Patent Publication Number: US-11641241-B2

Title: Optical communication system using mode-locked frequency comb and all-optical phase encoding for spectral and temporal encrypted and stealthy transmission, and for optical processing-gain applications

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Stage application of PCT/IL2020/050788, filed Jul. 14, 2020, which claims the benefit of U.S. Provisional Application No. 62/873,912, filed Jul. 14, 2019, both of which are incorporated by reference in their entireties herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of optical data encryption. More particularly, the invention relates to an optical communication system using mode-locked frequency comb and all-optical phase encoding for spectral and temporal encrypted and stealthy transmission, and for optical processing-gain applications. 
     BACKGROUND OF THE INVENTION 
     As the demand for bandwidth is scaling up to unprecedented levels, information confidentiality, integrity and availability are becoming increasingly important, particularly in sensitive applications such as financial transactions, military, medical records, and private information sharing. The optical infrastructure poses a major data breach, as it is exposed to various attacks such as fiber tapping, false data injection and jamming. To cope with these considerable security threats, common encryption protocols are implemented in all seven layers of the Open Systems Interconnection (OSI) model. Such protocols rely on prime factorization and high computational complexity algorithms. However, in the era of supercomputing and quantum computing, any digital encryption scheme can be decrypted in theory, as the valuable raw data can be recorded and processed offline by supercomputers. Moreover, in upper layers standard encryption techniques, metadata remains unencrypted, as well as the existence of the transaction itself, and might be used by an adversary for eliciting sensitive information on the users by data mining techniques. 
     Security schemes by all-optical means, also known as optical layer security or photonic layer security (PLS), have been proposed, in order to overcome this problem [5]. Such a solotion augments the security performances of the entire communication network, as it offers many unique advantages over digital security protocols. In particular, it enables real-time, low-latency, low-power operation, which cannot be provided by conventional security algorithms at current line rates, which exceed 1.3 Tbps per a single wavelength [6]. Additionally, side-channel attacks can be prevented since all-optical processing does not generate electromagnetic radio frequency (RF) signatures. Several approaches for optical layer security have been widely investigated, including chaos-based communications [7], quantum communication [8], all-optical logic [9, 10], and spread spectrum techniques [11-15]. 
     In chaos-based communications, the user message is transmitted using a chaotic signal carrier, which is very sensitive to initial hardware conditions, and is generated by a laser diode that contains optoelectronic feedback or optical feedback. At the receiver, the information is retrieved upon synchronization with the transmitter [7]. However, chaotic optical transceivers require complex laser structures and are extremely unstable. Furthermore, the throughput is greatly restricted by the intrinsic relaxation oscillation frequency of chaotic signal. 
     All-optical processing techniques such as optical XOR gates have been also suggested in [9, 10], utilizing nonlinearity phenomenons such as Four-Wave Mixing (FWM) in saturated Semiconductor Optical Amplifier (SOA). Though it complies with conventional implementations and WDM networks, in the sense of security performance, all-optical logic essentially does not provide substantial added value over digital Phusical layer (PHY) encryption. 
     For that reason, chaos-based communications and all-optical logic were not widely adopted in deployed fiber-optics networks. On the other hand, quantum key distribution (QKD) [16] is considered unconditionally secured (i.e. completely resilient against cryptanalytic attacks). However, its performances are still incomparable with the other encryption methods, as bitrate-distance (B·L) product is limited only to several Mbits/second×km [8]. Consequently, it is mainly considered for key exchange protocol, rather than for encrypting the data in a full line-rate. 
