Performance analysis of a hybrid optical ampli�er based 480-Gbps DWDM-FSO system under the effect of different atmospheric conditions

Free space optical (FSO) systems offer a desirable and affordable way of providing communication services in remote locations. They provide secure transmission without the requirement for licensing and with lower implementation costs. However, due to the impact of atmospheric turbulences on the effectiveness of FSO systems, their implementation faces signi�cant challenges. Dense wavelength division multiplexing (DWDM) has shown promise in addressing the growing bandwidth needs in optical networks. This paper introduces a hybrid optical ampli�er based system, combining DWDM and FSO technology. The proposed DWDM-FSO system operates at a rate of 480 Gbps and consists of 12 channels, each capable of transmitting data at 40 Gbps. The system’s performance is evaluated and compared by determining the Q-factor and bit error rate (BER) for both the cases when hybrid ampli�er is employed and with no hybrid ampli�er within the C-band, speci�cally focusing on wavelengths around 1550 nm. Moreover, the length of the FSO link is increased to assess the corresponding Q-factor and BER. Resultantly, the maximum distance for the FSO link is determined, ensuring that it remains within acceptable Q-factor and BER thresholds. Furthermore, the effectiveness of the FSO system is also evaluated and compared under different atmospheric conditions, considering both scenarios: when employing a hybrid ampli�er and when not utilizing one.


Introduction
A major revolution in communication services has been made possible by optical ber technology, which completely transformed the way huge amounts of data are effectively carried across very long distances [1].It serves as the backbone of modern telecommunications, allowing for the smooth transfer of data through large ber networks that span across wide geographic areas [2].Optical ampli ers play an essential role in this environment of optical transmissions by assuring continuous propagation of signals, which ensures reliable and effective communication [3].However, optical ber communication has some challenges related to point-to-point communication, cable faults, and physical connectivity costs [4].The adoption of free space optical communication (FSO) systems offer an appealing solution that overcomes these constraints and provides a number of bene ts, including enhanced exibility, reduced vulnerability, and lower infrastructure costs [5].
FSO is a state-of-the-art wireless optical communication system that uses light to transmit data through spaces, such as vacuum and air.FSO offers various advantages, including high bandwidth capacity, secure transmission, and immunity to electromagnetic interference [6].The capacity of FSO communication to transfer massive amounts of data over great distances without the requirement for licensing is one of its key advantages.This makes it an effective alternative in situations where traditional wired or radio frequency-based wireless communication may not be feasible or bene cial [7].FSO allows the transmission of data at signi cantly high speeds, reaching up to gigabits per second.Furthermore, remarkable developments in research have realized terabit per second speeds in certain environments [8].FSO links have a wide range of applications, including connecting aircraft, enabling satellite-to-satellite communication, establishing terrestrial connections, and serving the rigorous requirements of the military [9].FSO offers notable advantages in terms of maintenance and enhancement, when compared with other communication technologies.FSO systems often employ compact and easy-to-install devices, requiring minimal infrastructure [10].However, a direct line of sight between the transmitting and receiving equipment is one of the essential conditions for FSO to operate e ciently.This means that any obstacles, including buildings or trees, may interfere with the signal's ability to travel [11].Nevertheless, it is crucial to take into account how atmospheric conditions affect FSO systems.Conditions like fog, rain, or snow can degrade or disperse the optical signals, affecting the range and dependability of FSO links.These factors can considerably reduce the effectiveness of FSO systems during adverse weather conditions [12].
FSO technology has seen an enormous rise in interest from researchers as a result of the rapid expansion of wireless networks.Numerous research articles that highlight different FSO systems features and uses have been published in the literature [9].Hayal et al. [12] proposed a dense wavelength division multiplexing (DWDM) based FSO system operating in the presence of atmospheric turbulence.They proposed a hybrid modulation technique to address issues related to signal degradation and increased bit error rate (BER).The key performance metrics such as BER, signal-to-noise ratio (SNR), and system capacity are evaluated and the comparison is performed with conventional modulation techniques.Ambreen et al. [13] focused on the performance analysis of a FSO system working under chaotic conditions.The BER and channel capacity of the system are examined under different atmospheric conditions, including clear weather, fog, rain, and turbulence.They used chaotic modulation techniques while assessing the system's performance.Florence et al. [14] addressed the issue of signal degradation in FSO systems caused by rain attenuation, which can severely affect the transmission performance.They proposed a spectrum slicing wavelength division multiplexing (WDM) approach as a potential solution to lessen the negative effects of persistent rain on FSO communication.Fadil et al. [15] demonstrated a WDM-FSO system that utilizes optical chaotic signals for enhanced security.The system's performance is assessed while taking into account a number of factors, including BER, transmission distance, and SNR.Their ndings showed that the chaotic-based FSO system offers better security than conventional systems.Mehtab et al. [16] examined the performance of a hybrid transmission system that combines WDM, polarization-division multiplexing (PDM), and orthogonal frequency-division multiplexing (OFDM) techniques for FSO link.They realized a 1.6 Tbps transmission link and estimated its performance in terms of various parameters under different weather conditions.
Their ndings indicated that the hybrid WDM-PDM-OFDM-based FSO link realized high data rates with good signal quality and low BER.
Hybrid optical ampli er combines two or more different ampli cation technologies to obtain enhanced performance and exibility [17].Modern optical communication systems are nding hybrid optical ampli ers as a valuable asset.They can provide improved signal quality, wideband ampli cation, dynamic gain control, and network design exibility by combining the advantages of various ampli er technologies [18].Hybrid ampli ers have a substantial impact on the ability to transmit data e ciently and reliably over long distances due to their power e ciency and exibility with different network topologies.As optical networks continue to evolve, the signi cance of hybrid optical ampli ers is set to grow, enabling the development of advanced communication infrastructures [19].In this paper, a hybrid optical ampli er based 480-Gbps DWDM-FSO system is proposed and the performance is analyzed under different atmospheric conditions.The paper is organized in four sections.After introduction, the proposed system is described in Section 2, preceded by the results in Section 3. The research concludes with Section 4.

