Ad

KD

a

ARA

KFEDFS

1

sfici[iitltS[tsn[lttbfms

0d

Optik 122 (2011) 1407– 1411

Contents lists available at ScienceDirect

Optik

jou rna l homepage: www.elsev ier .de / i j leo

method of implementation of frequency encoded all optical encryptionecryption using four wave mixing

ousik Mukherjee ∗

epartment of Physics, B.B. College, Asansol, Burdwan, West Bengal 713303, India

r t i c l e i n f o

rticle history:eceived 23 March 2010ccepted 7 September 2010

a b s t r a c t

All optical encryption decryption method using frequency encoding is proposed based on semiconductoroptical amplifiers. The plain text and key are encoded in frequency encoding format i.e. the states ofinformation ‘0’ and ‘1’ are represented by two different frequencies in the c-band. The ultra fast speed ofoperation of the devices used for the implementation of this system makes it very attractive for future

eywords:requency encodingncryptionecryptionour wave mixing

all optical secure communication network. A simple method of conversion of frequency encoded datastream and intensity encoded data stream is also described, which enables us to use same technologyof production and detection of intensity encoded data signals until new techniques based on frequencyencoding comes out.

© 2010 Elsevier GmbH. All rights reserved.

emiconductor optical amplifier (SOA)

. Introduction

With the increase applications of all optical communication, theecurity is becoming an important issue for the researchers in theeld of all optical communication. To ensure secure high speed opti-al communication networks, the all optical encryption decryptions of prime importance for developing all optical packet switching1–3], identity verification and security clearance. An X-OR gates very suitable for encryption decryption and its fully developedmplementation attracted interest of many people in this field afterhe advent of image encryption based on X-OR gate [4]. After that aots of proposal and implementation based on X-OR based encryp-ion decryption are given using liquid crystal displays [5–7], usingOA [8,9] and by other method such as using chaotic dynamics10]. All the implementations have some advantages and disadvan-ages. They are first of all complex in hardware and require preciseynchronization of every bit and amplified spontaneous emissionoise sensitiveness puts a limit to the practical implementation9]. The polarization sensitivity and intensity loss dependent prob-ems of the previous proposals are the two major disadvantages ofhe schemes. The polarization states of the signals changes duringransmission of the signal and require polarization controllers toe used making the hardware complex. But this becomes difficult

or long distance communication. In intensity encoded system, the

ain problem is the loss of intensity during reflection or transmis-ion and thus a constant intensity should be maintained at each

∗ Corresponding author.E-mail address: lipton007@indiatimes.com

030-4026/$ – see front matter © 2010 Elsevier GmbH. All rights reserved.oi:10.1016/j.ijleo.2010.09.017

stage. Above all the intensity loss can cause problem in channelselection, because a non-linear material sends light of differentintensities in different directions. Recently frequency encodingtechnique [11,12] and hybrid encoding technique [13] free frompolarization and intensity loss dependent problems are proposed.In this communication the author proposes the frequency encodedall optical encryption decryption schemes using SOA non-linearity.

The basic principle of operation of the encryption and decryp-tion scheme is based on the X-OR gate. Here the property of theX-OR operation (A X-OR B) X-OR B = A will be used for encryptionand decryption. If P represents the plain text or message encodedin binary form and K represents the key, then the cipher text (C)i.e. the encrypted message will be given by C = P X-OR K, and inthe decryption part, the plaintext P is recovered by the operationC X-OR K = (P X-OR K) X-OR K = P, so the plain text is found back.The proper operation of the system requires that the key K shouldbe shared by the Alice and Bob. This type of cryptography is calledsymmetric key cryptography. In this technique the length of thekey and the plain text should be equal. But this technique is somehow very secured because more is the length of the key less is theprobability of Eavesdropping. For example using eight bit, one cangenerate 28 − 1 i.e. 255 different keys and for 16 bits the number ofkeys become 65,535, so for a standard size message it is impossiblefor Eave to guess a key to encode the message. In this commu-nication the plaintext, key, are in frequency encoded format andthe cipher is also frequency encoded which has due advantages

over other encoding systems in terms of information security also.For frequency encoding in this case we only need two frequen-cies from the broad range of frequencies in the C band of opticalcommunication.

