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WO1996007951A1 - Modulation de polarisation - Google Patents

Modulation de polarisation Download PDF

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Publication number
WO1996007951A1
WO1996007951A1 PCT/GB1995/002123 GB9502123W WO9607951A1 WO 1996007951 A1 WO1996007951 A1 WO 1996007951A1 GB 9502123 W GB9502123 W GB 9502123W WO 9607951 A1 WO9607951 A1 WO 9607951A1
Authority
WO
WIPO (PCT)
Prior art keywords
liquid crystal
modulator
polarisation
cell
signal
Prior art date
Application number
PCT/GB1995/002123
Other languages
English (en)
Inventor
Stephen Vincent Kershaw
Paul David Townsend
Original Assignee
British Telecommunications Plc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from PCT/GB1994/001952 external-priority patent/WO1995007582A1/fr
Application filed by British Telecommunications Plc filed Critical British Telecommunications Plc
Priority to AU34773/95A priority Critical patent/AU3477395A/en
Publication of WO1996007951A1 publication Critical patent/WO1996007951A1/fr

Links

Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1347—Arrangement of liquid crystal layers or cells in which the final condition of one light beam is achieved by the addition of the effects of two or more layers or cells
    • G02F1/13471—Arrangement of liquid crystal layers or cells in which the final condition of one light beam is achieved by the addition of the effects of two or more layers or cells in which all the liquid crystal cells or layers remain transparent, e.g. FLC, ECB, DAP, HAN, TN, STN, SBE-LC cells
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
    • H04L9/08—Key distribution or management, e.g. generation, sharing or updating, of cryptographic keys or passwords
    • H04L9/0816—Key establishment, i.e. cryptographic processes or cryptographic protocols whereby a shared secret becomes available to two or more parties, for subsequent use
    • H04L9/0852—Quantum cryptography
    • H04L9/0858—Details about key distillation or coding, e.g. reconciliation, error correction, privacy amplification, polarisation coding or phase coding
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/137—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering
    • G02F1/139—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering based on orientation effects in which the liquid crystal remains transparent
    • G02F1/141—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering based on orientation effects in which the liquid crystal remains transparent using ferroelectric liquid crystals
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/137—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering
    • G02F1/139—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering based on orientation effects in which the liquid crystal remains transparent
    • G02F1/141—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering based on orientation effects in which the liquid crystal remains transparent using ferroelectric liquid crystals
    • G02F1/1418—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering based on orientation effects in which the liquid crystal remains transparent using ferroelectric liquid crystals using smectic liquid crystals, e.g. based on the electroclinic effect
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F2203/00—Function characteristic
    • G02F2203/07—Polarisation dependent

