all optical signal processing of optical fiber raman amplifiers in advanced photonic communications...

JOURNAL OF TELECOMMUNICATIONS, VOLUME 25, ISSUE 2, JUNE 2014

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© 2014 JOT www.journaloftelecommunications.co.uk

All Optical Signal Processing of Optical Fiber Raman Amplifiers in Advanced Photonic

Communications Engineering Ahmed Nabih Zaki Rashed1*, Ibrahim M. El-Dokany2,

Abd El–Naser A. Mohamed3, and Sarah El-Tahan4

Abstract— This paper has presented the transmission systems with employing Raman amplifier technology have to put up with much higher level of design complexities, when compared to conventional transmission lines with doped fiber optical amplifier. Even for the construction of a fundamental, basic building block a unit of a fiber Raman amplifier (FRA), the designer have to struggle with the problems associated with the interactions between pump/signal waves mediated by Raman process, have to wander within the vast degrees of freedom given the choice of pumping directions/ratios, and have to contemplate with the wavelength dependent fiber loss/noise figure profiles in different fiber cable medias such as true wave reach fiber, freelight, and single mode fiber (SMF).

Index Terms— Optical signal processing, Performance signature, Raman Amplifiers, and Advanced photonic Communications Engineering.

—————————— u ——————————

1. INTRODUCTION

Wavelength division multiplexing (WDM) is basically frequency division multiplexing in the optical frequency domain, where on a single optical fiber there are multiple communication channels at different wavelengths [1]. A WDM system uses a multiplexer at the transmitter to join the signals together and a demultiplexer at the receiver to split them apart. By using WDM and optical amplifiers, they can accommodate several generations of technology development in their optical infrastructure [2]. Optical gain depends on the frequency of the incident signal and also on the local beam intensity. Dense wavelength division multiplexing (DWDM) is a technology that puts data from different sources together on an optical fiber, with each signal carried at the same time on its own separate light wavelength [3]. Optical amplifiers have several advantages over regenerators. Optical amplifiers can be more easily upgraded to a higher bit rate. In an optical communication system, as the optical signals from the transmitter propagate through optical fiber are attenuated by it and losses are added by other optical components, such as multiplexers and couplers which causes the signal to become too weak to be detected. Before this the signal strength has to be regenerated [4]. Most optical amplifiers amplify incident light through stimulated emission, its main ingredient is the optical gain realized when the amplifier is pumped to achieve population inversion. The optical gain, in general, depends not only on the frequency of the incident signal,

but also on the local beam intensity at any point inside the amplifier [5]. To understand how optical amplification works, the mutual or reciprocal action of electromagnetic radiation with matter must be understood [6]. Optical amplification uses the principle of stimulated emission same as used in a laser. Optical amplifiers can be divided into two basic classes: optical fiber amplifiers (OFAs) and semiconductor optical amplifiers (SOAs) [1]. An amplifier can boost the (average) power of a laser output to higher levels. It can generate extremely high peak powers, particularly in ultra short pulses, if the stored energy is extracted within a short time. It can amplify weak signals before photo detection, and thus reduce the detection noise, unless the added amplifier noise is large. In long fiber-optic links for optical fiber communications, the optical power level has to be raised between long sections of fiber before the information is lost in the noise. The combination of an erbium-doped fiber amplifier (EDFA) and a fiber Raman amplifier (FRA) is called a hybrid amplifier (HA), the Raman-EDFA. Hybrid amplifier provides high power gain. Raman amplifier is better because it provides distributed amplification within the fiber. Distributed amplification uses the transmission fiber as the gain medium by multiplexing a pump wavelength and signal wavelength. It increases the length of spans between the amplifiers and regeneration sites. So this provides amplification over wider and different regions [7]. Hybrid Raman/erbium-doped fiber amplifiers (HFAs) are an advance technology for future. Hybrid Raman/erbium doped fiber amplifiers are designed to maximize the long-haul transmission distance. In the present study, performance signature and all optical signal processing of optical Raman amplifiers in photonic communications engineering have deeply studied over wide range of the affecting parameters. Transmitted signal and pumping powers in forward, backward, and dual pumping configurations can be theoretically studied. Raman gain will be analyzed in

———————————————— • Ahmed Nabih Zaki Rashed (Corresponding author) is with

electronics and Electrical Communications engineering department, faculty of electronic engineering, menouf 32951, menoufia university, EGYPT.