1 ik 122 (2011) 1407– 1411

2

brr

2

rrwtiwqot

b�iit

2

opo

(

ttitoptnaicaacIb�t�i

2

i

Pump1TE

TM Signal

Θ

υυs υ2 υ1 υ1+υ2 – υs

Signal Converted

Pump1 pump2 Pump2

Fig. 1. Four wave mixing in orthogonal polarization scheme.

υ1, υ2, υ3 Reflec ted υ1

Dropped signalυ1

MQWamplifi er

Gratingfilter υ2, υ3

SOA, three ADMs, and two RSOAs. The implementation of the X-ORgate is shown in Fig. 4.

When both the inputs are �1, the converted signal of frequency2�1–�s is blocked and reflected back by the ADM1 and is dropped

SOA

Weak sig nal υ1

Highpowerinput υ2

Highly reflect ing coat ing Ultra low reflecting coating

408 K. Mukherjee / Opt

. Operation principle

The basic operation of the encryption decryption scheme isased on frequency encoding, four wave mixing in SOA, frequencyouting by add drop multiplexer and frequency conversion byeflective SOA (RSOA).

.1. Frequency encoding

In this encoding system the logical states ‘0’ and ‘1’ areepresented by signals of two different frequencies �1 and �2espectively. Thus any binary bit stream of the form 0101010. . ..ill be represented by �1 �2 �1 �2 �1 �2 �1. . .. The encoding

echnique is new and no devices for such encoded bit generations fully available, but the present devices for intensity modulation

ith some modification can be suitably used to implement the fre-uency encoding technique. Such bit pattern generation in the formf frequency encoding will be also discussed in this communica-ion.

In this communication the states of information are encodedy frequencies �1 and �2 corresponding to the wavelengths1 = 1545 nm and �2 = 1550 nm respectively. So when in any stage

f the detected signal corresponds to wavelength 1545 nm, the states zero or low or if the detected signal is of wavelength 1550 nm,he state is high.

.2. Four wave mixing

Four-wave mixing is a coherent non-linear process and canccur in SOA between two signals, a strong pump and a weakerrobe signal. There are different mechanisms behind the generationf four wave mixing:

(i) Modulation of the carrier density, in which the carrier–holerecombination between the conduction and valence band.

(ii) Spectral hole burning (SHB), is caused due to the creation ofhole in the inter band carrier distribution.

iii) Carrier heating which is caused by the stimulated emission andfree carrier absorption.

For efficient FWM, the polarization state of the pump andhe probe signals must be the same. So some polarization con-rol mechanism of either probe or pump will be necessary. Butn co-polarized and orthogonal polarized dual pump schemes,he FWM is polarization insensitive. In the implementation of allptical X-OR logic in this communication, orthogonal polarizedumps scheme to generate FWM will be used. In this schemehe orthogonal polarized pumps interact with the input data sig-al to generate a new conjugate signal, the power of which islso polarization independent. For these two orthogonally polar-zed pumps of frequencies �A and �B are combined by a 50:50oupler and the combined pump signal is combined again with

low power probe signal of frequency �s by a 90:10 couplernd injected into the SOA for FWM in all the SOAs used in thisommunication. The FWM converted frequency is �A + �B − �s.n this communication the states of information are encodedy frequencies �1 and �2 corresponding to the wavelengths1 = 1545 nm and �2 = 1550 nm respectively. The frequency ofhe probe signal is �s and the corresponding wavelengths iss = 1535 nm as used in this communication. The scheme is shown

n Fig. 1.

