] Figure 2A illustrates the linear polarization l of a photon P moving in a direction D. The linear polarization l is decomposed in this example on two axes, one vertical V and the other horizontal H.

[0139] Figure 2B illustrates the circular polarization of a photon P moving in a direction D. The circular polarization of the photon P rotates around the axis D, it can rotate clockwise like the polarization Cl or counterclockwise like the polarization C2.

[0140] Polarization direction modifier

[0141] The polarization modifier 2 may comprise a modifier of the direction of the polarization 7 or 8 as illustrated in Figures 3 and 4, on the path of the linearly polarized photon P, between the emitter and a modifier of the ellipticity of the polarization 20.

[0142] In a first embodiment illustrated in Figure 3, the polarization direction modifier 7 comprises a first quarter-wave plate 71 making it possible to transform the linear polarization of the photon P into a circular polarization. The polarization direction modifier 7 comprises, downstream of the plate 71, a transparent medium 72, followed by a second quarter-wave plate 73 making it possible to transform the circular polarization of the photon P into a linear polarization in a determined direction.

[0143] The rotation of the quarter-wave plate 73 is obtained for example by mechanical control to a sensor or to an electrically controlled device allowing its rotation, for example by rubbing on an axis set in motion by a piezoelectric material or by an electric motor device, for example direct current. In a second embodiment illustrated in Figure 4, the polarization direction modifier 8 rotates the polarization direction of a linearly polarized photon under the effect of an electrical command generating electric fields.

[0144] Polarization direction modifier 8 has several layers on top of each other.

[0145] The first layer 81 is for example a lithium niobate crystal subjected to a first electric field Eci in the y direction, inducing a variation An x of its refractive index for waves polarized in the x direction and a variation Any of its refractive index for waves polarized in the y direction.

[0146] When the incident light 800 enters this first layer 81, it is separated into two rays 811 and 812.

[0147] The second layer 82 is for example a second lithium niobate crystal oriented at 90° around the x axis relative to the first layer, subjected to a second electric field Ec2 of the same intensity U as the first, perpendicular to the plane of figure 4. The field Ec2 induces the variation An x of its refractive index for waves polarized in the y direction and the variation An y of its refractive index for waves polarized in the x direction. When rays 811 and 812 enter this second layer 82, they change direction again to become rays 821 and 822.

[0148] The two rays 821 and 822 are united into a single ray 830 at the exit of layer 82, when it enters layer 83 at point 87. The position of point 87 on layer 83 depends on the intensity of the electric fields Eci and Ec2. The direction of the ray 830 is independent of the intensity U of the electric fields Eci and EC2 which remain equal.

[0149] The layer 83 is for example a quarter-wave plate for the rays entering it with the direction of the ray 830 coming from the second layer 82. The light ray 830 enters at the exit of the layer 83 at an entry point 88 on a succession of quarter-wave plates 84 oriented in such a way that the light enters it circularly polarized in the direction of the ray 830 and leaves it linearly polarized in a direction depending on its entry point 88.

[0150] The light ray exiting the layer 84 passes through the layers 85 and 86 and exits the layer 86 at point 89 to form the ray 900. The layers 85 and 86 are of the same nature and orientation as the first and second layers 81 and 82 respectively, and are subjected to electric fields respectively in the y direction and in the direction perpendicular to the plane of Figure 4, the equal intensities U' of which are adjusted according to the electric fields Eci and Ec2, so that the light ray 900 exits at an invariant point 89 whatever the value of the intensity U of the electric fields Eci and Ec2.

[0151] The layers 81, 82, 85 and 86 are for example liquid crystals or Pockels cells arranged between two transparent electrodes. Alternatively, the refractive indices of the layers 81, 82, 85 and 86 are modulated by the application of intense light and the layers 81, 82, 85 and 86 are composed of non-linear refractive index materials.

[0152] Polarization Ellipticity Modifier

[0153] The polarization modifier 2 may include a polarization ellipticity modifier 20.

[0154] The polarization ellipticity modifier 20 allows the introduction of a phase shift between the two polarization components along two perpendicular axes, of one photon.

[0155] The polarization ellipticity modifier 20 illustrated in Figure 5 comprises a first birefringent crystal plate 21 dividing the light beam PI into two electromagnetic waves, of linear polarization E1 and E2, one along a first axis, the other along a second axis perpendicular to the first.