• Ibrahim el-dokany Abd elnaser A. Mohammed and Sarah El-tahan are with electronics and Electrical Communications engineering department, faculty of electronic engineering, menouf 32951, menoufia university, EGYPT..


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both forward and backward. Raman amplifier can be thought as more of a loss compensator than an amplifier. Hence we normally expect all the signals to come out of the amplifier with same signal strength with which they

entered it. For that we ideally require a flat gain profile. By adjusting the powers and wavelengths of multiple pumps employed this profile is achieved.

2. BASIC RAMAN AMPLIFIERS WITH MULTIPLEXING/DEMULTIPLEXING TECHNIQUES A schematic view of the configuration of Raman amplifiers with multiplexing and demultiplexing techniques are shown in Fig. A [7]. It is provided with arrayed waveguide grating (AWG) devices which act as multiplexing unit in the transmitting side, and act as

demultiplexing unit in the receiving side. Basically, pumped light and signal light are input to a single amplifier fiber, and amplification is effected by means of the stimulated scattering that occurs in the fiber.

Fig. A. Schematic view of Raman amplifier with multiplexing and demultiplexing techniques.

Figure A shows a configuration in which pumped light propagates bi-directionally in the distributed Raman amplifier, but sometimes it propagates in the same direction as the light signal (forward pumping) or the opposite direction (backward pumping). Moreover the system is provided with band pass filter (BPF) and AWG devices. Generally, speaking with forward pumping, the signal to noise ratio (SNR) can be kept high, while with backward pumping the saturation output power can be increased. In the case of a Raman amplifier the process of optical amplification takes place so rapidly that, unless the intensity noise of the forward pumping light is sufficiently small, the pumping light noise will be transferred to the signal light resulting in increasing transmission bit error rate (BER). Thus in many cases only backward pumping is used [8].

3. MODEL AND EQUATIONS ANALYSIS The evolution of the input signal power (Ps) and the input pump power (Pp) propagating along the single mode optical fiber in watt; can be quantitatively described by different equations called propagation equations. The rate of change of signal and pump power with the distance z, can be expressed as mentioned in [9]:

)()()( Re zPzPgzPdz

dPpsff

p

spLp

pλ

λα −−= (1)

)()()( Re zPzPgzPdzdP

psffp

ssLs

sλ

λα +−= (2)

Where λs and λp are the signal and pump wavelengths in µm respectively, z is the distance in km from z=0 to z=L, αLs and αLp are the linear attenuation coefficient of the signal and pump power in the optical fiber in km-1 respectively. Where α is the attenuation coefficient in dB/km. gReff is the Raman gain efficiency in W/km of the fiber cable length, L in km, which is a critical design issue and is given by the following equation:

( )18Reff10

=g −×eff

RA

g (3)

Where gR is the maximum Raman gain in km/W, Aeff the effective area of the fiber cable used in the amplification in µm2. Equation (1) can be solved when both sides of the equation are integrated. When using forward pumping, the pump power can be expressed as the following expression [10]:

( ) ( )zPzP LppoFPF α−= exp (4) Where PPoF is the input pump power in the forward direction in watt at z=0. In the backward pumping the pump power is given by [11]:

( ) ( )[ ]zLPzP LppoBPB −−= αexp (5) where PPoB , is the input pump power in the backward direction in watt at z=L. In the case of bi-directional pump both of the pump can be equal or different in the

λs1

λs3

λs(n-1)

λsn

λs1

λs2

λs3

λs(n-1)

λsn

Transmitters

Receivers

Optical fiber spans

Z L 0 Backward

Pump

BPF

Forward Pump

PP- (L) PP

+ (L)

PS(L) PS(0) λs2


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used wavelength or the used amount of power, therefore in this case the following equation can be used to calculate the pump power at point z [11, 12]:

( ) ( ) ( ) ( )[ ]zLPrfzPrfzP LppoBLppoFPFB −−−+−= αα exp1exp)( (6) Where rf is the percentage of pump power launched in the forward direction. If the values of PP are substituted in differential Eq. 2, and is integrated from z=0 to z=L for the signal power in the forward and the backward pumping the result mathematical equation can be written as mentioned in [13]:

( )⎥⎥

⎦

⎤

⎢⎢

⎣

⎡−

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛= zLP

Ag

PzP Lseffpoeff

RsoS αexp (7)

