【摘要】 传统掺铒光纤放大器（EDFA）有限的增益带宽已难以应对当前光网络的容量危机，而将放大波段从C波段或L波段扩展至C+L全波段，是解决这一问题的有效方案。提出了一种基于掺铒光纤（EDF）与高锗掺铋光纤（HiGe-BDF）组合结构的新型C+L波段光放大器，对该方案的增益特性与噪声特性进行了系统表征，并深入分析了增益谱形态的演化规律及调控机制。实验结果表明，在双向泵浦结构下，当输入信号功率为-25 dBm时，该放大器在1525～1625 nm波段范围内的平均增益达到18.04 dB，增益波动小于5.3 dB，噪声指数（NF）处于4.3 dB～7.2 dB区间，充分验证了该放大器在超宽带通信系统中的实用潜力。更多还原

【Abstract】 Objective Currently, the erbium-doped fiber amplifier（EDFA）, which is employed as the mainstream amplifier in optical fiber communication systems, can only provide a gain bandwidth of approximately 48 nm in both the extended C-band（1524-1572 nm） and extended L-band（1576-1624 nm）. Typically, two main types of technical solutions are adopted to address the gain bandwidth limitations of EDFAs. The first type comprises parallel optical fiber amplifiers of different bands that couple the amplified signals of different bands into the transmission optical fiber. However, this method exhibits various drawbacks, such as severe gain imbalance, the existence of gain “dead zones,” complex structures, and high costs. The second approach involves the development of ultrawideband amplification optical fibers to achieve C+L ultrawideband amplification in a single amplifier. High-germanium bismuthdoped fibers（HiGe-BDFs） show great potential for ultrawideband amplification because their wide emission spectrum overlaps with that of erbium-doped fibers（EDFs） near the L-band. Based on this feature, an erbium-bismuth codoped fiber（BEDF）, containing two luminescent centers, Er³⁺ and bismuth-associated active centers-germanium（BACs-Ge）, is proposed. Effective luminescence of the C + L broadband can be achieved by adjusting the concentration and proportion of the two luminescent centers. However, BEDF fabrication is complex and challenging, and this material exhibits several drawbacks, such as high unsaturated loss（UL）, poor noise performance, narrow emission spectrum of erbium, and challenges associated with controlling the concentration ratio of BACs-Ge and Er. Therefore, developing an optical amplifier with better noise performance and lower fabrication complexity is essential to achieve integrated amplification in the C+ L band.Methods EDF and HiGe-BDF are prepared using an improved chemical vapor deposition（MCVD） method combined with the solution-doping technology. The numerical apertures（NA） of EDF and HiGe-BDF are measured using a fiber preform analyzer, and the absorption spectrum of BDF is tested using a fiber analysis system and the truncation method. A single-stage bidirectional pump amplification structure is established. Two 1480-nm solid-state lasers（with front and rear pumping powers of 690 mW and 720 mW, respectively） are adopted to provide the pumping light, which is coupled with the signal light through a wavelength division multiplexer（WDM） before entering the active optical fiber. The amplification performances of EDF, HiGe-BDF, and their combinations are successively characterized. Finally, a spectrometer（OSA） is used to monitor the input signal after the signal source and output signal at the rear isolator（ISO） output port. The resolution is set to 0.1 nm, and the gain and noise figure（NF） are calculated and analyzed by the EDFA module of OSA.Results and Discussions First, the regulation laws of the EDF and HiGe-BDF lengths on the gain characteristics are explored [Figs. 3（a） and（b）]. Figure 3（a） shows the gain characteristics of the EDF at different lengths when the input signal is-25 dBm. While its gain range covers the C+L range, the gain decreases significantly in the L band, especially after 1605 nm. As shown in Fig. 3（b）, the net gain of HiGe-BDF is observed at ～1560 nm, and its positive gain covers the L band. The complementary distribution characteristics of this gain indicate that splicing the HiGe-BDF after the EDF can effectively increase the gain after 1560 nm—this strategy is suitable for achieving amplification in the C+L band. Figure 4 shows the results of the systematic tests conducted on the gain and NF of the C+L band amplifier（combined with the EDF and HiGe-BDF） under different EDF lengths. When the EDF length is adjusted to 2 m, the system achieves gain flattening in the range of 1525-1625 nm, with an average gain of 18.04 dB. The NF is 4.3 dB-7.2 dB, and the full-band gain flatness is controlled within 5.3 dB. Figure 5 shows the measured amplification performance of the combined optical fibers under different input signal powers（at the same optical-fiber length）. When the input signal power increases from-25 dBm to-6.5 dBm, the overall gain slightly decreases but still remains above 17.4 dB, and the NF and gain flatness are 4.0 dB-7.4 dB and 5.4 dB, respectively.Conclusions In this study, an EDF–HiGe-BDF combined structure is used to realize gain enhancement and noise optimization in a C+L band（1525-1625 nm） ultrawideband optical amplifier. The analysis shows that the EDF has a high gain efficiency in the C band（1525-1565 nm）, whereas its L-band gain significantly attenuates after 1605 nm owing to signal excited state absorption（SESA）. The gain of the HiGe-BDF in the L-band（1560-1630 nm） increases in the positive direction with the wavelength, and the gain characteristics of the EDF and HiGe-BDF exhibit excellent complementarity. Splicing the EDF（2 m long） and HiGe-BDF（235 m long） provides a planarization spectrum, with an average gain of 18.02 dB and a gain flatness of ≤5.3 dB, under small-signal conditions. The EDF prestructure utilizes its low-noise characteristics, and the HiGe-BDF post-structure absorbs the amplified spontaneous emission（ASE） noise of the former, yielding an improved NF of 4.3 dB-7.2 dB. This scheme circumvents the drawbacks of codoped optical fibers, such as concentration regulation requirements and poor noise performance. It offers a low-cost and highly compatible solution for ultrawideband amplification owing to its low process complexity, controllable costs, and a great potential for C+L ultrawideband amplification.更多还原