東海大学紀要情報通信学部
vol.6,No2,2013,pp.15-21
Proc. Schl. ITE Tokai Univ.

Paper

Paper

Vol.xx ,No. ,20xx,pp.xxx -xxx

Cryptography using dark-bright soliton to generate keys byorthogonal dark-bright soliton pairby

Xaythavy LOUANGVILAY *1 , Somsak MITATHA *2 , Masahiro YOSHIDA *3 , Noriyuki
KOMINE *4 and Preecha P. YUPAPIN *5(received on September 21, 2013&

accepted on February 6, 2014)

Abstract
A method of optical cryptography by using soliton as dark-bright conversion control within a modified
add/drop optical filter is proposed. In principle, the coincidence (orthogonal) dark-bright soliton pair has
shown promising behaviors, especially when they propagate into the π2 phase shifter, i.e. beamsplitter, π2phase shift between bright and dark solitons is occurred and separated. Such behaviors can be used to form
the orthogonal light modes (solitons), which can be useful for application of cryptography, where in this casethe long distance cryptography can be easily managed. In this work we have derived and presented a conceptof multi-orthogonal solitons generated by using dark-bright soliton pulses within the modified add/drop
optical filter, which is known as a PANDA ring resonator. A pair of optical keys is randomly generated andtransmitted into the transmission line, where finally the corrected keys between Alice (sender) and Bob(receiver) can be retrieved. By using the dark-bright soliton conversion control, the obtained output of the
dynamic states can be used to randomly form the multi-orthogonal soliton pairs, which can be available forcomputer and communication security applications, especially for long distance link.
Keywords: soliton cryptography; optical cryptography; optical security; PANDA ring.

In principle, the detection of dark soliton is extremely

1. Introductiondifficult. The dark soliton behavior has become the promising

Soliton communication has been a system fortheapplication when the transmission dark soliton can berequired minimum repeater is the advantage has become theconverted after passing through into the specific add/drop

key advantage of the system performance. However, infilter[6], which means that the transmission signals can bepractice, the problem of soliton-soliton interaction, soliton

transmitted in the form of dark soliton, which is hard tocollision, and dispersion management are required todetect, whereas the specific end user that connects to the link

long-distance

opticalcommunication

links,

where

solve.[1-3] Generally, there are two types of soliton known as

via the specific add/drop filter can obtain the signals

bright and dark solitons,[4] where the soliton behaviors andAlthough, the dark soliton applications have been widely

applications are well analyzed and described by Agarwal[5].

investigated in various applications[7-10], the searching for
further applications remains. The use of soliton, i.e., bright*1,2 King Mongkut’s Institute of Technology Ladkrabangsoliton in long-distance communication links has been in

Hybrid Computing Research Laboratory Faculty ofoperation for nearly two decades. However, some questions

Engineering Bangkok 10520, Thailand
*3,4

Tokai

University

Telecommunication

SchoolofInformation

andEngineering

Departmentofstill need to be answered in the area of communication safety,
whereas the use of a dark soliton pulse within a microring
resonator for communication security has been studied. The

Embedded Engineering, Tokyo, 108-8619, Japan*5

investigation of dark soliton behaviors[11] has been reported

King Mongkut’s Institute of Technology Ladkrabang
Nanoscale

ScienceandEngineeringand one has shown the interesting result that the dark soliton

Research

can be stabilized and converted into bright soliton and finally

Alliance (N’SERA) Faculty of Science Bangkok
10520, Thailand
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Cryptography using dark-bright soliton to generate keys by orthogonal dark-bright soliton pairTimes Times Times Times Times

detected. This means that we can use the dark soliton towith two sets of coupled waveguides, as shown in Fig. 1. The

perform the communication transmission for security,

relative phase of the two output light signals after couplingwhereas the required information can be retrieved by the

into the optical coupler is π2. This means that the signals

dark-bright soliton conversion, in which the use of optical

coupled into the drop and through ports have acquired a

microcavities[12-13] or micro ring resonators[14-20] havephase of π with respect to the input port signal. In application,

shown the promising applications for dark soliton, where awhen the coupling

PANDA ring resonator is a new model of them, which has
been successfully used to investigate the dynamic behavior ofdark-bright soliton collision. In this paper we proposed a newconcept of optical cryptography by using dark-bright soliton
conversion control within a modified add/drop filter (aPANDA ring), which is useful for cryptographic application.
The simulation results obtained have shown that the proposedsystem can be used to form the security keys, where thesecret keys can be retrieved and achieved, which is useful forcomputer and communication applications, especially, wherein addition the high capacity and long distance link can berealized by using the soliton communication.

