Qiguang Feng

Impacts of backscattering noises on upstream signals in full-duplex bidirectional PONs

The backscattering noises introduced by Rayleigh and stimulated Brillouin scattering have been experimentally studied by means of their spectrum broadening, the scattering power variation and their impacts on upstream signals with different transmission fiber lengths and incident powers in a single-fiber bidirectional passive optical network (PON) communication system. The results show that both spontaneous scattering and simulated scattering can take place. The power and spectrum of backscattering noises are determined by the downstream launch power, laser linewidth and transmission fiber length. With the transmission length increasing, the power of backscattering noises gets higher, the spectrum of the backscattering noise broadens and the simulated threshold power decreases. The backscattering noise can beat with uplink light to modulate envelop of upstream signal resulting in degradation of BER greatly. Under the condition of one single channel for the second next generation PON (NG-PON2), the fiber length is 40km and downstream launch power is up to 11dBm. At this time, the backscattering noises are easy to be stimulated and the scattering power rises up from -20dBm to 10dBm, which can overwhelm the US signal. The spectrum of the optical beat interference noise also rises up with fiber length, which causes the uplinks BER degradation. The experimental results are significant for mitigation of backscattering noises under the condition of bidirectional PONs.

Colorless Long-Reach Duplex WDM-PON With Rayleigh Backscattering Noise Mitigation Using Orthogonal Codes

We propose and experimentally demonstrate a novel colorless full-duplex passive optical network (PON) access architecture. Utilizing orthogonal codes and correlation receiving methods, the novel PON can mitigate the optical beat interference (OBI) noise induced by Rayleigh backscattering (RB). A pair of electrical orthogonal codes is generated and modulated at the same wavelength in the optical line terminal (OLT). Then, modulated optical signals are transmitted from the OLT to the optical network units (ONUs). One of the codes is for downstream signal coding, and the other is used as the upstream seed. In the ONU, the upstream signal is remodulated without erasing the downstream signal. Neither extra centralized continuous wave (CW) light sources nor gain-saturated reflective semiconductor optical amplifier is required. By using these orthogonal codes, the spectral overlap between upstream signal and downstream signal in the full-duplex system is reduced, which can mitigate the OBI noise significantly. The performance of transmission and power margin is investigated through the experiments with different transmission distances. The remodulated upstream signals are recovered by correlation algorithm in the OLT. By correlation algorithm, we can get coding gain, which is important in long-reach transmissions. Because of coding gain and the OBI noise mitigation, a link power margin of 4-10 dB can be achieved for 5 Gb/s downstream and 1.25 Gb/s upstream, when the transmission distance is from 20 to 70 km.

Investigations of Backscattering Effects in Optical Fibers and Their Influences on the Link Monitoring

In this paper, first, we employed the coupled-mode theory to investigate the backscattering power characteristics of Rayleigh backscattering (RB) and stimulated Brillouin scattering (SBS) both synchronously and precisely. We discovered how the RB and SBS power changed with pump powers and fiber lengths. The scattering power curves with different pump powers could be divided into three regions. The first was called “the linear region of RB,” in which the pump power was mainly converted into RB and the RB power grew linearly with pump powers. The second region was called “the stimulation region of Brillouin scattering,” in which the power was mainly converted into SBS. In “the gain-saturated region,” when the pump power was much higher than the SBS threshold, both the RB and SBS power were gain-saturated. We also obtained the power curves with different fiber lengths. The power curves were helpful to set up proper launch powers in practical engineering applications, such as optical time-dome reflectometry systems and distributed fiber sensors. Then, we verified our theoretical results by experiments. Finally, according to the measurements and numerical solutions of the coupled-mode equations, we discussed the application of backscattering and gave advice for pump power selections in different fiber link monitoring systems.

Ring-Based Colorless WDM-PON With Rayleigh Backscattering Noise Mitigation

We propose and experimentally demonstrate a novel ring-based wavelength-division-multiplexing passive optical network (WDM-PON). By using an orthogonal coding and correlation technique, the ring-based WDM-PON can efficiently mitigate the optical beat interference noise induced by Rayleigh backscattering. In our proposed orthogonal coding scheme, no extra light sources are required for the upstream (US) transmission, which means the number of light sources can be halved and the saved wavelengths can be used for more optical network units. In addition, compared to previous ring-based WDM-PONs, the proposed ring-based WDM-PON requires fewer fiber Bragg gratings and no blue/red-band filter, which means the proposed orthogonal coding scheme costs less. The performance of the proposed scheme is investigated through an experiment. In the experiment, 2.5 and 5 Gb/s US data rate and 5 Gb/s downstream data rate are investigated in two different ring architectures. According to the experimental results under different situations, it can be concluded that the US orthogonal coding scheme performs better than the normal US on–off keying in a ring-based WDM-PON. The power margin can be up to 1.8 dB when the US data rate is 2.5 Gb/s, and the power margin can be up to 1.69 dB when the US data rate is 5 Gb/s.