ABSTRACT SECURE OPTICAL LAYER COMMUNICATIONS SYSTEM INTRODUCTION

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1 SECURE OTICAL COMMUNICATIONS UTILIZING SK MODULATION, OLARIZATION MULTILEXING AND COHERENT DIGITAL HOMODYNE DETECTION WITH WAVELENGTH AND OLARIZATION AGILITY Aviv Saamon, Guy Levy-Yurista, Michae Tseytin, a S. Cho, and Isaac Shpantzer, CeLight Inc., 100 Tech Road, Suite 300, Siver Spring, MD 0904 evy.yurista@ceight.com ABSTRACT Data Encryption and User Authentication are the major communications security (ComSec) approaches used in today s communications networs. Whie these methods provide an efficient means to secure the sensitive information, they do not prevent a hostie opponent from jamming the signa or tapping the in. We present here an anaysis of a coherent optica communications system providing secure communications in free-space or over fiber. After reviewing the basic principes of Coherent Optica Communications we present a system using poarization/frequency agiity to secure the data in at the optica ayer. The ey buiding bocs wi be presented, incuding a QSK transmitter, and an optica hybrid combined with a coherent digita receiver. A description of CeLight s LiNbO 3 integrated impementation of the transmitter and the receiver wi be detaied. Next, a theoretica anaysis demonstrating the capabiities of such a secure communications system to send data over fiber and in free space eiminating the need for optica fitering and ambda-based de-mutipexing. Finay, we present an experiment demonstrating the abiity of the proposed frequency hopping soution to avoid an eavesdropper from intercepting the data channe. INTRODUCTION Techniques used today to secure communications networs consist of Data Encryption and User Authentication. Data encryption maes the deciphering of information that has been accessed without authorization more difficut, whie user authentication procedures are used to prevent unauthorized users from gaining access to the data in the first pace. Whie these methods provide ways to secure sensitive information, they do not prevent physicay tapping into the in and coecting the transmitted data. Commerciay avaiabe off the shef equipment provides such optica in tapping capabiities, enabing the interception and coection of data. Increasing computation power and efficient agorithms may enabe the eventua deciphering of this maiciousy coected data. We present here a method and system based on CeLight s VectorWAVE patform to provide a more fundamenta eve of security to the communications in at the optica ayer, whether free-space or fiber, maing the maicious coection of data consideraby harder. The proposed method incudes using phase shift eying (SK) moduation and coherent digita homodyne detection. This moduation and detection scheme provides inherent frequency seectivity and enabes the incorporation of waveength agiity into the communications in without reiance on narrowband tunabe optica fiters. In addition, the preservation of the signa phase information in the receiver enabes the impementation of a digita poarization diversity receiver without reiance on optica components, thus maing poarization mutipexing and poarization agiity impementabe. As a resut, a potentia eavesdropper wi have to use more compex receivers, covering a much wider bandwidth of optica spectrum and deveop a nove cass of devices accounting for the rapid signa poarization changes of the mutipexed signas. SECURE OTICAL LAYER COMMUNICATIONS SYSTEM Figure 1 shows a singe transmitter, one of many to be used in such a secure optica Communications system. A ow inewidth tunabe aser is optionay intensity moduated using a Mach-Zehnder Interferometer (MZI) and shaped into a train of RZ (return to zero) puses. The carrier is spit into two paths; each intended to carry data on a different poarization state (H and V). The data is encoded in the phase of each of the optica carrier poarization component using a phase moduator or a quadrature moduator, e.g. CeLight s LiNbO 3 quadrature moduator shown in Figure. The two signas are then added on two orthogona poarization states using a poarization beam combiner (BC) and transmitted. A 1 of 9