Definitions

  • the present invention relates to a method and -apparatus for polarisation modulation suitable for use, for example, in an optical communications system in which optical signals are encoded in different polarisation states. It is particularly, but not exclusively, concerned with a communications system employing quantum cryptography for the secure distribution of a key to be used in subsequent encryption/decryption of signals carried by the system.
  • quantum cryptography may be carried out using, e.g., modulation of a single-photon signal with different phase or polarisation states.
  • lithium niobate or other crystalline electro-optic modulators are birefringent devices which potentially offer high switching rates.
  • they suffer the significant disadvantages of being expensive to manufacture, and tend to produce high insertion losses.
  • Many lithium niobate devices also suffer from variable frequency response in the lower frequency ranges due to piezo-electric resonances.
  • a method of modulating an optical signal including passing the signal through a liquid crystal modulator and thereby modulating the polarisation state of the signal, characterised in that in the liquid crystal modulator the signal passes through a plurality of individually switchable liquid crystal cells arranged in series between an input and an output of the modulator, and in that for different input signals, the modulator is set to a different one of a multiplicity of configurations corresponding to different combinations of settings of the individual liquid crystal cells, thereby applying a corresponding one of a multiplicity of discrete polarisation modulation states to the signal.
  • the present invention provides a method of modulation which is able to apply a set of discrete polarisation states using relatively simple components with low insertion-loss, namely a stack of liquid crystal cells.
  • each liquid crystal cell includes a ferroelectric liquid crystal having a direction of polarisation which switches by 2 ⁇ in response to an applied electric control signal, where ⁇ is the characteristic smectic layer tilt angle of the material.
  • ferroelectric LC materials are preferred as providing cells which are potentially bistable and switched between two discrete states in response to an applied control signal.
  • a series of these cells together with appropriate combinations of control signals can therefore generate the required set of discrete polarisation states.
  • the signal modulated by the cell is a single-photon signal and the signal is subsequently detected at a single-photon detector as part of a method of key distribution using quantum cryptography.
  • the modulator of the present invention was developed in particular to meet the need which arises in quantum cryptography for modulation using sets of discrete polarisation states.
  • the single-photon signal may take the form, for example, of single photons obtained from a parametric optical amplifier source, or, alternatively, may comprise weak pulses of light from an attenuated laser which in general contains no more than one, and on average substantially less than one photon per pulse. Either signal-type exhibits the required quantum properties, and the term "single-photon pulse" is used herein to denote all such signals exhibiting quantum properties, irrespective of how they are produced.
  • quantum cryptography provides a method for the distribution of a key for use in subsequent encryption/decryption of transmitted data.
  • the security of such a system depends upon the key to the encryption algorithm being available only to authorised users.
  • the key is distributed over a secure quantum channel carried by single-photon signals and exhibiting non-classical behaviour.
  • the transmitter and the receiver then communicate over a separate channel, known as the public channel, to compare the transmitted and received data.
  • the presence of any eavesdropper intercepting the transmitted single-photon signals encrypted with the key results in a change in the statistics of the received data, which can be detected. Accordingly, in the absence of any such change in the statistics of the data, the key is known to be secure.
  • the secret key thus established can then be used for encryption/decryption of subsequent communications.
  • the existing key may periodically be replaced by a newly generated key.
  • a system using quantum cryptography may take any one of a variety of configurations, including those described in our above-cited International application.
  • a multiple- access network may be used for distribution of the key.
  • both the transmitter including the source of single photon signals, and the receiver including the single-photon detector may be located at a head-end station connected to such a multiple access network, with subscribers connected to the network being provided with modulators. In such a system the subscribers modulate the single-photon signal which then passes via a looped-back- path to the head-end station for detection.
  • the present invention is particularly concerned with systems where the key is encrypted on single-photon signals using a modulation alphabet formed from different polarisation states.
  • the modulator comprises two liquid crystal cells arranged in series.
  • the four polarisation states comprise four linear states with planes of polarisation which are respectively vertical, horizontal, 45° and 135°
  • the four polarisation states may comprise two linear states and two circular states, e.g. linear vertical, linear horizontal, right circular and left circular.
  • the encryption scheme may use six polarisation states formed by a superposition of the two four-state sets discussed above.
  • a modulator for an optical signal including a liquid crystal modulator, characterised in that the liquid crystal modulator comprises a plurality of individually switchable liquid crystal cells arranged in series between an input and an output, and by control means arranged to set the liquid crystal modulator in different ones of a multiplicity of configurations corresponding to different combinations of settings of the individual liquid crystal cells, thereby enabling corresponding ones of a multiplicity of discrete polarisation states to be applied to an optical signal.
  • a communications system using quantum cryptography including a source of single-photon signals, means for modulating the single-photon signals from the source, and means for detecting the modulated signals, characterised in that the means for modulating the single-photon signals comprise a modulator in accordance with the second aspect of the present invention.
  • Figure 1 is a schematic of a polarisation modulator embodying the present invention
  • Figure 2 is a diagram illustrating the general case of a stack of n liquid crystal cells
  • Figure 3 is a diagram illustrating the electric field induced reorientation of molecules in a ferro-electric liquid crystal material shown in bistable states before and after switching;
  • Figure 4 shows a first example of a quantum cryptography network;
  • Figure 5 shows a second example of a quantum cryptography network
  • Figure 6 is a flow diagram for the operation of the network of Figure .
  • a polarisation modulator 101 comprises a stack 102 of liquid crystal cells.
  • the stack comprises two chiral smectic-C cells SI, S2.
  • Each cell comprises a pair of glass substrates gl, g2 with an ITO electrode E formed on each substrate.
  • the thickness of the ITO electrode is chosen to provide a resistivity of around 50 Ohms/square.
  • Appropriate pre-coated ITO electrodes are available commercially in the UK from Balzers.
  • a polyimide coating, rubbed in one direction is formed on each of the electrodes. The rubbing directions on top and bottom substrates are usually opposed to minimise the stored splay elastic energy arising from the slight surface pretilt of the liquid crystal molecules.
  • polyimide alignment layers are preferred, other planar alignment materials are possible e.g. rubbed nylon, rubbed polyvinyl alcohol, or obliquely evaporated silicon oxide or magnesium fluoride. On balance polyi ides have so far tended to give the best all round performance.
  • the polyimide coating may have a thickness in the range 0.1 to 0.2 microns.
  • the alternative inorganic coatings referred to above may have thicknesses in the range 200 to 500 Angstroms.
  • a single double-sided substrate may replace the upper substrate of SI and the lower substrate of S2.
  • a control processor 103 sets the state of the modulator 101.
  • Spacers SP separate the substrates and define a volume in which the liquid crystal material is confined. Spacer material may be located at the periphery or scattered, possibly randomly throughout the cell.
  • An adhesive seal may be applied to one of the substrates to define the lateral boundary of the cell. The seal will have one or more gaps to permit capillary filling (under vacuum if only one gap is used) with liquid crystal.
