Introduction

In the modern era, we are surrounded by data hungry devices with data demand rising exponentionally day by day. Conventional radio frequency (RF)-based systems cannot fulfill this surge of data demand because, as per convention of communication theory, increase in data capacity must be accompanied with higher radio frequency bands. Higher radio bands have their own limitations. Firstly, such expansion will require complete overhaul of existing RF infrastructure and secondly, higher operating frequency may although enhance tranmission speeds but at the same time serviceable coverage area is also drasctically reduced. So in order to serve large geographical regions, the service providers will have to install massive infrastructure and this will lead to escalated service costs. Considering the current situation, optical fiber communication seems optimum and costs effective mechanism to provide high speed data services at reasonable costs. Wavelength division multiplexing (WDM) is one such phenomenon which not only enhances link capacity but also improves the spectral efficiency [1,2,3]. Over the years the lightwave communication hardware like the lasers, photodiodes and optical amplifiers have witnessed significant performance refinement. This evolution in lightwave comunication brought out by advancement in equippment has allowed WDM networks to be used for long haul applications [4, 5]. On the transmitter side, the WDM system contains a multiplexer which combines a number of signals having different operating wavelengths, and at the receiver side there is a demultiplexer to split the signals apart and thus decode user data. Based on the extent of multiplexing, the WDM systems are further categorized  as coarse wavelength division multiplexing (CWDM) and dense wavelength division multiplexing (DWDM) [2, 3]. If the wavelength spacing between the channels of the WDM system is of the order of 20 nm, then such links fall under the category of CDWM networks and if the wavelength spacing is less than 1 nm, then it is called as DWDM system. Thus, to achieve higher capacity, DWDM systems are more useful because it allows simultaneous propagation of large number of channels due to lesser frequency separation among the signals [6].

Signal transmission over a long distance causes increased path loss; thus, the signal power gets attenuated after a certain distance and the signal becomes too weak to be detected correctly. Earlier, optical links employed the combination of electro-optical devices to transmit and recover the signal. For example, electrical regenerators convert the optical signal into electrical one, clean it up, and convert back into the optical form for further transmission. For long haul applications, such devices are placed at regular distance intervals between the source and destinations; thus, each node is required to perform this complex action of O–E and then E–O conversion. The intermediate E–O or O–E conversion has its own set of limitations which include complex circuitry, noise accumulation (thermal and shot noise) in the system due to necessary photo-detection for O–E conversion and significant delay in signal propagation because of the time lag produced by the O–E/E–O conversion. With maturity and constant research in the field of optical communication today we have all optical devices that eliminate the need to intermediate O–E/E–O. Nowadays, in place of regenerators, optical amplifiers are used which directly amplify the signal into optical domain, thus eliminating any need for signal conversion. Based on the principle of operation, the optical amplifiers are generally of three types: (1) semiconductor optical amplifier (SoA), (2) doped fiber amplifiers like, erbium doped amplifier (EDFA) and (3) amplifiers based on material non-linearity, e.g. Raman amplifier [7]. SoA requires an external injection current to produce the gain at the output while EDFA requires an external pump power from a laser at a wavelength of 980 nm or 1480 nm along with the reference signal to amplify the signal. The SoAs are comparatively less expensive and consume low power. But, the major disadvantage of SoA is that it provides low output gain, high noise figure and low immunity against cross talk. On other hand, EDFA provides amplification with less signal impairments as compared to SoA and at the same time EDFA also reduces the cross talk and noise in the system. However, the major drawback of EDFA amplifier is that it requires high external pump power [8] and long fiber length to ensure that appropriate amplification is achieved. The amplification action in the Raman amplifier is based on the non-linear principle of stimulated Raman Scattering (SRS). In SRS, light is scattered by molecules from a lower wavelength to higher wavelength using an external light source (pump). Raman amplifiers also require a high power pump laser to produce waveguide non-linearity, and this process releases and transfers the energy from the pump to the signal. By altering the wavelength and the pump power, the gain of the Raman amplifiers can be tailored to achieve the requisite gain of a wide range of wavelength bands. The Raman amplifier also provides low noise figure, however, it suffers from high coupling loss and high non-linearity in amplification [9,10,11].

Hence, we may conclude that each amplifier type discussed here has certain limitations and advantages which can be intelligently used to design long haul networks. To overcome these limitations, we have proposed the use of gain optimization in the hybrid optical amplifier with an objective of achieving a considerable flat gain over large operating spectrum and delivering power gain enough to steer communication over distances. Such an arrangement helps in designing a cost convenient and tailored class of flat gain amplifiers in comparison to otherwise readily available expensive plug and play amplifiers. The rest of the paper is organized as follows: the proposed link design and simulation parameters have been discussed in “Proposed methodology” section, while, results and discussions are presented in “Results and discussions” section of this paper with concluding remarks in “Conclusions” section.