     An additional existing approach is optical code-division multiple access (OCDMA) [17]. It is a Spread Spectrum (SS) technique proposed for OCDMA passive optical networks (OCDMA-PONs) [18] application. In OCDMA, different users whose signals may be overlapped both in time and frequency share a common fiber and laser source. Multiple asynchronous access is achieved by minimal multi-user interference (MUI) of the optical codes (OCs). The broadband spectrum in OCDMA is provided by Optical Frequency Combs (OFC) which are in general considered highly attractive for next generation optical communications [19, 20]. First, they can serve as multi-carrier sources in WDM systems for both the signal&#39;s carrier and the local oscillator (LO), replacing tens of tunable continuous-wave lasers by a single source. Secondly, the OFC enables to significantly simplify the coherent receiver by realizing a joint digital signal processing for phase recovery. 
     In turn, OCDMA has been suggested for encryption purposes as well [11-15], since eliminating the MUI provides a certain level of authentication. The generation of Optical Code (OC) is provided by Spectral Phase Encoding (SPE) or by a multiplication with direct sequences (DS) in the time domain. In SPE, the OC can be inscribed by either a FIBER BRAGG GRATING (FBG) or obtained by a dynamic Spatial Light Modulator (SLM) [21]. However, the FBG is passive and cannot fit to encryption needs, where dynamic key changing is required. Therefore, for encryption application, a dynamic, short settling time optical engine should be provided such as SLM based-devices or demutiplexer and an array of phase shifters. 
     Traditionally, OCDMA has been classified into incoherent OCDMA and coherent OCDMA (Co-OCDMA), where in coherent OCDMA, coherent detection is obtained by mixing with a pulsed Local Oscillator (LO) [22, 23]. It was proven that intensity modulated (incoherent) OCDMA signals are significantly vulnerable to attacks [11, 24] using an energy detector with bandwidth limited to the bit rate, which is unable to resolve the fine structure of the noise-like waveforms, but integrates the energy in a bit period. On the other hand, a Co-OCDMA offers many benefits of the complex modulation scheme such as improved sensitivity, polarization diversity and more. Nevertheless, given the advance in coherent detectors and digital signal processing, Co-OCDMA may become vulnerable to bandwidth limited detection, same as incoherent OCDMA. 
     The gap between the data security needs and the available encryption techniques requires a new encryption method that is immune to traffic analysis, provides a stealthy transmission and enables scalable higher key change rates and low cost implementation. 
     It is therefore an object of the present invention to provide an optical communication system using mode-locked frequency comb and all-optical phase encoding for spectral and temporal encrypted and stealthy transmission, and for optical processing-gain applications. 
     It is another object of the present invention to provide an encryption method that is immune to traffic analysis, provides a stealthy transmission and enables scalable higher key change rates and low cost implementation. 
     Other objects and advantages of the invention will become apparent as the description proceeds. 
     SUMMARY OF THE INVENTION 
     A method for transmitting data carrying optical information over an optical channel, comprising the steps of:
         a) Providing an optical transmitter consisting of:
           a.1) a light source being a Mode-Locked Optical Frequency Comb (MLFC) for generating a frequency comb of multiple carriers, each of which being modulated by a baseband signal;   a.2) an optical modulator for modulating each and all of the multiple carriers in a modulation bandwidth extending up to the modes&#39; frequency spacing between the multiple carriers;   a.3) performing all-optical encoding of the modulated carriers by manipulating the optical amplitude and/or phase and/or polarization of all optically modulated carriers; and   
           b) transmitting, by the optical transmitter, the encoded modulated carriers to an optical receiver, over an optical channel.       

