Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/50—Transmitters
  • H04B10/516—Details of coding or modulation
  • H04B10/548—Phase or frequency modulation

Definitions

• the present invention relates to an optical transmission system and a device for receiving an optical signal comprising at least one optical signal modulated by an electrical signal whose phase varies as a function of the value of at least one data bit to be transmitted.
• the present invention is more particularly applicable in the field of securing information transfers and more particularly in the field of quantum cryptography.
• the information is encoded at the transmitter and decoded by the receiver using a predetermined algorithm known to the transmitter and the receiver.
• the security of the system depends on whether the key used by the algorithm is known only to the authorized sender and receiver.
• Quantum cryptography distributes the key of the algorithm to ensure that if a third party device captures the signals carrying the key, the transmitter and the receiver can determine if the key has been picked up by the third party device.
• a communication channel is used for the transmission, in the form of photons, of the quantum key.
• a second communication channel called public channel, is used by the transmitter and the receiver to exchange data to check whether the transmission of the key on the quantum channel has been distorted, picked up by a third party device or not.
• the transmission device transmits on the quantum channel a sequence of photons by randomly choosing the quantum state of each photon.
• the state of each photon is chosen according to a known rule of the transmitter and receiver devices. Some of the chosen states are non-orthogonal, that is to say that it is not possible to differentiate them in a certain way.
• the receiving device selects randomly and independently from that used by the transmitting device, a decision rule among at least two decision rules. If the receiving device uses the same decision rule as the transmitting device, the receiving device unequivocally determines the value of the transmitted bit. If the receiving device uses a decision rule that is not compatible with the state chosen by the sender or the decision rule chosen by the sender, the result obtained does not make it possible to determine the value of the transmitted bit. The probability of concluding at a bit 1 or a bit 0 is then equiprobable. The measure is therefore inconclusive.
• the receiver device When the photon transmission is complete, the receiver device discloses the decision rule for each received photon via the public channel to the transmitting device. The result of the measurement remains naturally secret.
• the transmitter and receiver devices eliminate by this method all the inconclusive results. Finally, they share a random sequence of bits that can be used as a cryptographic key.
• Various quantum cryptography techniques have been proposed. Some use the polarization state of the photon to encode a binary information, others a phase modulation.
• a first solution consists in introducing a phase shift carrying the information by introducing an optical path difference between the different optical signals between at least two temporally separated optical signals.
• a second solution consists in introducing a phase shift carrying the information between "
• This phase shift is performed by periodically modulating an optical signal.
• the aforementioned cryptographic techniques are sensitive to polarization variations mainly related to the medium used for the transmission of photons.
• the photon transmission medium is, for example and without limitation, the atmosphere or an optical fiber.
• These polarization variations are related to the environment of the medium such as, for example, temperature variations thereof.
• the invention solves the drawbacks of the prior art by providing a reception device which is insensitive to polarization variations and which thus enables key transmission according to quantum cryptography technique over long distances and / or high reliability in the time.
• the invention proposes a device for receiving an optical signal comprising at least one optical pulse signal ⁇ 0 modulated by an electrical pulse signal ⁇ whose phase ⁇ l varies as a function of the value. at least one bit of data to be transmitted, characterized in that the reception device comprises:
• a polarization separator for separating the optical signal of angular frequency ⁇ 0 modulated first and second optical signals propagating in the same direction, the first optical signal having a first polarization and the second optical signal having a second polarization,
• the invention also relates to a system for transmitting an optical signal comprising at least one optical pulse signal ⁇ > o modulated by an electrical pulse signal ⁇ whose phase ⁇ l varies as a function of the value of at least one bit of data to be transmitted, characterized in that the system comprises: a transmitting device capable of forming the optical pulse signal ⁇ 0 modulated by the electrical pulse signal ⁇ whose phase ⁇ 1 varies as a function of the value of at least one bit of data to be transmitted, a receiver device comprising: polarization splitter for splitting the optical signal of angular frequency ⁇ 0 modulated first and second optical signals propagating in the same direction, the first optical signal having a first polarization and the second optical signal having a second polarization,
• means for modulating the second optical signal from the second electrical ⁇ and ⁇ 2 phase pulsation signal means for combining the first modulated optical signal and the second modulated optical signal to form a recombined optical signal.
• the receiver device further comprises photon detection means included in the optical signal, counting means for counting the number of photons detected over a predetermined time interval, and means for transferring data to the photon.