  • ferroelectric liquid crystal (FELC) and liquid crystal mixtures available commercially that may be used in such devices, for example a suitable material is that available from Merck as ZLI-4318. A complete listing of available ferroelectric mixtures is available from Merck Ltd. at Merck House, Poole, Dorset, BH15 1TD, UK.
  • the spacing between the glass substrates in each cell is typically in the range 1.5 to 3 ⁇ m.
  • the thickness of each cell and refractive index anisotropy of the liquid crystal is chosen so that at the wavelength of the input' beam the cell functions e.g. as a half-wave or quarter-wave plate.
  • the liquid crystal material is assumed to have a typical birefringence of 0.15. Therefore, for operation at a wavelength of 830mn, for operation as a half-wave plate the cell is arranged to have a thickness of 2.8 ⁇ m, while a quarter-wave plate uses a cell of half that thickness.
  • the cell thickness is scaled accordingly.
  • Selected FELC materials may have birefringences as high as 0.5. However, more typically, FELCs have birefringences in the range 0.15-0.13 at 589nm.
  • Ferro-electric chiral smectic C materials possess a strong spontaneous polarisation at an angle approaching 90° to the longitudinal direction of the molecule. It is this spontaneous polarisation that interacts with the electric field.
  • the surface stabilisation and the chirality of the material ensures that when the spontaneous polarisation reverses during switching, then the layer tilt flips from + 0 to - 0 in the plane orthogonal to the applied electric field.
  • the magnitude of the layer tilt is not altered by the field, but only its rotation sense. It should be understood that there is no tendency for the longitudinal axis of the molecules to line up with the electric field, i.e. for them to tilt relative to the cell walls. The layer tilt is always induced in the plane along the field direction.
  • Bistable FE switching can be observed at voltages typically up to +/- 50V ⁇ m '1 or more before they break down.
  • the dotted lines separate smectic layer planes, and the solid slanted lines mark the molecular axes.
  • Figure 2 shows schematically a stack of LC devices (1- n) for polarisation state control.
  • the input polarisation state can be described by the Jones vector.
  • the properties outlined above enable a stack of switched cells such as that shown in Figure 1 to function as a polarisation modulator for selecting predetermined discrete polarisation states.
  • Input cell 1 state cell 2 state Output linear 0 0 linear vertical vertical linear ⁇ r/4 0 linear vertical horizontal linear 0 ⁇ /8 linear 135° vertical to horizontal (ccw) linear ⁇ r/4 ⁇ /8 linear 45° vertical to horizontal (ccw)
  • a liquid crystal modulator for implementing such a scheme again comprises a stack of two cells.
  • the following table shows the different states for this modulator: TABLE 2
  • a further alternative encoding scheme comprises six states being a superposition of the states used in the first two schemes.
  • the states for this modulator are shown in the following table: TABLE 3
  • the left circular pair are essentially degenerate, as are the right circular pair. While the absolute phase of each of the circular polarisations in each pair differs, the fact that the intensity is time averaged over a period many times the oscillation period of the wave means that the absolute phase is irrelevant. One is therefore left with four linear polarisation states and a left and right circular polarisation state. Effectively, when cell 2 is on, it does not matter what state cell 3 is in.
  • a number of other configurations are possible for a stacked liquid crystal modulator.
  • the half- * wave cells in the examples described above could be split into pairs of quarter-wave cells.
  • the order of some of the cell combinations could also be changed.
  • a further possible modification is the use of electroclinic devices to provide continuously tunable wave plates providing further coding flexibility.
  • electroclinic is used to denote chiral smectic A liquid crystals which have a continuously variable molecular layer tilt angle, the angle being linearly dependent on the cell drive voltage.
  • electroclinic materials are available commercially such as 764E, 854E, etc from Merck Ltd.
  • liquid crystal modulators as described above is found to be highly advantageous, enabling switching at relatively high rates with, for example, lO ⁇ s pulse spacing and offering the possibility of compact, cheap devices.
  • FIG 4 shows a specific example of a broadcast network containing two receivers and a transmitter.
  • the transmitter consists of a gain-switched semiconductor laser 9, which may be a DFB or Fabry-Perot device, an attenuator or intensity modulator 7, and a polarisation modulator 8 and control electronics 10.
  • the single-photon detectors in the receivers may be avalanche photodiodes (APDs) biased beyond breakdown and operating in the Geiger mode with passive quenching, as discussed in P. D. Townsend, J. G. Rarity and P. R. Tapster, Electronics Letters, 29, 634 (1993) .
  • APDs avalanche photodiodes
  • Each receiver includes a microprocessor control unit 2, which receives the output of the APD via a discriminator/amplifier circuit 3.
  • the control unit 2 also controls an electronic filter 4 and local oscillator 5, as well as the APD bias supply 6.
  • the electronic filter 4 isolates the first harmonic of the frequency spectrum of the signal output by the APD in response to synchronising > pulses received via the network. This generates a sinusoidal signal at the pulse frequency which locks the local oscillator 5.
  • the output of the local oscillator 5 is received at the control unit 2 to provide a timing reference during quantum transmissions.
  • the use of multi-photon signals on the transmission medium to calibrate the system prior to or during quantum transmission is described in further detail in our co- pending International patent application PCT/GB93/02637, the contents of which are incorporated herein by reference. This makes it possible to compensate, e.g., for changes in fibre polarisation resulting from environmental effects.
  • key distribution is initiated by the transmitter sending a stream of timing pulses into the network. These are bright, multi-photon pulses. Accordingly these pulses behave classically, and are received by both of the terminals connected to the network.
  • the receivers may set the reverse bias on their single-photon detectors to be well-below breakdown so that internal gain is low.
  • the APDs can detect the multi-photon timing pulses without suffering from saturation.
  • Each APD output signal then contains a frequency component at the fundamental repetition rate of the pulsed source, and this is used to lock the local oscillator in the receiver as described above.
  • the optical source in the transmitter is set to produce single-photon signals, for example by connecting an attenuator in line with a multi-photon source.
  • the output pulses from the transmitter then contain on the order of 0.1 photons on average.
  • the APDs are biased beyond breakdown so that internal gain is high enough to achieve detection sensitivity at the single-photon level.
  • encoded pulses are sent from the transmitter onto the network using, e.g., one of the encoding alphabets described above.
  • a public discussion phase is entered in which the transmitter and receivers communicate on a public channel provided, for example, by the transmission of multi-photon signals on the network.
  • the sent and received sequences are compared, data for time slots in which different encoding/measurement bases were used discarded, and error correction carried out.
  • any residual discrepancy in the transmitted and received data is compared with a predetermined threshold. If the discrepancy level is acceptably low, then the shared data sequences are known to be secure and are used to provide a shared secret key. The key is then available for subsequent encryption/decryption of data transmissions between the transmitter and receivers.
  • the polarisation modulator of the present invention may be used in a variety of different network topologies.
  • Figure 5 shows a quantum cryptographic system in which the receivers, rather than detecting the photon destructively, modulate it and pass it back to the head-end station or "controller".
  • This approach is described in further detail in our pending International application PCT/GB93/02637.
  • a possible attack upon such an implementation requires an eavesdropper to intercept the quantum channel on both sides of a given user Bob. Then by transmitting and detecting a multi-photon signal, Eve (the eavesdropper) can determine unambiguously the state of Bob's modulator. In practice however it is likely to be very difficult for Eve to establish connections to two or more points in the network.
  • This photon detector need not be of the sensitivity of the single-photon detectors employed conventionally in receivers, nor need every user have such a detector. The presence of such a detector in the network facilitates the detection of any multi-photon probe used by Eve.