Proposed methodology

Our proposed design of a 100-channel 40-Gbps (100 × 40 Gbps) DWDM fiber link using symmetric hybrid optical amplification is shown in Fig. 1a, b. The channel spacing between the DWDM channels is maintained at 0.1 nm while the link design and analysis have been done using the specialized design tool, OptiSystem 14.2. As shown in Fig. 1a, the setup of the proposed DWDM optical fiber design can be said to consist of three main sections. Each channel from the 100-channel WDM transmitter consists of a light source (laser) which modulates the user data using an external modulator. The data source generates the input signal, and the NRZ encoder formats the user data into an electrical signal. The light signal is generated by the continuous wave laser (CW) and finally modulated by the external modulator, which in our case is the Mach–Zehnder modulator. With an interchannel separation of 0.1 nm, all of the 100 optically modualted channels are then combined together using a 100:1 optical multiplexer. The multiplexing forms the second stage of our proposed design. Figure 1b shows the receiver section of the proposed link where the DWDM signal is initially amplified, followed by optical filtering to separate the spectrum into constituent channels. Each channel is then used to receive the original user data by optical to electrical conversion achieved using PIN photo-detectors. Figure 2a, b provides insight into the data modulation and data recovery sections that are used for each channel. The user data, generated from PRBS (pseudo random bit sequence), is electrically encoded into non-return to zero (NRZ) format before undergoing optical modulation using the laser source and the Mach–Zehnder modulator. On the receiver side, optical filter separates the spectrum into individual channel followed by photo-diode and low pass filter which helps recover original data.

Fig. 1
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Block diagram of proposed DWDM system with symmetric hybrid amplification: a transmitter section and b receiver section

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Fig. 2
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Block diagram representing a single-channel modulation and b single-channel detection

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Third stage in the transmitter is that of amplification which is has been achieved in the proposed link through an optimized combination of SoA followed by Raman amplifier and EDFA. Once all of the 100 optically modulated signals have been combined by the multiplexer, the multiplexed signals then pass through the amplification subsystem and later through 100 km long single mode fiber (SMF). After propagation through the fiber, the DWDM signal passes through a 100-channel demultiplexer which along with an optical filter (Gaussian) separates the spectrum into respective wavelengths. This separation of channels into different wavelengths is achieved through an optical filter and then converted into its original form with the help of a photo detector. A dual port analyzer is used to record the gain of the system, while bit error rate (BER) and eye patterns were analyzed using the BER analyzer. The details of different simulation parameters are further elaborated in Table 1. The amplifier subsection can either have a hybrid amplifier combination or any stand-alone amplifier (non-hybrid) for the purpose of link analysis.

Table 1 Transmitter simulation parameters
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Results and discussions

In this section, we have presented the results obtained with the help of the simulation setup shown in Figs. 1 and 2. The performance of the proposed link is recorded in terms of link gain, bit error rate (BER), spectrum analysis and eye patterns. As per the parameters shown in Tables 1 and 2, Fig. 3 depicts the gain analysis of the hybrid optical amplifier for the 100-channel DWDM link operating in a wavelength window of 1540 nm to 1549.9 nm. The gain analysis is carried out for different fiber lengths, i.e. 50 km, 75 km and 100 km. Table 3 illustrates the gain analysis of the link as observed from Fig. 3. From Fig. 3 and Table 3, we can see that as the fiber length increases the absolute maximum available gain decreases. The maximum gain (dB) in the case of a 50-km link of 15.67 dB is reduced to 14.22 dB in the case of a 100-km fiber link. However, it is the 100-km link that observes the best flat gain recorded among all three cases discussed in Table 3. The difference between maximum and minimum gain in the 100-km fiber link is 0.54 dB in contrast to 1.82 in the 50-km link.

Table 2 Amplifier simulation parameters
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Fig. 3
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Flatness of gain of hybrid amplifier at different fiber lengths

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Table 3 Gain analysis of hybrid optical amplifier operated link
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As per parameters listed in Table 4, Fig. 4 illustrates the gain comparison achieved through hybrid, Raman and SoA amplifiers. It can be seen here that, for the Raman amplifier, a maximum gain of 4.99 dB occurs at a fiber length of 10 km and a minimum gain of − 15 dB is obtained at a fiber length of 100 km. The maximum gain of SoA 5.19 dB is observed at a fiber length of 10 km and then decreases to − 13.71 dB at 100 km fiber length. The gain of both SOA and Raman amplifier linearly decreases as we increase the fiber length. The hybrid optical amplifier provides the maximum gain of 16.29 dB at a fiber length of 10 km and the lowest gain of 13.75 dB at a fiber length of 100 km. As we increase the length of the fiber, we find the gain of the amplifier remains almost flat in contrast to Raman and SoA amplifier configurations. Table 5 presents the gain analysis as seen from Fig. 4. We can observe the changes in the gain as the link increases from 10 to 100 km, while the absolute change is just 2.54 dB in the hybrid amplifier as compared to 18.99 dB and 19.9 dB in SoA and Raman amplifier-based links.