• transmitter device for a modification of the pulsation ⁇ > o of the optical signal.
• the receiving device is insensitive to frequency variations of the optical signals related for example to temperature or variations in time.
• the modulating means of the first optical signal and the second optical signal are phase modulators or intensity modulators or electro-absorbent modulators.
• the amplitude and / or phase of the first and second optical signals are adjusted independently.
• the data is a cryptographic key and the optical signal consists of at least one modulation sideband comprising a photon.
• the optical signal further comprises an optical pulse signal ⁇ s modulated by the electric pulse signal ⁇ and the means for obtaining the ⁇ and phase ⁇ 2 electric pulse signal comprise:
• a wavelength demultiplexer (140) which separates in the optical signal the modulated optical pulse signal ⁇ 0 from the pulsating optical signal ⁇ s,
• a detector which detects the photons of the pulsed optical signal ⁇ s modulated to form an electrical pulse synchronization signal ⁇
• phase shifter of the phase synchronization electrical signal ⁇ 2 a phase shifter of the phase synchronization electrical signal ⁇ 2.
• the receiving device has a synchronization signal which is insensitive to variations related to variations in the optical path of the optical signal received.
• the device further comprises at least one filter for forming an optical signal whose pulsation corresponds to the pulsation of one of the modulation sidebands resulting from the modulation of the pulsating optical signal ⁇ 0 and at least one detector for detecting at least one photon in the optical signal comprising the modulation sideband.
• the filter is a Fabry Pérot cavity and the device further comprises means for modifying the characteristics of the Fabry Pérot cavity.
• the optical signal consists of two lateral modulation bands and the means for modifying the characteristics of the Fabry Pérot cavity modify the characteristics of the Fabry Pérot cavity to form an optical signal comprising the one or other of the modulation sidebands.
• the means for modifying the characteristics of the Fabry Pérot cavity modify the characteristics of the Fabry Pérot cavity as a function of the number of photons detected over a predetermined time interval.
• the receiving device is insensitive to frequency variations of the optical signals related for example to temperature or variations in time.
• the Fabry Pérot cavity is associated with a temperature control device and the means for modifying the characteristics of the Fabry Pérot cavity comprise means for modifying the control temperature.
• FIG. . 1 represents the architecture of the optical transmission system according to the present invention
• FIG. 2 represents a Fabry Pérot cavity according to the present invention
• FIG. 3 represents a system for controlling the temperature of the Fabry Pérot cavity according to the present invention.
• Fig. 1 represents the architecture of the optical transmission system according to the present invention.
• the optical transmission system as shown in FIG. 1 is particularly suitable for transmitting a cryptographic key.
• a transmission device 160 transmits via a transmission medium 150, a cryptographic key to a receiving device 100.
• the transmission medium 150 is the quantum channel and is, for example, an optical fiber.
• the transmission medium 150 may also, according to an alternative embodiment, be the atmosphere.
• the transmission device 160 is also connected to the receiver device 100 via a public channel 170.
• the public channel 170 is for example included in a public communication network such as, for example, an IP type network or a telephone type communication network. Through the public channel 170, the transmitting device 160 and the receiving device 100 exchange information for the exchange of a key as previously described.
• the transmission device 160 comprises a sine wave oscillator 161 ⁇ .
• the sinusoidal electrical signal delivered by the oscillator 161 is then separated into two signals S1 and S2 by a power splitter 162 or "power splitter” in English.
• the signals S1 and S2 are preferably of the same amplitude.
• the signal Sl is then phase-shifted by a phase shift circuit 163.
• the phase shift of the signal Sl makes it possible to code the bits of information to be transmitted.
• the phase shift ⁇ l is equal to 0 or ⁇ / 2 when the two-state protocol B92 is used or is equal to 0 or ⁇ / 2, ⁇ or 3 ⁇ / 2 when the protocol BB84 is used.
• the BB84 protocol is described in the publication of CH Bennett and G. Brassard entitled “Quantum cryptography: Public key distribution and coin tossing", “Proceeding of IEEE International on Computers, Systems and Signal Processing, Bangalore, India” (IEEE New York, 1984). ), pp 175-179.
• the Sl-phase electrical signal is then transferred to an emission source 164 of an optical signal which modulates the optical signal of angular frequency ⁇ 0 by the phase shifted signal Sl.