2 poarization scramber may be used to change the poarization state of the signa. Tuanbe Laser use Shaping MZI Spitter H V H Data SK Moduator SK Moduator V Data BC oarization Scramber Figure 1. SK dua-poarization transmitter schematic. A ow inewidth tunabe aser is shaped into a puse train using a MZI, then spit into two paths each moduated with an independent data stream using a SK moduator, and combined again on orthogona poarization using a poarization beam combiner (BC). A poarization scramber is used to rapidy change the poarization of the transmitted signa enhancing its immunity to tapping. Optica in I Data Q Data hase contro to in Optica out Figure. CeLight s LiNbO 3 QSK moduator schematic. The carrier signa is divided into two arms, carrying the inphase (I) and quadrature-phase (Q) parts of the moduation. A MZI on each arm moduates the carrier s itude according to the data, and a 90 phase shift is induced between the two arms using a phase moduator (M). Finay, the two arms are combined using a couper. In the receiver, schematicay shown in Figure 3, the signa is spit into two (arbitrary) orthogona poarizations (H and V ) and each is mixed with a oca osciator in an optica 90 hybrid. The hybrid, shown in Figure 4, taes the signa S and the oca osciator L and produces four outputs: (i) S+L, (ii) S-L, (iii) S+jL and (iv) S-jL. Each of the optica output pairs (i), (ii) and (iii), (iv) is coected by a pair of baanced diodes whose photocurrents are subtracted, as shown in Figure 5, to produce currents proportiona to S+L - S-L 4 Re{SL * } and S+jL - SjL 4 Im{SL * }, together constructing the compex vaue SL *. Foowing this (inear) transformation the signas are eectricay fitered, sed and then processed. The oca osciator (aser) L, which is aso tunabe, is supposed to be phase-oced to the incoming signa S. The actua impementation of the phase-oc oop (LL) is done in the digita part of the receiver, eiminating the need for a compex optica LL. The optica hybrid and baanced diodes act as a mixer, down-converting the signa from the optica band to the eectrica baseband. It is important to note that since the transformation is inear, a of the fitering (for noise reduction and separation of carriers) can be performed in the eectrica domain, with no reiance on narrow optica fiters. For the same reason the signa received by the DS on arbitrary poarizations [S H S V ] T can be rotated (by mutipying it with the appropriate recovery matrix, M) to recover the origina poarization state of the signa M [S H S V ] T [S H S V ] T, and the data on each poarization can then be demoduated. BS H' V' 90 Optica Hybrid DC 90 Optica Hybrid Baanced Receiver H' I Baanced Receiver H' Q Tuanbe Laser Baanced Receiver V' I Baanced Receiver V' Q DS H, V Data out Figure 3. SK dua-poarization digita homodyne receiver schematic. The incoming signa is arbitrariy spit into two orthogona poarizations using a poarization beam spitter (BS), each poarization is input into a 90 optica hybrid, which mixes the signa S with a oca osciator L, shared by the two hybrids using a directiona couper (DC). Each hybrid output pair {(S+L), (S-L)} and {(S+jL), (S-jL)} is input into a baanced receiver that yieds a signa proportiona to Re{SL*} and Im{SL*} respectivey. The digitized ses are processed to recover the origina poarization state of the signa to compensate for the phase and frequency offsets of the asers, and to demoduate the data. The system presented above consists of many such transmitters and receivers sharing the same bandwidth, a changing waveengths and poarization states synchronousy or asynchronousy (where the number of waveength sots is much bigger than the number of carriers). Notice, that whie it can be reativey easy to foow a carrier when there are no other carriers present, the presence of a significant number of carriers changing waveengths synchronousy or asynchronousy maes the tas harder. of 9