Fig. 1 A schematic diagram of a proposed PANDA ring

Information technology(IT) has become the main

resonator.

key that can be used to develop and change the world
economy; therefore the finding of new techniques for morecoefficients are formed appropriately, the optical field

capacity and available network is remaining. One of them iscoupled into the through port with the resonant condition

the use of high performance network that is able to offer bothwould completely extinguish the resonant wavelength, and allhigh security and larger bandwidth. There are many

the power would be coupled into the drop port.

techniques have been reported,[21-24] where they have

Ein t   A0 tanh  TT01  exp z 2 LD  i0t
shown that the user demand can be improved. To date, opticalcommunication has been investigated in almost every layer of
information technology infrastructure: continent-to-continent,





(1)





(2)

Ein t   A0 sec h TT01  exp z 2 LD  i0tcomputer-to-computer,

Here A0 and z are the optical field amplitude and propagation

board-to-board, chip-to-chip, and finally intrachip, where all

distance, respectively. T  t  1 z , where 1 and  2 are theoptical devices have been used as the integrated components

coefficients of the linear and second-order terms of Taylor

city-to-city,

server-to-server,

for advanced optical technique in a lot of applications,

expansion of the propagation constant. LD  T02 /  2 is theespecially in optical communication, optical sensor, and
the

world

information

dispersion length of the soliton pulse. T0 in equation is asecurity

segmenthas

beensoliton pulse propagation time of initial input (or soliton

recognized as a promising tool for information necessitatespulse width), where t is the soliton phase shift time, and thefrequency shift of the soliton is 0 .

signalprocessing,

etc.

technology,communicationInthe security and privacy requirements due to the large
demand of the world networks, where there have been widelyused in many applications; for instance, sensors,[25]E1   1E4  jEi(3)computing[26],E 2  exp( jT / 2) exp(L / 4) E1(4)communication,[27]andsignalprocessing[28]. One of them used an optical device forE 3   2 E 2  j 2 E a(5)communication security,[29-32] which can be fabricated andE 4  exp( jT / 2) exp(L / 4) E3(6)made the integrated components for advanced opticalEt   1 E i  j 1 E 4(7)technology in applications.

Ed  2Ea  j 2 E2(8)

2. Operating Principle
In operation, the input and control fields at the
input and add ports are formed by the dark and bright

Here E1 , E 2 , E3 , E4 are the fields in the ring at points 1 to 4,
Ea is the add (control) field, Ed is the drop field, Ei is the

input field, Et is the through field, 1 is the field coupling

optical solitons and described by Eqs. (1) and (2),coefficient between the input bus and ring,  2 is the field

respectively. A dark-bright soliton conversion system using acoupling coefficient between the ring and output bus, T isring resonator optical channel dropping filter is composed

the time taken for one round trip (round trip time), L is the

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Xaythavy LOUANGVILAY・Somsak MITATHA・Masahiro YOSHIDA・Noriyuki KOMINE and Preecha P. YUPAPIN

Times Times Times Times Times

circumference of the ring, and  is the power loss in the ring

Similarly, the output circulated light field, E0L, for the left

per unit length. We assume that this is the lossless coupling,

ring at the left side of the add/drop optical multiplexingi.e.,  1, 2  1  12, 2 , T  Lneff / c .

system is given by

The output intensities at the drop and through ports are L1 ( 2 1   jk n )   (1   3 )(1   )  (1   3 )e 
E0 L  E3 1
 1  (1   )(1   ) e  L1 ( 2 1   jk n ) 

3

 


given byEd2

 1 2 A1 / 2 1 / 2   A
 Ea 2 1 Ei
1   1 2 A
1   1 2 A

Et

2

 Ei

2

(9)

  A  2   1 A Ea 1 2 1 / 2 1 / 2
1   1 2 A
1   1 2 A(14)

3. Results(10)

A schematic diagram of dark-bright soliton generationconsists of two parts such as transmission and receiver parts
as shown in Fig. 2.