Proposed Setup
Figure 1 shows the schematic of the proposed hybrid optical ampli er based DWDM-FSO system.This optical communication system combines the advantages of both DWDM and FSO technologies and designed to transmit data over a span of wavelengths, speci cally covering the 1546-1555 nm range with 12 channels spaced 0.8 nm apart.The optical sources for these channels are continuous wave lasers, which provide stable and coherent signals.The channels are driven by a data stream of 40 Gb/s, consisting of NRZ modulated signals, each carrying an input power of − 20 dBm.A wavelength division multiplexer is used to effectively multiplex different channels.The use of this component enables the simultaneous transmission of all data channels over a single link by combining the optical signals from various data channels.The modulated signals undergo initial ampli cation through hybrid optical ampli er before being transmitted via the FSO link.The hybrid ampli er consists of a combination of erbium-doped ber ampli er (EDFA) and Er-Yb co-doped ber ampli er (EYDFA).On the receiver side of the FSO link, a demultiplexer is responsible for dividing the combined optical signal back into its respective channels.This procedure permits the receiver to separate and extract the individual data streams carried by each channel for additional processing.After the signal has been demultiplexed, it is directed to a PIN photodetector.E cient conversion of incoming optical signals into electrical currents is ensured using a photodetector with a responsivity of 0.9 A/W and a dark current of 1 nA.A BER tester is employed to assess the quality of the received data.Simulations of the proposed system are conducted using OptiSystem, a powerful software tool particularly designed for optical communication systems.Moreover, MATLAB is used for detailed analysis, enabling further understanding of the behavior of the system.
The signal rst enters the EDFA stage in the hybrid ampli er, and then it undergoes further ampli cation as it is fed into the cascaded EYDFA.EDFA is a vital ampli er widely used in optical communication networks, providing high gain, wide bandwidth, and low noise ampli cation for optical signals in the Cband [20].Its exibility and effectiveness make it an integral component in building reliable and highcapacity ber-optic networks.The EDFA theoretical model is demonstrated in detail in [21] and the parameters are listed in Table 1.Following ampli cation from EDFA, the signals are subsequently fed into a forward pumped EYDFA.For C-band communication, an EYDFA is commonly utilized because it offers signi cant gain and a low noise gure (NF).Moreover, it offers additional advantages of higher pump e ciency and wide pump absorption band of 800 nm to 1100 nm [22].The broad absorption range allows for exibility in selecting the pump wavelength.In our proposed setup, a 1057 nm co-propagating pump with a power of 90 mW is employed to induce ion excitation within the EYDFA.The theoretical model for the EYDFA's operation is thoroughly discussed in [23] and the parameters are summarized in The FSO link comprises of three main components: the transmitter, FSO channel, and the receiver.
Determining the power received at the receiver is a crucial step in analyzing FSO link's performance.To ensure reliable communication, it is necessary to take into account a safety margin due to the many environmental conditions that can weaken the signal during transmission.This safety margin is known as the "link margin".Link margin is de ned as the magnitude relation of the received power and the receiver's threshold or sensitivity.Alternatively, it indicates the difference between the received power and the minimum power necessary for the receiver to accurately detect and decode the transmitted data.The link margin can be expressed in decibels as [24] 1 where LM is the link margin; PR is the received power at the receiver and S is the receiver threshold or sensitivity.For successful signal recovery at the receiver, it is essential that the power of the incoming signal exceeds the receiver's sensitivity level.Power at receiver is given as [24] 2 where P T and P R are, respectively, the power at the transmitter and receiver; α denotes atmospheric attenuation; L is the distance between transmitter and receiver; A RX denotes receiver aperture area and θ represents divergence angle.Eq. (2) shows that the power received at the receiver depends directly on the transmit power and receiver aperture area, whereas it is inversely related to the link range and divergence angle.Therefore, by raising the transmitter power and receiver area, or by reducing the beam divergence of the transmitter beam, the received power can be increased.The exponential component of the equation is associated with atmospheric attenuation and has the greatest impact on the link quality.
Table 3 shows the various system and FSO link parameters used in the simulation of proposed system, while Table 4 provides a summary of the attenuation values associated with various atmospheric conditions [25].