3 S L 90 Hybrid M M (a) DC DC S+L S-L S+jL S-jL transmitter (serving as the signa carrier) and receiver (serving as the oca osciator). Some additiona time (in the order of microseconds) wi be needed to reoc the receiver onto the incoming data stream. A major advantage of using such coherent detection scheme is that there is no need for narrow tunabe optica components, whose tuning and setting times are on the order of severa miiseconds, because the oca osciator transfers the optica signa ineary to the eectrica domain. OLARIZATION AGILITY (b) Figure optica hybrid. The hybrid taes two input signas S and L, and mixes them with four reative phase reations, adjusted by the phase moduators (Ms): (i) S+L, (ii) S-L, (iii) S+jL, and (iv) S-jL. DC Directiona Couper. (a) a schematic representation (b) CeLight s actua device. S+L S-L Baanced Diodes Re{SL*} TIA Fiter ADC Figure 5. A baanced receiver schematic. A pair of baanced diodes detects the two ight signas; their output is subtracted and ified by a trans-impedance ifier (TIA), fitered and sed in an anaog to digita converter (ADC). ADVANTAGES OF THE SECURE OTICAL COMMUNICATIONS SYSTEM WAVELENGTH AGILITY The main characteristic enabing the secure optica system discussed above is its waveength agiity. Each carrier can change its waveength by tuning the asers in the Another major advantage of the secure optica system discussed above is that no optica components are required in order to recover the poarization state of the signa in the receiver. The inear transformation of the signa from the optica to the eectrica domain and its digitization enabes the performance of digita poarization compensation, thus avoiding the probematic use of sow, non-endess optica poarization compensators, and the incurring osses [ 1]. Furthermore, an enhancement in the security eve can be achieved by appying rapid changes to the poarization state of the signa using a scramber in the transmitter, foowed by the appropriate counter adjustment of the digita poarization compensation matrix in the receiver. Moreover, aowing the transmission of two different data streams on the same carrier frequency, using the two orthogona poarization states, consideraby increases the security eve of the transmitted data, whie doubing the channe capacity and spectra efficiency. ERFORMANCE VS. CHANNEL NOISE The secure optica system discussed above utiizes coherent phase shift eying (SK) demoduation, in particuar the preferred moduation scheme wi be QSK (quadrature SK), where each symbo is encoded with two () bits per poarization, as shown schematicay in Figure 6, [ ]. It shoud be noted that CeLight s quadrature moduator 3], designed for QSK moduation, is abe to perform any moduation scheme (msk, QAM, etc.) given the appropriate driving eectronics. Figure 7 shows cacuated BER vs. SNR per bit curves for QSK/BSK, DQSK, DBSK [ 3] and intensity moduation direct detection (IMDD) [ 5]. The noise is considered to be additive white gaussian noise (AWGN) and is added before the receiver (optica hybrid). It is cear from these resuts that using QSK moduation, the system can toerate more than twice the noise compared to using IMDD. Note that the QSK moduation scheme has doube the spectra efficiency of BSK. 3 of 9

4 V I V Q (a) (b) Figure 6. oarization mutipexed QSK signa consteation. The signa is divided into two orthogona poarization states (a). On each poarization two bits are encoded, one on the in-phase (I) and one on the quadrature-phase (Q) of the signa (b). H I H Q cosey, reying on eectrica fiters to perform the separation. Accordingy, a spectra efficiency of 1.6 bits/sec/hz (40Gbps over 5GHz channe spacing) can be readiy demonstrated with these methods and devices. ERFORMANCE VS. CHANNEL DISTORTION Since the signa is down-converted ineary to the eectrica baseband and digitized, the entire arsena of digita signa processing methods conventionay used in RF systems becomes avaiabe for impementation in the system. articuary, impementation of channe equaization and compensation for inear channe distortions (e.g. chromatic dispersion, MD, some atmospheric effects etc.) is attractive. ANALYSIS OF QSK TRANSMISSION WITHOUT OTICAL FILTERS Assume that the incoming signa S is composed of N carriers, each with power s. The th carrier s anguar frequency is ω, and its phase data is ϕ. An AWGN noise n is added to the signa. s j( ω t ) S e +ϕ + n (1) The oca aser osciator whose power is is tuned to receive the th carrier L jωt e () The current in the I channe after the TIA without fitering is: ( 1+ ε) S L ξ( 1 ε) + nshot n therma i S + L ξ + (3) Figure 7. Cacuated BER vs. SNR per bit for