All-Optical 40 Channels Regenerator Based on Four-Wave Mixing

We have proposed a novel multi-channel regeneration scheme for wavelength division multiplexed systems, which is based on four wave mixing in a highly nonlinear ﬁber. A 40-channel wavelength division multiplexed signal having data rate of 10 Gbps per channel is divided into ﬁve groups. Each group is composed of eight channels and requires a single pump laser source and two segments of highly nonlinear ﬁbers to regenerate the eight channels. Therefore, our proposed scheme requires four times lesser number of highly nonlinear ﬁbers compared to the previously proposed techniques. The regeneration performance for all the forty channels is presented through bit error rate analysis at low optical signal to noise ratio of 15 dB . Simulation results show that an average improvement of 4 . 246 dB , 3 . 935 dB , 3 . 72 dB , 2 . 71 dB and 2 . 593 dB in receiver sensitivities has been observed for all the ﬁve groups of channels, respectively.


Introduction
Optical fiber technology has revolutionised the concept of information transfer over long distances during the past years. There has been an enormous generation of a phase-noise degraded non-return-to-zero differential phase-shift keying (DPSK) signal using bismuth oxide highly nonlinear fiber (Bi-HNLF). Recently, four channel phase regeneration of quadrature phase shift keying (QPSK) signals using phase sensitive amplification is demonstrated in [16], [17]. The technique uses Four wave mixing (FWM) in a HNLF to generate the corresponding three harmonic conjugates that are optically combined to realize phase regeneration by coherent addition. Experimental results showed that the OSNR can be improved up to 3 dB. Contestabile et. al. experimentally investigated all-optical regeneration of constant envelope alternate modulation format signals at 40 Gbps using hard limiting amplification in a saturated semiconductor optical amplifier (SOA). This technique is efficient when Polarization Shift Keying and Frequency Shift Keying signal formats are used, whereas, the improvement of OSNR is limited for Non-Return-to-Zero (NRZ) DPSK signal format [18]. Sixteen channels are regenerated using a Groupdelay-managed (GDM) medium that is made by concatenation of N number of fibers and periodic group-delay device (PGDD) unit cells [19]. The proposed design has a complex structure that is implemented in different stages.
Most of the techniques discussed above regenerate a single optical channel. The techniques proposed for multiple channels are generally complex and require specialized equipment. Therefore, there is a need for a multi-channel regeneration technique that uses off-the-shelf components and simple optical signal processing. In this study, we have proposed such a technique by employing FWM in a HNLF. Our proposed technique requires a single span of dispersion shifted HNLF to simultaneously regenerate four optical channels. We have divided the incoming WDM signal into groups of eight channels. Each group requires a single continuous wave (CW) pump source and two spans of HNLFs. The center frequency of the pump laser source in each group is chosen such that the multiple sidebands generated as a result of FWM do not overlap with each other at the output of the HNLF. Our proposed technique is suitable for WDM signals having large number of channels such as a forty channel system. For example, for a forty channel WDM system, the total number of pump sources and HNLF segments required for regeneration based on our technique are five and ten, respectively, compared to forty pump sources and forty HNLF segments required for previously proposed techniques. Therefore, the proposed technique is cost-efficient, employs off-the-shelf components and is simple to implement. Section-2 of the paper discusses the working principle of the proposed technique while Section-3 presents the simulation setup that is designed using the commercial tool OptiSystem 17. Section-4 of the paper presents the performance analysis while Section-5 discusses the conclusions.