• the source of emission 164 of an optical signal consists, for example and without limitation, of a laser diode 164a and a modulator 164b electro-optical integrated on a substrate of lithium crystals niobate (LiNbO 3 ) or with electro absorption preferably integrated on the chip of the laser diode 164a.
• the emission source 164 of the optical signal modulates the optical signal by the out-of-phase signal Sl with a modulation ratio denoted In 1 which is preferably much less than unity. It should be noted here that, the intensity phase modulation ratio of the laser diode 164 being negligible, the optical signal SI 1 formed by the emission source 164 is approximated as follows: o -
• E 0 is the peak amplitude of the signal E ⁇ (t).
• the spectral power density of the signal E ⁇ (t) consists of a carrier line of frequency at ⁇ 0 / 2 ⁇ , a frequency modulation sideband at ((Oo + ⁇ ) / 2 ⁇ , and a band lateral frequency modulation at ( ⁇ - ⁇ ) / 2 ⁇ .
• the laser diode 164a is a DFB diode, acronym for "Distributed Feed Back" whose pulse ⁇ 0 is modified, for example by means of a change in its operating temperature, according to an instruction received from the reception device 100 via the transmission medium 150 or the public channel 170.
• the electrical signal S2 is transferred to a transmission source 165 of an optical signal which modulates the optical pulse signal ⁇ s different from the pulsation ⁇ 0 by the signal S2 to form a synchronization signal S 12.
• the emission source 165 of an optical signal is constituted, for example and without limitation, a laser diode 165a and an integrated electro optic modulator 165b on a substrate of lithium crystals niobate (LiNbO 3 ) or electro absorption preferably integrated on the chip of the laser diode.
• optical signals SI 1 and S 12 are then multiplexed by a wavelength multiplexer 166 and transmitted on the quantum channel 150.
• the transmitter device 160 does not include a power divider 162, a transmission source 165 and a wavelength multiplexer 166.
• only the signal SI 1 is formed and transferred on the quantum channel 150.
• the receiving device 100 comprises a wavelength demultiplexer 140 which separates in the received signal the optical signal Sl I 1 or quantum signal Sl 11 from the optical signal S 121 or reference signal S 121.
• the reference signal S 121 avoids having, at the level of the reception device 100, a local oscillator synchronized with the pulsation signal ⁇ of the transmission device 160.
• the reference signal S 121 of pulsation ⁇ s is transferred to a detector 102, such as, for example, an avalanche photodiode.
• the detector 102 produces an electrical signal S 122 of the same pulsation ⁇ as the signal delivered by the oscillator 161 of the transmission device 160.
• the receiver device 100 instead of obtaining the electrical signal S 122 of the ⁇ pulsation of the received optical signal, the receiver device 100 comprises a local oscillator of pulsation ⁇ as well as means for synchronizing its sound. local oscillator with the local oscillator 161 of the transmitter device 160.
• the electrical signal S 122 is then phase shifted by a phase shift circuit 103.
• the phase shift circuit 103 shifts the electrical signal S 122 by a phase shift ⁇ 2 + ⁇ / 2.
• the phase shift ⁇ 2 is equal to 0 or ⁇ / 2 when the two-state protocol B92 is used or is equal to 0 or ⁇ / 2, ⁇ or 3 ⁇ / 2 when the BB84 protocol is used.
• the out-of-phase electric signal S 123 is then separated into two electrical signals S 123a and S 123b of the same amplitude by a power divider 104.
• the phases and amplitudes of the electrical signals S123a and S123b are adjusted so as to equalize the amplitude variations. and phase related to the characteristics of the active elements such as amplifiers (not shown in Fig. 1) or passive, such as the lengths of the tracks carrying the electrical signals S123a and S123b, so as to obtain a degree of modulation m 2 at the level of phase modulators 110a and HOb equal to nt / 2.
• the electrical signals S 123a and S 123b are used as modulation signals respectively by the modulators 110a and 110b.
• the quantum signal Sl I l is transferred, according to the invention, to a polarization splitter 105.
• the polarization splitter 105 makes it possible to separate the received quantum signal Sl I l from any polarization into two optical signals Sl l ia and Sl 1 Ib propagating in the same direction but in different polarizations.
• These polarizations are preferably orthogonal.