Here A is the effective mode core area of device and  isnonlinear phase shift of device

When light propagates within the nonlinear material(medium), the refractive index n of light within the

To form the broad soliton spectrum output, twononlinear ring resonators are introduced as shown in Fig. 1.

medium is given by

We have γ the fractional coupler-intensity loss,k n  2 /  the wave propagation number. λ the inputn  n0  n2 I  n0 

n2
P
A

(15)

wavelength light field. The circulated round-trip light fields
at the point 1(after through  4 ) and 2 (before through  4 )indexes, respectively. I and P are the optical intensity andof the right ring radii, Er1 and Er2, are given in Eqs. (11) andpower.

Here n0 and n2 are the linear and nonlinear refractive(12), respectively.

The basic proposed PANDA ring resonator parametersareEr1  ( j 1    4 E1 ) 1  1   1   4 e  L1 ( 2   jk n ) 


1

1

be

n0  3.34(InGaAsP/InP),and  0.2dBmm , the coupler intensity loss is   0.1 . The(11)bright soliton input pulse with center wavelength of 1.4 μm,

1

 1  1   1   4 e  L1 ( 2   jk n ) 



to
1

and an amplitude signal of 0.5 W is input into the system.

Er 2   j 1    4 E1e  L1 ( 2   jk n 


1

fixed

The coupling coefficient of the microring resonator is varied

(12)

1

from

0.1

to

0.98,

the

nonlinearrefractiveindex

is n2  2.2  10 17 . The other used parameters of the first
add/drop filter (optical multiplexer, PANDA ring resonator)

Thus the output circulated light field, E0, for the right ring

1  0.15 ,  2  0.35 ,  3  0.7 ,and  4  0.2 , respectively. The ring radii are Rad  15m ,and Rr  Rl  5m 2 5. Aeff are 0.50, 0.25 and 0.25 m 2 ,respectively. Moreover, the selected parameters have beenchosen closely to the practical device parameters, in which
the optical key suppressed and recovery system shouldareis given byE0  E1  (1   4 )(1   )  (1   )e  L1 ( 2   jk n ) 


1

1
 1  (1   4 )(1   ) e  L1 ( 2   jk n ) 



1

(13)

fixed

to

be

be possible to be fabricated.

(a)Fig. 2. Schematic diagram of dark-bright soliton generation,
where D n: photodetectors, n : coupling coefficients,
En: electric field, BS: Beamsplitter.

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Cryptography using dark-bright soliton to generate keys by orthogonal dark-bright soliton pairTimes Times Times Times Times

The simulation results of generating signals in a
PANDA ring resonator are as shown in Fig. 3, which is usedin the transmission part, where (a) is the input pulse (brightsoliton), (b) is the through port ( Et1 ) signal, (c) is the drop
port signal, and (d) is the comparison between though port
and drop port signals. Here (b) and (c) show the amplitude at
the through and drop ports for signal recovery, respectively.
The referencing multisoliton pairs are generated as shown in
Fig. 4(a)–4(c), in which the multisoliton pairs detected by
D1 and D2 are “1010101010101” for D1 and “bbbbbb” forD2 . Similarly, D1 and D2 at the through ports are detected andthe multisoliton pairs seen, where the D3 and D4 codes such as“1010101010101” and “bbbbbb” are obtained as shown in(b)Fig. 2, respectively. At the receiver part, theadd/drop filter parameters are fixed to ben0  3.34 (InGaAsP/InP), and   0.2dBmm1 , the coupler

intensity loss is   0.1 . The other used parameters of thefirst add/drop filter is fixed to be  5   6  0.13 . The

coupling coefficient of the microring resonator isvaried from 0.1 to 0.98 and the nonlinear refractive
index is n2  2.21017 .
Generally, when a bright or dark soliton is
propagated into a  / 2 phase shifter device (beamsplitter oran optical coupler), the dark-bright soliton conversion (pair)
can

(c)

be

randomly

generated

anddetected.