IMDD, DQSK, DBSK and QSK/BSK moduation schemes. SECTRAL EFFICIENCY Using QSK and poarization mutipexing enabes transmitting four (4) bits of information per symbo (puse). Accordingy, for the same signa bandwidth the secure optica communications system can deiver four times the information compared to an OOK or BSK signa transmitted on a singe poarization. In addition, since no optica fiters are used to separate carriers in the receiver, we can pac the channes more Here ξ is the average responsivity of the two baanced diodes, ε is the common mode rejection ratio (CMRR), n shot is the tota shot noise (induced by the two baanced diodes) and n therma is the therma noise induced by the TIA. After ow pass fitering we have i 4ξ ( cosϕ + R{ n }) s + ξε Ns n' + n' shot therma s + R { n } + n + (4) 4 of 9

5 With n being the noise fitered with the receiver oca aser osciator tuned to ω, and n, n shot and n therma are the fitered n, n shot and n therma. number of carriers in the optica fiter passband for a BER of x10-3 (9.dB SNR) and 10-9 (15.6dB SNR). Equation (4) consists of the foowing terms: the desired signa 4ξ cosϕ, the -n beating noise { } s 4ξ R n, the s-s beating interference that is (amost) constant with itude ξε(n s + ), the s-n beating noise 4 ξε s R{ n }, the n-n interference ξε n, the shot noise, and the therma noise. Assuming that (NSR s-n, NSR n-n )<<NSR -n the tota noise wi be minima when N s ½, where is the ima power aowed on each diode. The above inequaities are true if ε ε BWo <<1, and << 1, where 8N BWe BW e and BW o are the receiver eectrica and optica bandwidths, respectivey. Aso, NSR s-s may be quite big (around 1), and must be owered through common DC remova techniques. (a) LINK MEDIUM ANALYSIS FIBER In optica communications fiber ins the ifier noise comes mainy from the in in-ine ifiers. The eve of the incoming signa can be controed by a preifier, with a noise contribution that is assumed to be negigibe compared to the noise aready coming from the in. The tota noise to signa ratio NSR, negecting NSR s-n and NSR n-n and removing NSR s-s, is NSR NSR N + NSR shot therma n q SDth ( ξ + ) BWe + NSR n ( ξ ) + NSR Where SD th is the spectra density of the therma noise and q the eectron charge. The SNR penaty (in db) is SNR NSR ( ) penaty 10og n10 ( ξ ) NSR NSRshot NSRtherma q SDth ( + ) BWe SNR N ξ Figure 8 (a) shows the therma and shot SNR as a function of the number of carriers in the optica fiter passband with ξ0.75a/w, BW e 10GHz, SD th ( ) A /Hz, and 3dBm. Figure 8 (b) shows the SNR penaty vs. the (5) (6) (b) Figure 8. (a) Combined Shot and Therma noise SNR vs. no of carriers in optica fiter; (b) Combined Shot and Therma noise SNR penaty vs. no of carriers in optica fiter at a BER of x10-3 (9.dB SNR) and 10-9 (15.6dB SNR). The resuts show that even when 160 channes (!) are present in the optica fiter passband, the penaty in the receiver wi be sma. Accordingy, the secure optica system discussed above can indeed enabe the direct eectronic fitering of a heaviy popuated C-band with hundreds of channes hopping simutaneousy, deivering a capacity of 6.4 Tb/s. FREE-SACE With in the power at the input of the receiver ing (preifier), we have s L N, and (for the I channe), 5 of 9

6 NSR n hνnf hνnf ( N ) BWe (7) in L where NF is the preifier noise figure, L is the oss from the receiver input to the diode and hν the carrier photon energy. Ignoring the s-s, s-n and n-n noises we get NSR NSR + NSR + NSR shot therma n q SDth hν NF hν [ N( ) ] NF ξ + L + BWe ( ξ) in (8) Figure 9 (a) shows the sensitivity as a function of the number of carriers at a BER of x10-3 with the parameters used previousy before. Figure 9 (b) shows the receiver L preifier gain G for the same conditions. N in The resuts show that the sensitivity is hardy affected, even when 160 channes (!) are present in the optica fiter passband. CELIGHT TRANSEC DEMONSTRATION Soving for in in With hν NF BW SNR N e q SDth hν NF ( 1+ N ( ξ + L ) BWe SNR) ( ξ) ( ξ ) ( ) q SDth hνnf + L BWe SNR << 1 ξ. (a) (9) Foowing the above discussion we have performed a test to demonstrate the TranSec capabiities of the CeLight soution and the provision to integrate the ComSec and TranSec features into a tota data security pacage. Specificay the purpose