• the electric field of the received quantum signal Sl I 1 is represented in an orthogonal coordinate system whose axes u and v are the axes of the polarization separator 105 in the form: in which A and B are the respective projections of the electric field ⁇ sm on the axes û and v.
• the quantum signal Sl I l is divided into an optical signal Sl 1 or the quantum signal Sl 1 la whose electric field is:
• the polarization separator 105 is, for example and without limitation, a polarization separator marketed by General Photonics Corporation under the name "Polarization Beam Splitter PBS-001-P-03-SM-FC / PC”.
• the quantum signals Sl I la and Sl 11b are respectively transmitted to a phase modulator HOa and to a phase modulator HOb.
• HOa and HOb modulators are intensity modulators or electro-absorbent modulators.
• the modulator 110a modulates the quantum signal S11a by the electrical signal S123a
• the phase modulator HOb modulates the quantum signal Sl 11b by the electrical signal S 123b.
• Modulators 110 are modulators for example marketed by the company "EOspace” under the name “Very-Low-Loss Phase Modulator”.
• the pulsation modulation sideband ⁇ > o + ⁇ is maximum and the lateral band of pulse modulation ⁇ 0 - ⁇ is zero.
• the pulsation modulation sideband ⁇ 0 - ⁇ is maximum and the pulsation modulation sideband ⁇ 0 + ⁇ is zero.
• the intensity of the quantum signal Sl 12a in the pulsation band O) 0 ⁇ ⁇ at the output of the phase modulator 11 Oa is proportional to:
• the intensity of the quantum signal Sl 12b in the pulse band O) 0 ⁇ ⁇ at the output of the phase modulator 11 Ob is proportional to:
• the quantum signals Sl 12a and Sl 12b are then recombined by a polarization separator 115, identical to the polarization separator 105 and used in reverse. After recombination, the total intensity in the pulsation band O) 0 ⁇ ⁇ of the quantum signals Sl 12a and Sl 12b is proportional to:
• the recombined signal Sl 13 is filtered by a filter 120 to form a signal Sl 14 which comprises only one of the two modulation sidebands.
• the filter 120 consists of Bragg filters, multilayer filters, AWG filters, acronym for Array Wave Guide, etc.
• the filter 120 is a Fabry cavity
• the recombined signal Sl 13 consists of three frequencies: the frequency at ⁇ 0 11%, a frequency modulation sideband at ( ⁇ o - ⁇ ) / 2 ⁇ and a frequency modulation sideband at ( ⁇ 0 + ⁇ ) / 2 ⁇ .
• the filter 120 filters the recombined signal
• the signal Sl 14 is then processed by a quantum detector 130 consisting of a photo detector which detects each photon transmitted in the frequency sideband ( ⁇ 0 + ⁇ ) / 2 ⁇ .
• the receiver device 100 comprises two filters that filter the recombined signal Sl 13 so as to obtain respectively a first optical signal comprising the sideband at the frequency ( ⁇ 0 - ⁇ ) / 2 ⁇ and a second optical signal comprising the sideband at the frequency ( ⁇ 0 + ⁇ ) / 2 ⁇ .
• the first optical signal is then processed by a first photo detector which detects each photon transmitted in the frequency sideband ( ⁇ 0 - ⁇ ) / 2 ⁇ and the second optical signal is then processed by a second photo detector that detects each photon transmitted in the frequency sideband ( ⁇ 0 + ⁇ ) / 2 ⁇ .
• Fig. 2 represents a Fabry Pérot cavity according to the present invention.
• the Fabry Pérot cavity 120 consists of two Bragg mirrors 24a and 24b inscribed on an optical fiber 21 consisting for example of a core of 9 microns and a sheath of 125 microns.
• the cavity thus formed is held in a support composed of two parts 22a and 22b.
• the two parts 22a and 22b are shown spaced from each other in FIG. 2 in order to allow the representation of the optical fiber 21.
• the parts 22a and 22b are in contact to allow good thermal conduction.
• a temperature control module 23 such as, for example, a Peltier effect module 23, is placed on the upper part of the support 22a to allow heating or cooling the optical fiber 21.
• a heat sink 26 is placed on the Peltier effect module 23 and optimizes the temperature difference that exists between the external environment and the temperature of the Fabry Pérot cavity 120.
• a temperature sensor 25, for example a thermistor, is placed on the lower part. 22b of the support and makes it possible to determine the temperature of the optical fiber 21.