     Masking encoded modulated carriers before transmission may be performed by:
         a) attenuating the power of encoded modulated carriers; and   b) optionally, adding noise to the attenuated encoded modulated carriers, in a level being optionally above the power.       

     All-optical decoding of the modulated carriers may be performed by de-manipulating the optical phase and/or the amplitude and/or phase of all optically modulated carriers. 
     Processing-gain may be obtained by performing optical coherent addition of all the optical modes of the frequency comb of multiple carriers using “multi-homodyne” detection. 
     Multi-homodyne detection may be performed by:
         a) combining the received signal with a mode-locked optical frequency comb which operates as the optical multi-frequency local oscillator;   b) applying opto-electronic coherent detection of the combined optical signal;   c) performing baseband filtering of the “multi-homodyne” mixed signal between the identical frequencies of the two frequency combs at the receiver and the transmitter, by passing the photo-detected signal through a receiver of bandwidth narrower than the frequency spacing between the modes;   d) filtering out all the detected cross-term mixing products between the two frequency combs.       

     Multi-direct-detection may be performed by:
         a) applying self-homodyne opto-electronic direct-detection of the optical signal;   b) Performing baseband filtering of the “multi-direct-detection” signal;   c) passing the photo-detected signal through a receiver of bandwidth narrower than the frequency spacing between the modes, thereby filtering out all the cross-term mixing between the multi-frequencies of the comb.       

     In one aspect, multiple spectral replicas of the signal are coherently added to the baseband, to reinforce the signal-to-noise ratio. 
     The opto-electronic receiver may have a baseband bandwidth, rather than a full optical bandwidth. 
     A spectral may be applied phase mask being an encryption key, for obtaining secure transmission. 
     Processing-gain may be used for optoelectronic amplification at the receiver, for limited link budget applications and/or limited output power transmission. 
     The optical channel may a wired or wireless channel, selected from the group of: 
     glass; 
     plastic fiber; 
     Free-Space Optics (FSO); 
     random media. 
     The modulation is complex or intensity modulation. 
     The optical bandwidth of the all-optical encoder may cover the applicable spectrum of the MLFC, and can induce a temporal phase retardation to spread each symbol over multiple symbol&#39;s unit intervals (Uls). 
     The system may operating at high optical-noise conditions at very low, possibly negative, Optical-Signal-To-Noise-Ratio (OSNR), followed by performing high optical processing-gain at the receiver side. 
     The detected cross-terms may contain energy at frequencies larger than the baseband filter. 
     An optical system for transmitting data carrying optical information over an optical channel, comprising:
         a) an optical transmitter, that comprises:
           a.1) a light source being a Mode-Locked Optical Frequency Comb (MLFC) for generating a frequency comb of multiple carriers, each of which being modulated by a baseband signal;   a.2) an optical modulator for modulating each and all of the multiple carriers in a modulation bandwidth extending up to the modes&#39; frequency spacing between the multiple carriers;   a.3) an optical encoder for all-optical encoding of the modulated carriers by manipulating the optical amplitude and/or phase of all optically modulated carriers,
 
wherein the transmitter is adapted to transmit, by the optical transmitter, the encoded modulated carriers to an optical receiver, over an optical channel; and
   
           b) an optical receiver comprising means for performing coherent addition by:
           b.1) combining the received signal with a mode-locked optical frequency comb which operates as the optical multi-frequency local oscillator;   b.2) applying opto-electronic coherent detection of the combined optical signal;   b.3) performing baseband filtering of the “multi-homodyne” mixed signal between the identical frequencies of the two frequency combs at the receiver and the transmitter, by passing the photo-detected signal through a receiver of bandwidth narrower than the frequency spacing between the modes;   
           b.4) filtering out all the detected cross-term mixing products between the two frequency combs.       

     An optical system for transmitting data carrying optical information over an optical channel, comprising:
         a) an optical transmitter, that comprises:
           a.1) a light source being a Mode-Locked Optical Frequency Comb (MLFC) for generating a frequency comb of multiple carriers, each of which being modulated by a baseband signal;   a.2) an optical modulator for modulating each and all of the multiple carriers in a modulation bandwidth extending up to the modes&#39; frequency spacing between the multiple carriers;   a.3) an optical encoder for all-optical encoding of the modulated carriers by manipulating the optical amplitude and/or phase of all optically modulated carriers,
 
wherein the transmitter is adapted to transmit, by the optical transmitter, the encoded modulated carriers to an optical receiver, over the optical channel; and
   
           b) an optical receiver comprising a photodiode for performing incoherent detection by:
           b.1) detecting the signal&#39;s intensity by applying self-homodyne opto-electronic direct-detection of the modulated optical signal;   b.2) filtering the baseband of the “multi-direct-detection” signal;   b.3) filtering out all the cross-term mixing products between the multi-frequencies of the comb by passing the photo-detected signal through the optical receiver, the optical receiver having a bandwidth being narrower than the frequency spacing between modes.   
               

