【摘要】 具有低差分模式增益（DMG）的少模掺铒光纤放大器（FM-EDFA）对于模分复用（MDM）结合波分复用（WDM）系统的长距离传输尤为重要。采用环形芯少模掺铒光纤（FM-EDF）和高性能少模器件，构建了一种纤芯泵浦四模掺铒光纤放大器（4M-EDFA）。通过基模泵浦，实现了21.65 dB的平均增益和1.51 dB的低DMG。在高功率泵浦下，可获得高于25 dB最大增益。此外，对FM-EDFA同时放大多波长信道信号进行了评估。实验结果表明，整个C波段的平均增益为23.38 dB,DMG小于3.63 dB。更多还原

【Abstract】 Objective So far, numerous investigations on gain equalization for few-mode erbium-doped fiber amplifier（FM-EDFA） have been reported. Previous research on FM-EDFA has primarily focused on the amplification of single-wavelength signals. The multi-wavelength characteristics of FM-EDFA have been discussed theoretically in several papers. And there is a paucity of experimental research on FM-EDFA for multi-wavelength amplification. As demonstrated in reference, the reported FM-EDFAs only support two-mode groups(LP₀₁, LP_{11a, b}). In order to achieve enhanced transmission capacity, it is essential to investigate the potential of multi-wavelength amplification with a greater number of higher-order modes. Additionally, several studies on core-pumped FM-EDFAs employ complex free-space components to realize gain equalization, but with a cost of low system integration. In view of this, to simplify the amplifier structure and achieve the practical transmission of long-haul mode division multiplexing-wavelength division multiplexing（MDM-WDM） systems, experimental investigations of all-fiber FM-EDFA with low differential modal gain（DMG） for amplifying multi-mode and multi-wavelength signals are urgently required.Methods By employing simple fusion coupling between devices and fibers, we experimentally construct a compact core-pumped FM-EDFA. The few-mode erbium-doped fiber（FM-EDF） used in this study is fabricated in-house using the modified chemical vapor deposition（MCVD） and chelate pure vapor deposition process. The few-mode multiplexer employed in the experiment is developed through the fused-taper method, which is constructed by cascading four individual mode-selective couplers（MSCs）. The 980-nm single-mode laser diode（LD） is applied as the pump source. The pump light and signal light are coupled together into the FM-EDF through a few-mode isolator wavelength division multiplexer（FM-IWDM）. The FM-IWDM also ensures unidirectional transmission of signal light and prevents the adverse effects of oscillating light on amplifier stability. An FM-WDM is connected to the output port of FM-EDF to filter out the residual pump light. The FM-WDM can also be utilized to improve modal gains by connecting an extra pump for backward pumping, if a single pump provides insufficient gain. The FM-EDFA proposed in this paper has the advantage of highly compact. On the one hand, it adopts an all-fiber structure to enhance operational stability and ensure robust performance. On the other hand, the LP₀₁ mode core-pumping scheme can simplify the system structure without introducing additional phase plates or pump mode converters.Results and Discussions To investigate the gain characteristics of FM-EDFA, it is crucial to determine the optimum length of FM-EDF. In few-mode fibers, it is essential to take into account the impact of FM-EDF length on DMG. When the signal input powers of 4-mode are equally fixed at-12 dBm and the signal wavelengths are set to 1550 nm, the modal gains and DMG of FM-EDFA under five EDF lengths（3, 2.5, 2, 1.5, 1 m） are experimentally measured as a function of pump power. It can be seen that the modal gains for several fiber lengths gradually improve with the increase of pump power and eventually tend to be saturated. As the EDF length increases, the absorption of signal light by the EDF becomes stronger, requiring higher pump power to achieve positive gain（>0） of signal modes. When the EDF length is within the range of 1 m to 2.5 m, starting from the pump power that can obtain positive modal gain, the DMG of four modes generally decreases as the pump power increases. The result is that, starting from a length of 1.5 m, selecting a shorter EDF length will significantly reduce the modal gain. Although a lower DMG can be obtained with an EDF length of 1 m, it is still difficult to achieve a modal gain of 20 dB at a high pump power. At an EDF length of 2.5 m, the lowest DMG can be achieved at a pump power of 700 mW, and high modal gains can be also obtained.In general, the amplification characteristics of FM-EDFA also vary with different signal input powers. The relationship between DMG and different signal input powers is more complex and requires more in-depth experimental research to illustrate. The gain and DMG of the 4-mode FM-EDFA under different signal input powers when the FM-EDF length is selected as 2.5 m. It can be observed that there is a decrease in modal gain as the signal input power increases. For an input power of-10 dBm, the DMG is relatively high. At an input power of-5 dBm, despite the low DMG at a high pump power, the modal gain remains relatively low. When the signal input power is fixed at-15 dBm/mode and the pump power is set to 400 mW, a low DMG of 1.51 dB can be achieved between the four modes. Meanwhile, the gains of the four modes are all greater than 20 dB, with an average gain of 21.65 dB.Finally, the WDM signal amplification characteristic of FM-EDFA is also evaluated. In MDM-WDM transmission system, wavelength gain flatness is an important index in amplifiers for amplifying multiple wavelength signals simultaneously. The wavelength gain flatness is closely related to the length of EDF due to the gain differences and competition among different wavelengths. Generally, longer EDF lengths result in higher gains for the long wavelength signals, while shorter EDF lengths can achieve higher gains for shorter wavelength signals. To achieve gain balance in multi-wavelength channel amplification, the optimum length of the FM-EDF in this experiment is selected as 2 m. The modal gain and DMG characteristics of four modes in the whole C-band（1530 nm to 1565 nm）, where eight equalized signals with a total power of-15 dBm/mode are selected from the amplified spontaneous emission（ASE） source as the WDM signal input source. The amplified output spectrum of the LP₀₁ mode with 8-channel WDM signals at wavelengths of 1530.182, 1535.064, 1539.965, 1544.904, 1549.871, 1554.893, 1559.94, 1565.012 nm. The trend of the amplified signal output spectrum and the gain spectrum with wavelength is almost consistent. The amplification results of 8-channel WDM signals indicate that the average gain of four modes is 23.38 dB, the DMG is lower than 3.63 dB, and the wavelength gain flatness is below 4.3 dB over the C-band. While introducing an extra gain flattening filter（GFF） at the output of the FM-EDFA, it is expected to achieve lower wavelength flatness.Conclusions By using self-developed few-mode passive and active fibers, as well as few-mode devices, an all-fiber 4-mode FMEDFA is designed. Through the doping profile optimization of FM-EDF and simple LP₀₁ mode pumping, an average gain of 21.65 dB and an ultra-low DMG of 1.51 dB for the 4-mode are obtained experimentally. And the maximum modal gain of 25.43 dB can be achieved at a high pump power setting of 700 mW. Additionally, when the FM-EDFA is applied into WDM system for the amplification of multiple wavelengths, the average modal gain is 23.38 dB, the DMG is lower than 3.63 dB and the wavelength gain flatness is less than 4.3 dB over the C-band. These research findings demonstrate much potential for the transmission prospects of long-haul MDM-WDM systems.更多还原