A Novel Method for Analyzing the Performance of Free Space Optical Communication in WDM Using EDFA

This paper intends to design an artificial model of WDM Optical Network in respect of length, pump power and simple free space optical link which is designed and analysis of several FSO (Free-space optical communication) factors such as Bit Error Rate (BER), Q factor and received power have been measured with regard to the variants in the beam divergence and attenuation. The Simulation process of the System has been done using Optic simulation software to achieve gain enhancement and decrease noise factor of Erbium Doped Fiber Amplifier (EDFA) through optimized fiber length and pump power. This work focused to construct artificial design and analyses of 16 channels WDM system using EDFA and the influence of irregular atmospheric Motions on the link performance has been inspected by fluctuating beam divergence, atmospheric disturbances and modulation format at the bit rate of 10 Gb/s. As slicing in WDM schemes, in the transmitter channel with the frequency of 1558 nm and chip spacing 0.4 nm, power value is equal to + 23.5dBm, whereas the optical power meter plus can be used to predict the output power in the form of NRZ modulation, while the input power is produced with a help of CW laser, the gain level has been enriched from 1564 to 1558 nm wavelength band with bit rate of 10GbpS.This system is prepared by using Optic simulation software version 7.


Introduction
Optical fiber communication is unquestionably a very high capability, trustworthy and protected system for current and upcoming communication, which essentials WDM technique has been made the transmission bit rate to develop exponentially since the arrival of EDFA in1980s. EDFAs are desired as an optical amplifier in multichannel WDM scheme for their 1 3 unique Characteristics such as wide gain bandwidth, capable of simultaneously amplifying a large number of channels exclusively in C-band frequency (1530 to 1570 nm). The ancient method which are unrelatedly to the aspect of sub-carrier frequency on the multiplexing signal, which may cause to signal power leakage. In order to overcome that, three different carriers multiplexing has been considered for reducing the number of optimization equations approximately by 66% [1]. This paper pronounces an optical code-division multiple access (OCDMA) with the help of buffering structure by which each packet has been encrypted with an optical signature code to diminish the packet loss. In the absence of an extra encoder spectral Amplitude Coding signal and its complementary counterpart may be produced simultaneously [2].
EDFA contains Er + 3 ions doped optical fiber which is acted as a gain medium to amplify an optical signal when pump signal either at 980 nm or 1480 nm is used for excitation of Er + 3 ions. The optical signal will be enhanced correspondingly a pump laser are multiplexed into the doped fibre, and with the help of doping ions the signal gets amplified over the Transmission. EDFA is the prominent optical amplifier suitable to small loss optical window utilized in silicon-based fiber. This paper accomplishes enhancement of gain and diminished the EDFA noise power by varying the doped fibre length and the pump power with an applied input power of + 23.5dBm. EDFA of diverse Lengths (10m, 50m and 120m) with variable Gain and retained noise figure (NF)shall be done with the help of single pumping includes the wavelength of 980nm or 1480nm and the results are drawn between the effect of various Pump power and FSO communication link with the analysis has been initiated by various atmospheric weather conditions for a bit rate of 10Gb/s using NRZ and RZ technique [3][4][5][6][7].
The link performance has been investigated for the impact of atmospheric turbulence by fluctuating beam divergence, atmospheric disturbances and modulation format [8][9][10]. This paper discussed about the attenuation triggered during the rain on free space optical(FSO) links, which has been predicted by developed model interacted between the simulation of FSO links (path length up to 5 km) and precipitation maps are utilized to create the analytical equations [11]. Today's internet facility involves huge bandwidth so EDFAs are frequently operated with WDM technology to accomplish huge bandwidth and which is able to provide worthy services in terms of quality to users with low cost [12][13][14][15][16][17]. Also, once optical signal has been propagated over long distances which is required to utilize an optical amplifier due to the signals are mitigated at long distances propagation. EDFA is desired the reason behind that the low noise enclosure and great gain. Because of the wideranging bandwidth EDFA'S are very much consistent for long distance propagation with the help of single and multi-wavelength sources. It provides amplification of multichannel in absence of crosstalk and cost effective networks [18,19].
The impact of atmospheric conditions in the wireless optic systems gets varied in important parameters such as gain, bit error rate has been analyzed and rectified. Even though the noise of the wireless optic system has been increased as the gain increases [20][21][22][23][24]. This paper proposed that, the importance of Software Defined Data Center Networks (SDDCNs) to provide continuous services for clients for obtaining high reliability. Also it represents that the network recovery method during the failure occurs in data transmission [25]. WDM networks are used to splits their huge bandwidth into an optical channel, which allows several data stream to be transmitted along with a same fiber at the same time. The key element of the transmitter is an optical source that embrace Light Emitting Diodes (LED) or Semiconductor Laser Diode (SLD). Often the laser sources are acted as an optical source in optical Communication stream with respect to maximum efficiency, compressed size, decent reliability, slight spectrum and lesser emissive area well-suited with fiber core dimensions. Each channel allocated with a different wavelength at transmitter side these are multiplexed by using multiplexer onto a single fiber. WDM Technology which is able to provide so many benefits such as Upgrading Capacity, High level Transparency, Reuse in Wavelength, More Scalability, High Reliability etc. [26][27][28][29][30][31].
This paper focused on the impact of variation in EDFA's pump power and fiber length using 16 channels WDM system with the wavelength range lies in between 1558 to 1570 nm and channel separation of 0.8 nm which is designed and analyzed for data rate of 10 Gb/s. Performance of the proposed system has been analyzed by various atmospheric conditions in the aspects of gain, bit error rate, Q factor.