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Nonlinear Science (AREA)
  • Signal Processing (AREA)
  • Computer Security & Cryptography (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Theoretical Computer Science (AREA)
  • Mathematical Physics (AREA)
  • Chemical & Material Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Electromagnetism (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)

Abstract

Un procédé de modulation d'un signal optique consiste à faire passer le signal dans un modulateur à cristaux liquides, ce qui module l'état de polarisation. Le modulateur à cristaux liquides comprend une pluralité de cellules à cristaux liquides pouvant être commutées individuellement. Ces cellules sont disposées en série entre une entrée et une sortie du modulateur. Le modulateur est réglé selon différentes configurations correspondant à différentes combinaisons d'orientation des cellules à cristaux liquides individuelles. Le modulateur applique un des états distincts de modulation de polarisation au signal d'entrée en fonction de la configuration choisie. Le matériau cristallin liquide peut être un matériau ferroélectrique et le modulateur peut comporter deux ou trois de ces cellules à cristaux liquides configurées telles que des cellules demi-onde ou des cellules quart d'onde. Le modulateur peut être utilisé pour moduler un signal à photon unique dans un procédé de répartition de clés utilisant la cryptographie quantique.
PCT/GB1995/002123 1994-09-08 1995-09-07 Modulation de polarisation WO1996007951A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU34773/95A AU3477395A (en) 1994-09-08 1995-09-07 Polarisation modulation