Table 4 Parameters of different amplifiers when used in stand-alone condition (non-hybrid amplification)
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Fig. 4
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Gain analysis of different amplifiers for varying link lengths

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Table 5 Gain analysis of different amplifiers
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Figure 5 reveals that the gain analysis of different amplifiers when transmission power per channel is varied from 1 to 5 mW while the rest of the simulation parameters used in this analysis can be seen in Table 6. For the hybrid optical amplifier, the maximum gain of 6.76 dB occurs at 1 mw power and as the power increases, the gain of the amplifier decreases and reduces to a mere 0.84 dB at 5 mW. The gain of the SOA amplifier is higher than the gain of the Raman amplifier. For the SOA amplifier, the maximum gain is − 4.99 dB at 1ow power and then decreases to − 5.68 dB at a power of 5 mW. The gain of the Raman amplifier is the lowest among all the amplifiers and remains flat at − 8.9 dB for varying transmission power (1–5 mW).

Fig. 5
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Gain analysis of different amplifiers for varying transmission power

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Table 6 Parameters used to analyze the gain of the hybrid and non-hybrid optical amplifiers for varying transmitter power
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It is quite evident from Fig. 6 that when it comes to the comparison of the bit error rate (BER), the hybrid amplifier delivers a better error performance as compared to the optical link using the Raman amplifier. A general trend seen is that as the length of the fiber increases, the BER of the link also increases. For the Raman amplifier, the BER remains constant at 10−6 for fiber lengths varying from 10 to 30 km but then it degrades sharply rendering the link useless at a fiber length of 70 km. However, for the hybrid amplifier, the BER at a fiber length of 10 km is 10−10 and then it is slowly increased and becomes constant for a interval of 10–20 km, 30–40 km and 50–60 km and then again increases and becomes 10−4 at a fiber length of 100 km.

Fig. 6
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BER performance of hybrid and Raman amplified link for varying fiber lengths

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Figure 7 describes the gain of the proposed link when the 100 channels are operated in C (1540–1549.9 nm) and L (1565–1574.9 nm) bands, respectively. In other words, Fig. 7 reflects the behavior of the proposed design in these two bands and this analysis is crucial because, in the entire link design, the amplifier segment will be very much sensitive to the choice of operating bands. For the C band (1540–1549.9 nm), the gain observed in the link is higher as compared to the L band (1565–1574.9 nm). At a fiber length of 10 km, the gain of the hybrid amplifier is 16.29 dB and 12.19 dB for C band and L band, respectively. Similarly, for a fiber length of 100 km, the gain of the hybrid amplifier is 13.75 dB and 1.40 dB for C band and L band, respectively. From Fig. 7, it is clearly seen that the gain of the hybrid amplifier for the L band decreases rapidly as we increase the length of the fiber, while, for the C band, the amplifier gain remains significantly flat even after the propagation of a 100-km distance.

Fig. 7
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Gain analysis of hybrid amplifier in C and L bands for varying fiber lengths

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Figure 8 provides the spectral analysis of the 100-channel DWDM link at different segments of the link. Figure 8a shows the power versus wavelength analysis of the 100-channel link when operated in 1540–1549.9 nm range, while Fig. 8b shows similar analysis after achieving hybrid amplification in the link. Figure 8c, d illustrates the received signal spectrum after propagating a fiber length of 75 km, Fig. 8c shows the received signal when the transmitter is amplified using SoA, and lastly, Fig. 8d depicts the received spectrum of the hybrid amplified link. Thus, we can clearly conclude here that the hybrid amplified signal achieved a greater degree of flatness over longer distances. Figure 9a shows the eye patterns for the 100-channel DWDM link when only SoA is used for amplification, while Fig. 9b reflects the eye patterns of the hybrid amplified DWDM link.

Fig. 8
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Spectrum analysis (power vs. wavelength) of the 100-channel DWDM fiber link: a just after multiplexing, b after hybrid amplification, c received signal when transmitter uses SoA for amplification and d received signal when the hybrid amplifier is used

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Fig. 9
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Eye patterns: a SoA-based link and b hybrid amplifier-based link

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Conclusions

A high-capacity DWDM optical fiber link with 100 channels, a bit rate of 40 Gbps and a wavelength separation of 0.1 nm has been investigated for gain flatness. With the help of qualitative parameters like BER, gain and eye patterns, the performance of the link using hybrid amplification has been compared with that of stand-alone amplifiers. The gain of the non-hybrid optical amplifiers (SoA, Raman amplifier) decreases linearly with the increase of the fiber length and transmitted power. For the hybrid optical amplifier, as the length of the fiber increases the gain becomes flat. In a wavelength window of 1540–1549.9 nm, the maximum to minimum gain difference for the hybrid optical amplifier is recorded to be a mere 0.54 dB. This study reveals that the use of the hybrid optical amplifier aided with gain optimization in the DWDM transmission system supports a drastic expansion of internet traffic for high capacity and a longer distance transmission.