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results up to 3 Gbit/s with a 7 В 10А9 bit error rate are reported. The potential for operating at several 10s Gbit/s is demonstrated. Introduction: Communicating ...
Electro-optical chaos for multi-10 Gbit=s optical transmissions N. Gastaud, S. Poinsot, L. Larger, J.-M. Merolla, M. Hanna, J.-P. Goedgebuer and F. Malassenet A delayed nonlinear electro-optic oscillator producing ultra-high bandwidth and high complexity chaos is used for chaos encoded optical communication. The principle of operation and experimental results up to 3 Gbit=s with a 7  109 bit error rate are reported. The potential for operating at several 10s Gbit=s is demonstrated.

Introduction: Communicating with the chaotic carrier has attracted much attention in the past ten years. It appeared as a complementary solution to data protection at the physical layer for high-speed telecommunication systems. Three main competing schemes are being explored for fast optical chaos generation: the all-optical external cavity feedback [1–3], optoelectronic feedback [4, 5], and electrooptic (EO) feedback [6]. Through substantial modifications to the last scheme, we report a record bit-rate encryption up to 3 Gbit=s with acceptable masking efficiency and excellent decoding quality. emitter

+ DC offset RF driver

amplified photodiode

G

Se(t )

RF power combiner variable fibre coupler

fibre delay line

Fr [y (t )] +

Vp-shifted DC offset

Gr receiver

dB

2 ¥ 2 fibre coupler

F [x (t )] CW laser diode

where the subscript r denotes receiver quantities. Under perfect emitter– receiver matching conditions, one can easily see that y ¼ x, and Fr[y(t  Tr)] ¼ F[x(t  T)], thus performing chaos replication. The replicated chaos is subtracted from the direct detection signal se(t), thus performing message decoding. For experimental convenience, the subtraction uses an electronic RF combiner (DC–26.5 GHz), together with a receiver Mach-Zehnder bias adjusted so that the actual interference function is out of phase with respect to the emitter one [Fr ¼  F].

+

integrated EO Mach-Zehnder

ð2Þ

message

a.m(t ) laser diode

Fr ½yðt  Tr Þ ¼ cos2 ½yðt  Tr Þ þ jr  with yðtÞ ¼ br ½hr  Se ðtÞ

amplitude, a.u.

message

efficiency of the message within the chaotic carrier. If the message is not sufficiently masked (a too large), message recovery is possible through a direct detection. For an authorised receiver, the aim is to cancel the chaotic noise through a knowledge of the chaos generation process. The receiver is hence composed of the same components (matched pairs are used), with operating conditions adjusted carefully according to those in the emitter. The receiver is organised in an openloop architecture, which consists of two branches (see Fig. 1): one for chaos replication [4, 8], and the other for a direct detection of se(t). Following the same principles as for the emitter, the equations at the receiver are:

Fig. 1 Experimental system setup

The parameter b ¼ pP0SG=2VpRF represents the overall feedback loop gain of the chaotic oscillator. The parameter a determines the masking

-10 -20 -30 -40 -50

time, 100 ps/div. a

1

2

3

4 GHz b

5

6

Fig. 2 PRBS test at 3 Gbit=s for a ¼ 1.4 (7  109 BER) a Eye diagrams (upper: original; middle: encoded; lower decoded) b Corresponding RF spectra (black: original; grey-upper: encoded; grey-lower: decoded)

10-2

direct detection (eavesdropper)