.3. Frequency routing by ADM

The frequency routing is achieved using ADM by suitably adjust-ng the driving current. The function of ADM is to separate a

Fig. 2. Add/drop multiplexer.

particular frequency channel without interference from adjacentchannels. This is achieved by a frequency demultiplexer by inte-grated tunable SOA filter as in Fig. 2.

The filter can be tuned by changing injection current. The fre-quency channel selected is reflected by the filter, amplified secondtime by the MQW section and extracted to drop port using cir-culator. The remaining frequency channels pass through the filtersection.

2.4. Frequency conversion by RSOA

It utilizes a high reflecting coating on one facet and ultra lowreflecting coating (or anti reflecting coating) on the other facet toproduce a highly versatile reflective gain medium. The basic struc-ture is shown in Fig. 3 as used in this communication. A weak signalof frequency �1 can be converted to a high power signal of a dif-ferent frequency �2 using a signal of frequency �2 from outside.Inside the RSOA multiple reflections occurs and the power of theweak signal is converted to the high power signal. The basic phe-nomenon behind this type of operation is the well known cross gainmodulation (XGM) of converting information at one frequency toanother frequency. The converted signal is of high power due toamplification caused by the RSOA [14].

3. Working of the X-OR gate and the encryption/decryptionsystems

For implementation of the X-OR gate one has to use FWM in an

Conve rte d output υ

Fig. 3. Reflective semiconductor optical amplifier.

K. Mukherjee / Optik 122

SOA

υs

υB

υAADM1

ADM2

ADM3

RSOA1

RSOA2

2υ1- υs

υ1υ2

X-OR

2υ2- υs υ1+ υ2- υs

boScbvaaRtottXedtdT

twaar

apeaAi

Fig. 4. Frequency encoded X-OR gate.

y the circulator to the RSOA1 and is further converted to a signalf frequency �1 by the RSOA1 and extracted as output �1 (LOW).imilarly when any one of the two inputs are either �1 or �2, theonverted signal of frequency �1 + �2 − �s is blocked and reflectedy ADM3 and dropped by the circulator to the RSOA2. RSOA2 con-erts it to a signal of frequency �2 (HIGH). When both the inputsre �2, then the converted signal of frequency 2�2–�s is blockednd reflected by ADM2 and dropped by the circulator to the RSOA1.SOA1 converts it to a signal of frequency �1. In middle two condi-ions the output are �2 (HIGH) and rest are �1 (LOW). So this is theperation of an X-OR gate. With the X-OR gate as the basic block,he encryption decryption system is shown in Fig. 5 below. At firsthe frequency encoded plain text and the frequency encoded key is-OR’ed to get the encrypted cipher text which is also in frequencyncoded format. This is called encryption of the message. This isone in the transmission end of the communication system and isransmitted. In the receiving end, the decryption of the message isone by sending the cipher and the key through another X-OR gate.his cipher is also in the frequency encoded format.

Based on the block diagram the experimental arrangement ofhe encryption decryption scheme is shown in Fig. 6. Here fourave mixing in SOA is used. For frequency routing ADM are used

nd for frequency conversion RSOA are used. In the arrangement Pnd K are two frequency encoded signals of frequencies �1 and �2espectively.

The plain text signal and key are synchronized and coupled with signal of frequency �s and injected into the SOA1. The state ofolarization of the P and K are maintained to be orthogonal to

ach other. The output of the SOA1 is passed through circulatorsnd three ADMs 1, 2 and 3. The blocked signals by ADM 1 andDM2 are dropped and injected into the RSOA1 and that of ADM3

s dropped and Injected into RSOA2 for frequency conversion. The

P

K X-OR

Encryption

K

Cipher textC

Plain text P X-OR

Decryption

Fig. 5. Block diagram of the encryption/decryption scheme.