Experimental Details
The Optisystem software is used to construct and analyze the EDFA which is suitable for WDM system. The Developed system consists of 16 input signals (channels), an ultimate multiplexer, a pump laser and EDFA. The 16 equalized wavelength multiplexed signals are applied as the input of the system with wavelength region of 1558-1570 nm, the channels are spaced by 0.8 nm. Each channel has its power of + 23.5dBm. The pumping process occurs at 980 nm which is used to excite the doped atoms to a higher energy level and analyzed for data rate of 10gb/s. If frequency increases from (1558 to 1570 nm), gain increases and the noise is decreased. The values are attained through WDM analyzer when the input power is + 23.5dBm and the pump wavelength is about 980 nm.
The gain level enhancement of EDFA is acquired via through EDFA'S pump power, input power and the output power are increased as the pump power increases. The output of the signal will be normally amplified in the lowest loss with value of 1550 nm.The design of EDFA in the WDM system has been made through Optisystem simulation software. The system consists of 16 input signals (channels), an ideal multiplexer, a pump laser, erbium doped fibre as shown in Fig. 1 NRZ pulse generator has an advantage on controlling bandwidth. It occurs due to the characteristic of the generator that the returning signals to zero between the bits. The data signals are scrambled in terms of bit rates by Pseudo-random bit sequence generator. Mach Zehnder Modulator (MZ) comprised two inputs in terms of optical signal and electrical signal and one output which is related to optical signal. After that the input signal will be modulated with the help of semiconductor laser that can be represented by Continuous Wave (CW) laser Frequency 193.1THz, through Mach-zehnder modulator. Continuous laser diode (CW) to generate optical signals supplies input signal with 1550 nm wavelength and input power of + 23.5dBm which is externally modulated at 10Gbits/s, in a Mach-Zehnder modulator with 30 dB of extinction ratio will be applied with a non-returnzero (NRZ) pseudorandom binary sequence. The optical fiber is used as a single mode fiber because which provides a high data rate while transmitting over a long distance. In order to operate as the optical transmission system: Input power + 23.5dBm, Reference wavelength 1550 nm, fiber length is varied from 5 m, 6 m, to 7 m and the pump power is varied from 200 mW, 300 mW to 400 mW. This system consists of WDM transmitter, dual port WDM analyzer, EDFA, pump laser, optical spectrum analyzer are the key components used in the wavelength range between 1558 and 1570 nm with 0.8 nm channel spacing.
WDM is low noise, high saturation output power even though any data channels operated at variety of wavelengths within the gain region gets amplified with the help of single EDFA. These features provide a small module with low power consumption which is expressed in terms of few Watts. The good reliable performance can be achieved through this combination. Frequency in WDM (1530 to1570), this range gives a broad gain spectrum EDFA has L and C band. In this simulation C band is chosen because it has a quantum conversion efficiency i.e. direct measure of efficiency by photons transfer from pump power to signal power. EDFA is the most extensively used one in WDM system. It customs the EDFA as an optical amplification medium which is used to directly enhance the signals. EDFA is commonly used to compensate for fibre loss in long-haul optical communication, in which several optical channels shall be propagated simultaneously at different wavelengths on a single optical fiber.
Optical network constructed by WDM is commonly used to commonly used in telecommunication infrastructures to increase the rate of speed of data. WDM techniques combined with EDFA to enhance the capability of light waves propagation, which delivers high capability and enhanced flexibility of optical network technology. EDFA plays a vital and prized role in optical communication systems which is mentioned in Table.1. The EDFA components are used as a booster and amplifier. As an optical amplifier utilized in this formation which does not suffer from losses and attenuation. By this, which is possible to provide gain enhancement with a large range of dynamic gain, decreased noise, increased saturation output power. This combination provides trustworthy performance.
Frequency in WDM (1530 to 1570), this range gives a broad gain spectrum. EDFA has L and C band. In this simulation C band is chosen because it has a quantum  Fig. 2. The generated pseudo-random bit sequence (PRBS) signals are divided into two equal parts, encoding process will be through two modulation techniques using NRZ and RZ. CW laser acts as the main source, a mach-zehnder modulator (MZM) is used to modulate the data, Multiplexing and Demultiplexing systems can be used for optimization purpose. The optical signal generated by the transmitter that can be propagated over a FSO link which has different turbulence characteristics. An Optical amplifier is used to amplify the desired signals, especially suited in a long-haul system. Thereafter, the optical signal can be received with the help of an Avalanche photodiode and followed by low pass Bessel filter, based on the cut-off frequency of 0.75 * bit rate. The data is analyzed through the BER analyzer at 10 km link range and the beam divergence is varied from 0.3, 0.6, 0.9, 1.2, 2, 3 and then variations under climatic conditions they are clear air, ideal, light mist, very light mist is performed and analysis the Quality factor and BER and the received power. FSO technology is also irrespective to electromagnetic interference and has low power consumption if it is maintained properly. Important applications of free space optical technologies are Wireless outdoor access, Storage Area Networks, defence based works, point to point communication, multipoint communication, ship to ship communication, undersea communication and space communications. The beam of light from the optical source is the only change occurs that is sent through the atmosphere instead of sending it through the fiber optic cable. This process indicates that the major difference between the Free Space Optical Communication and Optical Fiber Communication. Data is permitted to propagate from the optical transmitter to the optical receiver once the modulation has been done. With the impact of non-ideal weather conditions, FSO is suffered from the atmospheric attenuations always FSO works in the lower part of troposphere and degradation occurs.