Where Pso and Ppo denotes to the input signal and pump power respectively. This means that Ppo= PpoF in case of forward pump and Ppo=PpoB in case of backward pump, and Leff, is the effective length in km, over which the nonlinearities still holds or stimulated Raman scattering (SRS) occurs in the fiber and is defined as [14]:

( )Lp

Lpeff

zL

α

α−−=

exp1 (8)

Recently, there have been many efforts to utilize fiber Raman amplifier in long-distance, high capacity WDM systems. The net gain [15] is one of the most significant parameters of the FRA. It describes the signal power increase in the end of the transmission span and presents the ratio between the amplifier accumulated gain and the signal loss. It can be simply described by the expression:

,)0()(

S

Snet P

zPG = (9)

If the Raman gain is not sufficient to overcome fiber losses, it is useful to introduce the concept of the on–off Raman gain using the definition [16]:

offpumpWiths

onpumpWithsR LP

LPLG

)()(

)( = (10)

Clearly, GR (L) represents the total amplifier gain distributed over a length Leff .

4. RESULTS AND PERFORMANCE ANALYSIS The optical FRAs have been modeled and have been parametrically investigated in different fiber cable medias such as true wave reach fiber, Freelight, and single mode fiber (SMF). In fact, the employed software computed the variables under the following operating parameters as shown in Table 1.

Table 1. Proposed operating parameters for performance signature of Raman amplifiers.[3, 5, 12, 13,17] Operating parameter

Symbol Value and unit

Operating signal wavelength

λs 1.45 ≤ λs, μm ≤ 1.65

Operating pump wavelength

λp 1.2 ≤ λp, μm ≤ 1.28

Input signal wavelength

PSo Pso=0.4 mW

Input pump power

Ppo Ppo =0.2:1 W

Forward pump ratio

rf 0.5

Signal attenuation αs αs = 0.2 dB/km Spectral linewidth of optical source

∆λ 0.1 nm

Transmission distance

z 0 ≤ z, km ≤ 100

Types of fiber cable media Freelight SMF Truewave-RS

Pump attenuation (dB/km)

αp 0.260 0.263 0.256

gR (1/W.km) gR 0.54 0.42 0.69 Then the set of the series of the following figures are shown below as the following results can be obtained: Fig. (1) has clarified that in the case of forward direction as the distance z increases, the pumping power decreases exponentially, In case of backward direction as distance z increases, the pumping power increases exponentially. As well as in case of bi-directional: For z ≤ 50km, the pumping power decreases exponentially, and for z > 50 km, the pumping power increases exponentially. As displayed in the series of figs. (2-6) have assured that without any amplification with increasing the transmission distance, z, the output signal power decreases exponentially. While in case of forward directional for certain value of initial pumping power: Initial pumping power = 0.5 W, for distance 1 < z, km ≤ 35, the output signal power increases, for 35 < z, km ≤ 100 the output signal power decreases, and in backward directional for distance 1 < z, km ≤ 75 km, the output signal power decreases, for 75 < z, km ≤ 100 the output signal power increases. The same like at: Initial pumping power = 0.7 W, for distance 1 < z, km ≤ 40 km, the output signal power increases, for 40 < z, km ≤ 100 the output signal power decreases, and in backward directional for distance 1 < z, km ≤ 80 km, the output signal power decreases, for 80 < z, km ≤ 100 the output signal power increases. After using different media of optical fiber cable, it is indicated that the true wave reach fiber presented the best results in comparison with other fiber media. Fig. (7) has demonstrated that the variation of gain with pump power for different fiber lengths at a constant signal input power. The results showed that forward and backward pumping have the same result. The gain of the FRA linearly increases with pump power. As a result; the gain coefficient in dB/W reduces for high pump powers. In addition, a higher gain can be obtained at a longer Raman fiber with sufficient pumping. The Truewave fiber has a higher gain than the two other fiber types. Figs. (8-10) have assured that in the backward directional the net gain decreases with the fiber length until it reaches a certain level depending on the pump power and then increases until it intersects with axis (approximately reaches zero). It is clear that, in all cases, the gain increases with the pump power. Figs. (11-13) show that in the forward directional the net gain increases with the fiber length until it reaches a certain level depending on the pump power and then decreases until it intersects with axis (approximately reaches zero). It is clear that, in all cases, the gain increases with the pump power.