of this experiment was to demonstrate CeLight s waveength hopping feature that prevents an eavesdropping receiver, empoying either sefhomodyne or homodyne detection from decoding the transmitted data. The test setup is depicted in Figure 10. The Tx transmitter consists of a tunabe aser source (TLS), a return-to-zero (RZ) puse moduator and a data moduator that produce a binary phase-shift-eyed (BSK) optica signa at 1.5 Gb/s. The waveength of the TLS is periodicay hopped to designated waveengths based on input from the hopping controer. Note that the Tx ComSec unit was not avaiabe for this experiment. A commercia pattern generator was used to create an unframed pseudo-random bit sequence (RBS) data to simuate the Tx ComSec unit s functions. The word ength of the RBS pattern used was Two continuous-wave (CW) asers (λ 1 and λ 3 ) were combined with the TLS to simuate DWDM channes in optica networs. A three asers were moduated with the same RBS data. The TLS waveength was tuned to nm whie λ 1 and λ 3 are tuned to ±5 GHz from nm. The BSK signa was transmitted through approximatey 1 m of singe-mode optica fiber. The transmitted BSK signa and a oca aser osciator (LO) are directed to the CeLight s coherent Rx. (b) Figure 9. (a) Sensitivity vs. number of carriers in the optica fiter passband at a BER of x10-3 ; (b) Receiver preifier gain vs. number of carriers in the optica fiter passband. The LO is derived from the TLS to ensure phase and frequency ocing. The BSK signa and the LO counterpropagated through the same 1-m of fiber using optica circuators to maintain phase coherency and reduce phase drifts. The transmitted BSK signas and LO were ified and directed to the CeLight s Lithium Niobate (LN) 90 o optica hybrid. oarization states of the input signa and LO were adjusted to aign to the axis of the hybrid using manua poarization controers. Two of the outputs of the hybrid were connected to a baanced detector. The baanced detector eectrica output was 6 of 9

7 connected to either a sing oscioscope or an error detector for eye pattern dispay or BER measurement. In order to obtain imum eye opening a phase contro oop was constructed to stabiize and maintain the proper optica phase shift between the BSK signa and the LO. The output of the hybrid was tapped and detected to produce a feedbac signa proportiona to the opening of the eye pattern. The feedbac signa was processed by a destop computer, which produces phase contro signas connected to the phase shifter and the optica hybrid as shown in Figure 10. Ony two phase contro signas were used. Due to the imited output votage range of the phase contro signa, reset of the oop occurred from periods of 30 seconds to 10 minutes depending on factors such as the contro oop parameters, phase and poarization drifts as a resut of environmenta perturbations. Note that a coc recovery at the receiver was not empoyed in the test. The coc output from the puse pattern generator was connected to the error detector for synchronization. The sef-homodyne and homodyne eavesdropper setups are shown in Figure 11. The sef-homodyne eavesdropper used an asymmetric Mach-Zehnder (AMZ) interferometer to demoduate the BSK signa. The AMZ interferometer was an optica deay-and-add device with a differentia deay corresponds to one symbo period (80 ps). The homodyne eavesdropper combined a LO with the tapped BSK signa using a fiber 3-dB couper. The LO was a CW aser at nm. The outputs for the two eavesdroppers are dispayed on a sing oscioscope. Figure 1 shows a typica eye diagram of the 1.5 Gb/s RZ-BSK at the CeLight s Rx baanced detector output for one transmitted channe (λ 1 and λ 3 turned off). The measured BER for this case is better than Note that a momentariy cosing of the eye occurs during waveength hopping of the BSK signa. This is due to the setting time of the TLS and the phase contro oop. The waveength hopping of the BSK signa was monitored with an optica spectrum anayzer (OSA) and a waveength meter. Figure 13 and Figure 14 show the oscioscope dispay of the sef-homodyne eavesdropper output for one and three transmitted BSK signas, respectivey. No stabe eye pattern can be obtained at the eavesdropper s Rx as the waveength of the BSK signa hops. This is the case if the