IJRREST: International Journal of Research Review in Engineering Science and Technology (ISSN 2278-6643) Volume-2 Issue-1, March 13 Forward Pumping Based Fiber Optical Raman Amplifiers in Different Optical Fiber Transmission Medium Systems *Ahmed Nabih Zaki Rashed *Electronics and Electrical Communications Engineering Department, Faculty of Electronic Engineering, Menoufia University, Menouf, EGYPT, E-mail: ahmed_733@yahoo.com Abstract This paper has presented the transmission systems with employing Raman amplifier technology in forward pumping directions in order to have put up with much higher level of design complexities, when compared to conventional transmission lines with doped optical amplifier. Even for the construction of a fundamental, basic building block a unit of a 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 loss/noise figure profiles. Optimizing optical signal to noise ratio (OSNR) and designing ultralong haul links with best signal quality factor performances and minimum bit error rates, while adjusting variables in the length, Rayleigh penalty, pump noise, nonlinear penalty, dispersion and gain distribution is a problem which can be easily stated, but in reality is not a process which can be easily achieved. Keywords: Optical signal processing, Performance signature, Raman Amplifiers, and Photonic Communications Engineering. 1. INTRODUCTION Wavelength division multiplexing (WDM) is basically frequency division multiplexing in the optical frequency domain, where on a single optical 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 1 P a g e technology that puts data from different sources together on an optical, 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 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 []. 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 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 -optic links for optical communications, the optical power level has to be raised between long sections of before the information is lost in the noise. The combination of an erbium-doped amplifier (EDFA) and a Raman amplifier (FRA or RA) 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
IJRREST: International Journal of Research Review in Engineering Science and Technology (ISSN 2278-6643) Volume-2 Issue-1, March 13 the. Distributed amplification uses the transmission 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 amplifiers (HFAs) are an advance technology for future. Hybrid Raman/erbium doped amplifiers are designed to maximize the long-haul transmission distance [8]. 2. MODEL AND EQUATIONS ANALYSIS The evolution of the input signal power (P s ) and the input pump power (P p ) propagating along the single mode optical 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]: dpp s LpPp gre ff Ps Pp (1) dz p dps s LsPs gre ff Ps Pp (2) dz 16 P a g e p Where λ s and λ p are the signal and pump wavelengths in µm respectively, z is the distance in km from z= to z=l, Ls and Lp are the linear attenuation coefficient of the signal and pump power in the optical in km -1 respectively. 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 []: PPF z P z pof exp Lp (3) where P PoF is the input pump power in the forward direction in watt at z=. If the values of P P are substituted in differential Eq. 2, and is integrated from z= to z=l for the signal power in the forward, then the result mathematical equation can be written as mentioned in []: P S z P so gr exp P po Lsz (4) Aeff where P so and P po denotes to the input signal and pump power respectively. This means that P po = P pof in case of forward pump and P po =P pob in case of backward pump, and L eff, is the effective length in km, over which the nonlinearities still holds or stimulated Raman scattering (SRS) occurs in the and is defined as [11]: 1exp Lp Lp z () Recently, there have been many efforts to utilize Raman amplifier (FRA) in long-distance, high capacity WDM systems. The net gain [12] 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: PS Gnet, (6) PS () The intensity of the stimulated scattered light grows exponentially once the incident pump power exceeds a certain threshold value. The threshold pump power P th is defined as the incident power at which half of the pump power is transferred to the Stokes field at the output end of a of length L. The threshold pump power satisfies the condition [13]: Pth 16 g Re ff, (7) For standard silica cable, the transmitted signal bandwidth per transmitted channel can be given by [14]:.4848 BW. sig, (8) N z ch Where N ch is the number of transmitted channels, τ is the total pulse broadening after distance z which is given by [1]: D z, (9) Where D is the total dispersion coefficient in link media in ps/nm.km, and Δλ is the spectral linewidth of the optical source. This is mainly because FRA can improve the optical signal to noise ratio (OSNR) and reduce the impacts of nonlinearities [16], that is the OSNR of the system after amplification can be expressed as: OSNR db PS log, () 2 h c BW. Sig. Where h is the Planck's constant (6.2 x -34 J.sec),
