Patent Description:
Free-space optical communications use light propagating in free space (i.e. air, outer space, vacuum, or similar medium) to wirelessly transmit data for telecommunications or computer networking. Free-space optical communication technologies are used where the physical connections are impractical due for example to high costs.

Free-space optical links can be implemented using mid-infrared laser light. In particular, the mid-infrared domain is of prime importance for free-space communications due to the high transparency of the atmosphere. The mid-infrared domain is also more adequate than near-infrared wavelengths because detrimental factors, such as divergence and scintillation, are reduced at higher wavelengths. With the recent accelerated advances in mid-infrared semiconductor lasers such as quantum cascade lasers (QCLs) and the progress in Mercury-Cadmium-Telluride (MCT) photo-diodes, low sized, low weight, and energy efficient optical systems operating at room temperature in the mid-infrared have been developed. Exemplary free-space optical links have been described for example in:.

Recent experiments have been conducted in relation with data transmissions in the mid-infrared domain. Communications up to <NUM> Mbits/s were demonstrated with interband cascade lasers as disclosed in "<NPL>". Transmissions at several Gbits/s were achieved with frequency down and up-conversion between <NUM> and <NUM> as disclosed in "<NPL>".

Transmissions at similar high speeds using quantum cascade lasers have been reported at room and cryogenic temperatures with a free-space television link application as disclosed respectively in "<NPL>" and "<NPL>".

In terms of secure communications, quantum cryptography systems with quantum key distribution (QKD) have been a candidate of choice with free-space sources emitting in the visible spectrum, as disclosed for example in "<NPL>". However, quantum cryptography is not versatile to every kind of secure free-space communication applications such as mid-infrared domain applications and faces several implementation challenges comprising data transmission rate, transmission distances, and the implementation costs. There is accordingly a need for secured free-space optical communication systems in the mid-infrared domain.

In order to address these and other problems, there is provided a free-space optical communication system comprising a transmitter and a receiver, the transmitter being configured to transmit an encrypted message to the receiver at the mid-infrared domain. The transmitter comprises a master mid-infrared optical source configured to generate a mid-infrared signal and a chaos generator configured to generate a chaotic signal by applying external optical feedback to the master mid-infrared optical source. The transmitter is configured to determine an encrypted message from an original message by applying a message encryption technique to the original message and to send the encrypted message to the receiver through an optical isolator. The receiver comprises a slave mid-infrared optical source similar to the master mid-infrared optical source. The slave mid-infrared optical source is configured to recover the chaotic signal from the encrypted message by applying chaos synchronization. The receiver further comprises a first detector configured to detect the encrypted message, a second detector configured to detect the chaotic signal, and a message recovery unit configured to recover the original message from the encrypted message detected by the first detector and the chaotic signal detected by the second detector, wherein said external optical feedback consists in re-injecting a part of the emitted light by the master mid-infrared optical source back into the master mid-infrared optical source and wherein the chaos generator comprises a feedback reflector and a mid-infrared polarizer, said external optical feedback being obtained using one or multiple round-trips between the feedback reflector and the emitting facet of the master mid-infrared optical source, said mid-infrared polarizer being adapted to tune the amount of the external optical feedback, the angle of the mid-infrared polarizer defining the feedback strength.

According to some embodiments, the chaos generator may further comprise an injector laser configured to perform optical injection on the mid-infrared signal.

According to some embodiments, external optical feedback may use phase-conjugate feedback.

According to some embodiments, external optical feedback may use rotated polarization feedback.

According to some embodiments, the message encryption technique may be chosen in a group comprising chaos masking, chaos modulation, and chaos shift keying.

According to some embodiments in which the message encryption technique is chaos masking, the transmitter may be configured to determine the encrypted message by adding the original message and the chaotic signal, the message recovery unit being configured to recover the original message by subtracting the chaotic signal detected by the second detector from the encrypted message detected by the first detector.

According to some embodiments, the master mid-infrared optical source and the slave mid-infrared optical source may be mid-infrared semiconductor lasers chosen in a group comprising mid-infrared Quantum Cascade Lasers, Interband Cascade Lasers.

According to some embodiments, the master mid-infrared optical source may be selected depending on the application of the free-space optical communication system and/or on the data rate of the original message.

According to some embodiments, the free-space optical communication system may further comprise a mid-infrared telescope.

There is also provided a method for free-space optical communication between a transmitter and a receiver, an encrypted message being sent from the transmitter to the receiver at the mid-infrared domain, the method comprising the steps consisting in:.

wherein said external optical feedback consists in re-injecting a part of the emitted light by the master mid-infrared optical source back into the master mid-infrared optical source and wherein the method comprises using a feedback reflector and a mid-infrared polarizer, the method comprises obtaining said external optical feedback using one or multiple round-trips between the feedback reflector and the emitting facet of the master mid-infrared optical source, and adapting said mid-infrared polarizer to tune the amount of the external optical feedback, the angle of the mid-infrared polarizer defining the feedback strength.

