中红外侧面泵浦合束器及全光纤超荧光光源的研制(特邀)

Mid-infrared side-pumping combiner and all-fiber superfluorescent fiber source(invited)

中红外超荧光光源具有光谱范围宽、空间相干性好、时域稳定性高等特点,应用前景广泛,但受限于中红外侧面泵浦合束器,目前普遍利用空间结构泵浦产生。文中根据拉锥光纤侧面耦合的原理,在125μm包层直径的无源双包层氟化物光纤上实现了中红外光纤侧面泵浦合束器的研制,该合束器泵浦光耦合效率达82.3%,可承受的最大泵浦功率达87.5 W。通过在中红外增益光纤上制得侧面泵浦合束器,实现了全光纤中红外超荧光光源产生,前后向输出的中红外超荧光最高功率和为91.09 mW(后向输出53.67 mW,前向输出37.42 mW),输出光谱范围从2702 nm覆盖至2830 nm。在中红外超荧光总输出功率为33.03 mW时,获得了108 nm的最宽20 dB带宽。文中实现的中红外全光纤超荧光光源克服了以往空间泵浦复杂度高、调节难的问题,对推动中红外超荧光光源的进一步功率放大具有重要意义。

Objective Mid-infrared superfluorescent fiber sources(SFS),working between fluorescent and laser,not only have good spatial coherence,wide emission spectrum and high brightness,but also have no mode competition,no relaxation oscillation and high temporal stability compared to laser.It has been applied to gas detection,optical coherence tomography,and optical fiber sensing.At present,most of the reports on SFS focus on the near infrared band of 1-2μm,while in the mid-infrared band,there are few reports.Besides,all the works on mid-infrared SFS is based on the space pumping structure,which is mainly caused by the lack of mid-infrared side-pump combiner.In the scheme of space pumping,pump laser is collimated and then focused into the end face of the gain fiber to realize the coupling,and the mid-infrared SFS filtering is realized through a dichroic mirror and a long-wave filter.The structure of this scheme is relatively complex and the system stability is poor.Besides,the end face of the fluoride fiber is easy to be damaged due to the end face pumping,so the injected pump power is limited.Therefore,the development of the mid-infrared side pump combiner can not only realize all fiber structure of midinfrared SFS and overcome the problems caused by space pumping,but also realize high-power mid-infrared SFS output through backward pump.For this purpose,a home-made mid-infrared fiber side-pumping combiner and mid-infrared all fiber SFS are designed and realized in this paper.Methods Firstly,a home-made mid-infrared fiber side pumping combiner is developed on a passive double clad fluoride fiber with 125μm cladding diameter by tapered fiber side coupling principle(Fig.1).Influence of different tapering fiber profiles on combiner’s coupling efficiency has been studied.The output power and heating condition of the combiner as the pump power increasing have also been studied.Secondly,a home-made midinfrared fiber side pumping combiner is developed directly on a Er^(3+)-doped double clad fluoride fiber to obtain mid-infrared all-fiber SFS by the same way.Both ends of the gain fiber are cut at 12°to reduce the threshold of laser self-excited oscillation and increase the output power of mid-infrared SFS.When the output is measured,the output is collimated through a calcium fluoride lens at first and then filtered through a 2.4μm long wave filter(LF)to remove the residual pump light at 976 nm and SFS near 1550 nm to obtain pure mid-infrared SFS(Fig.2).Both forward and backward output power and spectrum are measured.Results and Discussions Optimized tapering parameters of 5 cm taper length,1.5 cm waist length and 15μm waist diameter has been chosen.At the maximum pump power of 87.5 W,the output power of the combiner reaches 71.3 W,and the highest hot spot of combiner reaches 105℃.The corresponding coupling efficiency and the maximum pump power are up to 82.3%and 87.5 W,respectively.By fabricating side-pumping combiner on the Er^(3+)-doped double clad fluoride fiber directly,the generation of all-fiber mid-infrared SFS source is achieved.The mid-infrared SFS power sum is 91.09 mW(backward output of 53.67 mW,forward output of 37.42 mW),and the output spectrum ranges from 2702 nm to 2830 nm.The maximum 20 dB bandwidth reaches 108 nm when SFS power is 33.03 mW.This proposed scheme overcomes the problems of spatial pump’s high complexity and difficult adjustment,and is of great significance for further power amplification of mid-infrared SFS.However,the gain fiber used in this paper has a low doping concentration and a short fiber length,which limits the improvement of the output power and efficiency of mid-infrared SFS.In the future,the power can be further improved by increasing the doping concentration of fiber to alleviate the self-terminating phenomenon of Er^(3+)level and optimizing the length of gain fiber to improve the absorption efficiency of pump laser.Conclusions This paper reports the development of mid-infrared side-pumping combiner and all-fiber SFS source.Based on the side-coupling principle of tapered fiber,a mid-infrared side-pumping combiner is developed on the passive double-clad fluoride fiber with a cladding diameter of 125μm.The coupling efficiency of the combiner is up to 82.3%,and the maximum available pump power is 87.5 W.By directly fabricating the combiner on the gain fiber,this paper realizes the generation of all-fiber mid-infrared SFS source for the first time.The maximum power sum of mid-infrared SFS output forward and backward is 91.09 mW(backward output of 53.67 mW,forward output of 37.42 mW),and the output spectrum ranges from 2702 nm to 2830 nm.When the total output power of mid-infrared SFS is 33.03 mW,the maximum bandwidth of 20 dB at 108 nm is obtained.The mid-infrared side-pumping combiner and the all-fiber SFS source developed in this paper can not only improve the compactness and reliability of the mid-infrared SFS source,but also provide a good solution for further power amplification of the mid-infrared SFS source.