IJRREST: International Journal of Research Review in Engineering Science and Technology (ISSN 2278-6643) Volume-2 Issue-1, March 13 P S (z) is the transmitted signal power after z distance, c is the speed of light (3x 8 m/sec), λ is the operating signal wavelength in μm, and B.W sig is the transmitted signal bandwidth. According to modified Shannon theorem, the maximum bit rate per optical channel for supported number of users, or the maximum capacity of the channel for maximum subscribers is given by [17]: B Sh 17 P a g e BW. log2 1 OSNR, (11) sig Based on MATLAB curve fitting program, the relationship between the signal quality factor (Q) with both number of transmitted channels (N ch ) and effective length L eff in km and transmitted signal power after distance z can be expressed as the following formula: 12.4 2.78 187. Q 28.6 P 2 2 3 3 S Nch Nch Nch L eff, db (12) Then the bit error rate (BER) can be expressed as a function of Q in the following formula [18]: 2 Q BER.exp. Q 8 3. RESULTS AND PERFORMANCE ANALYSIS, (13) The optical FRAs have been modeled and have been parametrically investigated in different cable medias such as true wave reach, non return to zero dispersion shifted (), and single mode () with employing different multiplexing techniques namely ultra wide wavelength division multiplexing (UW-WDM) based on the coupled differential equations of first order, and also based on the set of the assumed of affecting operating parameters on the system model. In fact, the employed software computed the variables under the following operating parameters as shown in Table 1. Operating parameter Operating signal wavelength Symbol λ s Value and unit 1.3 μm Operating pump wavelength Input signal wavelength Input pump power Forward pump ratio Signal attenuation Pump attenuation Spectral linewidth of optical source UW-WDM channels Transmission distance λ p P So P po 1.28 μm dbm 3 dbm r f. α s α p Δλ N ch(uw- WDM) Types of cable media.2 db/km.3 db/km.1 nm channels z z, km 4 reach Effective area A eff μm 2 72 μm 2 8 μm 2 Raman gain efficiency Dispersion coefficient g Reff.6 W -.4 W -.38 W - 1 km -1 1 km -1 1 km -1 D 2 ps/nm.km ps/nm.km 16 ps/nm.km Table 1. Proposed operating parameters for performance signature of Raman amplifiers [3,, 12, 18]. Then the set of the series of the following figures are shown below as the following can be obtained: (i) Fig. (1, 2) have assured that transmitted signal power and pump power decrease with increasing transmission distance. It is observed that true wave reach has presented transmitted signal and pump powers with compared other transmission mediums. (ii) Fig. (3, 4) have assured that signal gain and threshold pump power decrease with increasing transmission distance. It is observed that true wave reach has presented transmitted signal gain and threshold pump power with compared other transmission mediums. (iii) Fig. has indicated that transmitted signal bandwidth decreases with increasing transmission distance. It is theoretically found that single mode medium has presented the highest transmitted signal bandwidth with compared to other transmission
Signal gain, G db Optical signal to noise ratio, OSNR, db Pump power, Pp, dbm Transmitted signal bandwidth, BWsig., GHz Transmitted signal power, Ps, dbm Threshold pump power, Pth dbm IJRREST: International Journal of Research Review in Engineering Science and Technology (ISSN 2278-6643) Volume-2 Issue-1, March 13 mediums. 14 12 8 6 4 2 reach Fig. 1. Variations of transmitted signal power against variations of transmission distance at the assumed set of the 3 27. 2 22. 17. 1 12. 7. 2. reach Fig. 2. Variations of pump power against variations of transmission distance at the assumed set of the 4 3 3 2 1 reach T 4. 4 3. 3 2. 2 1. 1. reach Fig. 4. Threshold pump power in relation to 9 8 7 6 4 3 reach Fig.. Transmitted signal bandwidth in relation to 3 3 2 1 reach Fig. 3. Signal gain in relation to transmission distance at the assumed set of the operating parameters. Fig. 6. Optical signal to noise ratio in relation to 18 P a g e
Signal bit error rate, BERx -12 Signal transmission quality, Q, db Shannon transmission bit rate, BSh, Tb/s IJRREST: International Journal of Research Review in Engineering Science and Technology (ISSN 2278-6643) Volume-2 Issue-1, March 13 9 8 7 6 4 3 reach Fig. 7. Shannon transmission bit rate in relation to 4 3 3 2 1 reach Fig. 8. Signal transmission quality in relation to 9 8 7 6 4 3 2 1 reach Fig. 9. Signal bit error rate in relation to iv) Fig. 6 has indicated that optical signal to noise ratio increases with increasing transmission distance. It is theoretically found that true wave reach medium has presented the highest optical signal to noise ratio with compared to other transmission mediums. v) Fig. 7 has assured that Shannon transmission bit rate decreases with increasing transmission distance. It is observed that single mode has presented the highest transmitted signal bit rate with compared other transmission mediums. vi) Fig. 8 has indicated that signal transmission quality decreases with increasing transmission distance. It is theoretically found that true wave reach medium has presented the highest signal transmission quality with compared to other transmission mediums. vii) Fig. 9 has indicated that signal transmission bit rate increases with increasing transmission distance. It is theoretically found that true wave reach medium has presented the lowest signal transmission bit rate with compared to other transmission mediums. 4. CONCLUSIONS In a summary, the model has been investigated forward pumping based optical Raman amplifiers in different optical transmission medium systems over wide range of the affecting parameters. It is observed that transmitted signal power, pump power and its threshold value, signal gain, optical signal to noise ratio, transmitted signal bandwidth, signal transmission quality and transmission bit rates decrease with increasing transmission distance. As well as true wave reach has presented the highest systems transmission performance compared to other transmission mediums under the same operating of conditions.. REFERENCES [1] Abd El-Naser A. Mohammed, Mohamed M. E. El-Halawany, Ahmed Nabih Zaki Rashed, and Mohamoud M. Eid Optical Add Drop Multiplexers with UW-DWDM Technique in Metro Optical Access Communication 19 P a g e
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