Working Principle
The working principle of the proposed regeneration scheme is based on FWM in a HNLF. A pulsed optical signal is coupled with a CW high power pump, as illustrated in Fig. 1. The combined optical signal whose field envelope is denoted as E(z, t) is transmitted through the HNLF where its propagation may be represented by the following expression [20]: The constants β (2) and β (3) represent the dispersion and dispersion slope of the HNLF. The pulsed optical signal can be treated as a CW signal since the time duration of Kerr nonlinearity in the HNLF is much shorter than the intensity fluctuations induced over the signal by the optical link. Therefore, the input field can be represented in terms of the complex amplitude A j (z) of the pump and signal as: Here, w j is the frequency offset of the signal from the pump. The number of sidebands generated at the output of the HNLF are dependent upon the power of the CW pump signal. In our scheme, the pump is operated in the depletion regime while its power is chosen such that the sidebands generated beyond f 1 and f −1 , as shown in Fig. 1, have almost negligible powers at the output of the HNLF. It has been shown in [20] that for the sideband centered at f 1 , the output power on ones saturates for high input powers and scales as square of the input power for low input power values. As a result, we get reduced fluctuations on ones of the input pulsed signal while any power appearing over the zeros is strongly attenuated. To take advantage of the regenerative properties of the signal centered at f 1 , it is filtered out using a bandpass filter at the output of the HNLF. If we neglect the sidebands beyond f 1 and f −1 , the evolution of the remaining four waves along the HNLF may be represented by the following four coupled ordinary differential equations [20]: Here, δ represents the Kronecker delta and is either 1 or 0, depending upon the values of the frequencies ω, and β j represents the propagation constants that may be expressed as: It may be observed from Eq. 3 that FWM between the pump and the signal results in the generation of multiple sidebands at the output of the HNLF. These sidebands are spaced at frequencies that are equal to the frequency difference between the pump and the signal. For single channel regeneration, a simple bandpass filter centered at f 1 may be used to filter out the regenerated signal. However, the situation becomes complex when we have more than one optical channel at the input of the HNLF. In this scenario, the equally spaced optical sidebands of different channels will overlap in frequency and will result in channel distortion. Therefore, to the best of our knowledge, FWM based regeneration for multiple channels has not been demonstrated until now. To cope with this issue, we have proposed a novel scheme that regenerates four optical channels simultaneously by employing a single dispersion shifted HNLF and a CW pump. The center frequency of the CW pump is chosen such that each channel is spaced at a different frequency from the CW pump before FWM is performed. Therefore, the multiple sidebands generated due to FWM would be centered at different frequencies and can be easily filtered. Furthermore, the power of the CW pump is chosen such that it is almost depleted while generating the sidebands f 1 and f −1 .
3 System Architecture to introduce dispersion over the optical signals. After inducing noise as well as dispersion over the WDM signal, we now implement our proposed multichannel regeneration scheme. The WDM signal received at the regeneration block is demultiplexed by employing a 1x40 demultiplexer having a channel spacing of 100 GHz and channel bandwidth of 25 GHz. The forty optical channels received at the output of the demultiplexer are divided into five groups, where each group has eight channels. The first group called Group 1 has eight optical channels having center frequencies between 192.1 T Hz to 192.8 T Hz with a frequency spacing of 100 GHz. Therefore, we have a total of five groups whose range of frequencies are mentioned in Table. 1. Each group of channels is further divided into two sub-groups, as shown in Fig. 2. For Group 1, the first sub-group is composed of channels having frequency span of 192.1 T Hz to 192.4 T Hz, while the second sub-group has a frequency span of 192.5 T Hz to 912.8 T Hz. Similarly, each group that is composed of a total of eight channels is divided into two sub-groups, each having four optical channels. The wavelengths of each sub-group are combined using an optical combiner, as shown in Fig. 2. The combined optical channels are amplified, coupled with a CW pump source and given to the input of a HNLF. It may be observed from Fig. 2 that for each group, a single CW pump source is amplified, split in two paths and coupled with the channels of the two sub-groups. The center frequencies of the pump laser sources employed for Group 1 to Group 5 are 192.45 T Hz, 193.25 T Hz, 194.05 T Hz, 194.85 T Hz and 195.65 T Hz, respectively. As discussed earlier, the frequencies of the pump laser sources are chosen such that each channel that is coupled with the pump source has a different frequency spacing from the pump. Consequently, there is no overlap among the optical sidebands generated at the output of the HNLF due to FWM. The HNLF has a length of 1 km, an attenuation of 0.22 dB/km, dispersion slope of 0.07 ps.nm −2 .km −1 and nonlinearity coefficient of 10.61 W −1 km −1 . For each group, the zero dispersion frequency of the HNLF is kept the same as the frequency of the CW pump laser for that particular group. For example, the zero dispersion frequency of the HNLF employed for Group 1 is 192.45 T Hz, that is the same as the center frequency of the  pump signal used for Group 1. While propagating through the HNLF, the optical channels and the pump signal interact nonlinearly to generate multiple sidebands due to FWM. Fig. 3 shows the spectral plots at different points of the link for the four channels of the first sub-group of Group 1 that has a frequency range of 192.1 T Hz to 192.4 T Hz. Fig. 3(a) shows the four channels before the addition of noise and transmission over the 40 km SMF. Fig. 3(b) shows the four channels after noise is added and the channels are transmitted over the SMF. Fig. 3(c) shows the spectral plot of the signal at the output of the HNLF. It may be observed from Fig. 3(c) that the spectral plot is composed of multiple sidebands that are generated due to FWM between the four channels and the CW pump. These sidebands are not overlapping due to the choice of a suitable frequency for the pump source.
As discussed earlier, the optical sidebands generated as a result of FWM have lesser noise power compared to the actual signal when the pump is completely depleted. Therefore, we use optical bandpass filters at the output of the HNLF to extract the optical channels at frequencies that are offset from their original values. For the Group 1, the channels centered at frequencies of 192.1 T Hz to 192.8 T Hz are filtered at frequencies of 192.8 T Hz to 192.1 T Hz, respectively by using optical bandpass filters each having a bandwidth of 25 GHz. Similarly, the data modulated sidebands generated at the output of the HNLFs for the channels of the remaining groups are also filtered using bandpass filters that are centered at offset frequencies. Table 1 shows the center frequencies of the channels before and after regeneration for Group 1 to Group 5, respectively. Fig. 4 shows the eye diagrams for Channel 1 before and after regeneration. Fig. 4(a) shows the signal before regeneration where it may be observed that the eye opening is narrow due to the addition of ASE noise to the signal. Fig. 4(b) shows the eye diagram of the regenerated signal where it may be seen that the eye opening is wide due to lower amplitude fluctuations. These eye diagrams are used to calculate the bit error rate (BER) performance of the link, as discussed in the next section.

