Abstract
1. Induction
As the significant development of data center, real-time video, and online social networks, there are rapid growth of the transmission capacity in the optical fiber communication system[
The data centers are extremely sensitive to power consumption, footprint and cost. In order to satisfy the requirements, we should consider the reliability and power consumption of devices. The power consumption of electro-absorption modulated laser (EML) is larger compared to the DMLs, which will cause the larger power consumption in the data centers. Besides, it is difficult for us to characterize the reliability of EMLs due to its increased complexity[
In our previous work, we have experimentally demonstrated that PAM4 transmission system over 40 km SSMF with the complex electrical equalization processing of DFE and FFE, which will result power consumption in the practical application[
2. Analysis of the performance of DML
In our laboratory, we have packaged a batch of DMLs and the paper[
Figure 1.(Color online) Assembly schematic of proposed DML.
Fig. 2(a) shows the center wavelength of the DML (1310.19 nm) is in the range of zero dispersion point, which is able to greatly reduce the complexity of DSP. Besides, the Fig. 2(a) also shows the side mode suppression rations (SMSR) is able to reach at 50 dB, which it is advantageous for DML to work in single longitudinal mode (SLM). As shown in Fig. 2(b), the degradation reaches at 3-dB at 23 GHz. We can reduce the degradation of bandwidth in the high frequency range[
Figure 2.(Color online) (a) Measured optical spectrum of DML. (b) Frequency response of DML. (c) Measured
3. Experiment setup and results
The experimental setup is shown in Fig. 3, which shows the transmission of the single channel 56 Gbps PAM-4 signal based on DML over 35 km SSMF. Four copies of pseudo-random bit sequences (PRBS) of length 215–1 was generated by arbitrary waveform generator (Keysight M8195A) operating at 65 GSa/s with 23 GHz analog bandwidth. In order to reduce the inter-symbol interference (ISI), the roll-off coefficient of root rising cosine was set at 0.35 by its attached software. The signal was amplified by electric amplifier (EA), then this signal directly drives the DML. The output of DML was launched into SSMF, and the received optical power (ROP) was controlled by a variable optical attenuator (VOA). Then the signal was detected by a P–I–N type photodiode with a transimpedance amplifier (TIA). The signal was sampled at 80 GSa/s by a real-time digital sampling oscilloscope (Keysight DSOZ634A) with 32 GHz analog bandwidth. The stored signal was processed by FFE with different tap numbers offline compensating ISI induced by bandwidth limitation. It is worth to note that the network has not adopt any optical amplification.
Figure 3.(Color online) Experimental setup of single wavelength PAM-4 signal transmission.
We tested that the BER performances versus ROP for the proposed scheme. The Fig. 4 shows the BER against the ROP for BTB, 25 km, 35 km distance. From this figure, it can be seen that the BER performance degrades as the ROP decreases and the back to back (BTB) PAM-4 transmission and 25 km PAM-4 transmission have the some change tendency. From the Fig. 4 the BTB PAM4 transmission and 25 km PAM4 transmission almost have the same BER of different ROP and this phenomenon can be explained by the fact that a small amount of negative chirp in distributed feedback (DFB) laser can reduce ISI[
Figure 4.(Color online) BER performances versus ROP for different distance.
The Fig. 5 plots BER curves versus tap number of 35 km 56 Gbps PAM4 transmission with ROP of –5.7 dBm. As expected, BER reduces with the FFE Tap number increase. From this graph, we can learn that BER will stable at around 3.5 × 10–4 with FFE tap coefficients reaches at 27, considering the DSP complexity and BER performance, 27 taps were fixed during the off-line processing. Fig. 6 presents the BER against different ROP of PAM4 35 km transmission system with FFE and without FFE, respectively. We can see that the BER of the system has greatly decreased greatly after FFE equilibrium, When the ROP is lower, the role of FFE equilibrium is more obvious but the effect of FFE equilibrium has degraded as the ROP decreased due to the sensitivity of the P–I–N photodiode. Fig. 7 shows that the eye diagram of different distance (BTB, 25 km, 35 km) of 56 Gbps. After the 25 and 35 km transmission, we are able to see the horizontal eye opening and vertical eye opening becoming worse showing that the BER of the system is terrible. After the FFE Equilibrium we can see that the horizontal eye opening and vertical eye opening have been improved and it also makes lower jitter time, which can obviously show that the FFE equilibrium have better equilibrium effect and It effectively reduces the bit error rate.
Figure 5.(Color online) BER versus FFE tap number.
Figure 6.(Color online) BER performance with FFE and without FFE versus ROP for 35 km.
Figure 7.(Color online) The eye diagram performance for different distance.
4. Conclusion
In this paper, we have experimentally demonstrated that the 56 Gbps single wavelength PAM-4 signal transmission system transferred over 35 km SSMF only using the different size of FFE equilibrium without any optical amplification. We experimentally demonstrated that the BER of BTB, 25 km, 35 km PAM-4 transmission system is below the 7% FEC limit of 3.8 × 10–3 with the receiver sensitivity of –8 dBm and is below the KP4-FEC 2.4 × 10–4 with the receiver sensitivity of –6 dBm. Our DMLs with high bandwidth play a significant role in reducing ISI induced by device bandwidth, which significantly help to reduce the complexity of the algorithm. The DML made in our laboratory with good linearity is suitable for PAM-4 modulation due to high nonlinear tolerance of PAM-4 modulation. Besides, our DML also owns high output power and low threshold current playing an important role in simplifying transmission system, which will be a low cost choice in the practical application.
Acknowledgments
This work was supported by National Key Research and Development Program of China (No. 2018YFB2201101) and the National Natural Science Foundation of China (Nos. 61635001 and 61575186).
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