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other characteristics and advantages of the invention will be better understood through the following illustrative and non-limitative detailed description of preferred embodiments thereof, with reference to the appended drawings, wherein: 
         FIG.  1    illustrates the system functional block diagram with complex modulation and homodyne detection; and 
         FIG.  2    illustrates the system functional block diagram with intensity modulation and direct detection. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention proposes an optical communication system using Mode-Locked Frequency Comb (MLFC-which is a laser source that before modulation, generates multiple optical carriers) and all-optical phase encoding for spectral and temporal encrypted and stealthy transmission, and for optical processing-gain applications. 
     The Mode-Locked Frequency Comb (MLFC) generates equally spaced optical carriers based on the same source, such that all the carriers have the same optical phase. This way, the symbol (the electrical signal with a particular spectrum) to be transmitted by modulating a carrier signal is replicated n times, such that its bandwidth becomes n times wider. 
     The electrical signal then modulates an al the multiple optical carriers generated by the MLFC, such that its baseband PSD is replicated in the frequency domain. Practically, the modulating signal is multiplied by all carriers (which is a sum of multiple sine waves). The total bandwidth of the replicated signal is broadened to the optical bandwidth of the frequency comb, which is much wider than the capabilities of the electronics. 
     After modulation, a spectral phase encoder is used to manipulate the phase of each replica by a unique pattern, in order to apply a phase mask, which is used for encryption. The spectral phase encoding can be done for example, by using a Spatial Light Modulator (SLM). An encoding key is used for phase encoding of all modulated carriers, such that the information carried by each carrier is masked by shifting the phase of each (modulated) carrier by a different unique phase, which may have a random value, for example. 
     At this stage, the signal is spread in the time domain in a form that corresponds to the unique pattern encoded by the phase mask. The combination of frequency comb and phase encoding, generated an encrypted optical signal. 
     At the next step, all the modulated the optical information is covered below a predetermined noise level. Practically, in the presence of noise, the signal cannot be detected without applying an inverse phase mask (phase decoding). This can be done by substantially attenuating the encrypted optical signal power (e.g., by 30 dB) and then, optionally adding additive noise to the attenuated encrypted optical signal, such that its magnitude will be optionally below the noise level, so as to obtain stealthy transmission. It should be noted that masking may be obtained only by attenuating the encrypted optical signal. Additive noise may be obtained, for example, by adding the output of an optical amplifier to the optical channel (the optical amplifier does not amplify the signal—it is used only as a white noise source, for masking the modulated signal). Therefore, the transmitted signal comprises the optical information masked by noise. 
     An eavesdropper who wishes to decode the transmitted data will not be able to do it, since he will have to record the masked optical information, for further analysis. However, as soon as the masked optical information is recorded (by electrical means), the optical phase encoding pattern ceases, and there will be no way to recover it. This way, the optical information masked by noise becomes immune to eavesdropping. 
     Only the authorized receiver can retrieve the transmitted data, since it uses all-optical means. 
     Decoding by the receiver can be implemented by two detection schemes: homodyne detection or direct detection. For secure applications, the authorized user has to decode the phase mask by optical means. Wrong phase masks leads to a permanent destruction of the signal due to incoherent addition process. In addition to encryption application, the same optical system (without the phase encoding), can be used to generate a broadband signal that can be coherently combined into narrow signal with larger SNR. The advantage of such transmission scheme can be either stealthiness, or for limited link budget applications and/or limited output power transmission cases, for example, to avoid regulatory constrains (where the ratio between power and wavelength is much lower). 
     Two possible implementations of the proposed technique are depicted in  FIGS.  1  and  2   . 