Results and Discussion
The performance of the proposed system is determined in terms of Q-factor and BER under two different scenarios: when using a hybrid ampli er and when not using one.Furthermore, the length of the FSO link is increased to evaluate the corresponding Q-factor and BER.Also, the performance of the FSO system is examined under various atmospheric conditions.

Gain and noise gure
Figure 2(a) shows the gain spectra of the hybrid ampli er as a function of signal wavelengths, along with the gains of individual EDFA and EYDFA stages at an input power of − 20 dBm per channel.The hybrid ampli er achieves a gain of more than 30.16 dB at each wavelength, with the highest gain occurring at 1554.8 nm.Gain ripple of 1.28 dB is attained over the selected wavelength range.The obtained better gain atness ensures consistent and reliable ampli cation across the wavelength range of interest.Figure 2(c) shows a comparison of the average gain for the proposed ampli ers.The EDFA and EYDFA ampli cations resulted in average gains of 18.51 dB and 12.42 dB, respectively.However, when combined in the hybrid con guration, the average gain signi cantly increased to 30.93 dB.Therefore, it is observed that the hybrid con guration offers superior performance, delivering high gain while maintaining the lowest gain ripple.
The noise gure (NF) pro les of the suggested ampli ers as a function of channel wavelengths are displayed in Fig. 2(b).It shows that the hybrid ampli er maintains an NF value of less than 3.99 dB across the selected wavelength range.The average NF depicted in Fig. 2(c) indicates that the EDFA exhibits the lowest value of 3.35 dB.The hybrid ampli er also exhibits better results, achieving an average NF of 3.91 dB.These results highlight the effectiveness of the hybrid con guration, demonstrating its potential as a means of improving ampli cation performance and signal quality in optical communication systems.

Q-factor and BER performance
In a communication system, the estimation of transmission quality and degradation is critical to ensure reliable data transfer.The Q-factor and BER are two commonly used metrics to evaluate this performance [26].The crosstalk plays a signi cant role in impacting the Q-factor and BER of a system.Crosstalkinduced interference can result in signal degradation, increasing the likelihood of errors in the transmitted data.Maintaining optimal communication performance requires minimizing crosstalk [27].
For proposed system, the Q-factor and BER of the received electrical signals have been investigated and the results are presented in Fig. 3(a) and (b), respectively.These values are measured at FSO length of 1600 m.In the scenario where no hybrid ampli er is utilized before the FSO link, the obtained Q-factor is greater than 2.01 dB, and the BER is less than 10 -02 .However, these values are regarded as unsuitable for reliable and high-performance FSO communication.The option to place a hybrid ampli er before the FSO link has a considerable positive impact on the system's performance.With the same FSO parameters, the results attained with the hybrid ampli er are highly satisfactory.At each wavelength, a much-improved Qfactor of greater than 6 dB and low BER of less than 10 -09 are obtained.Consequently, the overall performance of the proposed system shows a signi cant improvement when the hybrid ampli er is employed.
The impact of increasing the FSO range on Q-factor, BER and received power are investigated and the obtained values are plotted in Fig. 4.These values are measured at the rst channel of the proposed system.In the rst scenario (without a hybrid ampli er), the results showed that as the FSO link length is increased from 400 m to 2000 m: the Q-factor decreases from 7.65 dB to 1.83 dB, BER increases from 10 -12 to a much higher value of 10 -02 and the received power at the output of the FSO link decreases from − 17.61 dBm to − 31.54dBm.In the second scenario (with a hybrid ampli er), the results showed that as the FSO link length is increased from 400 m to 2000 m: the Q-factor decreases from 15.45 dB to 4.07 dB, BER increases from 10 -31 to10 -05 and the received power at the output of the FSO link decreases from 6.42 dBm to − 7.51 dBm.In summary, increasing the FSO range had adverse effects on the performance of the system.The system with hybrid ampli er offers some improvement in mitigating these effects, resulting in better Q-factor, BER, and received power compared to the system without an ampli er.