The

multi-dark-bright soliton conversion is also seen when the
input soliton is chopped to be many soliton pulses by thenonlinear effects, where in this case the nonlinear devices areformed by the two small ring resonators of the center ring
(add/drop filter), which is formed a PANDA ring. In
application, the signal is sent into the input port by thetransmitter as shown in Fig. 3. Where (a) is the input pulse(bright soliton), (d) is the comparison between Though port
and Drop port signals which are used as key and data forencryption. in Fig. 3(b) is the Through port (Et) signal, 3(c)is the Drop port signal.Then both of them are sent into the
input port at the receiver part, where the signal is decrypted
to separate key and data. The decryption key is used for the
ciphertext decryption, which is also sent by the transmitter.

(d)

By using the orthogonal (coincidence) dark and bright soliton
Fig. 3. A schematic diagram of dark-bright soliton generation,

pair; thus our proposed system can be claimed as a new

where (a) is the input pulse (bright soliton), (b) is the

security technique for optical cryptographic design in which

Through port (Et1) signal, (c) is the Drop port signal,

the information either analog or digital signals can beand (d) is the comparison between Though port andencoded and formed the secret keys introduced by a random

Drop port signals.

dark-bright soliton pair.

The key suppressed and recovery system should be possible
to be fabricated.

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Xaythavy LOUANGVILAY・Somsak MITATHA・Masahiro YOSHIDA・Noriyuki KOMINE and Preecha P. YUPAPIN

Times Times Times Times Times

(a)

(d)

(b)

(e)

(c)

(f)
Fig. 4. Simulation results that can be used for encryption key
and data, where (a) and (c) are the keys, (b) and (d)are the data signals, (e) is the orthogonal dark-brightsoliton pair(key), (f) are shown Full Width at Half
Maximum, (FWHM)(FSR).

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free spectrum range

Cryptography using dark-bright soliton to generate keys by orthogonal dark-bright soliton pairTimes Times Times Times Times

Moreover, this triple security functions canbe realized when the security is formed by using the
suppressed optical key, in which the optical key
changing in every data frameand new optical
cryptography technique. Fig.4 shows the simulation
result of optical cryptography at the receiver side,
where (a) and (c) are the keys, (b) and (d) are the data
signals, (e) is the orthogonal dark-bright soliton pair(keys), where the selected wavelength center can bemade by using the designed add/drop filter, whereas therequired spectral width (full width at half maximum,
FWHM) and free spectrum range (FSR) are obtained asshown in Fig. 4(f) The channel spacing and channel
capacity are represented by FSR and FWHM,
respectively; for instance, the FSR and FWHM of 0.002
μm and 0.00007 μm are obtained, which means that
there are many channels are filtered by PANDA ring
resonator

the Eq. (16), in which the information can be retrieved.
Otherwise, it is not the required information. In
addition, the different wavelengths and codes can beapplied to large network applications.

4. Soliton Cryptography Protocol
Generally, when a bright or dark soliton is

Acknowledgments
We would like acknowledge the AUN/SEED-Net for the full

5. Conclusion
We have proposed a novel design optical cryptography,
which is the interesting concept of the optical key generationthat can be used in either analogue or digital communication,
where

the

long-distance

link

by

using

a

solitoncommunication is the other advantage. The random keys are
generated by the dark-bright soliton pair within a PANDA
ring resonator, in which the high-capacity keys are alsoavailable due to the nonlinear behavior of the PANDA ring.
In applications, the required information between Alice andBob can be obtained privately without eavesdropping byusing the proposed system, where the use of such a system
for large scale and long-distance networks is also available.

propagated into a π2 phase shifter device (beamsplitter or an

financial support of Mr. Xaythavy Louangvilay in higher

optical coupler), the dark-bright soliton conversion (pair) can

education

be randomly generated and detected. Dark-bright soliton pairTechnology Ladkrabang (KMITL), Bangkok, Thailand. forhas shown the interesting concept in the same way ofproviding the excellent research facilities.

andthanksKing

Mongkut’s

Instituteoforthogonal signal behaviors [Eqs. (16)]. By using theorthogonal dark-bright soliton pair, the random binary bits

Reference

can be established, which can be described by the following[1]

relations.

diffraction and dispersion managed soliton in a

B D  1

(16a)

B  D  0, B  D

(16b)

2

2

B  D 1

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bits (codes) are initially generated by the transmission
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