10-4

BER

Principle of operation: A functional block diagram of the system is shown in Fig. 1. The nonlinear component is a 10 Gbit=s lithium niobate integrated EO Mach-Zehnder interferometer, acting as a nonlinear intensity modulator. The primary optical source is a 5 mW DFB laser at 1550 nm, modulated electrically by a voltage V, providing a transfer function F(x) ¼ cos2(x þ F). The normalised variable x ¼ pV=2VpRF corresponds physically to the electro-optically induced optical path difference (OPD), where VpRF ¼ 4.5 V is the RF half-wave voltage; F ¼ pVb=2Vp is a static OPD phase shift controlled by a bias voltage Vb (Vp: DC half-wave voltage). The modulator output is fed into a fibre delay line of approximately 5 m, then into a 2  2 fibre coupler. One of the coupler outputs sends the signal into the transmission line, while the other output is fed back to drive the modulator, after an optical-to-electrical conversion with a sensitivity S ¼ 2 V=mW (BW 10 GHz), and also after an 18 dB amplification (G ¼ 8, BW 25 GHz); the peak detected optical power is approximately P0 ¼ 0.5 mW. The actual EO voltage swing ensures an intensity modulation in a highly nonlinear regime, i.e. MZ transmission curve with multiple extrema, thus allowing for a high complexity chaos and wideband nonlinear spectrum spreading of the optical chaotic carrier. Through the free input of the coupler, a fast direct-modulated laser diode seeds inside the EO oscillator an NRZ binary sequence m(t) ¼ 0 or 1, with an adjustable weight determined by the peak binary optical power aP0. The encoding process can be described as an Ikeda-type nonlinear delayed dynamics in chaotic regime [7], which oscillation is significantly perturbed by the seeded message. Assuming linear operation of the electronics, the chaos dynamics are governed by the filtering process of the electronic feedback: let h(t) be the impulse response of the electronic feedback circuit (input: intensity detected by the photodiode, output: OPD thus generated). Then the variable x(t) is expressed as the convolution product between h(t) and the intensity light beam consisting of the chaos superimposed to the message, i.e. the optical signal transmitted to the receiver se(t):   xðtÞ ¼ b hðyÞ  ½F½xðy  T Þ þ amðyÞ ðtÞ ¼ b h  se ðtÞ ð1Þ

-10 -20 -30 -40 -50

10-6 10-8 10-10

chaos decoder (authorised receiver)

10-12 1

2 a

3

Fig. 3 3 Gbit=s experimental BER against masking coefficient a Chaos replication: —— authorised receiver; —— direct detection, eavesdropper

Results: Robust temporal replication has been observed at 5 GHz (oscilloscope analogue bandwidth), with a chaos-to-synchronisation error ratio of 18 dB. The message is obtained from an external laser direct modulation using an NRZ pseudorandom bit sequence (PRBS) of 27  1 bits up to 3 Gbit=s (maximum bit rate with the available equipment); the latter is used as the direct modulation signal for the message laser. Typical eye diagrams are shown in Fig. 2 with their corresponding spectra, for a masking efficiency a ¼ 1.4. This situation allows for a 7  109 BER. A message direct detection limit was estimated experimentally with the transmitted signal in terms of a-value. Below a ¼ 1.7 (see Fig. 3), no BER could be measured (>102), thus defining a security threshold. As depicted in Fig. 3, a

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better masking efficiency (lower a) can be adjusted, but resulting in a BER degradation. Finally, testing bit rate up to more than 10 Gbit=s should be easily feasible using a faster signal analyser. The masking capability of the chaotic optical carrier (MZ output) was indeed observed to cover a flat RF spectrum over more than 20 GHz (which is shown flat over 6 GHz in Fig. 2b, upper). This is explained by the strong nonlinear operation of the MZ function F[x], thus providing a spectrum spreading of the input EO voltage into the RF spectrum of the output optical intensity. Conclusions: We have described an experimental chaos encoding communication system based on nonlinear EO dynamics and involving standard 1550 nm telecom components. The effective chaotic masking bandwidth extends over more than 20 GHz, and secure communication has been tested up to 3 Gbit=s with a 7  109 BER. Faster commercially available components should allow encoding and decoding with the same setup at several 10s Gbit=s. Work is in progress to further increase the quality of the synchronisation, thus improving both the transmission security and the decoding quality. Dispersion effects on long transmission distances are also being explored.

N. Gastaud, S. Poinsot, L. Larger, J.-M. Merolla, M. Hanna, J.-P. Goedgebuer and F. Malassenet (GTL-CNRS Telecom, UMR FEMTO-ST 6174, Georgia Tech Lorraine, 2-3 rue Marconi, 57070 Metz, France) E-mail: [email protected] References 1

2 3 4 5 6

Acknowledgments: This work is supported by the European Community (OCCULT project, IST-2000-29683), and by the French Ministry of Research (ACI-SI program). # IEE 2004 Electronics Letters online no: 20045072 doi: 10.1049/el:20045072

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ELECTRONICS LETTERS 8th July 2004 Vol. 40 No. 14