(2011) 1407– 1411 1409

RSOA1 converts the signals into a frequency �1 and RSOA2 convertsto a frequency �2. The outputs of the RSOA1 and RSOA2 are coupledto generate cipher text C. This cipher text is the frequency encodedversion of the encrypted plain text. In the decryption part the ciphertext and the same key are again synchronized and injected to theSOA2 with the assist signal of frequency �s. In the decryption partthe same type of operation happens with the C and K as happenedwith P and K in encryption part. This is the beauty of the device thatif we couple the outputs of the RSOA3 and RSOA4 we will get theplain text in the frequency encoded format. If we consider a mes-sage (pain text P) of the form �1 �2 �1 �1 �1 �2 �1 �2 (01000101)and Key (K) of the same length �1 �2 �2 �2 �2 �1 �1 �1 (01111000),the cipher text will be obtained by bit-wise XOR operation betweenP and K and will be of the form �1 �1 �2 �2 �2 �2 �1 �2 (00111101).The result of encryption and decryption is shown in Fig. 7 below.

In the decryption part similarly we can summarize the results asshown in Fig. 8 below. It is interesting to note that, the same plaintext is obtained in the frequency encoded format.

4. Conversion of intensity encoded signal and frequencyencoded signal into one another

Since frequency encoding technique is very new idea in the fieldof optical communication and computation, the available devicesfor all optical computation and communication are mostly workon the principle of intensity encoding. So in this communication, amethod of conversion of these two types of signals into one anotheris described.

4.1. Conversion of intensity encoded signal to frequency encodedsignal

Among the lots of technique, the most suitable correspondingto this communication is based on cross gain modulation (SOA)in SOA. One of the important non-linearity of the SOA is crossgain modulation (XGM). The material gain spectrum of an SOA ishomogeneously broadened in it. The temporal response of the car-rier density depends on the lifetime of the carrier. As shown inFigs. 9 and 10 below, a weak CW probe signal and a strong pump sig-nal, with a small signal harmonic modulation at angular frequencyω, are injected into an SOA. XGM in the SOA will transpose informa-tion at one wavelength to another signal at different wavelengths.This XGM is a way to affect the gain of a weak by a strong signal.When the pump signal is absent, the probe is transmitted and theoutput is high and when the pump is present, there is cross gainmodulation and in the output no signal is present.

Using this type of XGM unit one can convert an intensityencoded signal to frequency encoded signal if direct generation offrequency encoded signal is not available in hand. In Figs. 11 and 12below the conversion of the intensity encoded signal to frequencyencoded signal is shown. The intensity encoded signal is used aspump to the SOA1, and output of the filter is divided into two partsone is used as frequency encoded output and another part is usedas pump in the SOA2. It is interesting to note that when the out-put of the SOA1 will be high that of the SOA2 is low but when theoutput of the SOA1 is low then the output of the SOA2 is low. Soa low intensity encoded input data corresponds to high output forSOA1 and the corresponding frequency encoded data output is atfrequency �1, similarly a high intensity encoded data input corre-sponds to a low output at SOA2, and this results in a high output atthe SOA2. So the frequency encoded data will be at frequency �2.

The result of conversion from intensity encoded data stream100110 to frequency encoded form is shown in Fig. 10 below. It isfound that the frequency encoded output has the form �2 �1 �1 �2�2 �1.

1410 K. Mukherjee / Optik 122 (2011) 1407– 1411

P

Plain text P

Decryption part

SOA1υs

KADM1

ADM2

ADM3

RSOA1

RSOA2

2υ υ

υ1υ

Cipher text C

2υ - υυ + υ - υ

SOA2

ADM4

ADM5

ADM6

RSOA3

RSOA4

2υ - υ

υ1υ2

2υ υυ υ - υ

K

υ

Encryption part

Fig. 6. Frequency encoded encryption/decryption scheme.

1 2 1 1 1 2 1 2

1 2 2 2 2 1 1 1

1 1 2 2 2 2 1 2

Plaintext(P)

Key(P)

Cipher(C)

Fig. 7. The frequency encoded encryption operation.