Advantageously, the embodiments of the invention provide secured communications at mid-infrared domain using free-space optical chaos synchronization and communications.

Advantageously, the embodiments of the invention enable combining the high degree of protection offered by chaos with the possibility of transmission into the atmosphere, including under degraded conditions, offered by mid-infrared optics.

Advantageously, Quantum Cascade Lasers and Interband Cascade Lasers under external optical feedback provide sustained and complex chaos at mid-infrared wavelengths.

Advantageously, combining optical injection and external optical feedback techniques enable increasing the bandwidth of chaos of mid-infrared semiconductor lasers.

Further advantages of the present invention will become clear to the skilled person upon examination of the drawings and the detailed description.

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate various embodiments of the invention and, together with the general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the embodiments of the invention.

Embodiments of the invention provide secured free-space communication system and method enabling secured free-space communications with optical transmission in the mid-infrared domain using mid-infrared optical sources. Secured free-space communications according to the embodiments of the invention rely on the use of chaotic optical communications that provide a high degree of protection.

The embodiments of the invention may be implemented in any free-space optical system operating in the mid-infrared domain and employed for mid-infrared data transmission in a diverse range of communication applications comprising, without limitations:.

Referring to <FIG>, there is illustrated a free-space optical communication system <NUM> according to the embodiments of the invention, in which a transmitter <NUM> is configured to transmit mid-infrared data to a receiver <NUM>. The communication between the transmitter <NUM> and the receiver <NUM> is performed at mid-infrared wavelengths varying from <NUM>, above the visible spectrum, to <NUM> which corresponds to the limit with the terahertz regime.

According to some embodiments, the transmitter <NUM> and the receiver <NUM> may be implemented in devices or systems or objects configured to operate in the mid-infrared domain such as, without limitation, ground stations, vehicles, aircrafts, space-crafts, and computer peripherals. The devices implementing the transmitter <NUM> and the receiver <NUM> may be fixed or mobile (e.g. aircrafts or vehicles).

Secured free-space communications according to the embodiments of the invention rely on chaos synchronization and communications. Accordingly, messages generated at the transmitter <NUM> are hidden within chaotic signals generated at the transmitter <NUM> so that the messages do not disturb the larger chaotic fluctuations and remain well concealed from an eavesdropper. Chaos synchronization and communication according to the embodiments of the invention is based on two mid-infrared optical sources: a master mid-infrared optical source <NUM> implemented at the transmitter <NUM> and a slave mid-infrared optical source <NUM> implemented at the receiver <NUM>. The master mid-infrared optical source <NUM> is configured to generate a mid-infrared signal. The slave mid-infrared optical source <NUM> is similar to the master mid-infrared optical source <NUM> and differs from the master mid-infrared optical source <NUM> in that the slave mid-infrared optical source <NUM> may or may not be chaotic. The messages to be transmitted by the transmitter <NUM> are thus encoded within a chaotic carrier (also referred to as a 'chaotic signal') denoted by c(t). The chaotic signal may be then injected into the slave mid-infrared optical source <NUM> that operates under similar conditions as the master mid-infrared optical source <NUM>. The slave mid-infrared optical source <NUM> synchronizes to the chaos of the master mid-infrared optical source <NUM>. Then the original message may be recovered from the input and the output of the receiver <NUM>.

According to some embodiments, the master mid-infrared optical source <NUM> may be modulated using a near-infrared laser (not illustrated in <FIG>).

More specifically, with reference to <FIG>, the transmitter <NUM> may comprise a message generator <NUM> configured to generate a message and a message modulator <NUM> configured to determine a modulated message by applying a modulation scheme to the message generated by the message generator. The transmitter <NUM> comprises a current source <NUM> that may be configured to determine an original message denoted m(t) from the modulated message. The modulation scheme may be chosen among a group comprising On-Off-Keying (OOF) modulation schemes and Pulse Amplitude Modulation (PAM) schemes.

According to some embodiments, the current source <NUM> may be a low-noise source delivering a continuous bias that may be modulated with an external signal (not illustrated in <FIG>) from a waveform generator. The low-noise source may further comprise filters (e.g. low pass filter).

According to some embodiments, the modulated message may be introduced through a bias-tee (not illustrated in <FIG>) in parallel to the current source bias <NUM>.

According to the embodiments of the invention, the master mid-infrared optical source <NUM> may be chaotic by applying external optical feedback. Accordingly, the transmitter <NUM> may further comprise a chaos generator <NUM> configured to generate a chaotic signal c(t) by applying external optical feedback to the master mid-infrared optical source.

The transmitter <NUM> may be then configured to determine an encrypted message denoted e(t) from the original message m(t) by applying a message encryption technique to the original message m(t).

