Introduction

The tremendous expansion of high speed digital networks and internet services required the large gain bandwidth for next generation’s dense wavelength division multiplexed (DWDM) optical communication systems [1]. Basically DWDM is wavelength division multiplexed (WDM) at reduced channel spacing. In which a large number of optical signals are transmitted simultaneously at reduced channel spacing over the same fiber [2]. This technology can easily increase the communication channels at demand without installing new fibers. But it is important to maintained the required level of the communication system as multichannel communication system are highly sensitive to non-linearity, chromatic dispersion and attenuation [3]. To maintain the level of the data signals, optical amplifiers are used in the channel. Conventional optical amplifiers such as semiconductor optical amplifier (SOA), EDFA and Raman amplifier are the major components of DWDM systems [4]. In EDFA and SOA, optical gain is given by stimulated emission while Raman amplifier used non-linearity for amplification. In optical amplifiers, spontaneous emission also occurs, which amplified the input signal which in turns produced the amplified spontaneous emission (ASE) noise which reduced the transmission distances. These amplifiers are very important for DWDM systems as they covered the broad wavelength range in the communication system with large distance.

In recent years, Raman amplifier has attracted huge attention. This is due to fact that Raman amplifier can amplifies a large number of signals of any wavelengths simply by adjusting the pump wavelength [5]. As these amplifiers are operating at particular wavelength; therefore these amplifiers are not suitable for large capacities long distance transmission. In order to fulfil this requirement conventional amplifiers (such as SOA, EDFA, Raman amplifier) with different gain bandwidth were combined to extend the bandwidth of hybrid optical amplifier. Hybrid optical amplifiers (HOAs) are the enabling and promising technology for future DWDM systems. HOA is a combination of two or more conventional amplifiers [6]. HOA covers all the wavelength bands for amplification of the signals, which increase the transmission distance and capacity of the network. Hybrid optical amplifiers were designed to improve the gain flatness/gain bandwidth and also to diminish the nonlinearities of the systems. Conventional optical amplifiers can exhibit high levels of nonlinear crosstalk, because they require accurate phase matching for proper operation. A hybrid optical amplifier (HOA) can help in reducing the internal nonlinear crosstalk, while exhibiting a low noise figure. HOA have some properties which can potentially be exploited for improving the performance of optical communication systems such as: the ability to perform wavelength conversion and phase conjugation; large gain bandwidth and a noise figure as low as possible. It is very important to use these properties to amplify optical signals for optical communication systems.

In hybrid optical amplifiers, broad gain spectrum could be achieved for any wavelength range by changing the number of pump and pump wavelength of the amplifier. For multiterabit DWDM optical systems, hybrid optical amplifier gave a cost effective solution without using any complex and large circuitry. Several gain flattering schemes have been proposed in the literature such as using gain flatting filter, fiber acousto optic filter and fiber bragg grating filters [7], [8].

Raman-EDFA hybrid optical amplifier has been designed for flat gain bandwidth of 90.5 nm up-to transmission distance of 50 km [9]. Singh et al. [10] demonstrated the different dispersion compensation configurations (such as pre, post and symmetrical) for the placement of optical amplifiers. Jeenanjyoti et al. [11] determined the different modulation formats in single channel light-wave system and also found the effects of amplified spontaneous emission on CSRZ, RZ and NRZ. Paul et al. [12] proposed a wideband amplifier based on EDFA in which erbium doped zirconia fiber is used as gain medium. Bobrovs et al. [13] determined the performance of hybrid optical amplifiers in DWDM systems. The performance of Raman-EDFA and Raman-SOA hybrid optical amplifiers has been compared and found that Raman-EDFA gave better results for long distance transmission system as compared to SOA-Raman amplifier. Singh et al. [14] proposed a flat gain hybrid optical amplifier using distributed raman amplifier (DRA) and EDFA for 160 × 10 Gbps DWDM system at 0.2 nm channel spacing. And it is found that as input power is increased, the gain over the bandwidth also increases without using any gain flattering technique. The performance of hybrid optical amplifiers for 0.8 nm channel spacing has been studied for DWDM systems [15,16,17,18]. Sharma et al. [19] proposed a gain flattened hybrid optical amplifier and EDFA with gain flattening filter for WDM passive optical network having 16 channels within C band. Further by optimizing the length of EDFA, pump frequency and pump power, the hybrid optical amplifier reduced the gain variation. In addition to this, the gain deviation was diminished to 0.0015 dB using GFF and noise figure was maintained at < 6 dB in both the schemes. Malik et al. [20] investigated the performance of conventional and hybrid amplifiers at reduced channel spacing of 0.4 nm. From the observation it was seen that Raman-EDFA gave improved results as compared to SOA-EDFA up to 100 km. Onwards 100 km SOA-EDFA gave improved results as compared to Raman-EDFA for small number of channels.