[0156] The polarization ellipticity modifier 20 comprises, downstream of this blade 21, a Pockels cell 22, for example made of lithium niobate, arranged on the second axis. The Pockels cell 22 is crossed by an adjustable electric field generated by electrodes 220, for example perpendicular to the direction of propagation of the light. The Pockels cell makes it possible to make the wave oriented along the second axis, of polarization E2, acquire a predetermined phase shift relative to the wave oriented along the first axis, of polarization EL

[0157] Finally, the polarization ellipticity modifier 20 comprises, downstream of the Pockels cell 22, a second birefringent crystal plate 23 which unites the two waves E1 and E2 along the same axis, which creates a single PL beam.

[0158] Receiver Figure 6 illustrates a receiver 3 comprising an amplifier 31 for amplifying a light P, thus creating a luminous flux nP. The amplifier 31 is, for example, a doped fiber optical amplifier.

[0159] The receiver 3 further comprises an optical device comprising two semi-reflecting mirrors 32 and 33 and a mirror 34. The first semi-reflecting mirror 32 reflects and deflects a portion of the light flux nP, for example a third, at the output of the amplifier 31, towards a first birefringent prism 321. The birefringent prism 321 separates the deflected light polarized into two light fluxes along the two specific axes of the prism 321. The two light fluxes are then sent respectively to sensitive photo sensors 322 and 323 which measure their respective intensity.

[0160] A second semi-reflecting mirror 33 reflects a portion of the light flux, for example half, not reflected at the output of the first mirror 32, and deflects this reflected flux towards a second birefringent prism 331. The birefringent prism 331 separates the deflected light polarized into two light fluxes along the two proper axes of the prism 331. The first light flux is then sent to a camera 333 to illuminate a surface Si, the second flux is sent to a polarization direction modifier 332 which modifies the polarization direction of the second flux by 90° before sending this flux towards the same surface Si of the camera 333. Interference fringes then appear on the camera, the position of which makes it possible to measure the phase shift between the fluxes oriented along the two proper axes of the prism 331.

[0161] The light flux not reflected by the second semi-reflecting mirror 33 then passes through a polarization direction modifier 37 which modifies the polarization direction of the flux by 45°.

[0162] A third reflecting mirror 34 then reflects the light flux having passed through the polarization direction modifier 37, and deflects this reflected flux towards a third birefringent prism 341. The birefringent prism 341 separates the deflected light polarized into two light fluxes along the two proper axes of the prism 341. The first light flux is then sent towards a surface Si' of a camera 343, the second flux is sent towards a polarization direction modifier 342 which turns the polarization direction of the second flux by 90° before sending this flux towards the same surface Si' of the camera 343. Interference fringes then appear on the camera, which testifies to the phase shift between the fluxes oriented along the two proper axes of the prism 341.

Claims

Claims 1. Optical communication system(s) comprising: • A polarized light emitter comprising a source (1) configured to generate at least one photon polarized according to a predetermined polarization on a propagation path; • A complex polarization modifier (2) placed on the propagation path of the light generated by the emitter, configured to modify the direction of the polarization of said light and introduce a phase shift between two perpendicular components of the electric field of the light; • A receiver (3) arranged on the light propagation path, downstream of the complex polarization modifier (2), comprising a measuring instrument (35) making it possible to measure the ellipticity and the orientation of the polarization of the light according to the Jones formalism.

2. System according to claim 1, the emitter being configured to emit light with a predetermined polarization, in particular with a linear polarization.

3. System according to the preceding claim, the emitter comprising, downstream of the source (1), a linear polarizer (4) configured to select a direction of polarization of the photon.

4. System according to any one of the preceding claims, the polarization modifier (2) comprising a polarization ellipticity modifier (20), arranged on the propagation path, between the transmitter and the receiver (3).

5. System according to the preceding claim, the polarization modifier (2) comprising a polarization direction modifier (7; 8), arranged on the propagation path, between the transmitter and the polarization ellipticity modifier (20) or between the polarization ellipticity modifier (20) and the receiver (3).

6. System according to the preceding claim, the polarization direction modifier (7) comprising a first quarter-wave plate (71) configured to modify the linear polarization of the light flux incident on the polarization direction modifier (7) into circular polarization, followed by a second quarter-wave plate (73) transforming the circular polarization into linear polarization of the light flux oriented along an axis depending on the direction of the axis of the second quarter-wave plate.

