IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 9, SEPTEMBER 2005
1995
Optical Chaos Masking of Video Signals
Valerio Annovazzi-Lodi, Senior Member, IEEE, Mauro Benedetti, Member, IEEE, Sabina Merlo, Senior Member, IEEE,
Michele Norgia, Member, IEEE, and Biagio Provinzano
Abstract—In this letter, we report on what we believe is the first
demonstration of a cryptographic technique, based on optical
chaos, applied to “real world” high-frequency signals. A standard
TV signal at 2.4 GHz has been transmitted through an optical
fiber link. The output from a chaotic laser, added to the signal
at the transmitter side, strongly reduces its signal-to-noise ratio,
and prevents an eavesdropper tapping the fiber from decoding the
message. At the receiver side, the signal is extracted from chaos
using a master–slave synchronization scheme. This requires a pair
of lasers with strictly matched parameters, which represent the
hardware cryptographic key of the method.
Index Terms—Chaos, communication systems, cryptography,
optical fibers.
I. INTRODUCTION
O
PTICAL chaotic cryptography is a technique for secure
transmission, implemented at the physical layer, which
has been studied for over a decade, though it has been experimentally demonstrated only recently. This technique [1]–[11]
makes use of a couple of lasers routed to chaos, usually by
back-reflection from a remote mirror.
A chaotic distributed feedback (DFB) laser generates a very
complex, apparently random, modulation waveform, which is
highly sensitive to starting conditions and to system parameters,
and can, however, be predicted by a deterministic model. In
the cryptographic schemes, two chaotic lasers are used: one
(called “master”) at the transmitter, to hide the signal to be sent
(the message) within the chaotic waveform; the other (called
“slave”), at the receiver, for message extraction. In most cases,
chaos is simply added to the message to strongly reduce its
signal-to-noise (S/N) ratio, thus implementing the so-called
“chaotic masking.” To this purpose, different solutions are possible [1], [3], [8]. If, as shown in Fig. 1, the message modulates
the pump current of a third laser, whose emission is combined
to that of the master, the scheme is called “additive chaos
masking” (ACM).
Extraction of the hidden message from chaos is based on synchronization of transmitter and receiver, i.e., on the generation
of the same chaotic waveform at both ends of the channel. Synchronization is obtained by injection of a part of the output of
Manuscript received April 22, 2005; revised May 24, 2005. This work was
supported by the European Union under Contract ST-2000 29683 (OCCULT
Project).
V. Annovazzi-Lodi, M. Benedetti, S. Merlo, and B. Provinzano are
with the Dipartimento di Elettronica, Universita’ di Pavia, Pavia I-27100,
Italy
(e-mail:
valerio.annovazzi@unipv.it;
mauro.benedetti@unipv.it;
sabina.merlo@unipv.it; biagio.provinzano@unipv.it).
M. Norgia is with with the Dipartimento di Elettronica, Universita’
di Pavia, Pavia I-27100, Italy, and also with the Dipartimento di Elettronica e Informazione, Politecnico di Milano, Milan I-20133, Italy (e-mail:
michele.norgia@unipv.it).
Digital Object Identifier 10.1109/LPT.2005.853267
Fig. 1. Setup for transmission with ACM cryptographic scheme.
the master laser into the slave laser, which, under suitable conditions, replicates the chaotic regime of the master but not the
message. Thus, message extraction can be simply performed by
making the difference between the signal coming from the transmitter (message chaos), and the chaotic waveform replicated
at the receiver. However (and this is the core of the method), it
is very difficult for an eavesdropper to obtain synchronization
even holding a system similar to that of the authorized listener.
In fact, effective synchronization relies on the two lasers being
closely matched, and, typically, the two devices must be selected
from the same wafer. The cryptographic key, in this case, consists of the parameters of the two matched lasers. It is worth
noting that the knowledge of the key, by itself, is not sufficient
to extract the message, because to do that one should also select
(or build) the laser. An important characteristic of this method
is its compatibility, in principle, with a standard network.
Though optical chaotic cryptography has been widely
studied theoretically [1], [2], [4], [5], and already demonstrated
experimentally, [1], [3], [6]–[9], as far as we know, no cases
of transmission of a real-world message have appeared in the
literature. In the following, we report on experiments of secure
transmission of a composite TV signal, generated by a standard surveillance camera, along a 1.2-km fiber span including
splitters, joints, couplers, and an optical amplifier.
II. EXPERIMENTAL SETUP
Experiments have been performed on the ACM setup of
Fig. 1. The master laser at the transmitter is driven to chaos
1041-1135/$20.00 © 2005 IEEE
1996
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 9, SEPTEMBER 2005
by back-reflection from the fiber tip positioned in front of
its launching lens [8], which defines an external cavity of
about 3 cm. This short-cavity scheme [5] is compact and
suitable to future integration, and offers a continuous and flat
chaotic spectrum, where the message can be easily hidden.
