Based on the experimental results (Fig. 3), a comparison between the performance of the two-stage double-pass structure and the two-stage single-pass structure in the L-EDFA shows that the two-stage double-pass configuration yields significantly higher gain than its single-pass counterpart, particularly beyond the 1558 nm mark, with all maintaining the same pump power for the main amplifier. In contrast to the main amplifier's double-pass amplification structure, while the gain in the wavelength range above 1614 nm experiences a minor reduction, the two-stage double-pass structure exhibits substantial enhancement across the broader span of 1555-1614 nm. Meanwhile, the noise figure of the L-EDFA in the two-stage double-pass structure demonstrates minimal increase when compared to the two-stage single-pass structure, and it remains lower than that of the main amplifier's double-pass structure across the majority of the wavelength range. These findings underscore the effectiveness of the two-stage double-pass amplification structure in significantly boosting both gain level and gain bandwidth, exhibiting clear advantages over both the two-stage single-pass amplification and the main amplifier's double-pass structures. Additionally, the noise figure remains within a lower range. This achievement is primarily attributed to the pre-amplifier incorporation, which serves the dual purpose of elevating gain levels and mitigating the noise figure. During comparing the key parameters of L-EDFAs at home and abroad in recent years (Table 1), the two-stage double-pass L-EDFA achieves optimal gain levels and gain bandwidth with the lowest total pumping power. Furthermore, the noise figure remains consistently low, even though there is a slight overall improvement in the noise figure.

Due to the increasing demand for information transmission capacity, traditional wavelength-division multiplexing (WDM) optical fiber transmission technology cannot meet the requirements of communication technology advancement. In recent years, L-band extended erbium-doped fiber amplifiers (L-EDFAs) have become a research hotspot due to their ability to cover a wider wavelength range, enabling the transmission of more wavelength channels and thus a larger information amount. Consequently, this type of device has caught significant research attention. However, the initial research on L-EDFAs often results in relatively low gain levels, which could not satisfy the requirements for transmitting large information amounts. Thus, we propose a solution that may achieve high-gain and low-noise L-EDFAs. Experimental results demonstrate that the proposed L-EDFA outperforms similar devices in other references. This advancement could provide a foundation for further research, potentially leading to the industrialization of such devices. The proposed solution has promising prospects for meeting the growing demand for high-performance optical amplification in L-band applications.

By utilizing 1480 nm lasers to pump Er/Yb/P co-doped fiber and incorporating double-pass amplification and pre-amplification technologies, an enhanced L-EDFA is developed. Ytterbium (Yb) ions and phosphorus (P) ions are initially introduced into the erbium-doped fiber to mitigate excited state absorption (ESA) of erbium ions in the longer wavelengths of the L-band, extending the L-band for the EDFA. Subsequently, a two-stage double-pass experimental setup for the L-EDFA is implemented to further enhance its gain. This experimental arrangement comprises a main amplifier and a pre-amplifier cascaded through a circulator to form a two-stage amplification structure. Once amplified through a single pass, the signal light is reflected to the main amplifier for secondary amplification. The initial testing involves evaluating the single-pass and double-pass amplification performance of the main amplifier. Pump power optimization for the single-pass structure of the main amplifier is conducted and compared with that of the double-pass structure. To minimize noise, we form a two-stage single-pass or double-pass amplification structure by cascading the pre-amplifier and the main amplifier. The pump power for the two-stage single-pass structure is also optimized and compared with that of the double-pass structure to reduce noise and improve performance.

Based on the experimental results in the single-pass and double-pass amplification performance of the main amplifier in the L-EDFA (Fig. 2), it is evident that the main amplifier achieves an extended gain bandwidth for the L-band in both single-pass and double-pass amplification configurations. Notably, within the wavelength range of 1560-1620 nm and equivalent pump power, the double-pass amplification structure demonstrates a significantly higher gain compared to the single-pass structure. The gain enhancement effect of the double-pass configuration is particularly remarkable. By progressively increasing the reverse pump power of the single-pass structure to 250 mW and maintaining the forward pump power at 300 mW, the EDFA gain is improved. However, it remains substantially lower than that of the double-pass structure, even with a 200 mW total pump power reduction across a substantial portion of the gain bandwidth. This further underscores the advantageous gain enhancement properties of the double-pass amplification structure. Regarding noise figure comparisons, the single-pass amplification structure of the L-EDFA maintains a lower noise level, even when the reverse pump power is elevated to 250 mW. Notably, the noise figure remains relatively stable. Conversely, the double-pass amplification structure exhibits a higher noise figure, suggesting a trade-off between gain level and noise performance. Possessing favorable noise characteristics, the single-pass structure presents a modest gain level. Conversely, the double-pass structure provides a higher gain level but compromised noise performance.

We utilize a 1480 nm laser-pumped Er/Yb/P co-doped fiber and combine pre-amplification and double-pass amplification techniques to develop a two-stage double-pass L-EDFA. Within the wavelength range of 1556-1621 nm, a gain exceeding 20 dB is achieved, and within the range of 1557-1615 nm, a gain surpassing 30 dB is realized. Specifically, gains of 48 dB and 39 dB are respectively attained at 1566 nm and 1605 nm. Remarkably, the saturated output power reaches 20.58 dBm at 1605 nm. The noise figure remains below 5.8 dB within the 1580-1610 nm range, with a minimum of 4.6 dB. Comparing with L-EDFAs reported domestically and internationally in recent years, we achieve the optimal gain level and gain bandwidth while operating under the lowest total pumping power. This approach has the potential to become the mainstream technical solution for the next generation of L-EDFAs and can be extensively applied to future high-capacity fiber optic transmission systems.