7. System according to claim 5, the polarization direction modifier (8) comprising: - one or more first blades or one or more first prisms whose refractive index(es) are dynamically adjustable, - downstream of the first blade(s) or first prism(s), an intermediate device, in particular comprising a material that is at least partly chiral or rotating, and - downstream of the intermediate device, one or more second blades or one or more second prisms, and possibly one or more third blades or one or more third prisms, the refractive index(es) of which are dynamically adjustable and for which the refractive index(es) are in particular adjusted symmetrically with respect to those of the first blade(s) or the first prism(s).

8. System according to any one of the preceding claims and claim 4, the polarization ellipticity modifier (20) comprising a first birefringent plate or prism (21) dividing the beam into two electromagnetic waves of linear polarization, one (El) along a first axis, the other (E2) along a second axis, and a variable refractive index retardation plate arranged on the second axis (22).

9. System according to any one of the preceding claims, the measuring instrument (35) of the receiver (3) comprising at least one photon detector (322; 323; 333; 343), arranged to measure the intensity of the luminous flux along two perpendicular axes and the phase shift of the light between these same two axes.

10. System according to any one of the preceding claims, the receiver comprising an optical amplifier (31) upstream of the measuring instrument (35) of the receiver (3) and the measuring instrument (35) of the receiver (3) comprising a succession of semi-reflecting plates (32; 33; 34) arranged downstream of the optical amplifier (31), said plates (32; 33; 34) directing the luminous flux, in predefined proportions, towards instruments making it possible to characterize the ellipticity of its polarization.

11. System according to any one of the preceding claims, the receiver comprising an optical amplifier (31) upstream of the measuring instrument (35) of the receiver (3) and the optical amplifier (31) being a doped fiber amplifier.

12. System according to any one of the preceding claims, the transmitter being configured to successively generate a plurality of photons.

13. Photonic communication method transmitting coded information (A) on a light flux, using the system of any one of claims 1 to 12, comprising the steps of: (1) Generating a polarized light flux of predetermined polarization from an emitter, the light flux being emitted towards a receiver (3), (2) Encode the information on the luminous flux by introducing, using a polarization modifier placed between the transmitter and the receiver, a modification of the direction of the polarization of the luminous flux and a phase shift between the polarization components of the luminous flux along the two specific axes of the polarization modifier, the modification of the direction and the phase shift depending on the information to be transmitted, (3) Measure the average phase shift between the polarization components of the luminous flux along two perpendicular axes as well as the proportion of the luminous intensity along these same axes, and determine according to these measurements the information transmitted by the transmitter.

14. Method according to the preceding claim, comprising between step (2) and step (3), in particular if the light flux is composed of only one or a few photons: duplicating the photon into a flux of photons at the receiver (3), using an amplification device (31), the light thus created having retained the polarization state of the photon received at the receiver (3).

15. Method according to one of claims 13 and 14, the phase shift between the polarization components of the luminous flux along two perpendicular axes being chosen from phase shifts spaced 9° apart between -90° and +81°, and / or the direction of polarization of the luminous flux being chosen from directions spaced 9° apart between -90° and +81°, the state of polarization of the luminous flux then being chosen from 361 distinct states of polarization.

16. Method according to any one of claims 13 to 15, the same information (A) being coded on a predetermined number N of photons emitted successively by the transmitter.

17. Method according to the preceding claim, the receiver considering having received the information after having measured in step (4) a predetermined number n of photons received by the receiver (3) carrying the same information.

18. Method according to any one of claims 13 to 17, the transmitter transmitting a coded message comprising a plurality of information (A), in particular a plurality of letters (A), each coded on one or more photons.

19. Method according to the preceding claim, a transmitted information (A), in particular a transmitted letter (A), coded on one or more photons, being separated from another transmitted information, in particular from another transmitted letter, by the transmission of a separation information, in particular from a separation letter, coded on one or more photons, preferably the transmission of two identical pieces of information, in particular two identical letters, being separated by the transmission of separation information, in particular a separation letter.

20. Device for modifying the direction of polarization of a luminous flux belonging to the optical communication system described in any one of claims 1 to 12, the device comprising: one or more first blades or one or more first prisms whose refractive index(es) are dynamically adjustable, downstream of the one or more first blades or one or more first prisms, an intermediate device arranged to rotate the direction of polarization of the luminous flux by an angle depending on the point through which the luminous flux penetrates said intermediate device, and downstream of the intermediate device, one or more second blades or one or more second prisms, and optionally one or more third blades or one or more third prisms, whose refractive index(es) are dynamically adjustable.

21. Device according to the preceding claim, the intermediate device comprising a material that is at least partly chiral or rotatable.