The transmitter <NUM> may be then configured to send the encrypted message e(t) to the receiver <NUM> through an optical isolator <NUM> so that back-reflections are avoided. The transmitter <NUM> may comprise a beam splitter <NUM> used to split the laser beams in the directions of the chaos generator <NUM> and the optical isolator <NUM>. The beam splitter <NUM> may be a non-polarizing beam splitter.

According to some embodiments, the transmitter <NUM> may further comprise a lens (not illustrated in <FIG>) in front of the master mid-infrared optical source <NUM>. In such embodiments, the beam splitter <NUM> may be configured to split the focused laser beams into the directions of the chaos generator <NUM> and the optical isolator <NUM>.

The receiver <NUM> may be configured to receive the encrypted message e(t). More specifically, the receiver <NUM> comprises a slave mi-infrared optical source <NUM> similar to the master mi-infrared optical source <NUM>, the slave mid-infrared optical source <NUM> being configured to recover the chaotic signal c(t) from the received encrypted message e(t) by applying chaos synchronization. Chaos synchronization occurs when the output of the master mid-infrared optical source is uni-directionally injected into the slave mid-infrared optical source <NUM>. The slave mid-infrared optical source <NUM> synchronizes only with the chaotic fluctuations. The synchronization describes how the receiver <NUM> is capable of following the dynamical properties of the transmitter <NUM>. The receiver <NUM> may further comprise a first detector <NUM> configured to detect the encrypted message e(t) and a second detector <NUM> configured to detect the chaotic signal c(t). The first detector <NUM> and/or the second detector <NUM> may be a mid-infrared detector (e.g. a Mercury-Cadmium-Telluride (MCT) detector or a Quantum Well Infrared Photodetector (QWIP)). The receiver <NUM> may further comprise a message recovery unit <NUM> configured to recover the original message m(t) from the encrypted message e(t) detected by the first detector <NUM> and the chaotic signal c(t) detected by the second detector <NUM>.

According to some embodiments, the chaos generator <NUM> may comprise a feedback reflector and a mid-infrared polarizer (not illustrated in <FIG>), external optical feedback being obtained using one or multiple round-trips between the feedback reflector and the emitting facet of the master mid-infrared optical source <NUM>. The feedback reflector and the emitting facet of the master mid-infrared optical source <NUM> define the external cavity length L. External optical feedback consists in re-injecting a part of the emitted light by the master mid-infrared optical source <NUM> back into the master mid-infrared optical source <NUM>. The mid-infrared polarizer is a device adapted to tune the amount of optical feedback, the angle of the mid-infrared polarizer defines the feedback strength. According to some embodiments, optical feedback may be combined with optical injection to improve non-linear dynamics of the master mid-infrared optical source <NUM> and as a consequence improve the bandwidth of the chaos determined from the master mid-infrared optical source <NUM> and the speed of secure transmissions. In such embodiments, the chaos generator <NUM> may further comprise an injector laser (not illustrated in <FIG>) configured to perform optical injection on the master mid-infrared optical source <NUM>.

According to some embodiments, external optical feedback may use phase-conjugate feedback techniques.

According to other embodiments, external optical feedback may use rotated polarization feedback techniques.

Phase-conjugate feedback techniques and rotated polarization feedback techniques enable modifying the feedback light when it travels in the external cavity.

In embodiments in which the message encryption technique is chaos masking, the transmitter <NUM> may be configured to determine the encrypted message e(t) = m(t) + c(t) by adding the original message m(t) and the chaotic signal c(t), the message recovery unit <NUM> being configured to recover the original message m(t) by subtracting the chaotic signal c(t) detected by the second detector <NUM> from the encrypted message e(t) = m(t) + c(t) detected by the first detector <NUM>.

According to some embodiments, the master mid-infrared optical source <NUM> and the slave mid-infrared optical source <NUM> may be mid-infrared semiconductor lasers chosen in a group comprising mid-infrared Quantum Cascade Lasers, Interband Cascade Lasers.

According to some embodiments, the master mid-infrared optical source <NUM> may be selected depending on the application of the free-space optical communication system <NUM> and/or on the data rate of the original message m(t).

According to some embodiments, the free-space optical communication system <NUM> may further comprise a mid-infrared telescope (not illustrated in <FIG>) used to propagate the laser beams from the master mid-infrared optical source <NUM> over long distances.

According to some embodiments, the free-space optical communication system <NUM> may further comprise forward error correction encoders/decoders and/or further components such as future mid-infrared optical fibers (e. g chalcogenide optical fibers), optical amplifiers (e.g. mid-infrared SOA), and filters (e.g. low-pass filters).

With reference to <FIG>, the embodiments of the invention provide a method for free-space optical communication between a transmitter and a receiver, an encrypted message being sent from the transmitter to the receiver at the mid-infrared domain, encrypted transmission being achieved using chaos synchronization and communications.