Result and Discussion
The simulation layout is shown in Fig. 3 consist of WDM transmitter system is connected to ideal mux then the output of the mux is fed into the EDFA and the output is measured in optical power meter and optical spectrum analyzer and the gain enhancement is displayed in dual port analyzer. Table 2.
In this design Table 3 the wavelength value starting from 1558.0 nm and slicing into 16channels every chip slicing is 0.8 nm. so, the distributed is illustrated as the following Table 3If frequency increases, gain increases and noise decreases in the above-mentioned table Output Power at + 23.5dBmWhich is shown in Fig. 4.The input power is changing from + 23.5 to + 25.5, the gain and noise figure is measured in the Table 3, Output signal and noise power variation at + 23.5dBm which is shown in Fig. 5. Results shows that the gain, noise and output power is measured by varying the pump power and length of the  Table 2 Gains and NF at pump power 20dBm & input power + 23.5dBm 1 3 fiber with the same input power of + 23.3dBm using optimized erbium doped fiber of 5 m length. The parameters evaluation in terms of max gain and max power are also made considering three pump powers at 200mW, 300mW&400mW and length of the fiber is varied from 6 m,7 m and 8 m at + 23.3dBm and the power at 200mW and both.length and pump   power is varied at + 23.3dBm. Figure 6 shows the Output Power at + 25.5dBm and Fig. 7 represents the Output signal and noise power variation at + 25.5dBm In first case, pump power is varied from 200 to 400mW with the same input signal power at 23.3dBm and the fiber length is about 5 m which is mentioned by Table 4. In another case Table 5, fiber length is varied from 6 to 8 m with the same input power at + 23.3dBm and the pump power is fixed about 200mW and then both pump power and length of the fiber is varied with fixed input power at + 23.3dBm in Table 6. In these all cases, gain is enhanced and the noise is decreases and the output power is also   (i) Beam divergence at 0.3mrad Figure 8 shows that the NRZ outputs for diverse light parameters. In case (i) beam divergence is considered as one of the parameter with a value of 0.3mrad, as a result of this NRZ outputs are measured such as BER, Quality factor and Power.
• BER = 1.82e-027 • Q-factor = 10.75 • Power = -16.1dBm • Beam divergence at 0.6 mrad  Figure 9 shows that the NRZ output with the modified beam divergence (Case ii) value of 0.6mrad. As a result of this, the NRZ outputs gets varied like BER, Quality factor and Power level of the FSO with respect to the beam divergence.
• BER = 8.67e-027 • Q-factor = 10.61 • Power = − 22.1dBm • Beam divergence at 0.9mrad Figure 10 shows that the NRZ output with the modified beam divergence (Case iii) value of 0.9mrad. As a result of this, the NRZ outputs gets varied like BER, Quality factor and Power level of the FSO with respect to the beam divergence.
• BER = 1.65e-023 • Q-factor = 9.88 • Power = − 25.5dBm • Beam divergence at 1.2mrad  Figure 11 shows that the NRZ output with the modified beam divergence (Case iv) value of 1.2mrad. As a result of this, the NRZ outputs gets varied like BER, Quality factor and Power level of the FSO with respect to the beam divergence.
• BER = 3.95e-020 • Q-factor = 9.07 • Power = − 28.0dBm • Beam Divergence at 2mrad Figure 12 shows that the NRZ output with the modified beam divergence (Case v) value of 2mrad. As a result of this, the NRZ outputs gets varied like BER, Quality factor and Power level of the FSO with respect to the beam divergence.
• BER = 1.67e-012 • Q-factor = 6.92 • Power = − 32.4dBm • Beam Divergence at 3mrad Figure 13 shows that the NRZ output with the modified beam divergence (Case vi) value of 2mrad. As a result of this, the NRZ outputs gets varied like BER, Quality factor and Power level of the FSO with respect to the beam divergence.

