The rapid development of emerging technologies such as 5G, the Internet of Things, and artificial intelligence has created an ever-growing demand for high-capacity data transmission, increasing the pressure on current optical fiber systems. However, the bandwidth limitations of optical fiber amplifiers constrain the number of usable channels, creating a bottleneck in expanding system capacity. Enhancing the gain bandwidth of optical fiber amplifiers is therefore an effective approach to increase the transmission capacity. In long-distance fiber transmission systems, the C-band/L-band Erbium-Doped Fiber Amplifier (EDFA) configuration is often employed to amplify optical signals across parallel structure. However, traditional L-band EDFAs generally cover a gain bandwidth of only 40 nm (1 565~1 605 nm), while the International Telecommunication Union defines the L-band as 60 nm (1 565~1 625 nm). This has driven significant interest in extending the L-band gain bandwidth toward longer wavelengths. As a result, recent years have seen growing research efforts toward developing L-band extended EDFAs. However, the development of L-band extended EDFAs still faces challenges such as limited gain bandwidth, insufficient gain levels, and poor gain flatness.In this study, we explore the gain extension properties of bismuth-erbium co-doped fiber (BEDF), which offers promising potential for extending the gain bandwidth of L-band EDFAs. We designed and developed a two-stage L-band extended BEDF amplifier, which incorporates a double-pass amplification structure in the main amplifier stage. This configuration not only aims to broaden the gain bandwidth but also enhances the gain performance and suppresses noise figure effectively. Our research confirmed that BEDF can effectively extend the L-band gain bandwidth, providing new insights for further advancements in L-band extended EDFA technology. To achieve the L-band gain extension, co-doping erbium-doped fibers with elements such as phosphorus (P), ytterbium (Yb), and aluminum (Al) can shift the Excited State Absorption (ESA) spectrum of erbium ions, effectively broadening the gain spectrum into the longer L-band wavelengths. The addition of bismuth (Bi) ions into the fiber composition further enhances the emission cross-section of erbium ions, contributing significantly to the extended gain bandwidth. By incorporating Bi ions, the BEDF amplifies more effectively, as these ions both broaden the gain bandwidth and increase emission intensity.To enhance gain and suppress noise figure, we implemented a two-stage L-band extended bismuth-erbium co-doped fiber amplifier, where the main amplifier and pre-amplifier stages are connected via a circulator. The pre-amplifier uses a 2 m-long Er-doped fiber pumped by a 980 nm semiconductor laser at 100 mW. The main amplifier consists of a segment of BEDF that is bidirectionally pumped by two 1 480 nm semiconductor lasers through a Wavelength Division Multiplexer (WDM), with forward and backward pump powers of 400 mW and 500 mW, respectively. A Faraday Rotator Mirror (FRM) is positioned at the end of the amplifier, which reflects the amplified signal back through the BEDF, effectively enhancing the gain performance.The study tested BEDF lengths of 3.6 m, 5.8 m, and 6.8 m, analyzing gain and noise figure characteristics for each configuration. Results indicated that the gain spectra for all three fiber lengths appeared in the range of 1 560~1 620 nm, with notable shifts as fiber length increased. Specifically, longer BEDF lengths resulted in a gain peak shift towards longer wavelengths, while gain at shorter wavelengths diminished, leading to a narrowing of the gain bandwidth. At a BEDF length of 6.8 m, the gain level was significantly lower than that at 5.8 m, suggesting an optimal BEDF length for effective amplification. Across all BEDF lengths, the amplifier achieved a gain level above 11.36 dB at 1 620 nm, demonstrating effective L-band gain bandwidth extension, which is attributed to two intrinsic mechanisms: the broadening of the emission cross-section of erbium ions by Bi ions, and the energy transfer from Bismuth Active Centers (BACs) to erbium ions. This transfer enhances emission intensity and efficiency, especially in the presence of BAC-Ge, which exhibits an emission peak at 1 610 nm, directly supporting L-band gain bandwidth extension. The double-pass amplification structure further enhances gain and suppress noise figure. The signal reflected by the FRM undergoes a second amplification in the BEDF, effectively increasing the overall length of the gain fiber. Testing with double-pass BEDF lengths of 3.6 m and 4.8 m showed that the 3.6 m BEDF achieved a wider 20 dB gain bandwidth, higher maximum gain, and a lower minimum noise figure, although with slightly lower gain flatness compared to the 4.8 m configuration. The 4.8 m BEDF showed a slight reduction in 20 dB gain bandwidth and maximum gain, but with improved average gain and gain flatness.Comparison of the single-stage main amplifier and the two-stage amplifier within the 1 560~1 615 nm range revealed that the two-stage amplifier delivered a higher gain than the main amplifier, with comparable gains in the 1 615~1 620 nm range. This is because the pre-amplifier increases the overall pump power, resulting in a shift of the central wavelength towards shorter wavelengths and thus enhancing gain at shorter wavelengths while slightly reducing gain at longer wavelengths. In terms of noise performance, the two-stage amplifier exhibited a lower noise figure within the 1 560~1 610 nm range compared to the main amplifier, with similar noise levels in the 1 610~1 620 nm range. The noise figure improvement is attributed to the primary role of the first stage in determining noise figure, where the pre-amplifier contributes to increased gain and noise suppression.This study achieved a 20 dB gain bandwidth of 63 nm (1 555~1 618 nm), a maximum gain of 52.84 dB, a minimum noise figure of 4.23 dB, and a 3 dB flatness gain bandwidth of 35 nm (1 565~1 600 nm). Compared with recently reported L-band extended EDFAs, our configuration achieved a broader 3-dB flatness gain bandwidth, while maintaining high gain and wide high-gain bandwidth. These results underscore the potential of BEDF for advancing optical amplifier technology and supporting high-capacity, long-distance optical transmission systems. Further optimization in the co-doping composition and ratio of elements in BEDF is expected to extend gain bandwidth even further, paving the way for innovations in optical amplifier design.