• Bragg 24 corresponding to the maximum reflection is variable depending on the temperature.
• a system for controlling the temperature of the Fabry Pérot cavity is made in such a way as to adjust the frequency band or the frequency bands filtered by the Fabry Pérot cavity 120.
• the Fabry cavity is made in such a way as to adjust the frequency band or the frequency bands filtered by the Fabry Pérot cavity 120.
• Perot 120 is not temperature controlled to adjust the frequency band or the filtered frequency bands as a function of the number of photons detected within a predetermined time interval.
• the pulsation ⁇ 0 of the laser diode 164a is controlled so that one of the two modulation bands is included in the frequency band or the frequency bands filtered by the Fabry Pérot cavity 120.
• Fig. 3 represents a system for controlling the temperature of the Fabry Pérot cavity according to the present invention.
• the recomposed signal Sl 13 is filtered by the Fabry Pérot cavity 120 described above.
• the resulting signal Sl 14 consists of a single frequency and contains on average less than one photon.
• the quantum detector 130 is preferably a cooled avalanche photodiode.
• the avalanche photodiode operates as an active trigger and / or a feedback trigger.
• the quantum detector alternatively comprises means for transposing the frequency of the resulting signal Sl 14 into a double frequency, so as to increase the performance of the quantum detector.
• the quantum detector 130 makes it possible to detect the passage of a photon.
• the quantum detector 130 emits an electric pulse which is shaped by an adaptation circuit 31 so as to be processed subsequently by conventional digital electronic components.
• the adapted signal S300 is transferred to a processing unit 30.
• the processing unit 30 is for example a microprocessor or a DSP, acronym for "Digital Signal Processing", or a computer.
• the processing unit 30 comprises a communication bus 301 to which a processor 300, a non-volatile memory 302, a random access memory 303, a filter interface 305 and a counter 307 are connected.
• the processing unit 30 further comprises a communication interface, not shown in FIG. 3, which allows the transfer of data allowing the control of the pulsation ⁇ 0 of the laser diode 120.
• the non-volatile memory 302 stores the frequency control program of the filter according to the present invention.
• the programs are transferred to the RAM 303 which then contains the executable code of the invention as well as the data necessary for the implementation of the invention.
• the pulses of the adapted signal S 300 are counted by the counter 307 for a predetermined time of the order of a few microseconds to a few seconds.
• the predetermined time is defined inter alia depending on the performance of the detector, the attenuation of the transmission channel.
• the processor 300 obtains the number of pulses counted by the counter 306. When the filter 120 is not tuned to the frequency ( ⁇ 0 + ⁇ ) / 2 ⁇ , the number of pulses counted decreases.
• the processor 300 determines, from a predetermined formula or a correspondence table stored in the non-volatile memory 302, the electrical signal to be delivered to the Peltier effect module 23 so as to modify the temperature of the optical fiber.
• the processor 300 determines, from a predetermined formula or a correspondence table stored in the non-volatile memory 302, data that is transmitted to the transmission device 160 so as to modify the pulsation ⁇ 0 of the laser diode 120 so that one of the two modulation bands is included in the frequency band or the frequency bands filtered by the Fabry Pérot cavity 120.
• the processor 300 transfers the determined electrical signal to the filter interface 305 which delivers the electrical signal corresponding to the Peltier effect module 23.
• the temperature modification makes it possible to displace the frequential characteristics of the Fabry Pérot cavity 120 and to correct the drifts. wavelength of the sinusoidal filter or oscillator 161 of the transmitter device 160.
• the processor 300 transfers the determined data to the transmission device 160 via the communication interface and the transmission medium 150 or the public channel 170.
• the filter interface 305 is able to receive the electrical signal delivered by the thermistor 25 to check whether the temperature of the optical fiber 21 is in accordance with the regulation temperature and to correct the variations in the wavelength or transmission frequency of the emission source 164.
• the processor 300 is able to transfer an electric signal to the Peltier effect module so as to bring the temperature of the optical fiber 21 to two different setpoint temperatures. These setpoint temperatures modify characteristics of the Fabry Perot cavity 120 to obtain an optical signal Sl 14 comprising one or other of the modulation sidebands. This allows you to choose the modulation sideband.
• the processor 300 is also able to process the pulses of the adapted signal S300 to use them for the negotiation of the encryption key and to transfer it to a decryption device and / or encryption or further processing.

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