铋铒共掺光纤实现C+L+U波段超宽带放大

Bismuth Erbium Co⁃Doped Fiber for C+L+U⁃Band Ultra⁃Wideband Amplification

传统掺铒(Er)光纤放大器的增益带宽已无法满足日益增长的通信带宽需求。为了对现有通信系统进行升级扩容,迫切需要开发出可实现多波段同时放大的光纤放大器。然而,国内尚未成功研制出能够实现多波段同时放大的增益光纤。基于改进的化学气相沉积技术,成功制备了可实现C+L+U波段同时放大的铋(Bi)铒共掺光纤,通过比较不同Bi、Er掺杂浓度对增益谱的影响,分析了其影响机制。讨论了不同泵浦方式(前向泵浦、后向泵浦和双向泵浦)和不同泵浦波长(1460 nm和1480 nm)对增益谱的影响。结果显示,基于双向泵浦结构,当输入信号功率为-25 dBm时,采用自制铋铒共掺光纤搭建的放大器在1530~1670 nm范围内实现了11.8 dB以上的增益,噪声指数为6.3 dB~9.3 dB。

Objective Existing communication systems continue to use traditional erbium (Er)-doped fiber amplifiers,which can amplify only the C or L band,severely limiting further expansion of communication transmission bandwidth.To expand the transmission bandwidth,a common method is to connect fiber amplifiers in different bands in parallel and then couple the amplified signals in different bands into a single fiber for transmission.However,this method also faces problems such as severe gain imbalance,the existence of gain“dead zones,”and complex structures with high equipment costs.Therefore,achieving simultaneous amplification of multiple bands in a single fiber is considered a major solution for expanding gain bandwidth and improving transmission capacities.Due to their ultra-wideband emission characteristics,bismuth (Bi) Er co-doped fibers (BEDFs) are considered the most feasible solution for achieving simultaneous multiband amplification and have attracted widespread attention from academia and industry.With reliance placed on the joint action of Er and germanium-related bismuth active centers (BACs-Ge),ultra-wideband flat amplification can be achieved in the C+L+U band.In 2017,the Fiber Optic Research Center of Russia (FORC) successfully prepared for the first time a BEDF with a gain of over 15 dB at 1515?1775 nm.However,to date,no studies have been reported in China on BEDFs that can simultaneously achieve a positive gain in the C+L+U band.Methods Preforms with different concentrations of Bi and Er are prepared using a modified chemical vapor deposition (MCVD)method.The GeO_(2) content in the BEDF core is measured using an electron probe micro analyzer (EPMA),and the Er and Bi contents in the BEDF core are measured using inductively coupled plasma mass spectrometry (ICP-MS).The absorption spectrum of the BEDF is measured using a fiber analyzer based on the truncation method.The amplification testing system is built based on a selfmade BEDF using three different pumping schemes,namely,forward,backward,and bidirectional.The input signal is provided by a32-channel comb light source in the range of 1515?1670 nm with a wavelength interval of 5 nm,and the total power is set to-25 dBm.Laser diodes (LDs) of 1460 nm and 1480 nm are used as pump light sources at powers of 500 mW and 583 mW,respectively.Results and Discussions The gain performance of the BEDFs is tested using a forward pump amplification testing system,and the results are shown in Fig.3.The ratio of unsaturated loss to absorption coefficient of the BEDFs is tested at a pump wavelength of1460 nm,as shown in Table 2.Figure 3 shows that the optimal length is shortened from 240 m (BEDF-1) to 45 m (BEDF-4) as the absorption coefficient at 1650 nm increases from 0.24 dB/m (BEDF-1) to 1.6 dB/m (BEDF-4),which are mainly affected by the concentration of BACs-Ge.The gain of BEDF-1 at 1670 nm is significantly lower than those of other BEDFs,mainly because its ratio of unsaturated loss to absorption coefficient (28%) at 1460 nm is significantly higher than those of other BEDFs.The difference in the ratio of unsaturated loss to absorption coefficient also leads to an opposite trend in the gain of BEDF-1 and other BEDFs between1580 nm and 1670 nm.A higher ratio of unsaturated loss to absorption coefficient leads to greater pump energy loss.Although the absorption coefficients and usage lengths of BEDF-2 and BEDF-3 differ,their maximum gains at 1670 nm are similar.This is mainly due to the similar ratios of unsaturated loss to absorption coefficient of BEDF-2 and BEDF-3 at a pump wavelength of 1460 nm,which significantly affects the maximum gain.The ratio of unsaturated loss to absorption coefficient of BEDF-4 at 1460 nm is 15%,which is lower than those of BEDF-2 and BEDF-3,but its gain at 1670 nm is not significantly different from the latter.This is because the concentration of Er in BEDF-4 is significantly higher than those in BEDF-2 and BEDF-3,and the loss of Er at long wavelengths is greater than the gain,which reduces the luminescence intensity of BACs-Ge at long wavelengths.To observe the effects of pump wavelength changes on the gain spectrum,a 1480-nm pump light is also used.Based on the 1460-nm and 1480-nm LDs,the gain and noise spectra of BEDF-4 are tested using the three schemes of forward,backward,and bidirectional pumping,as shown in Fig.4.The gain under backward pumping at 1480 nm decreases in the range of 1515?1580 nm as compared with the gain spectrum under forward pumping at 1460 nm,whereas it increases in the range of 1580?1670 nm.This is because the absorption coefficient of BACs-Ge increases,whereas that of Er for the pump light decreases after the pump wavelength shifts from 1460 nm to1480 nm.This results in more pump light energy being transferred to longer wavelength direction.Compared with that under unidirectional pumping,the gain under bidirectional pumping increases,and the corresponding noise coefficient decreases.A bidirectional pumping scheme is also adopted for BEDF-2,as shown in Fig.5.The figure shows that the gain is over 11.8 dB in the range of 1530?1670 nm,and the corresponding noise figure is 6.3 dB?9.3 dB.Conclusions This study prepares a BEDF with C+L+U-band amplification capabilities based on MCVD combined with liquidphase doping technology.An ultra-wideband amplification testing platform is built to test and analyze the BEDF.The BEDF-2 achieves a gain of over 11.8 dB in the range of 1530?1670 nm with a noise figure of 6.3 dB?9.3 dB under the conditions of an input signal power of-25 dBm and bidirectional pumping.