1 1 2 2 2 2 1 2

1 2 1 1 1 2 1 2

1 2 2 2 2 1 1 1

Ciphertext(C)

Key(P)

Plaintext(P)

Fig. 8. Frequency encoded decryption operation.

SOA

Pump

CWProbe at υ1

Modulated output

υ1 pass Filter

Fig. 9. XGM in SOA.

4.2. Conversion of frequency encoded data to intensity encodeddata

The advantage of using frequency encoding system is that it canbe easily converted to intensity encoded format just by a singlefilter. So if frequency detection technique is not available in handit may be converted to intensity variation and the conventionalmeasurement of intensity for the logic ‘0’ and ‘1’ may be used. Ifthe frequency encoded signal is passed through a �2 pass filter,the output of the filter is simply intensity encoded signal. This isdone on the principle that since the frequency �1 corresponds to

low, it is totally rejected and in the intensity encoded format thissignal is absent i.e. the information state represented is ‘0’. Againthe signal of frequency corresponds to high state and this signal

SOA1

Pump

CWProbeat 1

1 passFilter

SOA1

CWProbeat 2

Frequency encoded output

Fig. 10. Intensity encoding to frequency encoding converter.

K. Mukherjee / Optik 122

Intensityencoded data

Frequency of the output of SOA1

Frequency of the output of SOA2

Frequencyencoded data at the output of the converter

ν2 υ1 υ1 ν2 ν2 ν1

1 0 0 1 1 0

0 υ1 υ1 υ1

υ2 υ2 υ2

Fig. 11. Conversion of 100110 to frequency encoded �2 �1 �1 �2 �2 �1.

υ2 υ1 υ1 υ1υ2υ2

υ2 pass filter

1 0 0 1 1 0

ai

eot

5

sRvvhiroSfogifffS

[

[

[

[

[

[

Fig. 12. Frequency encoding to intensity encoding.

llowed to pass through the filter to represent high state in thentensity encoded format.

So the conversion of frequency encoded signal to intensityncoded one is simpler and requires no active element like SOA butnly a filter. This is another advantage of using frequency encodingechnique.

. Conclusion and discussion

Frequency encoded encryption decryption using polarizationensitive four wave mixing, ADM based frequency routing andSOA based frequency conversion is proposed. The method of con-ersion from frequency encoding to intensity encoding and viceersa are also discussed. All the devices and systems are of ultraigh speed and have THz speed of operation. The frequency encod-

ng has many advantages over other encoding techniques in manyespects as described in Section 1. With these advantages the usef frequency encoding has advantage with respect to security also.ince in the C band of optical communication a large number ofrequencies are available, the exact frequency by which the statesf information are encoded is not possible for an Eavesdropper touess. This is an extra advantage of frequency encoding. So if theres enough research in the field of frequency encoding technique,

uture all optical computation and communication will be moreast, error free and secure. For the proper operation of the deviceor encryption decryption the driving current of the SOA1 andOA2 (both are Kamelian OPB-10-15-NC-FA) should be adjusted at

[

(2011) 1407– 1411 1411

200 mA. The input powers of the SOAs are 4–10 dBm for pump and−2 to −4 dBm for probe signal. Both the up and the down conver-sion of frequencies (extinction ratio is better for down conversion)are done by RSOAs (Opto speed SOA 1550MRI/Especial) becausetheir efficiency is more than conventional SOAs [14]. The power ofthe probe beam for this frequency conversion is kept in the range−4 dBm to −2 dBm, whereas the power of the input pump beam ismaintained in the range 4–10 dBm. It is also remembered that theprobe power of the SOA should not fall below −8 dBm for faithfulfrequency conversion. The frequency selection by the ADMs is doneby simple electronic tuning of the filter i.e. by adjusting the injec-tion current. So the same ADM can be used for selecting differentfrequency channels, just by adjusting the current. In the encod-ing conversion the driving current of the SOAs should be kept at200 mA. The pump power in these SOAs is kept between 2 and4 dBm and the probe power is kept between −2 dBm and −4 dBmfor proper operation of the device. In the RSOAs used in the devicesin the Figs. 3, 4 and 6, there are circulators at the output (not shown).The working of the ADM is explained in [15]. The FWM used in thiscommunication is described in [16].