22. Device according to one of claims 20 or 21, the refractive index(es) of the one or more second blades or of the one or more second prisms being adjusted symmetrically with respect to those of the one or more first blades or of the one or more first prisms.

23. Device for modifying the direction of polarization (8) of a luminous flux (800) belonging to the optical communication system described in any one of claims 1 to 12, the device comprising several superimposed layers (81; 82; 83; 84; 85; 86): a first layer (81) being a lithium niobate crystal subjected to a first electric field Eci of intensity U in a direction y inducing a variation An xof its refractive index for luminous fluxes polarized in an x ​​direction perpendicular to the y direction, and a variation An y of its refractive index for light fluxes polarized in the y direction, a second layer (82) being a second lithium niobate crystal oriented at 90° around the x axis relative to the first layer, subjected to a second electric field Ec2 of the same intensity U as the first perpendicular to the xy plane, the field EC2 inducing the variation An x of its refractive index for luminous fluxes polarized in the y direction and the variation An yof its refractive index for light fluxes polarized in the x direction, a third layer (83) being a quarter-wave plate for the light flux from the second layer (82) which enters the third layer (83) at an entry point (87), the position of the entry point (87) on the third layer (83) depending on the intensity of the electric fields Eci and Ec2, a fourth layer (84) being a succession of quarter-wave plates oriented such that the light from the third layer (83) enters it circularly polarized and leaves linearly polarized in a direction depending on its entry point (88) in the fourth layer (84), a fifth and a sixth layer (85; 86) of the same nature and orientation as the first and second layers (81;82) respectively, and subjected to electric fields respectively in the y direction and in the direction perpendicular to the xy plane, the equal intensities U' of which are adjusted according to the electric fields Eci and Ec2, so that the light ray leaving the sixth layer (86) comes out at an invariant point (89) whatever the value of the intensity U of the electric fields Eci and Ec2.; 24. Device according to the preceding claim, the first, second, fifth and sixth layers (81; 82; 85; 86) being liquid crystals or Pockels cells arranged between two transparent electrodes, or the refractive indices of the first, second, fifth and sixth layers (81; 82; 85; 86) being modulated by the application of intense light and the first, second, fifth and sixth layers (81; 82; 85; 86) being composed of non-linear refractive index materials.

25. Device for receiving a luminous flux, arranged to measure the ellipticity and the orientation of the polarization of a light (P) according to the Jones formalism, the device belonging to the optical communication system described in any one of claims 1 to 12, the device comprising: an amplifier (31), for example a doped fiber optical amplifier, configured to amplify the light (P), thus creating a luminous flux (nP), an optical device comprising: two semi-reflecting mirrors (32; 33), a mirror (34), three prisms (321; 331; 341), two photosensitive sensors (322; 323), two cameras (333; 343) and three polarization direction modifiers (37; 332; 342), the first semi-reflecting mirror (32) being configured to reflect and deflect a part of the luminous flux (nP), for example a third, at the output of the amplifier (31),towards a first birefringent prism (321) separating the polarized deflected light into two light fluxes along the two proper axes of the prism (321), the two light fluxes being sent respectively to sensitive photo sensors (322; 323) which measure their respective intensity, the second semi-reflecting mirror (33) being configured to reflect a part of the light flux, for example half, not reflected at the output of the first mirror (32), and deflect this reflected flux towards a second birefringent prism (331) separating the polarized deflected light into two light fluxes along the two proper axes of the second prism (331), the first light flux then being sent towards a first camera (333) to illuminate a surface Si, the second flux being sent towards a first polarization direction modifier (332) which modifies the polarization direction of the second flux by 90° before sending this flux towards the same surface Si of the first camera (333),interference fringes then appearing on the camera, the position of which makes it possible to measure the phase shift between the fluxes oriented along the two specific axes of the second prism (331), the second polarization direction modifier (37) being configured so that the light flux not reflected by the second semi-reflecting mirror (33) passes through the second, polarization direction modifier (37) which modifies the polarization direction of the flux by 45°, the third mirror (34) being configured to reflect the light flux having passed through the second polarization direction modifier (37), and deflect this reflected flux towards a third birefringent prism (341) separating the deflected light polarized into two light fluxes along the two proper axes of the third prism (341), the first light flux being sent towards a surface Si' of a second camera (343), the second flux being sent towards a third polarization direction modifier (342) which turns the polarization direction of the second flux by 90° before sending this flux towards the same surface Si' of the third camera (343), interference fringes then appearing on the camera, which testifies to the phase shift between the fluxes oriented along the two proper axes of the third prism (341).

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