       FIG.  1    illustrates the system functional block diagram of a coherent transmission system with complex modulation and “Multi-Homodyne” detection, using Mode Locked Frequency Comb (MLFC) as both the laser transmitter and as the Local Oscillator (LO). At the transmitter (Tx) side, the MLFC feeds a multicarrier comb signal to a Dual Parallel Mach-Zehnder Modulator (DP-MZM) and which received complex modulating signal (using In-Phase and Quadrature channels) from the transmitter&#39;s DSP (Tx DSP). The modulated signal is then fed into a Polarization Controller (PC) to adjust the polarization of the modulated optical signal (to align the signal and the LO before they are mixed together at a coherent receiver) and then into a Spatial Light Modulator (SLM), which performs phase encoding. Then the optical modulated optical signal is input to a Variable Optical Attenuator (VOA), which attenuates it to a level that can be masked by noise, followed by an Erbium-Doped Fiber Amplifier (EDFA) that adjusts the OSNR levels before transmission, such that the encoded information will be covered by noise and then the encoded information is transmitted to the receiver (Rx) over an optical channel, which may be a wired channel such as glass or plastic fiber, as well as free-space optics (FSO) communication system or a random media. 
     It should be noted that following the MZM, an all-optical phase and/or amplitude processor is used for phase and/or amplitude encoding. The optical bandwidth of the encoder covers the applicable spectrum of the MLFC, and can induce a temporal phase retardation to spread each symbol over multiple symbol&#39;s Unit Intervals (Uls). In secure applications, the optical processor applies a spectral phase mask which in turn stands for the encryption key. In both coherent and direct detection schemes, a controlled noise mechanism is used to hide the signal below the noise level. Whereas in marginal link budget applications, the noise is introduced by the channel itself or by a pre/post-amplification devices. 
     Multi-Homodyne Detection 
     At the receiver side Rx, another SLM is used for decoding the phase masking using a decoding key which is the inverse of the encoding key used for phase encoding of all modulated carriers, such that the information carried by each carrier is no longer masked and the phase/amplitude encoding is removed. In “Multi-homodyne” coherent-detection, multiple spectral replicas of the signal are coherently added to the baseband, to reinforce the signal-to-noise ratio. Detection is implemented by mixing the incoming signal with a MLFC that functions as a LO. The LO is temporally adjusted by an Optical Delay Line (ODL) in order to synchronize its ultra-short pulses with the signal&#39;s modulated ultra-short pulses. 
     Subsequently, at the output of an Integrated Coherent Receiver (ICR), a Low-Pass electrical Filter (LPS) filters the baseband signal to be further sampled by an Analog-To-Digital Converter (ADC) and processed by a Digital Signal Processing (Rx-DSP) unit. 
     In “Multi-Homodyne” coherent detection, optical processing-gain is obtained by combining the received signal with a mode-locked optical frequency comb which operates as the optical “multi-local oscillator”, and applying opto-electronic coherent detection of the combined optical signal. Baseband filtering of the “multi-homodyne” mixed signal between the identical frequencies of the two frequency combs by is done by passing the photo-detected signal through a receiver of bandwidth that is narrower than the frequency spacing between the modes, and filtering out all the detected cross-term mixing between the two frequency combs (since the detected cross-terms contain energy at frequencies larger than the baseband filter). 
       FIG.  2    illustrates the system functional block diagram of an incoherent transmission system with intensity modulation and direct detection. 
     “Multi-Direct Detection” 
     In the case of “Multi-direct-detection”, a photodiode is used to detect the signal&#39;s intensity (rather than using the ICR in the system of  FIG.  1   ). Detection is done by applying self-homodyne opto-electronic direct-detection of the optical signal. The baseband of the “multi-direct-detection” signal is filtered and the photo-detected signal is passes through a receiver with a bandwidth that is narrower than the frequency spacing between the modes. This way, all the cross-term mixing products between the multi-frequencies of the comb (which contain energy at frequencies larger than the baseband filter) are filtering out. 
     In both systems, the detected electrical Signal-To-Noise (SNR) is considerably improved compared to the line optical-signal-to-noise ratio (OSNR). The ratio between the OSNR and the electrical SNR is referred to as “processing-gain”. The processing-gain is directly related to the number of applicable spectral modes of the MLFC. Also, in both systems, a MLFC is used as a multi-carrier source, modulated by the desired data. The data can be a complex symbols stream in the coherent case, or an intensity modulated data in the direct detection case. Modulation rate can be tuned up to the MLFC&#39;s repetition rate. The system supports high order constellations as well as dual-polarization modulation. 
     Experimental results are shown in “40 Gbps stealthy and encrypted optical coherent transmission using gain switched frequency comb and multi-homodyne coherent detection”, Wohlgemuth et. al., 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement. 
     The above examples and description have of course been provided only for the purpose of illustration, and are not intended to limit the invention in any way. As will be appreciated by the skilled person, the invention can be carried out in a great variety of ways, employing more than one technique from those described above, all without exceeding the scope of the invention. 
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