eavesdropper fais to eep trac of the AMZ s differentia phase shift (0 or π for BSK) with the signa waveength. For the muti-channe case (Figure 15), the eavesdropper wi need a tunabe optica fiter to reject other channes and the optica fiter must eep trac of the hopping signa waveength in order to recover the transmitted data. Figure 15 shows the oscioscope dispay of the homodyne eavesdropper output when the waveength of one BSK signa ( nm) is far from that of the eavesdropper LO ( nm). No eye pattern can be observed for this case. Figure 16 shows the oscioscope dispay of the homodyne eavesdropper output when the waveength of one BSK signa is approximatey equa to that of the eavesdropper LO. An eye pattern can be seen, however, the eye is cosed since phase-oced oop was not impemented. To sing oscioscope or error detector To phase contro oop To waveength meter and OSA CW aser, λ 1 CW aser, λ 3 Computer: waveength hopping TLS hase contro CeLight's ithium niobate 6-port 90 o optica hybrid 1.5 GHz 1.5 Gb/s Coc Data RZ mod BSK mod hase contro C 3 LO EDFA 1-m fiber Signa C EDFA hase 1 shifter 1 hase shifter hase contro Figure 10. Setup with sef-homodyne or homodyne eavesdroppers. 3 To Eavesdropper (sef-homodyne or homodyne) Eavesdropper setup: Sef-homodyne detection Signa Sing oscioscope Eavesdropper setup: Homodyne detection Signa LO CW Laser nm AMZ interferometer 3-dB couper Sing oscioscope Figure 11. Eavesdroppers setup. Top: Sef-homodyne detection with asymmetric Mach-Zehnder interferometer. Bottom: Homodyne detection with fixed waveength CW oca aser. 7 of 9

8 Figure 1. Homodyne detection of 1.5 Gb/s RZ-BSK signa. One channe transmitted. Figure 14. Eavesdropping with sef-homodyne detection. Three channes transmitted. No stabe eye pattern. Figure 13. Eavesdropping with sef-homodyne detection. One channe transmitted. No stabe eye pattern. Figure 15. Eavesdropping with homodyne detection. Loca aser waveength: nm. Signa aser waveength: nm. No eye pattern. 8 of 9

9 consideraby enhance communications security at the physica ayer. Further investigation and deveopment is sti required in order to deiver a fu soution that utiizes this technoogy for various defense and commercia appications. REFERENCES 1. M. Tseytin et. a., Digita, Endess oarization Contro for oarization Mutipexed Fiber-Optic Communications, OFC Shpantzer et a., Coherent Optica Fiber Communication Architecture, Modeing and Optimization, SCEE 00. Figure 16. Eavesdropping with homodyne detection. Loca aser waveength approximatey the same as the signa waveength. SUMMARY In summary, we have presented and anayzed an optica secure communications system in free-space or over fiber. We presented the system architecture, based on CeLight s VectorWAVE patform using waveength/poarization agiity to secure the optica ayer. The ey buiding bocs were presented, incuding a QSK transmitter, an optica hybrid and a digita homodyne receiver. 3. Arady Kapan, Kobi Achiam, Art Greenbatt, Geof Harston and a Shing Cho, "LiNbO3 Integrated Optica QSK Moduator and Coherent Receiver", Conference roceedings, ECIO '03, rague, Czech Repubic, pp. 79-8, J. G. roais, Digita Communications, 4 th edition, McGraw-Hi. 5.. A. Humbet, M. Azizogu, On the Bit Error Rate of Lightwave Systems with Optica Ampifiers, JLT, Vo. 9, No. 11, Nov We have reviewed the basic principes of Coherent Optica Communications and compared various demoduation methods to concude that the system presented above is poised to dispay superior performance. Next, we presented a theoretica anaysis showing that either in fiber ins or in free space ones the secure optica system discussed above eiminates the need for any optica fitering. Thus, aowing a considerabe cost reduction as we as extreme rapid hopping. Finay, we performed an experiment demonstrating the abiity of the proposed frequency hopping soution to avoid an eavesdropper from intercepting the data channe. The eavesdropper in this experiment was simuated for using either a sef-homodyne or a homodyne receiver. In both cases it was demonstrated that it coudn t decode the data transmitted over the frequency hopping secure channe. The above discussion and demonstration present an indication to the potentia of utiizing such a soution to 9 of 9