Currently, the erbium-doped fiber amplifiers (EDFA) operating in the C-band have been developed for many years and are widely used in wavelength division multiplexing (WDM) systems, especially in WDM passive optical network (PON) architectures. In most reported WDM-PON architectures, both the upstream and downstream transmission directions are used separately with their independent amplifiers, which undoubtedly increases the deployment cost. Considering the cost effectiveness of amplifiers, Jung et al. used a single-fiber amplifier with bidirectional amplification in a hybrid WDM long-haul PON architecture, which resulted in significant device cost savings. However, owing to the rapid growth in data volume, researchers have also been investigating fiber amplifiers for other bands. In particular, the O-band amplifiers have received significant attention because they are also in the low-loss region of silica fibers. After years of research, bismuth-doped fiber amplifiers (BDFA) have been proven to be the most promising candidates for O-band amplifiers. Thus, O-band BDFAs have advanced rapidly in recent years, and a series of BDFAs with a high gain and low noise figure (NF) have been reported one after another. However, to date, there have been few reports on the application of O-band BDFA in WDM-PONs. Therefore, investigating the performance of O-band BDFA in bidirectional amplification is of great significance for future applications of BDFA in PON.

In this study, we constructed a single-fiber bidirectional amplification system for an O-band BDFA via a coupler and circulator and compared the gain and NF obtained for both signals at the same wavelength. First, we evaluated the light leakage degree of the circulator and the limited isolation degree of the isolator to ensure the performance of the passive device. We constructed a single-fiber bidirectional BDFA based on a coupler to test the performance of the two signals when they were subjected to reverse light interference without isolators at the input and output of the amplifier. Finally, we reconfigured the system with a circulator instead of a coupler to verify the optimization of the circulator in bidirectional amplification and compared the results with those of the coupler structure. In the experiment, an isolator was connected between the pump and the WDM to prevent back reflection from damaging the pump. Couplers with different splitting ratios were connected to the tunable laser to ensure that the signals in both directions had the same wavelength and power. After the signal source was split into two by the coupler, the two signals from the left and right circuits passed through the circulator/coupler and the WDM, entered the bismuth-doped fiber to be amplified, and finally were output to an optical spectrometer analyzer (OSA) 1 and OSA 2 for detection.

We first tested the bidirectional amplification performance of the BDFA under a coupler structure, and the results are shown in Figs. 5(a) and (b). The results show that the maximum gain of both signals does not exceed 20 dB, and the NF is in the range of 8 dB?12 dB at an input power of -15 dBm. The gain and NF of both signals in the bidirectional amplification system are severely degraded compared to those when only one signal is amplified separately. We consider that one of the main reasons for the deterioration of the gain and NF of the signal is the high insertion loss of the 3 dB coupler, which leads to a significant reduction in the power of the signal after passing through the coupler twice during the amplification process, making it difficult to obtain a high gain. In addition, the absence of isolators at both the input and output of the BDFA in the coupler structure leads to a certain measure of reverse light power in the system, which competes with the signal light for amplified spontaneous emission (ASE) power and is prone to cross-gain modulation.

Figure 6 shows the bidirectional amplification performance of the BDFA with a circulator structure. The results show that the circulator plays an active role in the bidirectional amplification of the BDFA, as it effectively reduces system loss, and the unidirectional transmission characteristic prevents the reverse light from interfering with the signal light. Further, at a pump power of 1 W and an input power of -15 dBm, both signals obtain a gain of 24 dB and saturated output power of 16 dBm. However, the NF of the signal is also reduced compared with the results in Fig. 5(b). We also observe that the NFs of the two signals are not close to the same level; the NFs obtained from OSA 2 are, on average, approximately 1.5 dB higher than OSA 1. Theoretically, if the losses in the left and right channels are identical, the NFs of the two signals do not differ significantly. However, ensuring that the loss in each pathway is equal is difficult in practical systems. Therefore, we suggest that this may be due to different accumulated losses in the two branches. As a result, the signal power (-11.416 dBm) of the OSA 1 is only approximately 0.7 dB lower than that (-10.726 dBm) of the OSA 2. In conclusion, some differences in the NFs of the two channels result from a combination of signals obtaining different ASE powers and gains.

We constructed a single-fiber bidirectional amplification system for an O-band BDFA using a coupler and circulator and compared the gain and NF obtained in bidirectional amplification for two signals at the same wavelength. The experimental results show that high-performance amplification of two O-band signals can be effectively realized using a circulator structure. The use of the circulator in bidirectional amplification effectively reduces the loss of the system, and the unidirectional transmission characteristic of the circulator reduces the interference of the reverse light on the signal. Consequently, both signals obtain a gain of nearly 24 dB and an NF of 7 dB?9 dB with a total pump power of 1 W and an input power of -15 dBm. Compared to the coupler scheme, the signals obtain a 41% increase in gain and a nearly 50% decrease in NF with the circulator structure. This study provides a valuable reference for the practical applications of O-band BDFA in WDM-PONs.