of the effects of different atmospheric conditions on performance of FSO system
The Q-factor and BER values are determined for the proposed system under adverse weather conditions, and the results as a function of signal wavelengths are presented in Fig. 5(a) and (b), respectively.These values are calculated at FSO length of 1600 m.It is observed that under clear weather conditions, the minimum Q-factor achieved is 6 dB, which indicates a relatively good signal quality.The corresponding maximum BER value is 10 -09 .In the presence of little fog weather, the minimum Q-factor obtained is 4.84 dB.The Q-factor is slightly reduced compared to clear weather, indicating a decrease in signal quality due to the fog.The corresponding maximum BER value is 10 -06 .The minimum Q-factor drops even more to 4.09 dB in the presence of light rain.This drop in Q-factor indicates a signi cant degradation in signal quality as a result of rain attenuation.The maximum BER is measured at 10 -05 , indicating a higher occurrence of bit errors compared to both clear weather and little fog conditions.Under moderate fog conditions, the minimum Q-factor recorded is 2.9 dB.The maximum BER reaches 10 -03 , signifying a substantial increase in the probability of bit errors.In the case of heavy rain, the minimum Q-factor observed is 2.35 dB.The heavy rain causes severe signal degradation, resulting in a further reduction in signal quality.The maximum BER in this scenario is 10 -02 .Finally, under dry snow conditions, the minimum Q-factor observed is 1.39 dB, which is the lowest value among all weather conditions.The dry snow considerably reduces signal quality due to scattering and absorption effects.The maximum BER reaches 10 -01 , indicating a signi cant number of bit errors and greatly affected data transmission.In summary, these results demonstrate the sensitivity of the system's performance to challenging weather conditions.The system performs well in clear weather conditions, with a high Q-factor and low BER values.However, if the weather gets worse, both the Q-factor and BER are adversely affected, which lowers the signal quality and raises the probability of bit errors.
Table 5 and Table 6 list the maximum FSO link distance possible under various weather conditions, ensuring acceptable Q-factor and BER values for two scenarios: one without a hybrid ampli er and the other with a hybrid ampli er.In Table 5, it is clear that the maximum FSO distance attainable in clear weather conditions while maintaining acceptable Q-factor and BER values is 510 m.However, the maximum FSO distance reduces considerably under adverse weather conditions.The minimum feasible distance is only 50 m in dry snow conditions.According to Table 6, with the incorporation of a hybrid ampli er, the maximum FSO distance achievable under clear weather conditions with acceptable Q-factor and BER values increases to 1700 m.In a manner similar to the previous instance, the maximum FSO distance decreases under challenging weather conditions.The maximum possible distance in dry snow conditions is only 210 m.
Figure 6 shows the measured eye diagrams, obtained at the rst channel of the proposed system under clear weather conditions.In Fig. 6(a), the eye diagram is obtained for the system with the hybrid ampli er case.It can be seen that the eye diagram exhibits a wider opening, indicating a robust signal with reduced noise and distortion.On the other hand, Fig. 6(b) displays the eye diagram for the system without the hybrid ampli er case.Comparing this to Fig. 6(a), a visibly narrower eye opening can be observed.The reduced eye height in this case indicates increased signal degradation.Resultantly, the comparison of the two eye diagrams clearly demonstrates that employing the hybrid ampli er in the proposed system leads to signi cantly improved performance.

Conclusion
This paper demonstrates a hybrid optical ampli er based 480-Gbps DWDM-FSO system.The performance of the system is assessed by analyzing the Q-factor and BER in the C-band, with an emphasis on wavelengths around 1550 nm.In addition, the length of the FSO link is varied to analyze the corresponding Q-factor and BER.It is essential for determining the maximum distance that the FSO link can cover while maintaining acceptable Q-factor and BER values.Moreover, the effectiveness of the FSO system is also evaluated under varying atmospheric conditions.Consequently, the proposed system offers a robust and high-capacity solution for transmitting multiple data channels over the FSO link.By amplifying the signals using hybrid optical ampli er, this system can provide reliable and high-speed communication over long distances without the need for traditional optical bers.

Declarations
Ethical Approval: Not Applicable Funding: The declare that no funds, grants or other support were received during the preparation of this manuscript.
Competing Interest: The authors have no relevant nancial or non-nancial interests to disclose.
Con icts of Interest: On behalf of all authors, the corresponding author states that there is no con ict of interest Author Contributions: All authors contributed to the study, conception and design

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Table 3
System and FSO link parameters.

Table 5
Maximum FSO link distance possible with acceptable Q-factor and BER values for different weather conditions (system without hybrid ampli er).Maximum FSO link distance possible with acceptable Q-factor and BER values for different weather conditions (system with hybrid ampli er).