Optics and Photonics Journal, 2013, 3, 183-186
doi:10.4236/opj.2013.32B044 Published Online June 2013 (http://www.scirp.org/journal/opj)
Experimental Investigation of Improving the Performance
of the Chaotic Optical Communication with
Chaos-Masking through Wavelength Mismatch*
Wei Chang, Xiaolei Chen, Qingchun Zhao, Hongxi Yin, Nan Zhao
Lab of Optical Communications and Photonic Technology, School of Information and Communication Engineering,
Dalian University of Technology, Dalian, China
Received 2013
ABSTRACT
In this paper, the wavelength mismatch between the message and the chaotic carrier in the chaotic optical communica-
tion with c hao s-ma ski ng ha s b een e xpe ri mental ly i nve stiga te d. T he result s sho w t hat the d ec ryptio n p erfo rma nce o f t he
receiver can be improved when the message wavelength is greater than that of the chaotic carrier. When the wavelength
offset i s set to 0.12 nm, high-speed secure optical communication with a message of 2.5 Gbits/s is achieved.
Keywords: Chaos Masking; Chaotic Synchronization; Chaotic Communication; Wavelength Mismatch
1. Introduction
As a hardware-based encryption technique at the physical
layer, the chaotic optical communication has attracted
extensive interests from the community of secure and
optical communications during the past decade [1,2]. At
the receiver, the message can be filtered out by the re-
ceiver laser, and the chaotic carrier that synchronizes
with the carrier generated by the transmitter can be ob-
tained. Then, the message can be decrypted by subtract-
ing the output of the receiver laser from the encrypted
signal [3 ,4] . With the in-depth exploring the secure chao-
tic optical communication, the issues for secure chaotic
optical communication mainly focus on bandwidth en-
hancement [5], generation of novel chaotic carriers [6],
chaotic photonic integrated circuits (PICs) [7], and bidi-
rectional communications [8]. It is important to solve
these issues for which can improve the performance of
the system and promote the practical applications. How-
ever, the encryption schemes as well as the relationship
between the message and the chaotic carrier is the most
important factor, which constraints the performance of
the secure chaotic optical communication system direct-
ly.
At present, three encryption schemes have been pro-
posed for the secure chaotic optical communications,
name ly, chaos modulation (CM), chaos masking (CMS),
and chaos shift-keying (CSK) [9]. For simple physical
processes and convenient achievement, CMS is the most
common encryption scheme. Argyris et al. experimen-
tally investigated the performance of the three encryption
schemes with a message of 1.5 Gbits/s [10]. However, it
is still a problem being worthy further investigating how
to improve the performance of different encryption
schemes and to promote the data-rate of message.
In this p aper , the misma tch o f the wavel engt h between
the message and the chaotic carrier in the secure chaos-
masking optical communication has been experimentally
investigated. The relationship between the mismatch of
the wavelength and the performance of the system has
been quantitatively analyzed. When the wavelength off-
set is set to 0.12 nm, high-speed secure communication
with a message of 2.5 Gbits/s has been achieved experi-
mentally.
2. Experimental Setup
Figure 1 shows the experimental setup of the secure
optical communication system with chaos-masking. The
chaotic carrier is generated by optical feedback. The cir-
culator, optical coupler (OC), and the variable optical
attenuator (VOA) constitute the fiber-loop. The chaotic
carrier is then split into two paths by a 50:50 OC after
amplified by an Erbium-doped fiber amplifier (EDFA).
The upper path is used for encrypting the secure message
by CMS, and the lower path is transmitted as a secure
*This work is supported in part by the National Natural Science Foun-
dation of China (NSFC) under Grants 61071123, 61172059, and
6120122 4, the Funds for Ph. D. Studen t Academic New Investi gator of
Ministry of Education of China, and the Natural Science Foundation of
Changzhou City under CJ20 120015.