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Fig. 1. Pump power in different configurations in relation to transmission distance in forward, backward and bi-

directional at the assumed set of the operating parameters.

Fig. 2. The signal power in relation to transmission distance without pump power at the assumed set of the operating

parameters.

Fig. 3. The signal power in relation to transmission distance with input pump power Pp=0.5 W in SMF, Freelight and

true wave reach fiber in forward directional at the assumed set of the operating parameters.


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true wave reach fiber in forward directional at the assumed set of the operating parameters.


true wave reach fiber in backward directional at the assumed set of the operating parameters.


true wave reach fiber in backward directional at the assumed set of the operating parameters.


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Fig. 7. The Raman gain in relation to pump power SMF, Freelight and true wave reach fiber at the assumed set of the

operating parameters.

Fig. 8. The Raman net gain in relation to transmission distance for SMF fiber at different pump powers in backward


Fig. 9. The Raman net gain in relation to transmission distance for Freelight fiber at different pump powers in

backward directional at the assumed set of the operating parameters.


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Fig. 10. The Raman net gain in relation to transmission distance for true wave reach fiber at different pump powers in

backward directional at the assumed set of the operating parameters.

Fig. 11. The Raman net gain in relation to transmission distance for SMF fiber at different pump powers in forward


Fig. 12. The Raman net gain in relation to transmission distance for Freelight fiber at different pump powers in forward



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Fig. 13. The Raman net gain in relation to transmission distance for true wave reach fiber at different pump powers in

forward directional at the assumed set of the operating parameters.

5. CONCLUSIONS In a summary, we have deeply investigated fiber Raman amplifiers in different pumping configurations (forward, backward, and bi-directional). It is theoretically found that the dramatic effects on output signal power with increasing transmission distance in the absence of pumping power for different transmission medium such as SMF, Freelight and Truewave reach fibers. It is indicated that the increased pumping power, this leads to the increased Raman gain and therefore this results in the increased output signal power in different pumping configurations. It is verified with forward pumping configuration, that higher amplification ratio than backward direction configuration.

REFERENCES [1] J. Nagel, V. Temyanko, J. Dobler, E. Dianov, A. Sysoliatin, A.

Biriukov, R. Norwood, and N. Peyghambarian, “High Power, Narrow Linewidth Continuous Wave Raman Amplifier at 1.27

μm,” IEEE Photonics Technology Letters, Vol. 23, No. 9, pp. 453-466,

2011. [2] M. E. Dobbs, J. Dobler, M. Braun, D. McGregor, J. Overbeck, B.

Moore, E. Browell and T. S. Zaccheo, “A Modulated CW Fiber Laser-Lidar Suite for the ASCENDS Mission,” Proc. 24th International Laser Radar Conference, June 2008.

[3] Ahmed Nabih Zaki Rashed, Abd El-Naser A. Mohammed, Mohamed M. E. El-Halawany, and Mohamoud M. Eid “Optical Add Drop Multiplexers with UW-DWDM Technique in Metro Optical Access Communication Networks,” Nonlinear Optics and Quantum Optics, Vol. 44, No. 1, pp. 25–39, 2012.

[4] Ahmed Nabih Zaki Rashed, Abd El-Naser A. Mohammed, Mohamed M. E. El-Halawany, and Mohammed S. F. Tabour “High Transmission Performance of Radio over Fiber Systems over Traditional Optical Fiber Communication Systems Using Different Coding Formats for Long Haul Applications,” Nonlinear Optics and Quantum Optics, Vol. 44, No. 1, pp. 41–63, 2012.

[5] Ch. Headley, G. Agrawal, “Raman Amplification in Fiber Optical Communication Systems”, Elsevier, 2009.

[6] M. Islam, “Raman Amplifiers for Telecommunications and Physical Principles”, Springer, 2004.

[7] L. Binh, T. Lhuynh, S. Sargent, A. Kirpalani, “Fiber Raman Amplification in Ultra-high Speed Ultra-long Haul Transmission: Gain Profile, Noises and Transmission Performance”, Technical Report MECSE-1-2007, CTIE, Monash

University, 2007. [8] H. B. Sharma1,T. Gulati, and B. Rawat, “Evaluation of Optical

Amplifiers,” International Journal of Engineering Research and Applications (IJERA), Vol. 2, No. 1, pp. pp.663-667, 2012.