The characteristics of the chaotic regime of the laser depend
on working conditions, such as supply current, cavity length,
and back-injected power, which can be varied by changing
the alignment. The slave laser at the receiver is also routed
to chaos by building an external cavity identical to that of
the master. This is the so-called closed-loop (or symmetric)
configuration [2], which ensures a robust synchronization,
for efficient masking. The fiber path between transmitter and
receiver is of about 1.2 km and, besides splitters, couplers,
and joints, it includes also a semiconductor optical amplifier
to increase the maximum injection level from master into
slave. To prevent undesired back-reflection into the master,
photodiode PD1 has been slightly tilted, and the fiber facing
it has been angled cleaved at 10 ; this also prevents injection
from the message laser into the master. The optical isolator
in Fig. 1 ensures unidirectional injection. Polarizers in front
of the lasers select, for both reflection and injection, the same
polarization of the laser emission. Moreover, the polarizer on
the slave is used, together with the polarization controller, to
trim the injection level. The laser pair consists of standard DFB
telecommunication lasers, which have been selected between
first neighbors on the same wafer. Their difference of threshold
and of differential efficiency is less than 1%. Both external
cavities have been also trimmed in length with a resolution
of a fraction of wavelength. Master–slave synchronization is
obtained by adjusting the working point, the alignment, and the
temperature of both lasers, as well as the injection level. The
regimes of the two lasers can be compared from PD1, PD2 with
a fast oscilloscope or with a radio frequency (RF) spectrum
analyzer. Synchronization is better checked by observing the
spectrum of the difference between master and slave, obtained
by amplifying the outputs of PD2 and PD3 by one inverting
and one noninverting amplifier (Fig. 1), and then passively
summing their outputs. The difference should be minimized
for optimum synchronization. To compensate the differential
propagation delay of master and slave fields to PD3, PD2,
respectively (mainly due to the fiber pigtail from the slave to its
coupler), an RF delay line has been introduced.
III. EXPERIMENTAL RESULTS
In Fig. 2, time series of the chaotic waveforms of synchronized master and slave are shown. For both lasers, supply currents were 50% above threshold (master: 8 mA; slave: 8.5 mA).
Both back-injection and master–slave injection levels were of
the order of 1% of the laser output power ( 1 mW). The waveforms have been observed by a 4-GHz real-time oscilloscope,
reading the outputs of PD2, PD3; their correlation coefficient
is of about 0.8. In Fig. 3, the RF spectra of master and of the
master–slave difference are also reported, showing good chaos
cancellation over a large bandwidth. It is worth noting that the
chaotic spectrum in Fig. 3 is limited to 5 GHz because of photodiode and amplifier limitations; however, the actual chaos bandwidth, as measured by an optical spectrum analyzer, is of about
Fig. 2.
Time series of synchronized master and slave lasers.
Fig. 3. RF chaos spectra: master with hidden signal (upper trace) and
difference between synchronized master and slave (lower trace) with extracted
signal.
50 GHz. In Fig. 3, a 3-GHz sinusoidal signal applied to the
system input, and detected at the output, is also shown: it is
completely hidden in the master spectrum and, however, after
synchronization, it becomes visible in the difference signal at
the system output.
During alignment, all lasers were first tuned in temperature
at the same wavelength, within the resolution of our spectrum
analyzer (200 pm). Then, temperature was slightly changed for
optimum synchronization. Perturbation effects on the slave by
the message laser (injecting a power about five times lower than
the master) were also minimized by a small temperature change,
which resulted in a wavelength variation lower than the instrument resolution.
Later, preliminary transmission experiments have been performed with an amplitude-modulated carrier at 2.4 GHz applied
to the system input. The quality of the received signal has been
evaluated after synchronous detection and baseband filtering at
the receiver output node. The signal amplitude has been adjusted
as a compromise between efficient masking, low signal distortion, and good quality of the recovered message. For example,
in Fig. 4, the first diagram shows a message (a 1-kHz sinusoid)
as received at the system output without added chaos (master
and slave OFF). The second diagram is the encrypted message
ANNOVAZZI-LODI et al.: OPTICAL CHAOS MASKING OF VIDEO SIGNALS
1997
the picture hidden within chaos, and represents the message
as it would be recovered by an eavesdropper. Fig. 5(c) shows
the extracted message after synchronization. Again, the signal
level has been adjusted as a tradeoff between sufficient image
masking by chaos and acceptable image quality after chaos
cancellation. Fig. 5(b) was obtained by setting the AM sideband level at about 4 dB over chaos (a rapid deterioration of
the image quality was observed below 5 dB, probably due to
threshold in AM detection and to loss of TV synchronism).
– dB for the decoded
In these conditions, we get S/N
message [Fig. 5(c)].
We finally observe that the system performance is presently
limited by the chaos amplitude, which determines the allowable
RF signal amplitude. Increasing the chaos amplitude, by using
suitable lasers, would result in a larger S/N ratio of the decoded
signal.
Fig. 4. Transmission of a 1-kHz sinusoidal signal over a 2.4-GHz
carrier: (a) no encryption; (b) encrypted; (c) decrypted.
ACKNOWLEDGMENT
The authors would like to thank W. Hunziker and J. Shumaker
of Optospeed (CH) for supplying matched laser pairs, and Tektronix (Italy) for supplying the fast real-time oscilloscope.
REFERENCES
Fig. 5. TV frames of a still image transmitted with the setup of Fig. 1: (a) no
encryption; (b) encrypted; (c) decrypted.
(master ON, slave OFF); it has been obtained by setting the amplitude of the amplitude modulation (AM) sidebands (which carry
the information) at the same level as chaos. The third diagram
is the decrypted message (master and slave ON, and synchronized), as measured without changing either message or master
chaos amplitude. The obtained S/N ratio of the decoded signal
is of about 10 dB.
Finally, the cryptographic setup has been inserted in a transmission link connecting a standard surveillance TV camera
and its receiver. The camera output is a composite TV signal
at the carrier frequency of 2.4 GHz, and has been connected
to the cryptographic system input. The output of the system
was sent to the TV receiver to be displayed on a monitor. In
Fig. 5, three photographs of the monitor screen are shown,
taken while the camera was aimed at a still picture. Fig. 5(a) is
relative to transmission with no added chaos. Fig. 5(b) shows
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