W. CHANG ET AL.
184
key. At the receiver, the two optical signals are detected
by two photodetectors (Discovery Semiconductor, DSC-
R401HG-39). Then, the message can be decrypted by
subtracting the secure key from the encrypted message.
The radio-frequency filter here is used to filter the extra
high-frequency noise. The digital storage oscilloscope
(Agilent DSO90404A) at the receiver-end is used for
real-time record of the synchronization between the two
chaotic signals and the decrypted message.
3. Chaotic Synchronization
The waveforms of the two chaotic signals without mes-
sage at the receiver are shown in Figure 2(a) and (b). It
can be seen that the synchronization of the system is well
achieved. The correlation coefficient of the two chaotic
signals is 0.9485, which provides a guarantee for the rea-
lization of the secure chaotic optical communication.
4. Effect of Wavelength Mismatch on the
Decrypti on Perf ormanc e
In the experiment of secure chaos-masking optical com-
munication, the continuous wave (CW) emitted by a
tunable laser (Thorlabs, TL1550-B) is modulated by a
Mach-Zehnder modulator (MZM). Then a 1.25-Gbits/s
optical message is ge nera ted. T he messa ge is hidde n into
a chaotic carrier by an OC, and launched into the fiber
for secure transmission. At the receiver, the message can
be decrypted by subtracting the secure key from the en-
crypted message. The wavelength of the message can be
changed by adjusting the wavelength of the tunable laser,
and ∆
λ is defined as the wavelength offset between the
wavelength of the message and that of the chaotic carrier
Figure 1. Experimental setup of the secure optical communication system with chaos-masking. DFB: distributed feedback
laser diode, OC: optical coupler, VOA: variable optical attenuator, EDFA: Erbium-doped fiber amplifier, PC: polarization
controller, MZM: Mac h-Zehnder modulator, PD: photodetecto r.
Figure 2 . Time domain wavefo rms of the t wo chaot ic signals w ithout message at t he receiver-end. (a) c haotic carrier w ithout
message, ( b) s ecure key.

W. CHANG ET AL.
185
Figure 3 . Recovered messag es wi th different ∆ λ . (a) - (e) are the o ptical spectr a of the encry pted messages, and the ∆ λ is set
to -0.15nm, -0.08nm, 0nm, 0.08nm, 0.15nm, respectively. (f) - (j) are the recovere d messages corres pond to different ∆
λ .
Figure 4. Experimental results of the secure chaotic optical communication with a 2.5-Gbits/s message when the ∆ λ is set to
0.12 nm. (a) chaotic carri er with a 2.5-Gbits/ s message, ( b) t he recovered messag e at t he receiver.
∆
λ = λ
m −
λ
c,
where λ
m and
λ
c denote the wavelengths of the message
and that of the chaotic carrier, respectively. ∆
λ > 0
means the wavelength of the message is grater than that
of the chaotic carrier and vice versa. In the experiment,
the wa ve lengt h o f the messa g e is c ha n ge d t o ge t d i ffe rent
∆
λ , and then the decryption results are recorded
The experimental results are shown in Figure 3. It can
be seen that when ∆
λ = 0, which means the wavelength
of the message matches with that of the chaotic carrier,
the message recovered at the receiver is of bad quality.
When ∆
λ is less than zero, the quality of the recovered
message is improved. And a lager |∆
λ | means a better
result. W hen ∆
λ is greater than zero, the decryption per-
formance can also be improved. Comparing different
wavelength offset, it is found that the decryption perfor-
mance is better when ∆ λ is greater than zero.
5. High-speed Secure Chaotic Optical
Comm unication
In the secure chaos-masking optical communication sys-
tem, the larger the wavelength difference is, the better the
decryption performance is. However, a larger wavelength
difference will lead to less security of the message.
Therefore, it is necessary to consider both the security of
the message and the decryption performance of the re-
ceiver. A suitable wavelength offset will facilitate the
achievement of the high-speed secure communication.
When ∆ λ is set to 0.12 nm, the experimental results of
the secure chaotic optical communication with a 2.5-
Gbit s/s me ssa ge i s sho wn i n Figure 4. I t can be seen that
the message is safely hidden in the chaotic carrier and
well recovered at the receiver, which means the high-
speed secure communication is realized successfully.

W. CHANG ET AL.
186
6. Conclusions
In this paper, the mismatch of the wavelength between
the message and the chaotic carrier in the chaotic optical
communication with chaos-masking has been experi-
mentally investigated. The results show that the decryp-
tion performance of the receiver can be improved when
the wa velength of the message is greater than that of the
chaotic carrier. When the wavelength offset is set to 0.12
nm, high-speed secure communication with a message of
2.5 Gbits/s can be achieved. The results presented here
may provide some useful suggestions for the practical
applications of secure chaotic op tic a l c ommunications.
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