Proposed models

We have proposed different models for multichannel (16 channels) DWDM system using EDFA, Raman amplifier and E-R HOA configurations. Proposed configurations can easily be illustrated from Fig. 1. In Fig. 1a, we have proposed EDFA configuration, in which amplification is done by EDFA followed by gain flattening filter (GFF). Figure 1b explains Raman amplifier configuration, where we have used Raman amplifier with multiple pumping schemes. Whereas in Fig. 1c E-R hybrid optical amplifier configuration is shown, where EDFA is followed by Raman amplifier. We have transmitted the data at a bit rate of 20 Gbps with reduced channel spacing of 0.2 nm in all the configurations. Parameters, which we have used for EDFA are 5 m length of EDFA fiber, 1 × 1025 m−3 erbium doped ion density, 0.24 numerical aperture, 980 nm pump wavelength with 50 mW pump power. The parameter used for Raman amplifier are 10 km length of Raman amplifier fiber, 300 K operating temperature, with four backward pumps of wavelength 1462, 1464, 1466, 1468 nm with 500 mW pump power. NRZ modulation format is used in all the configurations at a channel bit rate of 20 Gbps. The transmitter section consists of CW laser, data source, modulator and electrical driver. Data source simulates a deterministic logical signal and generates pseudo random signal followed by an electrical driver. Electrical driver is an important component, which translates binary sequence and input logical signal into an electrical signal and generate the desired data. In all the configurations, 16 different sources at channel bit rate of 20 Gbps are used with ultra reduced channel spacing of 0.2 nm varying in the range of 1553.13–1556.15 nm. These optical signals are multiplexed with the help of wavelength division multiplexer as shown in models and are then forwarded to transmission channel having different optical amplifier configurations. The optical signals are received from the channel after de-multiplexing with the help of PIN photo detector. In this work, 16 channels DWDM system is proposed at reduced channel spacing of 0.2 nm using E-R HOA with multi-wavelength backward pumping schemes. The input power of the transmitter varied from “0” to 10 dBm experimentally with NRZ format. Different parameters which are used to analyze the performance of this system are output power, OSNR, noise figure and gain of the DWDM system. Optical spectrum analyzer (OSA) is used in Fig. 1a–c to measure spectrum power of the transmitted optical signals. That is further used at the receiver to analyze the output power received from the signals. In Fig. 1a, erbium doped fiber amplifier is proposed to determine the performance of the system. The performance is measured in terms of gain and noise figure; so, in order to obtain the maximum gain flatness, gain flattening filter (GFF) is used in this configuration. While in Fig. 1c, E-R hybrid optical amplifier is proposed to obtain the flat gain without using any gain clamping scheme. That’s why GFF is not used in this configuration.

Fig. 1
figure 1

Proposed models for 16 channels, 20 Gbps DWDM network a EDFA configuration, b Raman amplifier configuration, c (E-R) HOA configuration

Full size image

List of parameters used in different configurations are listed in Tables 1, 2 and 3.

The parameters used in the EDFA and Raman amplifier configurations are shown in Tables 1 and 2 respectively; whereas, the Table 3 covers the parameters used for the proposed E-R HOA configuration. Regrettably, earlier hybrid optical amplifier either used large channel spacing (> 0.4 nm) or complex components in the system to improve the performance of the system. Here we recommended hybrid optical amplifier for dense wavelength division multiplexed system at 0.2 nm ultra dense reduced channel spacing. The aim of this research paper is to propose a hybrid optical amplifier for next generation DWDM optical networks in order to obtained flat gain without using any gain clamping scheme. Organization of paper is as follows. This paper is segregated in four sections. In the first section the introduction and review of literature is covered. “Proposed models” section covers the proposed models for multichannel 20 Gbps DWDM systems “Results and discussion” section explained the results for different parameters and conclusion is reported in “Conclusion” section.

Table 1 EDFA parameters
Full size table
Table 2 Raman amplifier Parameters
Full size table
Table 3 Parameter used in proposed E-R HOA configuration
Full size table

Results and discussion

In this section, we will discuss the results obtained for proposed models depending upon output power, gain, noise figure and OSNR. We have analyzed the results for different optical amplifier configurations, which are proposed in this paper. On the basis on mentioned parameters, we have covered results for 16 channels, 20 Gbps DWDM optical networks. The input power of the transmitter is also varied from 0 dBm to 10 dBm and also compared the different configurations.