RZ Outputs
(i) Beam Divergence at 0.3mrad Figure 14 shows that the RZ output with the modified beam divergence (Case i) value of 0.3mrad. As a result of this, the RZ outputs gets varied like BER, Quality factor and Power level of the FSO with respect to the beam divergence.
• BER = 1.09e-17 • Q-factor = 27.80 • Power = − 22.9dBm • Beam Divergence at 0.6mrad Figure 15 shows that the RZ output with the modified beam divergence (Case ii) value of 0.6mrad. As a result of this, the RZ outputs gets varied like BER, Quality factor and Power level of the FSO with respect to the beam divergence.
• BER = 2.20e-060 • Q-factor = 16.32 • Power = − 28.9dBm • Beam divergence at 0.9mrad Fig. 13 Beam divergence at 3mrad Figure 16 shows that the RZ output with the modified beam divergence (Case iii) value of 0.9mrad. As a result of this, the RZ outputs gets varied like BER, Quality factor and Power level of the FSO with respect to the beam divergence.
• BER = 5.19e-028 • Q-factor = 10.88 • Power = − 32.4dBm • Beam Divergence at 1.2 mrad Figure 17 shows that the RZ output with the modified beam divergence (Case iv) value of 1.2mrad. As a result of this, the RZ outputs gets varied like BER, Quality factor and Power level of the FSO with respect to the beam divergence.
• BER = 9.13e-016 • Q-factor = 7.92 • Power = − 34.8dBm • Beam Divergence at 2mrad Figure 18 shows that the RZ output with the modified beam divergence (Case v) value of 2mrad. As a result of this, the RZ outputs gets varied like BER, Quality factor and Power level of the FSO with respect to the beam divergence.
• BER = 1.67e-005 • Q-factor = 4.11 • Power = − 39.3dBm • Beam Divergence at 3mrad Figure 19 shows that the RZ output with the modified beam divergence (Case vi) value of 2mrad. As a result of this, the RZ outputs gets varied like BER, Quality factor and Power level of the FSO with respect to the beam divergence.
• BER = 0.0143  With increase in beam divergence the beam directivity is reduced so that bit error rate increases and decrease in Quality factor and received power. From the above simulation results, concluded that when beam divergence is increases from 0.3 to 3mrad, the Quality factor decreases and the bit error rate increases and the received power is also decreases. If the bit error rate increases, there is a closure of eye diagram. With increase in beam divergence the beam directivity is reduced, so that the Bit Error rate increases and there is decrease in the Quality factor and the received power. Range = 10 km. Transmitter aperture diameter = 15 cm. Receiver aperture diameter = 20 cm. Data rate = 10 Gb/s. Weather condition outputs (NRZ).  Figure 20 shows that the NRZ output with the (Case i) beam divergence at perfect atmospheric conditions. As a result of this, the NRZ outputs gets varied like BER, Quality factor and Power level of the FSO with respect to the beam divergence.
• BER = 6.54e-032 • Q-factor = 11.66 • Power = 15.65dBm • Clear Air (0.6) Figure 21 shows that the NRZ output for the (Case ii) beam divergence at clear air with a value of 0.6. As a result of this, the NRZ outputs gets varied like BER, Quality factor and Power level of the FSO with respect to the beam divergence.
• BER = 7.30e-032 • Q-factor = 11.65   Figure 23 shows that the NRZ output for the (Case iv) beam divergence at light mist with a value of 4.6. As a result of this, the NRZ outputs gets varied like BER, Quality factor and Power level of the FSO with respect to the beam divergence.