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
GBPCT/GB94/01952 1994-09-08
PCT/GB1994/001952 WO1995007582A1 (fr) 1993-09-09 1994-09-08 Repartition de codes dans un reseau a acces multiple faisant appel a la cryptographie quantique
EP95301050 1995-02-20
EP95301050.1 1995-02-20

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Publication Number Publication Date
WO1996007951A1 true WO1996007951A1 (fr) 1996-03-14

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Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1998006002A1 (fr) * 1996-08-05 1998-02-12 Deutsche Telekom Ag Composant pour la commutation de modeles optiques dans l'espace de microsecondes
EP0975119A1 (fr) * 1998-07-24 2000-01-26 Deutsche Telekom AG Système de cryptographie quantique pour la transmission d'une clé aléatoire en utilisant le procédé de polarisation
US6438234B1 (en) 1996-09-05 2002-08-20 Swisscom Ag Quantum cryptography device and method
US7324647B1 (en) * 2000-10-23 2008-01-29 Bbn Technologies Corp. Quantum cryptographic key distribution networks with untrusted switches
US7627126B1 (en) 2002-10-15 2009-12-01 Bbn Technologies Corp. Systems and methods for implementing path length control for quantum cryptographic systems
EP4158800A1 (fr) * 2020-05-28 2023-04-05 SEW-EURODRIVE GmbH & Co. KG Système et procédé de transmission de données au moyen d'un flux lumineux, et unité technique

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Publication number Priority date Publication date Assignee Title
WO1988006303A1 (fr) * 1987-02-18 1988-08-25 The General Electric Company, P.L.C. Dispositif de commande de la polarisation
WO1990009614A1 (fr) * 1989-02-16 1990-08-23 S.T. Lagerwall S.A.R.L. Dispositifs a cristaux liquides utilisant un effet electro-optique lineaire
WO1994015422A1 (fr) * 1992-12-24 1994-07-07 British Telecommunications Public Limited Company Systeme et procede de distribution de code au moyen de la cryptographie quantique
JPH0777699A (ja) * 1993-09-07 1995-03-20 Fujitsu Ltd 偏光制御器

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1988006303A1 (fr) * 1987-02-18 1988-08-25 The General Electric Company, P.L.C. Dispositif de commande de la polarisation
WO1990009614A1 (fr) * 1989-02-16 1990-08-23 S.T. Lagerwall S.A.R.L. Dispositifs a cristaux liquides utilisant un effet electro-optique lineaire
WO1994015422A1 (fr) * 1992-12-24 1994-07-07 British Telecommunications Public Limited Company Systeme et procede de distribution de code au moyen de la cryptographie quantique
JPH0777699A (ja) * 1993-09-07 1995-03-20 Fujitsu Ltd 偏光制御器

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WO1998006002A1 (fr) * 1996-08-05 1998-02-12 Deutsche Telekom Ag Composant pour la commutation de modeles optiques dans l'espace de microsecondes
US6438234B1 (en) 1996-09-05 2002-08-20 Swisscom Ag Quantum cryptography device and method
EP0975119A1 (fr) * 1998-07-24 2000-01-26 Deutsche Telekom AG Système de cryptographie quantique pour la transmission d'une clé aléatoire en utilisant le procédé de polarisation
DE19833330A1 (de) * 1998-07-24 2000-01-27 Deutsche Telekom Ag Quantenkryptographiesystem zur gesicherten Übertragung zufälliger Schlüssel unter Verwendung des Polarisationsstellverfahrens
DE19833330C2 (de) * 1998-07-24 2001-03-15 Deutsche Telekom Ag Quantenkryptographiesystem zur gesicherten Übertragung zufälliger Schlüssel unter Verwendung des Polarisationsstellverfahrens
US6748081B1 (en) 1998-07-24 2004-06-08 Deutsche Telekom Ag Quantum cryptography system for a secure transmission of random keys using a polarization setting method
US7324647B1 (en) * 2000-10-23 2008-01-29 Bbn Technologies Corp. Quantum cryptographic key distribution networks with untrusted switches
US7627126B1 (en) 2002-10-15 2009-12-01 Bbn Technologies Corp. Systems and methods for implementing path length control for quantum cryptographic systems
EP4158800A1 (fr) * 2020-05-28 2023-04-05 SEW-EURODRIVE GmbH & Co. KG Système et procédé de transmission de données au moyen d'un flux lumineux, et unité technique

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