References

[1] O. Buskila, A. Eyal, M. Shtaif, Secure communication in fiber optic system viatransmission of broad-band optical noise, Opt. Exp. 16 (5) (2008) 3383.

[2] K.S. Li, Y. Xie, W.S. Zhang, Z. Chen, A novel algorithm for evolving encryptionsequences based on particle dynamics, in: CEC’2008, Hong Kong, 1–6 June,2008, p. 174, doi:10.1109/CEC.2008.4630874.

[3] F. Froehlich, F. Price, C.H. Turpin, T.M. Cook, All optical encryption for links at 10Gbps and above, in: MILCOM 2005, Atlantic City, New Jersey, 17–20 October,2005, 4, 2158.

[4] J.-W. Han, C.-S. Park, D.-H. Ryu, E.-S. Kim, Optical image encryption based onX-OR operations, Opt. Eng. 38 (1999) 47.

[5] A. Jeffrey, D.E. Davis, D.M. Mcnamara, T. Cottrell, Sonehara, Two dimensionalpolarization encoding with a phase only liquid crystal spatial light modulator,Appl. Opt. 39 (2000) 1549.

[6] P.C. Mogensen, J. Glücstad, Phase only optical encryption, Opt. Lett. 8 (2000)566.

[7] B. Zhang, M. Kah, Simulation of optical X-OR encryption using MATLAB, in: 7thAFRICON Conf. 2004, 15–17 September, 2, 2004, p. 995.

[8] Y.J. Jung, C.W. Son, S. Lee, S.K. Gil, H.S. Kim, N.K. Park, Demonstrated of 10 GbpsOptical Encryption and Decryption by using Semiconductor Optical Amplifiers.

[9] W. Ya-Ping, W.U. Chong-qing, W. Zhi, W. Yong-Jun, Y. Shuang-Shu, S. in-Zhi, Anencryption method based on the XPM between O band and C band light waves,Chin. Phys. Lett. 26 (2007), 074219-1.

10] L. Larger, J.P. Goedgebuer, Encryption using chaotic dynamics for opticaltelecommunications, C. R. Phys. 5 (2004) 609.

11] S.K. Garai, S. Mukhopadhyay, A method of optical implementation of frequencyencoded different logic operations using second harmonic and difference fre-quency generation technique in non linear material, Optik 121 (2010) 715–721.

12] K. Mukherjee, P. Ghosh, A novel frequency encoded all optical CNOT gateexploiting difference frequency generation and implementation of fast binaryadders using frequency encoding and non linear dielectric films, Optik (2010),doi:10.1016/j.ijleo.2009.11.006.

13] K. Mukherjee, Implementation of a novel hybrid encoding technique and real-ization of all optical logic gates exploiting difference frequency generationalone, Optik (2010), doi:10.1016/j.ijleo.2010.02.013.

14] L.Q. Guo, M.J. Connelly, A novel approach to all-optical wavelength conver-sion by utilizing a reflective semiconductor optical amplifier in co propagationscheme, Opt. Commun. 281 (2008) 4470.

15] G. Raybon, U. Koren, B.I. Miller, M. Chien, M.G. Young, R.J. Capik, K. Dreyer, R.M.Derosier, A wavelength-tunable semiconductor amplifier/filter for add/drop

multiplexing in WDM networks, IEEE Photon. Technol. Lett. 9 (1997) 40–42.

16] G. Hunziker, R. Paiella, D.F. Geraghty, K.J. Vahala, U. Koren, Polarization – inde-pendent wavelength conversion at 2.5 Gb/s by dual pump four – wave mixing ina strained semiconductor optical amplifier, IEEE Photon. Technol. Lett. 8 (1996)1633–1635.