Weather condition outputs (RZ)
(i) Ideal (0) Figure 24 shows that the RZ output with the (Case i) beam divergence at perfect atmospheric conditions. As a result of this, the RZ outputs gets varied like BER, Quality factor and Power level of the FSO with respect to the beam divergence.  Figure 27 shows that the RZ output for the (Case iv) beam divergence at light mist with a value of 4.6. As a result of this, the RZ outputs gets varied like BER, Quality factor and Power level of the FSO with respect to the beam divergence.
• BER = 7.83e-013 • Q-factor = 7.03 • Power = − 35.75dBm Due to the effect of climatic weather condition from ideal to light mist there is an increment took part in bit error rate and decrement in Quality factor, received power as well. The obtained simulation results represent weather Conditions outputs for NRZ and RZ which shows that the analysis of Various parameters such as BER, Q-Factor and power. For strong turbulence, NRZ format has a strong component compared with RZ. This is the cause that NRZ format is preferred over RZ format for strong turbulence. For air and very light mist turbulence, RZ format is preferred over NRZ format. Because RZ format shows minimum BER and then Quality factor is increased and the power is also increasing. so, for air and very light mist RZ format is suitable when compared to NRZ format. For light mist weather condition NRZ format gives the minimum BER when compared to RZ format. Because it gives good Quality factor and minimum BER.
In the Existing methods, the comparison proposed between the range of wavelength and the WDM-FSO between amplifier-assisted and relay-assisted system [32][33][34][35]. In this proposed work, the comparison between RZ and NRZ with various beam divergence at diverse environment condition in the aspect of air with mist is shown in the Table 7. In the perspective of ideal and clear air condition, the low BER rate has been occurred during the process of RZ data and high BER has been occurred at the process of NRZ data.

Conclusions
A 16 channels WDM system using EDFA in the wavelength range of 1558-1570 nm with channel spacing of 0.8 nm is designed and analyzed for data rate of 10 Gb/s. The gainenhancement of EDFA in such system is achieved through EDFA's pump power and fiber length variation. Results shows that the gains are enhanced at pump power of 400mW and input power of + 23.3dBm using optimized erbium doped fiber of 8 m length. The performance evaluation in terms of max gain and max power are also made considering three pump powers at 200, 300 &400mW. The simulation results show satisfactory system parameters to work with for a 16 channel WDM system. The performance analysis can be extended using more channels and higher bit rate cases of practical importance. Channel interference issues can also be addressed in future and This paper on FSO based WDM of 8 channel design is proposed, in which a signal  reception is examined to produce most efficient and error free output. A network is designed in which the input signal of 193.1 THz is sent from a transmitter to the receiver.
Optical amplifier is used for the amplification purpose of the design. The BER and Q factor of the input signal were analyzed once the signals are received by the receiver. The efficient result was achieved at 10 Gbps bit rate with 193.1 THz frequency. Free space optical link performance is analyzed by taking BER and Quality factor as a performance metric. WDM has been employed successfully and analyzed with the help of this simulation. The designed parameters are varied and simulated to analyze the impact of beam divergenceand atmospheric weather conditions on the BER performance. It has been observed that NRZ modulation gave us better performance in comparison to RZ modulation under light mist weather condition and RZ modulation gave us better performance under air and very light mist condition. When the atmospheric weather condition is increased, there is a distortion in the BER analyzer. As future work a greater number of channels could be employed. This system Permitsmultiple data stream to be Communicated along with a similar fibre at same time.

Future Work
The Simulation model of this system will be updated using recent technologies and also the parameters analyzed in the system will be enhanced such as SNR, PSNR and Gain. With the help of pseudo noise (PN) sequence the signal shall be propagated through the existing work.
Author Contribution All the authors contribute equally for the preparation of the manuscript.
Funding There was no financial support received from any organization for carrying out this work.

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.

Rajyalakshmi Kottapalli Assistant Professor of Mathematics, Koneru
Lakshmaiah Education Foundation, Guntur, A.P. has been working in the current position for the last seven years, total being13 years of teaching experience. She did her Post graduation and Doctor of Philosophy in Statistics from the prestigious and one of the State Universities in India, Acharya Nagarjuna University, Guntur, A.P. She published more than 50 research articles in well reputed and refereed international journals like Elsevier, Taylor& Francis, Emerald, AST international etc., indexed in SCI, Scopus, ESCI and web of science with good impact factor. She presented several research articles in in national and international conferences and also serving as the reviewer. To her credit, she has filed and published 4 Indian patents in IPR website. Her areas of interests include probability and statistics, Design of experiments and machine learning techniques.