Performance Analysis
This section discusses the performance of our proposed regeneration scheme.
As mentioned earlier, we transmitted 40 WDM channels, each having a data rate of 10 Gbps. To consider the degradation of OSNR of the optical channels due to multiple optical amplifiers in a long haul optical link, we add broadband ASE noise through an external source. The power of the ASE noise source is chosen such that it results in a low OSNR of around 15 dB for all the channels received at the regenerator. Furthermore, to induce the effect of fiber dispersion, the noisy WDM channels are passed through a 40 km SMF without dispersion compensation. BER analysis was performed for all the channels. The received optical power required to obtain a BER of 10 −9 is considered to observe the improvement in power penalty of all the channels due to the introduction of the regeneration scheme. Since we have a large number of channels, the improvement in their sensitivity due to regeneration have been shown in the form of a table denoted as Table 1. Table 1 also shows the center frequency of each channel before and after regeneration. For each channel, the sideband centered at frequency f 1 shown in Fig. 1, has been filtered out. Therefore, the frequency of the regenerated channel is shifted from that of the input channel. It may be observed from Table 1 that on average, the sensitivity of the regenerated signals has improved by a value of 4 dB compared to the noisy signal at the input of the regenerator. Fig. 5 shows the complete set of BER values at different received optical powers for the ten subgroups mentioned previously. For each subgroup, the BER curves of the four channels before and after regeneration have been  shown. It may be observed from the BER plots that the proposed regeneration scheme significantly improves the BER of the 40 channel WDM signal. It is worth mentioning here that the BERs were obtained by choosing a suitable optical power for each sub-group that is input to the regenerator. As discussed in [20], for low input signal powers, the power of the sidebands at the output of the HNLF remains very low. Therefore, the intensity of noise appearing on the zeros in the data stream is strongly compressed at the output of the regenerator. After crossing a certain value of input power, the output power suddenly increases and reaches a maximum point. This behaviour is similar to that of a nonlinear switch. After the output power reaches a maximum point, further increasing the input power does not result in a significant increase in the output power of a particular sideband. At this point, maximum compression of intensity fluctuations on ones in the data stream is obtained. Based on this behaviour of the generated sidebands, we have chosen a suitable operating point for each sub-group by adjusting the gain of the amplifier placed after the 4x1 multiplexer in Fig. 2. This operating point is close to the point of maximum output power and results in the lowest BER.

Conclusion
We reported an all-optical multi-channel regeneration scheme for WDM systems. A forty channel WDM signal was generated and passed through a standard SMF to induce dispersion and nonlinear effects over the channels. To reduce the OSNRs of the channels to 15 dB at the regenerator, broadband noise source was coupled with the WDM signal before transmission over the SMF. The forty channels were divided into five groups, each composed of eight channels. A single CW pump laser source and two segments of HNLFs were used to regenerate all the eight channels in a single group. The proposed scheme is very useful for implementation in current WDM systems that are using a single regenerator for a single channel. The introduction of our proposed regeneration scheme to such WDM systems would result in significant reduction in component count and therefore, an increase in cost efficiency. An average improvement in receiver sensitivity of 4.246 dB, 3.935 dB, 3.72 dB, 2.71 dB and 2.593 dB was observed for the first, second, third, fourth and fifth group, respectively. The proposed scheme is scalable to higher data rates since it is based upon ultra-fast nonlinear interaction inside HNLFs.

Declarations
Funding Not Applicable

Conflicts of interest/Competing interests
On behalf of all authors, the corresponding author states that there is no conflict of interest.

Availability of data and material
Available on request.