At step <NUM>, an original message m(t) may be generated. The original message may be previously determined from a message that is modulated by applying a modulation scheme and processed by a current source. The modulation scheme may be chosen among a group comprising On-Off-Keying (OOF) modulation schemes and Pulse Amplitude Modulation (PAM) schemes.

According to some embodiments, the modulated message may be processed by a bias-tee in parallel to the current source.

At step <NUM>, a mid-infrared signal may be generated at the transmitter using a master mid-infrared optical source. According to some embodiments, the master mid-infrared optical source may be modulated using a near-infrared laser.

At step <NUM>, a chaotic signal c(t) may be determined at the transmitter by applying external optical feedback on the master mid-infrared optical source.

According to some embodiments, external optical feedback may be performed using one or multiple round-trips between a feedback reflector and the emitting facet of the master mid-infrared optical source, external optical feedback being achieved using the feedback reflector and a mid-infrared polarizer adapted to adjust the strength of the feedback light. According to some embodiments, optical feedback may be combined with optical injection performed using an injector laser.

At step <NUM>, an encrypted message e(t) may be determined at the transmitter from the original message m(t) by applying a message encryption technique to the original message.

In embodiments in which the message encryption technique is chaos masking, the encrypted message e(t) = m(t) + c(t) may be determined at step <NUM> by adding the original message m(t) and the chaotic signal c(t).

At step <NUM>, the encrypted message e(t) may be sent to the receiver through an optical isolator.

At step <NUM>, to the chaotic signal may be recovered at a slave mid-infrared optical source comprised in the receiver by applying chaos synchronization, the slave mid-infrared optical source being similar to the master mid-infrared optical source comprised in the transmitter.

At step <NUM>, the encrypted message may be detected by a first detector comprised in the receiver.

At step <NUM>, the chaotic signal c(t) may be detected at source second detector comprised in the receiver.

At step <NUM>, the original message m(t) may be recovered at the receiver from the encrypted message e(t) detected by the first detector and the chaotic signal c(t) detected by the second detector.

In embodiments using chaos masking, the original message m(t) may be recovered at step <NUM> by subtracting the chaotic signal c(t) detected by the second detector from the encrypted message e(t) = m(t) + c(t) detected by the first detector.

According to some embodiments, the master mid-infrared optical source and the slave mid-infrared optical source may be mid-infrared semiconductor lasers chosen in a group comprising mid-infrared Quantum Cascade Lasers, interband cascade lasers.

According to some embodiments, the master mid-infrared optical source may be selected depending on the application of the free-space optical communication system and/or on the data rate of the original message m(t).

According to some embodiments, the laser beams sent by the transmitter may propagate through a mid-infrared telescope.

While the invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the Applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art.

Further, in certain alternative embodiments, the functions, acts, and/or operations specified in the flow charts, sequence diagrams, and/or block diagrams may be re-ordered, processed serially, and/or processed concurrently consistent with the appended claims.

Claim 1:
A free-space optical communication system (<NUM>) comprising a transmitter (<NUM>) and a receiver (<NUM>), the transmitter (<NUM>) being configured to transmit an encrypted message to the receiver (<NUM>) at the mid-infrared domain, wherein:
the transmitter (<NUM>) comprises a master mid-infrared optical source (<NUM>) configured to generate a mid-infrared signal and a chaos generator (<NUM>) configured to generate a chaotic signal by applying external optical feedback to said master mid-infrared optical source (<NUM>), the transmitter (<NUM>) being configured to determine said encrypted message from an original message, by applying a message encryption technique to said original message using said chaotic signal, and to send said encrypted message to said receiver (<NUM>) through an optical isolator (<NUM>), the receiver (<NUM>) comprising a slave mid-infrared optical source (<NUM>), said slave mid-infrared optical source (<NUM>) being configured to recover the chaotic signal from the encrypted message by applying chaos synchronization, the receiver (<NUM>) further comprising a first detector (<NUM>) configured to detect the encrypted message, a second detector (<NUM>) configured to detect the chaotic signal, and a message recovery unit (<NUM>) configured to recover the original message from the encrypted message detected by said first detector (<NUM>) and the chaotic signal detected by said second detector (<NUM>),
wherein said external optical feedback consists in re-injecting a part of the emitted light by the master mid-infrared optical source (<NUM>) back into the master mid-infrared optical source (<NUM>) and wherein the chaos generator (<NUM>) comprises a feedback reflector and a mid-infrared polarizer, said external optical feedback being obtained using one or multiple round-trips between the feedback reflector and an emitting facet of the master mid-infrared optical source (<NUM>), said mid-infrared polarizer being adapted to tune the amount of the external optical feedback, the angle of the mid-infrared polarizer defining the feedback strength.