Figure 2 demonstrates the gain spectrum of EDFA, Raman amplifier and E-R HOA configurations for 16 × 20 Gbps system; where the relationship of gain is shown in terms of wavelength. From the simulation results it is observed that flattened gain is attained along the whole wavelength range in all the configurations as shown in Fig. 2. It is clear from the figure that the gain received in E-R HOA configuration is always better than the other two configurations. At 0 dBm power, EDFA gives average gain in the range of 14.79 dB with noise figure of 7.52 dB. From EDFA configuration, gain flatness of 0.44 dB is achieved with better performance. Gain of the system increases linearly from 14.54 to 14.98 dB over the range 1553.13–1556.15 nm with maximum gain at 1556.15 nm. Raman amplifier gives better gain as compared to EDFA with average of 24.19 dB. In Raman amplifier, the observed gain flatness is 0.74 dB which is better than EDFA. The acceptable noise figure is 6.92 dB with noise figure variation is 0.20. E-R HOA gives better gain and noise figure than the EDFA and Raman amplifier configurations. The average gain observed from hybrid configuration is 38.85 dB with minimum noise figure of 4.22 dB, which are much better than other configurations as shown in figure. The gain flatness observed in proposed E-R hybrid optical amplifier configuration is 1.237 dB which is much better than EDFA and Raman amplifier configuration. The gain flatness of 0.44 and 0.74 dB were observed in case of proposed EDFA and Raman amplifier configurations. These results are clearly noticable from the results shown in Fig. 2. Figure 3 demonstrates the gain analysis of E-R HOA for 16 channels, 20 Gbps DWDM system with different input power Pin (0.0, 5.0, 10.0 dBm). Gain of 38.85, 37.64 and 34.13 dB was observed for 0.0, 5.0 and 10.0 dBm input power signal respectively. It is seen that gain of DWDM system starts decreases as increases the input power. This is attributed due to high input power that saturated the amplifier. The gain flatness was achieved due to optimization of input power, proper length of fiber and choosing appropriate pump power.

Fig. 2
figure 2

Gain spectrum for EDFA, Raman and E-R HOA

Full size image
Fig. 3
figure 3

Gain variation for E-R hybrid optical amplifier for input power Pin = 0, 5, 10 dBm

Full size image

Figure 4 shows the pattern for the noise figure for all configurations at 0 dBm input power. It is observed that E-R HOA gives minimum noise figure as compared to the other configurations. In case of EDFA configuration, the observed noise figure is 7.52 dB while Raman configuration gives 6.92 dB. Noise figure for E-R HOA is 4.22 dB, which is less than 5 and consider to be better for high speed DWDM systems. The noise figure variations for EDFA, Raman and E-R HOA are 0.39, 0.20 and 0.29 dB respectively. It is also observed that with the increase in the input power, the noise figure variations also increases.

Fig. 4
figure 4

Noise figure variation for EDFA, Raman and E-R HOA

Full size image

Figure 5 shows the Optical Signal to Noise Ratio (OSNR) variations for EDFA, Raman and E-R HOA. From the results, it is examined that the performance of E-R HOA is better than all other configurations and it gives maximum optical SNR as compared to EDFA and Raman amplifier configuration.

Fig. 5
figure 5

Optical SNR for all the proposed amplifiers configurations

Full size image

Figure 6 shows the output power variations for EDFA, Raman and E-R HOA. From the results, it is examined that E-R HOA gives maximum output power among all the other configurations. EDFA gives maximum output power of 1.99 dBm at 1555.75 nm wavelength. The input power used in E-R hybrid optical amplifier configuration is varied from 0 to 5 dBm and 10 dBm. It is observed that as the input power is increased, the gain started deceasing due to saturation being the optical amplifier. So the proposed configuration is being taken at minimum input power that is 0 dBm. Maximum output power for Raman amplifier is 11.51 dBm at channel wavelength 1556.15 nm and E-R HOA gives maximum output power at 1556.15 nm is 26.44 dBm. Keeping in view, the results shown in Figs. 2, 3, 4, 5 and 6, it is seen that EDFA-Raman hybrid optical is preferred for 16 channels 20 Gbps DWDM systems.

Fig. 6
figure 6

Output power for EDFA, Raman and E-R HOA

Full size image

Conclusion

This research paper proposed a E-R HOA for gain flatness in large capacity DWDM system at dense channel spacing. Amplifiers were operated for 16 × 20 Gbps at 0.2 nm reduced channel spacing. It is observed that, the gain received in E-R HOA configuration is always better than the other two configurations. The average gain observed from hybrid configuration is 38.85 dB with minimum noise figure of 4.22 dB, which is much better than other proposed configurations. The gain flatness of 1.237 is observed, which is also much better than other proposed configuration. Further, it is observed that output power increases and variation in output power decreases with increase in input laser power. In addition to this, it is observed that for “0” dBm input power, the system gives minimum noise figure with noise figure variation 0.818 dB. The proposed model could be an efficient solution having better performance for the large capacity next generation DWDM system.