[9] Q. Hen, J. Ning, H. Zhang, and Z. Chen, “Novel Shooting Algorithm for Highly Efficient Analysis of Fiber Raman Amplifiers,” IEEE J. Lightwave Technol., Vol. 24, No. 4, pp. 1946-1952, 2006.

[10] Ahmed Nabih Zaki Rashed, Abd El-Naser A. Mohammed, Abd El-Fattah Saad, and Hazem Hageen “Low Performance Characteristics of Optical Laser Diode Sources Based on NRZ Coding Formats under Thermal Irradiated Environments,” International Journal of Computer Science and Telecommunications (IJCST), Vol. 2, No. 2, pp. 20-30, 2011.

[11] M. N. Islam, “Raman Amplifiers for Telecommunications,” IEEE J. of Select. Topics in Quantum Electron., Vol. 8, No. 3, pp. 548–559, 2008.

[12] A. Galtarossa, L. Palmieri, M. Santagiustina, and L. Ursini, “Polarized Backward Raman Amplification in Randomly Birefringent Fibers,” J. Lightwave Technol., Vol. 24, No. 3, pp. 4055–4063, 2009.

[13] Ahmed Nabih Zaki Rashed, Abd El Naser A. Mohammed, Osama S. Fragallah, and Mohamed El-Abyad, “New Trends of Multiplexing Techniques Based Submarine Optical Transmission Links for High Transmission Capacity Computing Network Systems,” Canadian Journal on Science and Engineering Mathematics, Vol. 3, No. 3, pp. 112-126, 2012.

[14] X. Liu, J. Chen, C. Lu, and X. Zhou, “Optimizing Gain Profile and Noise Performance for Distributed Fiber Raman Amplifiers,” Opt. Express, Vol. 12, No. 24, pp. 6053–6066, 2011.

[15] G. P. Agrawal, Fiber Optical Communication Systems, New York, John Wiley and Sons, 2005.

[16] Ming-Seng Kao and Jingshown WU, “Signal Light Amplification by Stimulated Raman Scattering in an N-Channel WDM Optical Fiber Communication System,” J. Lightwave Technol., vol. 7, no. 9, pp. 1290-1299, 1989.

[17] N. M. Anwar, and M. H. Aly, “Backward Pumped Distributed Fiber Raman Amplifiers,” Academy of Scientific Research and Technology 27th National Radio Science Conference Faculty of Electronic Engineering, Menoufia Univ., Menouf, Egypt 16-18 March 2010.


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Dr. Ahmed Nabih Zaki Rashed was born in

Menouf city, Menoufia State, Egypt country in 23 July, 1976. Received the B.Sc., M.Sc., and Ph.D. scientific degrees in the Electronics and Electrical Communications Engineering Department from Faculty of Electronic Engineering, Menoufia University in 1999, 2005, and 2010 respectively. Currently, his job carrier is a scientific lecturer in Electronics and Electrical Communications Engineering Department, Faculty of Electronic Engineering, Menoufia university, Menouf 32951.

His scientific master science thesis has focused on polymer fibers in optical access communication systems. He has published more than 100 scientific published papers in impacted international journals. Moreover his scientific Ph. D. thesis has focused on recent applications in linear or nonlinear passive or active in optical networks. His interesting research mainly focuses on transmission capacity, a data rate product and long transmission distances of passive and active optical communication networks, wireless communication, radio over fiber communication systems, and optical network security and management. He has published many high scientific research papers in high quality and technical international journals in the field of advanced communication systems, optoelectronic devices, and passive optical access communication networks. His areas of interest and experience in optical communication systems, advanced optical communication networks, wireless optical access networks, analog communication systems, optical filters and Sensors. As well as he is editorial board member in high academic scientific International research Journals. Moreover he is a reviewer member in high impact scientific research international journals in the field of electronics, electrical communication systems, optoelectronics, information technology and advanced optical communication systems and networks. His personal electronic mail ID (E-mail:[email protected]). He has supervised four PhD students and three MSc. students successfully and four PhD students and Six MSc. students are currently pursuing their research under guidance. His published paper under the title "High reliability optical interconnections for short range applications in high performance optical communication systems" in Optics and Laser Technology, Elsevier Publisher has achieved most popular download articles in 2013.

all optical signal processing of optical fiber raman amplifiers in advanced photonic communications...

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